Industrial Hygiene

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The current OSHA permissible exposure limit (PEL) for nuisance dust is 15 mg/m3 as an 8-hour time-weighted average (TWA) exposure. The ACGIH has published a threshold limit value (TLV) for inhalable dust of 10 mg/m3. OSHA is required to consider economic and feasibility influences as well as health impacts for establishing or updating PELs. The ACGIH can consider only the health impacts when establishing TLVs. Discuss the extent to which you believe economic and feasibility impacts should be considered when establishing an occupational exposure limit (OEL).

Evaluation of Diesel Exhaust Continuous Monitors in Controlled Environmental Conditions

Chang Ho Yu,

Allison P. Patton,

Andrew Zhang,

Zhi-Hua (Tina) Fan,

Clifford P. Weisel &

Paul J. Lioy

Abstract

Diesel exhaust (DE) contains a variety of toxic air pollutants, including diesel particulate matter (DPM) and gaseous contaminants (e.g., carbon monoxide (CO)). DPM is dominated by fine (PM2.5) and ultrafine particles (UFP), and can be representatively determined by its thermal-optical refractory as elemental carbon (EC) or light-absorbing characteristics as black carbon (BC). The currently accepted reference method for sampling and analysis of occupational exposure to DPM is the National Institute for Occupational Safety and Health (NIOSH) Method 5040. However, this method cannot provide in-situ short-term measurements of DPM. Thus, real-time monitors are gaining attention to better examine DE exposures in occupational settings. However, real-time monitors are subject to changing environmental conditions. Field measurements have reported interferences in optical sensors and subsequent real-time readings, under conditions of high humidity and abrupt temperature changes. To begin dealing with these issues, we completed a controlled study to evaluate five real-time monitors: Airtec real-time DPM/EC Monitor, TSI SidePak Personal Aerosol Monitor AM510 (PM2.5), TSI Condensation Particle Counter 3007, microAeth AE51 BC Aethalometer, and Langan T15n CO Measurer. Tests were conducted under different temperatures (55, 70, and 80°F), relative humidity (10, 40, and 80%), and DPM concentrations (50 and 200 μg/m3) in a controlled exposure facility. The 2-hr averaged EC measurements from the Airtec instrument showed relatively good agreement with NIOSH Method 5040 (R2 = 0.84; slope = 1.17±0.06; N = 27) and reported ∼17% higher EC concentrations than the NIOSH reference method. Temperature, relative humidity, and DPM levels did not significantly affect relative differences in 2-hr averaged EC concentrations obtained by the Airtec instrument vs. the NIOSH method (p < 0.05). Multiple linear regression analyses, based on 1-min averaged data, suggested combined effects of up to 5% from relative humidity and temperature on real-time measurements. The overall deviations of these real-time monitors from the NIOSH method results were ≤20%. However, simultaneous monitoring of temperature and relative humidity is recommended in field investigations to understand and correct for environmental impacts on real-time monitoring data.

Sampling efficiency of modified 37-mm sampling cassettes using computational fluid dynamics

T.Renée Anthony,

Darrah Sleeth &

John Volckens

Pages 148-158 | Accepted author version posted online: 29 Oct 2015, Published online:08 Jan 2016

ABSTRACT

In the U.S., most industrial hygiene practitioners continue to rely on the closed-face cassette (CFC) to assess worker exposures to hazardous dusts, primarily because ease of use, cost, and familiarity. However, mass concentrations measured with this classic sampler underestimate exposures to larger particles throughout the inhalable particulate mass (IPM) size range (up to aerodynamic diameters of 100 μm). To investigate whether the current 37-mm inlet cap can be redesigned to better meet the IPM sampling criterion, computational fluid dynamics (CFD) models were developed, and particle sampling efficiencies associated with various modifications to the CFC inlet cap were determined. Simulations of fluid flow (standard k-epsilon turbulent model) and particle transport (laminar trajectories, 1–116 μm) were conducted using sampling flow rates of 10 L min−1 in slow moving air (0.2 m s−1) in the facing-the-wind orientation. Combinations of seven inlet shapes and three inlet diameters were evaluated as candidates to replace the current 37-mm inlet cap. For a given inlet geometry, differences in sampler efficiency between inlet diameters averaged less than 1% for particles through 100 μm, but the largest opening was found to increase the efficiency for the 116 μm particles by 14% for the flat inlet cap. A substantial reduction in sampler efficiency was identified for sampler inlets with side walls extending beyond the dimension of the external lip of the current 37-mm CFC. The inlet cap based on the 37-mm CFC dimensions with an expanded 15-mm entry provided the best agreement with facing-the-wind human aspiration efficiency. The sampler efficiency was increased with a flat entry or with a thin central lip adjacent to the new enlarged entry. This work provides a substantial body of sampling efficiency estimates as a function of particle size and inlet geometry for personal aerosol samplers.

Unit Homework Assignment

Welding fumes are a common occupational exposure. Several different welding fumes can cause similar adverse
health effects. Personal sampling of a welding operation at a manufacturing facility produced the following 8-hour
time-weighted average (TWA) results for individual metal fumes.

Metal Fume Result OSHA PEL ACGIH TLV
Antimony 0.05 mg/m³ 0.5 mg/m³ 0.5 mg/m³
Beryllium 0.00001 mg/m³ 0.002 mg/m³ 0.00005 mg/m³ (I)
Cadmium 0.025 mg/m³ 0.1 mg/m³ 0.01 mg/m³
Chromium 0.02 mg/m³ 1 mg/m³ 0.5 mg/m³

Copper 0.0

mg/m³ 0.1 mg/m³ 0.2 mg/m³
Iron Oxide 0.5 mg/m³ 10 mg/m³ 5 mg/m³ (R)

Magnesium Oxide 0.02 mg/m³ 15 mg/m³ 10 mg/m³
Molybdenum 0.003 mg/m³ 15 mg/m³ 10 mg/m³ (I)

Nickel 0.25 mg/m³ 1 mg/m³ 1.5 mg/m³ (I)
Zinc Oxide 0.3 mg/m³ 5 mg/m³ 2 mg/m³ (R)

(R) Respirable fraction (I) Inhalable fraction

Briefly summarize the primary health effects associated with overexposure to each type of metal fume, including both acute
and chronic health effects. Explain what analytical methods you would use for evaluating health hazards in the workplace.

Identify the types of metal fumes that would produce similar health effects on an exposed worker. Calculate the equivalent
exposure (in relation to OSHA PELS) for the metal fumes with similar health effects based on the “Result” column in the
table above. Discuss whether you believe any of the individual metal fume exposures or the combined exposure exceeds an
OSHA PEL or an ACGIH TLV.

Your homework assignment should be a minimum of two pages in length.

Information about accessing the grading rubric for this assignment is provided below.

http://www.cdc.gov/niosh/docs/2003-154/pdfs/2553

roscoearmstrong
Cross-Out

eration and Behavior
of Airborne Particles (Aerosols)

Paul Baron

Division of Applied Technology

National Institute for Occupational Safety and Health

Centers for Disease Control and Prevention

Overview
I. Particle size range

II. Inhalation & lung deposition

III. Particle behavior
– Settling, impaction, electrostatic

effects

IV. Particle generation
– Energy input, size, charge, humidity

Scenarios
• Letter release

• Carpet release

VI. Particle collection and measurement

What is an AEROSOL?

• Simply defined

–

tiny particles
or droplets suspended in air.

• The haze in the picture on the
right is caused by light
scattering from numerous
water/oil droplets and mineral
particles released into the air
from the drilling of rock.

Are Aerosols dangerous?

• The air we breathe always contains solid particles
or droplets and is therefore an aerosol.

• These aerosol particles can be from natural
sources or man-made sources

• Sometimes the particles are of type that, at
sufficient concentration, are toxic to our body.

• The organ in our body most sensitive to particle
exposure is the respiratory system

Toxic Aerosols!?
Our respiratory system is efficient at removing aerosols, but if they fall
within particular size ranges, are highly concentrated, or toxic, they may
cause adverse health effects. They may also deposit on skin or eyes,
generally only causing irritation, though more toxic effects may occur.
Very small particles may pass through the skin and enter the body that
way. Soluble particles may dissolve and pass through the skin.

Read on for more details on aerosol generation and behavior

Overall Scenario:
Evaluation of Exposure in Workplaces

Aerosol Transport
Based on Air Flow

Aerosol Sampling/
Measurement

Aerosol Inhalation

Aerosol
Generation from,

e.g., Grinding

Aerosol
Losses

Surfaces

Secondary
Sources

(Resuspension)

Loss Mechanisms
Settling,

Diffusion

,
Impaction,

Electrostatic
Deposition

Filter
Samplers

Direct
Reading
Instruments

Aerosol Assessment in the
Workplace: Types of Measurements

• Sampling, usually with a filter and pump, provides
a sample that can be analyzed in the lab for
specific chemicals, quantity of dust, particle shape
(fibers), etc.

• Direct reading

instrument

s allow continuous
observation of dust concentrations, e.g., mass or
concentration or size distribution, but do not
usually provide specifics of the aerosol type.

Aerosol Assessment in the
Workplace: Types of Measurements

• The most accurate assessment of worker’s
exposure is measurement with a personal sampler,
i.e, a collection or measurement device placed on
the worker’s chest.

• Techniques for control of exposures can use either
personal samplers or (fixed) area measurement
devices. Direct reading devices allow rapid
assessment of the effectiveness of dust control
devices or strategies.

I. Aerosol Size Range
Particle size is often determined by the process that generated the particle.
Combustion particles usually start out in the 0.01-0.05 m size range, but combine
with each other (agglomerate) to form larger particles. Powder is broken down
into smaller particles and released into the air; it is difficult to break down such
particles smaller than ~0.5 m. Biological particles usually become airborne from
liquid or powder forms, so these particles are usually larger than ~0.5 m.

Viruses

Bacteria

Spores

Mechanical Generation
(dust or mist)

Combustion Particles
(fume)

0.001 10 10

Particle Diameter ( m)

0.01 0.1 1

II. Respiratory System Deposition
ICRP Model, averaged
over males, females,
several respiration rates

Particle Diameter ( m)
0

0.6

0.8

1.0

0.4

0.2

Total

Head Airways

Alveolar

Alveolar
(Gas exchange)

Tracheo

–

bronchial

Tracheo-
bronchial

0.001 0.01 1010.1 100

D
ep

os
it

io
n

F
ra

ct
io

III. Aerosol Particle Behavior

• Settling

•

Impaction

• Charge effects

• Release from surfaces

• Agglomeration/
Deagglomeration

Particle Settling in Still Air

Time to settle 5 feet by unit density spheres

0.5 m 1 m 10 m 100 m3 m

41 hours 12 hours 1.5 hours
8.2 minutes

Aerodynamic diameter definition:
diameter of a unit density sphere that
settles at the same velocity as the particle
in question

5.8 seconds

Particle Settling in a Closed Room
Stagnant air Turbulent air

Time

Conc. Conc.

Time
Particles of the same size will settle
at the same speed in still or
stagnant air

Particles passing close to a
horizontal surface can settle, but
the rest will continue to be stirred.

Concentration
profiles using a

direct
measurement

instrument

Particle Settling in Turbulent Air

Half-life of particles in 8 foot high room

0.5 m 1 m 10 m 100 m3 m
41 hours 12 hours 1.5 hours
8.2 minutes

Particles settling in turbulent air will
have an exponential decay rate as
indicated in the previous slide 5.8 seconds

Particle Transport in Buildings

• Most large particle losses by settling
• Most small particle losses by exchange

with outdoor air
• Complex flow systems
• Turbulence production

Doors, people, fans. ventilation

Ventilation system

III. Aerosol Particle Behavior
• Settling
• Impaction
• Charge effects
• Release from surfaces
• Agglomeration/
Deagglomeration

Particle Impaction

• Impaction depends on
particle size, air
velocity, jet diameter

• Large particles deposit
more easily

• Even larger particles
can bounce from

surface

• Impaction surface can
be modified to improve
collection, e.g., add oil

Cascade Impactors

Pump

• Used for size distribution
measurement

• Commercial impactors
– Andersen

– MOUDI

Filter
>1

Virtual Impactors

• Used to reduce
particle bounce

• Used to concentrate
larger particles
– Commercial virtual

impactor up to 100:

– Contains smaller

particles in minor
flow

0.9Q0

0.1Q0

III. Aerosol Particle Behavior
• Settling
• Impaction
• Charge effects
• Release from surfaces
• Agglomeration/
Deagglomeration

Electrostatic Effects

+
+
+
+
+
+
+

+
–

• Particle-particle interaction small
• Particle-surface interaction large
• Particle charge depends mostly on

generation process, surface energy,
humidity, time in the air

• Airborne particle charge gradually
decreases due to ions in air
(particles are nearly neutral after
about 30 min)

Particle Charge Imparted During
Generation—Liquid Droplets

• In conductive solution,
ions equally distributed

• In nonconductive
solution, fewer ions

• Droplet charge
generally low

• When liquid
evaporates, the final
particle may have
relatively high charge –

+
–
–
+

–

Particle Charge Imparted During
Generation—Solid Particles

• Difference in surface
energy levels

• Separation energy

• Humidity creates
bridge between
particle and surface

+ + + + + + + +
–

Space Charge
Expansion of Aerosol

– High aerosol concentration

– Particles are highly charged

– All particles have same polarity

– Aerosol will expand because of particle-particle
repulsion

III. Aerosol Particle Behavior
• Settling
• Impaction
• Charge effects
• Release from surfaces
• Agglomeration/
Deagglomeration

Generation from Carpet

• Particles deposited in
carpet; acts as a sink

• Footstep crushes
fibers against each
other

• Footstep compresses
carpet, creating high
velocity air flow

Particle Transport from Sources

Small particles
through
ventilation
system Transport by

local turbulence

Direct settling
(larger particles
and clumps

Resuspension
by activity

Asbestos Fiber Generation
Effect of humidity on particle charge

and particle generation efficiency

Mean
Electrical
Mobility

Relative
Concentration

0 10 20 30
Relative Humidity (%)

Particle Removal from Surfaces
by Air Flow

• Boundary layer near
surface—produced
by motionless
surface

• Factors affecting
release: Air velocity,
particle attraction to
surface versus
particle cross
section

• Water (humidity)
can increase
adhesion

< 0.1 m virtually impossible > 20 m relatively easy

Vacuum Removal

• Suction
forces air
near surface
to remove
particles

• Variable
removal
efficiency

III. Aerosol Particle Behavior
• Settling
• Impaction
• Charge effects
• Release from surfaces
• Agglomeration/
Deagglomeration

Agglomeration/ Deagglomeration

• Particles in a powder are in close
contact, primarily agglomerates

• Shaken powder releases clumps
(agglomerates) and single particles

• Shear forces, caused by difference
in air velocity across the particle,
can break apart clumps

• Shear forces increase with
increasing energy (air velocity)

Particle Size Evolution

105

10-3

10-1

103

C
on

ce
nt

ra
ti

on
(

#/
cm

3 )

Particle Diameter ( m)

Grinding aerosol
T = 0

T = 25 min

T = 225 min
Settling/

Impaction

Diffusion

Coagulation (high conc.)

Surfaces
Surfaces

0.001 0.01 0.1 1 10 100

IV. Aerosol Generation

< 0.1 m virtually impossible > 20 m relatively easy

Energy Input
Air flow
Mechanical energy

Overcome adhesion
between particle and

surface

Airflow to entrain
particles

Release happens
in microseconds

Adhesion depends
mostly on micro-
roughness of
surface, also on
relative surface
energies

V. Particle Collection and Measurement

• Filter sampling
– Filter efficiency, pore size, filter type
– Sufficient volume for analysis
– Dries particles because of continuous air flow
– Removal from filter can be an issue

• Impactor sampling
– Cascade impactor: 3 to 8 stages, size resolution
– Sufficient volume for analysis
– Dries particles, though less than filter
– Inert (oiled?) surface or direct to growth medium

Filtration
• Air filtration different

from liquid filtration
• Pore size in air filters

generally meaningless as
indicator of efficiency

• Small particles collected
by diffusion, large ones
by impaction/interception

• Maximum penetration at
about 0.3 m

• Efficiency increases with
increasing air velocity

Particle diameter ( m)
0.01 0.1 1 10

E
ff

ic
ie

nc
y

Impaction/
Interception

Diffusion

Direct Reading
Aerosol Measurement

Optical particle counter
– Relatively inexpensive (~2K – $10K)
– Portable, battery operated
– Rapid detection
– Nonspecific for bacteria
– Toxic concentrations near or below

ambient particle concentrations
– Can be used for tracer studies

Direct Reading
Aerosol Measurement

Aerodynamic Particle Sizer
– Relatively expensive (~$40K)
– Movable, line operated
– Higher size resolution, possibly improved size

distribution signature
– Fluorescent detection version
– Can be used for tracer studies

Overall Scenario
Aerosol Sampling/
MeasurementAerosol Transport

Aerosol
Source

Characteristics

Aerosol
Losses

to Surfaces

Aerosol Inhalation
Secondary
Sources
(Resuspension)

Resources for Aerosol
Information

• Hinds, 1999, Aerosol Technology, Wiley

• Baron and Willeke, 2001, Aerosol Measurement,
Wiley

• Hurst, 1997, Manual of Environmental
Microbiology, ASM Press

• Spreadsheet: Aerosol Calculator available from
www.tsi.com or www.bgiusa.com

Brown et al. Particle and Fibre Toxicology 2013, 10:12
http://www.particleandfibretoxicology.com/content/10/1/12

RESEARCH Open Access

Thoracic and respirable particle definitions for
human health risk assessment
James S Brown1*, Terry Gordon2, Owen Price3 and Bahman Asgharian4

Abstract

Background

: Particle size-selective sampling refers to the collection of particles of varying sizes that potentially
reach and adversely affect specific regions of the respiratory tract. Thoracic and respirable fractions are defined as
the fraction of inhaled particles capable of passing beyond the larynx and ciliated airways, respectively, during
inhalation. In an attempt to afford greater protection to exposed individuals, current size-selective sampling criteria
overestimate the population means of particle penetration into regions of the lower respiratory tract. The purpose
of our analyses was to provide estimates of the thoracic and respirable fractions for adults and children during
typical activities with both nasal and oral inhalation, that may be used in the design of experimental studies and
interpretation of health effects evidence.

Methods

: We estimated the fraction of inhaled particles (0.5-20 μm aerodynamic diameter) penetrating beyond the
larynx (based on experimental data) and ciliated airways (based on a mathematical model) for an adult male, adult
female, and a 10 yr old child during typical daily activities and breathing patterns.

Results

: Our estimates show less penetration of coarse particulate matter into the thoracic and gas exchange regions
of the respiratory tract than current size-selective criteria. Of the parameters we evaluated, particle penetration into the
lower respiratory tract was most dependent on route of breathing. For typical activity levels and breathing habits, we
estimated a 50% cut-size for the thoracic fraction at an aerodynamic diameter of around 3 μm in adults and 5 μm in
children, whereas current ambient and occupational criteria suggest a 50% cut-size of 10 μm.

Conclusion

s: By design, current size-selective sample criteria overestimate the mass of particles generally expected to
penetrate into the lower respiratory tract to provide protection for individuals who may breathe orally. We provide
estimates of thoracic and respirable fractions for a variety of breathing habits and activities that may benefit the design
of experimental studies and interpretation of particle size-specific health effects.

Keywords: Size-selective sampling, Fine and coarse particles

Background
It has long been recognized that the regional pattern of
particle deposition in the respiratory tract affects the
pathogenic potential of inhaled aerosols. For example,
Morgan [1] concluded that respirable dusts likely caused
pneumoconiosis and silicosis in coal miners, whereas a lar-
ger size fraction caused bronchitis and obstructive changes
in pulmonary function. Sampling the total air concentra-
tion of particulate matter (PM) provides a crude estimate
of exposure that may not correlate with observed health

* Correspondence: Brown.James@epa.gov
1National Center for Environmental Assessment, U.S. Environmental
Protection Agency, MD B243-01, Research Triangle Park, Raleigh, NC 27711,
USA
Full list of author information is available at the end of the article

© 2013 Brown et al.; licensee BioMed Central
Commons Attribution License (http://creativec
reproduction in any medium, provided the or

effects if the risk is associated only with those particles that
may enter the thorax or penetrate beyond the ciliated air-
ways. The concept of size-selective particle sampling has
been employed as a means for effectively sampling the
particle sizes associated with specific pathologic outcomes
(e.g., the respirable fraction with parenchymal disease). If
an environmentally or occupationally related particle is
recognized to only affect the gas-exchange region of the
lung, then a sampling strategy that only collects the respir-
able fraction of airborne PM is preferable to sampling total
suspended particulate (TSP) or the thoracic fraction.
The human respiratory tract can be divided into three

main regions based on size, structure, and function,
namely, the head, tracheobronchial region (also known as

Ltd. This is an Open Access article distributed under the terms of the Creative
ommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and
iginal work is properly cited.

mailto:Brown.James@epa.gov

http://creativecommons.org/licenses/by/2.0

Figure 1 Thoracic particulate mass fraction criteria (relative to
total airborne particles) for size-selective sampling. Individual
data points are observed human head penetration efficiency during
oral inhalation for an inspiratory flow rate of 43.5 L/min, i.e., light
exercise [5-7]. As stated by ACGIH [3], the sampling criterion is offset
to the right of experimental data to overestimate the amount of
exposure to the lower respiratory tract, i.e., the lungs, and
correspondingly to provide a greater level of protection for exposed
workers. From ACGIHW, Particle Size-Selective Sampling in the
Workplace, Report of the ACGIHW Technical Committee on Air
Sampling Procedures. Copyright 1985. Reprinted with permission.
Courtesy: Dr. Otto G. Raabe.

Brown et al. Particle and Fibre Toxicology 2013, 10:12 Page 2 of 12
http://www.particleandfibretoxicology.com/content/10/1/12

the conducting airways), and the gas-exchange region (also
known as the parenchymal, alveolar, or pulmonary) region.
Size-selective sampling is intended to help discern the
amount of aerosol expected to be available for deposition
in a region. Most sampling conventions have been defined
in terms of particle penetration into respiratory regions ra-
ther than the expected particle deposition or dose to re-
gions. Specific definitions used herein, adopted from the
European Committee for Standardization (CEN), are [2]:

� Inhalable fraction – the mass fraction of total
airborne particles which is inhaled through the nose
and mouth.

� Extrathoracic fraction – the mass fraction of inhaled
particles failing to penetrate beyond the larynx.

� Thoracic fraction – the mass fraction of inhaled
particles penetrating beyond the larynx.

� Respirable fraction – the mass fraction of inhaled
particles penetrating to the unciliated airways.a

The above definitions are stated in terms of a mass
fraction. Relative to total airborne particles, the particle
size having 50% penetration for the thoracic and respir-
able fractions are 10 μm and 4.0 μm (all particle sizes
are aerodynamic diameter unless expressed otherwise),
respectively [2,3]. These criteria were specifically devel-
oped for workplace atmospheres. Since particles must
generally become deposited to exert biological effects,
these conventions, based on regional exposure (i.e., par-
ticles penetrating into a region of the respiratory tract),
are conservative by design in that they overestimate the
amount of inhaled material that becomes deposited and
thereby available to induce an effect.
In 1985, the American Conference of Governmental In-

dustrial Hygienists (ACGIH) recommended particle size-
selective sampling in setting threshold limit values for occu-
pational exposures [4].b The ACGIH specifically considered
a reference worker (weight, 70 kg; height, 175 cm) breath-
ing orally while engaged in light activity (minute ventilation,
21.75 liters/min). Criteria were established for Inspirable
(now Inhalable), Thoracic, and Respirable Particulate Mass
that were intended to be protective against materials that
were considered hazardous when deposited anywhere in
the respiratory tract, anywhere within the lungs, and in the
gas-exchange region, respectively. These criteria were based
on exposure of a respiratory tract region (based on particle
penetration into that region), not particle deposition in a re-
spiratory tract region. The ACGIH committee recognized
uncertainty related to individual biological variability in re-
spiratory health status, breathing patterns (rate and route),
and airways structure as well as differences in work rates,
all of which can cause differences in inhaled aerosol depos-
ition and dose. Facing these uncertainties, the committee
afforded extra protection to exposed workers by over

representing the true penetration of particles into regions of
the respiratory tract as illustrated in Figure 1 [4].
Size-selective sampling has also been employed by the U.

S. Environmental Protection Agency (EPA) in setting the
national ambient air quality standards (NAAQS) for par-
ticulate matter (PM). In 1987, the EPA changed the indica-
tor for PM from TSP (effectively an aerodynamic cut-size
varying from 25 to 40 μm, depending on wind speed and
direction) to PM10 (particles with a nominal mean aero-
dynamic diameter ≤ 10 μm) [8]. Consistent in concept with
the ACGIH thoracic particle fraction, PM10 delineates a
subset of inhalable particles (referred to as thoracic parti-
cles) that are thought small enough to penetrate to the
thoracic region (including the tracheobronchial and alveolar
regions) of the respiratory tract.c In 1997, the EPA extended
size-selective sampling to include fine particles indicated by
PM2.5 (particles with a nominal mean aerodynamic

Brown et al. Particle and Fibre Toxicology 2013, 10:12 Page 3 of 12
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diameter ≤ 2.5 μm) and retained PM10 as the indicator for
thoracic coarse particles [9]. The selection of PM2.5 by the
EPA was mainly to delineate the atmospheric fine (combus-
tion derived, aggregates, acid condensates, secondary aero-
sols) and coarse (crustal, soil-derived dusts) PM modes and
for consistency with community epidemiologic health stud-
ies reporting various health effects associated with PM2.5.
With consideration to the PM NAAQS, Miller et al. [10]
also specifically recommended a particle size cut-point of ≤
2.5 μm as an indicator for fine PM based on consideration
of particle penetration into the gas-exchange region and
the delineation of the fine and coarse particle modes.
Most recently, the International Organization for Stan-

dardization (ISO) has released recommendations for sam-
pling conventions based on particle deposition (rather
than exposure) in adult males and females engaged in ac-
tivities of sitting, light exercise and heavy exercise as speci-
fied in Table 1 [11]. The ISO estimates of deposition were
determined using the International Commission on Radio-
logical Protection (ICRP) human respiratory tract model
[12]. These new ISO conventions [11] are not considered
further herein as current sampling conventions for occu-
pational and non-occupational settings remain dependent
on the probability of particle penetration rather than de-
position in specific regions of the respiratory tract.
Conceptually, size-selective sampling better characterizes

PM exposure to regions of the respiratory tract and thereby
affords more appropriate avenues for protection of exposed
populations than TSP. Such a simple concept is not, how-
ever, without ambiguity in definitions and debate over ap-
propriate sampling approaches. For example, the definition
for the thoracic fraction specifies particles “penetrating

Table 1 Ventilatory and activity patterns for adult males,
adult females, and a ten year-old child

Sleeping Sitting Light Heavy

Exercise Exercise

Adult Male VT (mL) 625 750 1250 1920

Sedentary worker f (min-1) 12 12 20 26

t (hr) 8.5 5.5 9.75 0.25

Vdaily (L/day) 3825 2970 14625 749

Adult Female VT (mL) 444 464 992 1364

Sedentary worker f (min-1) 12 14 21 33

t (hr) 8.5 5.5 9.75 0.25

Vdaily (L/day) 2717 2144 12187 675

Child (10 yrs) VT (mL) 304 333 583 752

Male or Female f (min-1) 17 19 32 45

t (hr) 10 4.67 9.33 0

Vdaily (L/day) 3101 1772 10447 0

VT, tidal volume; f, breathing frequency; t, time spent engaged in specific
activity; Vdaily, total volume inspired in 24 hr. Data are from ICRP [12] Tables
B15 and B16A-B for breathing and activity patterns, respectively.

beyond the larynx,” whereas the ACGIH thoracic conven-
tion for sampling (Figure 1) clearly and intentionally overes-
timates the fraction of large particles penetrating into the
thoracic region to afford extra protection of occupationally
exposed individuals. The purpose of this paper is to provide
realistic estimates of thoracic and respirable particle frac-
tions for adults and children that may be used in the design
of experimental studies and interpretation of health effects
evidence.
The ICRP human respiratory tract model [12] was used

to estimate particle penetration through the extrathoracic
(ET) airways. The ICRP predictive equations for ET depos-
ition are based on experimental measurements in humans.
Although also based on human data, the ICRP model was
not used to estimate penetration through the tracheobron-
chial (TB) airways due to its reliance on measurements of
particle clearance from the TB airways and the ability to
target particle deposition into the ciliated airways. That is,
much of the available regional deposition data for the TB
and alveolar regions have been obtained from experiments
with radioactively labeled, poorly soluble particles or by use
of aerosol bolus techniques (see Sections D.9.2 and E.5.3 of
Ref [12]). Aerosol bolus (40 ml volume of 3.5 μm particles)
inhaled to a very shallow lung volume (70 ml, ~75% of
phase I inert gas washout) by healthy adults (10 M, 6 F;
20-43 yrs of age) show preferential left lung deposition and
23% retention at 48 hrs [13]. This suggests slow TB airway
clearance and/or some penetration into the alveolar region.
Given the above, coupled with uncertainty related to slow
TB clearance [e.g., 14,15], we utilized the publicly available
multiple path particle dosimetry (MPPD; ver 2.1, © 2009)
model to estimate penetration through the TB airways.

Methods
Once particles have entered the respiratory tract via the
nose or mouth, the primary factors affecting particle pene-
tration into the lower respiratory tract (i.e., beyond the lar-
ynx) are airways size and structure, breathing pattern (flow
and volume), route of breathing (nose vs. mouth), and in-
haled particle size. With regard to particle size, we have
considered particles whose deposition is governed by their
inertial properties, i.e., ≥ 0.5 μm. Breathing patterns vary
mainly by sex, age, and activity. Table 1 provides the breath-
ing patterns, subject groups, and activity patterns from the
ICRP [12] model that were used in our assessment.
Based on our comparison of the ICRP model [12] to

more recent data provided by Brochu et al. [16], the daily
ventilation rates and activity patterns provided in Table 1
overestimate typical daily ventilation rates. Table 2 pro-
vides daily ventilation rates (5th, 50th, and 95th percentiles)
reported by Brochu et al. [16]. The daily ventilation rates
from the ICRP [12] model’s recommended time budget
roughly correspond to the highly active 95th percentile
(see Table 1 vs. Table 2). To assess the effect of daily

Table 2 Daily ventilation rates (Vdaily).
a

Vdaily (m
3/day)

Very sedentary Median Highly active

5th percentile 50th percentile 95th percentile

Male b 12.86 17.48 22.11

Female b 9.91 13.67 17.42

Children c 7.20 10.22 13.24
a Data are from Table 2 of Brochu et al. [16]; b values for age range of 23 to <30 years; c average of male and female values for age range of 7 to <11 years.

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activity/ventilation rates on particle penetration into the
respiratory tract, we reduced the estimated time (given in
Table 1) that individuals spent engaged in light exercise
and correspondingly increased their time sitting to match
the daily ventilation rates in Table 2. To match the very
sedentary 5th percentile of daily activity, it was necessary
also to eliminate time spent by the adult female engaged
in heavy exercise and to attribute all of the child’s activity
to sleeping. Even when considering 100% of the child’s ac-
tivity equivalent to sleeping, the ICRP [12] breathing pat-
tern slightly overestimated (by 3%; 7.4 vs. 7.2 m3/day) the
5th percentile daily ventilation rates of Brochu et al. [16].
Route of breathing varies with inspiratory flows and gener-

ally shifts from nasal to oronasal breathing at higher flows.
The ICRP [12] model characterizes breathing habit based on
Niinimaa et al. [17], who examined the route of breathing as
a function of activity in healthy adults (14 males, 16 females).
Eighty-seven percent of the subjects breathed through the
nose at rest and switched to oronasal breathing with exer-
cise. These subjects were referred to as “normal augmenters.”
Thirteen percent breathed oronasally even at rest and were
referred to as “mouth-breathers.” For both of these breath-
ing habits (i.e., normal augmenters and mouth-breathers,
we estimated the fraction of a breath passing through the
oral and nasal pathways from regression equations for oral
breathing in Figure 1 of the Niinimaa et al. study [17]. The
ICRP [12] model utilizes this same general approach, but
for each breathing habit and activity (i.e., sleeping, sitting,
etc.), the same fraction of oral breathing was assumed
applicable to all ages and both sexes.
In considering breathing habit, we differed from the

ICRP, in that we assumed the fraction of oral breathing to
differ between adult males and females as a function of
their minute ventilation rather than their activity level.
However, children tend to have a greater fraction of oral
breathing than adults at rest and during exercise [18,19].
Therefore, consistent with the ICRP, we assumed the frac-
tion of the breath inhaled through the mouth (Fm) in the
child engaged in some specific level of activity was equal to
that of the adult male engaged in the same level of activity
despite the dramatically lower ventilation rates of the child.
We also considered recent breathing habit data not avail-
able for inclusion in the ICRP model [12]. Bennett et al.

[19,20] show a more gradual increase in oronasal breathing
than did Niinimaa et al. [12]. In addition to the normal
augmenter and mouth-breather breathing habits based on
the Niinimaa et al. [17] study, we also considered the more
gradual onset of oronasal breathing observed in adults and
children by the Bennett et al. [19,20] studies, herein termed
as “gradual augmenters.”
The gradual augmenter breathing habit for children was

estimated by linear regression of the observed minute ven-
tilation and Fm at rest and at 40% maximum physical work
capacity from data in Table two and Figure two of Bennett
et al. [19] for 12 children (9 M, 3 F; 6-10 yrs of age). The
gradual augmenter breathing habit for adult males and fe-
males was estimated by linear regression of the observed
minute ventilation and Fm at rest and at 60% maximum
physical work capacity from data in Table two and
Figure three of Bennett et al. [20] for 22 adults (11 M, 11
F; mean age, 22 yrs). In the adult females, the fitted Fm was
zero for the activity of sitting and so was also set to zero
for the activity of sleep. Table 3 provides the Fm for all
breathing habits (normal augmenters, mouth-breathers,
and gradual augmenters) used in our simulations. In a
study of 37 subjects from 7-72 years of age, James et al.
[21] reported that 2 subjects (5.4%) breathed orally only.
With this finding in mind, we have also considered purely
oral breathing in our estimates of particle penetration into
regions of the lower respiratory tract.
For air passing through the mouth, deposition of large

particles by impaction occurs mainly at the larynx. From
Eq D.30 of ICRP [12], laryngeal deposition efficiency,
η(ET)larynx, is given by:

η ETð Þlarynx ¼ 1− 1:1 � 10−4 d2a Qtotal SF3
� �0:6

Vt SFt
3

� �−0:2h i1:4 þ 1
� �−1

ð1Þ
where: da is aerodynamic diameter (μm); Qtotal is total
inspiratory flow rate (mL/s); VT is tidal volume (mL); and
SFt is a scaling factor of 1.0 for adult males, 1.08 for adult
females, and 1.26 for ten year-old children from Table
fifteen of ICRP [12].
For nasal breathing, ET deposition efficiency due to

impaction was calculated from Eq. D.32 and D.33 of
ICRP [12]. The ET deposition efficiencies for the anter-
ior, η(ET1)nose, and posterior, η(ET2)nose, nasal regions
are given by:

η ET1ð Þnose ¼ 0:5 1− 3 � 10−4 d2a Qnose SFt3
� �

þ 1
� �−1n o

ð2Þ
η ET2ð Þnose ¼ 1− 5:5 � 10−5 d2a Qnose SFt3

� �1:17 þ 1
h i−1

ð3Þ
where: Qnose is the inspiratory flow (mL/s) through the
nose. The use of SFt in Equations 2 and 3 presumes that

Table 3 Partitioning of breaths through the mouth and
nose

Sleeping Sitting Light Heavy
Exercise Exercise

Adult

Male

Normal
Augmenter a

0, 1.00 d 0, 1.00 0, 1.00 0.52, 0.48

Mouth-
breather a

0.29, 0.71 0.36, 0.64 0.59, 0.41 0.66, 0.34

Gradual
Augmenter b

0.12, 0.88 0.13, 0.87 0.29, .071 0.54, 0.46

Adult

Female

Normal
Augmenter a

0, 1.00 0, 1.00 0, 1.00 0.50, 0.50

Mouth-
breather a

0.12, 0.88 0.23, 0.77 0.57, 0.43 0.65, 0.35

Gradual
Augmenter b

0, 1.00 0, 1.00 0.22, 0.78 0.59, 0.41

Child Normal
Augmenter a

0, 1.00 0, 1.00 0, 1.00 0.51, 0.49

Mouth-
breather a
0.29, 0.71 0.36, 0.64 0.59, 0.41 0.66, 0.34

Gradual
Augmenter c

0.29, 0.71 0.31, 0.69 0.51, 0.49 0.77, 0.23

a From regression equations for oral breathing in Figure 1 of Niinimaa et al.
[17]; b Data based on Table 2 and Figure 3 of Bennett et al. [20] for 22
individuals (11 M, 11 F; mean age, 22 yrs); c Data based on Table 2 and
Figure 2 of Bennett et al. [19] for 12 children (9 M, 3 F; 6-10 yrs of age); d

fraction inhaled through mouth, fraction inhaled through nose.

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nasal deposition efficiency increases with decreasing body
size and increasing nasal resistance. Two studies [19,22]
suggest that the nasal deposition in children is less than that
of adults. These two studies, not considered in the ICRP
model [12], suggest that it may be inappropriate to apply a
scaling factor for nasal deposition of children. Accordingly,
we estimated the nasal deposition efficiency of the 10 yr old
child for a SFt of both 1.0 (child-A) and 1.26 (child-B). Add-
itionally, we estimated the upper and lower 95% confidence
bounds for inter-individual variability attributable to differ-
ences in deposition efficiency within the ET region predicted
by Equations 1-3 as specified in paragraphs D44 and D68 of
ICRP [12].
The deposition efficiencies along the ET pathways (i.e.,

nasal and oral) were assumed to be independent. As such,
total ET deposition was taken to be the sum of deposition
between the pathways weighted by the flow partitioning (see
Paragraph 161 of ICRP [12]). The thoracic fraction, defined
as particle penetration past the larynx, P(ET), is given by:

P ETð Þ ¼ 1− Fm η ETð Þlarynx− 1−Fmð Þ
� η ET1ð Þnose þ 1− η ET1ð Þnose

� �
η ET2ð Þnose �

�
ð4Þ

We estimated inspiratory deposition efficiency in the
TB region, ηTB, of particles (0.5-20 μm; 0.1 μm incre-
ments) using the publicly available multiple path particle
dosimetry (MPPD; ver 2.1, © 2009) model.d The model
considers deposition by the mechanisms of impaction,
sedimentation, and diffusion. The approach and formula
used to calculate particle losses in the MPPD model are
described by Anjilvel and Asgharian [23]. Physiological
input parameters (namely, tidal volume [VT], breathing
frequency [f], functional residual capacity [FRC], and
upper respiratory tract volume [URT]), necessary for
MPPD simulations are provided in Tables 1 and 4. FRC
and URT for each group are from Table fifteen of ICRP
[12]. The Yeh and Schum [24] typical path whole lung
model was utilized and scaled for FRC and VT. The ef-
fects of these physiologic parameters on deposition in
humans free of respiratory disease are described by de
Winter-Sorkina and Cassee [25].
The respiratory fraction, defined as particle penetra-

tion through the ciliated airways of the TB region, P
(TB), is given by:

P TBð Þ ¼ ∫
Tinh
0 C1 dt

∫Tinh0 C0 dt
≅P ETð Þ 1−ηTBð Þ ð5Þ

where: C0 and C1 are particle concentration passing the
larynx and terminal bronchioles, respectively; and Tinh is
the time of inhalation. Since conducting airway particle
concentration is nearly constant during inhalation, respira-
tory fraction can be expressed in terms of TB deposition

efficiency as given above. An

Additional file

1: Appendix
to this paper provides estimates of P(TB) based on the
ICRP [12] model rather than the MPPD model.
After calculating P(ET) and P(TB) for all activities and

individual groups, daily average estimates of P(ET)avg
and P(TB)avg weighted by daily ventilation (see Tables 1
and 2) were calculated as a function of particle size.
Ventilation-weighted averages of P(ET)avg for each par-
ticle size were calculated as:

P ETð Þavg ¼
Xn
i¼1

P ETð Þi

V daily−i

� �

=
Xn
i¼1

V daily−i

ð6Þ
where: P(ET)i is the ET fraction for activity, i; Vdaily-i is the
daily volume inhaled while engaged in activity, i; and n is
the number of activities. Ventilation-weighted averages of
P(TB)avg were computed similarly to those of P(ET)avg.
Finally, thoracic and respiratory particle fractions were also

calculated after applying the ICRP [12] inhalability criterion
assuming no ambient wind, 1−0.5 [1− (0.00076 da

2.8 + 1) −1].
The ICRP [12] criterion was utilized as it better represents
the inhalation of particles <10 μm than the ACGIH and CEN [2,3] criterion.

Results
We estimated particle penetration fractions into the thorax
and respiratory region of an adult male, adult female, and a

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8 10

P
(E

T
)

da (µm)

Figure 2 Thoracic fraction, i.e., particle penetration through the
extrathoracic region, P(ET), as a function of breathing route.
Penetration data are with respect to particle diameter as a function
of the fraction of air inhaled through the mouth (Fm) in an adult
male engaged in light exercise relative to particles entering the
respiratory tract. Curves are for the Fm of 0.00, 0.25, 0.50, 0.75, and
1.00 as indicated on the figure. Horizontal red line highlights
50% penetration.

0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10

P
(T

B
)

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10 yr old child. In the results described below, data do not
consider particle inhalability unless specifically stated.
Inhalability was not considered since, as the results will
show, it has a minimal affect on 50% cut-points for particle
penetration into the lower airways for all breathing habits
except the less probable condition of oral breathing.

Route of breathing
Of the factors we considered, route of breathing (or breath-
ing habit) had the greatest effect on estimates of P(ET) and
P(TB). Figure 2 and 3 illustrate P(ET) and P(TB) for an
adult male engaged in light exercise as a function of Fm. In
Figure 2, fifty percent P(ET) occurs at the da of 1.8 , 2.7,
4.4, 6.1, and 7.0 μm for the Fm of 0.00, 0.25, 0.50, 0.75, and
1.00, respectively. Note that the curve in Figure 2 for Fm=1
is for comparable conditions to those for which ACGIH [4]
thoracic fraction was based, i.e. an orally breathing adult
male engaged in light exercise. In Figure 3, fifty percent P
(TB) occurs at the da of 1.7 , 2.5, 3.8, 5.1, and 5.7 μm for
the Fm of 0.00, 0.25, 0.50, 0.75, and 1.00, respectively. As
ventilation is shifted to the lower removal efficiency oral
passages, there is an ever greater separation between the P
(ET) and P(TB) curves. By contrast, for purely nasal breath-
ing (Fm = 0, the case for normal augmenters during light
exercise), due to the vast removal of particles in the nasal
airways, there is nearly no difference between the P(ET)
and P(TB) curves in Figures 2 and 3, respectively.
Table 5 provides the 50% cut-points for particle penetra-

tion into the thorax and respiratory region for all the
breathing habits we evaluated. As may be expected based
on Fm (see Table 3), the predicted particle penetration for
the gradual augmenter breathing habit is enveloped be-
tween that of the normal augmenter and mouth-breathers.
Additionally, consistent with Figures 2 and 3, Table 5
shows that the largest 50% cut-points are observed during
the case of oral breathing. Table 6 provides data on the
penetration of 10 μm particles into the lower respiratory
tract which is generally less than 20%, except for the case
of oral breathing where penetration into the thorax can
approach 40%.

Age and sex
Daily weighted penetrations curves for P(ET)avg and
P(TB)avg are illustrated in Figures 4 and 5, respectively.
For normal augmenters and oral breathing, 50% cut-
points for P(ET)avg and P(TB)avg were generally similar

Table 4 Functional residual volume (FRC) and upper
respiratory tract volumes (URT)

FRC (mL) URT (mL)

Adult Male 3300 50

Adult Female 2680 40

Child (10 yrs) 1484 25

between adult males and females, but shifted to slightly
(<0.2 μm) smaller particle sizes in the females (see Table 5). There was a larger (<0.8 μm) difference in 50% cut-points between males and females for the mouth-breather and gradual augmenter breathing habits which is attributable to greater nasal inhalation by females than males. The data for child-A are more consistent, than child-B,

with experimental data [19,22] showing a lower nasal par-
ticle removal efficiency in children than adults. For all
breathing habits except oral breathing in Table 5, child-A
shows larger 50% cut-points than adults. Additionally, in
Table 6, the penetration of 10 μm particles into the thor-
acic and respiratory regions of child-A is generally greater
than or equal to that of adults for all breathing habits other

da (µm)

Figure 3 Respirable fraction, i.e., particle penetration through
the tracheobronchial region, P(TB), as a function of breathing
route. Penetration data are with respect to particle diameter as a
function of the fraction of air inhaled through the mouth (Fm) in an
adult male engaged in light exercise relative to particles entering
the respiratory tract. Curves are for the Fm of 0.00, 0.25, 0.50, 0.75,
and 1.00 as indicated on the figure. Horizontal red line highlights
50% penetration.

Table 5 Particle penetration (50% cut-point) through respiratory tract regions relative to particles entering the
respiratory tract

Vdaily (%-tile)

Normal Gradual

Augmenter Mouth-breather Augmenter Oral only

P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg

Male

5% 2.94 a 2.74 5.15 4.32 3.60 3.25 9.00 6.67

95% CI (1.72–5.03) (1.65–4.34) (3.12–8.48) (2.84–6.10) (2.11–6.11) (2.00–4.98) (5.86–13.8) (5.00–8.14)

50% 2.46 2.30 5.09 4.30 3.37 3.05 8.09 6.11

95% CI (1.44–4.20) (1.38–3.71) (3.14–8.26) (2.86–5.88) (1.99–5.67) (1.88–4.65) (5.26–12.4) (4.58–7.34)

95% 2.14 2.03 5.08 4.31 3.19 2.90 7.61 5.89

95% CI (1.25–3.66) (1.20–3.32) (3.16–8.16) (2.89–5.83) (1.90–5.33) (1.79–4.44) (4.95–11.7) (4.38–7.07)

Female

5% 2.92 2.71 4.32 3.78 3.10 2.86 8.78 6.50

95% CI (1.71–5.01) (1.63–4.28) (2.57–7.22) (2.40–5.52) (1.81–5.30) (1.73–4.48) (5.71–13.5) (4.88–7.91)

50% 2.42 2.27 4.46 3.87 2.91 2.68 7.88 5.96

95% CI (1.41–4.13) (1.36–3.64) (2.72–7.32) (2.51–5.45) (1.71–4.94) (1.63–4.19) (5.13–12.1) (4.47–7.17)

95% 2.10 1.99 4.58 3.96 2.74 2.54 7.44 5.77

95% CI (1.23–3.60) (1.18–3.26) (2.82–7.42) (2.60–5.49) (1.61–4.64) (1.53–4.00) (4.84–11.4) (4.29–6.94)

Child-B b

5% 2.77 2.61 4.31 3.78 4.30 3.78 8.34 6.46

95% CI (1.62–4.75) (1.56–4.18) (2.58–7.19) (2.40–5.57) (2.57–7.17) (2.40–5.56) (5.42–12.8) (4.74–8.04)

50% 2.25 2.13 4.37 3.81 4.07 3.59 7.50 5.81

95% CI (1.31–3.85) (1.27–3.48) (2.67–7.15) (2.47–5.35) (2.46–6.71) (2.29–5.15) (4.88–11.5) (4.32–7.08)

95% 1.89 1.81 4.34 3.76 3.98 3.49 6.91 5.33

95% CI (1.11–3.24) (1.07–2.98) (2.69–7.03) (2.48–5.11) (2.45–6.47) (2.27–4.88) (4.49–10.6) (4.01–6.36)

Child-A c

5% 3.92 3.56 5.60 4.72 5.59 4.72 8.34 6.46

95% CI (2.29–6.72) (2.18–5.44) (3.38–9.28) (3.09–6.63) (3.37–9.27) (3.09–6.62) (5.42–12.8) (4.74–8.04)

50% 3.18 2.94 5.37 4.53 5.12 4.36 7.50 5.81

95% CI (1.86–5.45) (1.77–4.58) (3.29–8.77) (3.03–6.10) (3.11–8.40) (2.88–5.95) (4.88–11.5) (4.32–7.08)

95% 2.68 2.50 5.16 4.34 4.85 4.14 6.91 5.33

95% CI (1.56–4.58) (1.50–3.97) (3.19–8.34) (2.94–5.66) (2.98–7.90) (2.76–5.51) (4.49–10.6) (4.01–6.36)

P(ET)avg, extrathoracic particle penetration which is the thoracic particle fraction averaged across all activity levels weighted by daily ventilation; P(TB)avg,
tracheobronchial particle penetration which is the respirable particle fraction averaged across all activity levels weighted by daily ventilation; 95% CI, ninety-five
percent confidence intervals for inter-individual variability attributable to differences in particle penetration through the extrathoracic region; a Aerodynamic
particle diameter in μm; b Scaling factor in Equations 2 and 3 equal to 1.26; c Scaling factor in Equations 2 and 3 equal to 1.0.

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than oral. For oral breathing, the penetration of 10 μm par-
ticles is lower in child-A than adults.

Activity level
Impaction in the nasal airways, larynx, and large bronchi in-
creases in conjunction with activity and increasing inspira-
tory flows across the range of da. Therefore, decreasing 50%
cut-points for both P(ET)avg and P(TB)avg are observed with
increasing activity. In general, the penetration of 10 μm par-
ticles into the thoracic and respiratory regions also showed
a small reduction with increasing daily activity level (see
Table 6). However, the small reductions in particle

penetration pale in comparison to the large increases in
ventilation and intake of particles that occur with increasing
activity level.

Inhalability adjustment
The thoracic and respirable fraction data in Table 5 were
relative to particles entering the respiratory tract. For com-
parison, Table 7 provides da associated with 50% penetra-
tion into the thorax and respiration regions after applying
inhalability criterion. Adjusting for inhalability shifts pene-
tration curves to smaller particle sizes, and this effect is
most evident where there is a low activity level and a

Table 6 Penetration of 10 μm (da) through respiratory tract regions relative to particles entering the respiratory tract
Vdaily

(%-tile)
Normal Gradual

Augmentera Mouth-breathera Augmentera Oral onlya

P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg
Male

5% 0.05b 0.03 0.21 0.10 0.11 0.05 0.43 0.21

50% 0.04 0.02 0.19 0.08 0.11 0.04 0.36 0.15

95% 0.03 0.01 0.19 0.06 0.10 0.03 0.32 0.12

Female

5% 0.06 0.03 0.15 0.07 0.06 0.03 0.41 0.20

50% 0.04 0.02 0.15 0.06 0.07 0.02 0.35 0.14

95% 0.03 0.01 0.16 0.05 0.07 0.02 0.31 0.11

Child-B c

5% 0.05 0.02 0.16 0.08 0.16 0.08 0.38 0.18

50% 0.03 0.01 0.15 0.06 0.14 0.06 0.31 0.13

95% 0.02 0.01 0.14 0.04 0.13 0.04 0.27 0.09

Child-A d

5% 0.10 0.05 0.21 0.10 0.21 0.10 0.38 0.18

50% 0.07 0.04 0.19 0.09 0.18 0.07 0.31 0.13

95% 0.05 0.02 0.17 0.05 0.16 0.05 0.27 0.09

P(ET), extrathoracic penetration for 10 μm particles; P(TB), tracheobronchial penetration for 10 μm particles; aData are daily averages, all activity levels weighted by
daily ventilation; bFraction penetration; cScaling factor in Equations 2 and 3 equal to 1.26; dScaling factor in Equations 2 and 3 equal to 1.0.

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substantial contribution of oral breathing. Table 6 provides
data on the penetration of 10 μm particles into the lower
respiratory tract. Those penetration data may be adjusted
for inhalability by multiplying by 0.84, the inhalability of
10 μm particles.

Discussion

We calculated thoracic and respirable particle fractions
for an adult male, adult female, and ten year-old child

0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10
P
(E

T
) a

v
g

da (µm)

10 yr-old Child-A

10 yr-old Child-B

Adult Male

Adult Female

Figure 4 Thoracic fraction, i.e., particle penetration through the
extrathoracic region, P(ET)avg, in adults and a 10 yr-old child.
Data are daily averaged values for a median activity level, gradual
augmenter breathing habit, and uncorrected for particle inhalability.
Child-A and child-B are for a scaling factor of 1.0 and 1.26 in
Equations 2 and 3, respectively. Horizontal red line highlights 50%
penetration which occurs at 3.1 μm (adult female), 3.4 μm (adult
male), 4.1 μm (child-B), and 5.1 μm (child-A).

engaged in typical daily activities ranging from sleep to
heavy exercise. Our estimates are intended to represent
full-day ambient and/or non-ambient exposures while
individuals are engaged in a variety of activities. This dif-
fers from the ACGIH and CEN criteria which are
intended to represent a workplace setting [2,4]. Similarly,
considering the need to provide protection for sensitive in-
dividuals who may breathe by mouth and/or oronasally, the
EPA [8] selected the nominal cut-point of 10 μm as an

0.0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10

P
(T
B
) a

v
g
da (µm)
10 yr-old Child-A
10 yr-old Child-B
Adult Male
Adult Female

Figure 5 Respirable fraction, i.e., particle penetration through
the tracheobronchial region, P(TB)avg, in adults and a 10 yr-old
child. Data are daily averaged values for a median activity level,
gradual augmenter breathing habit, and uncorrected for particle
inhalability. Child-A and child-B are for a scaling factor of 1.0 and
1.26 in Equations 2 and 3, respectively. Horizontal red line highlights
50% penetration which occurs at 2.7 μm (adult female), 3.1 μm
(adult male), 3.6 μm (child-B), and 4.4 μm (child-A).

Table 7 Inhalability adjusted particle penetration (50% cut-point) through respiratory tract regions

Vdaily
(%-tile)

Normal Gradual
Augmenter Mouth-breather Augmenter Oral only
P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg P(ET)avg P(TB)avg
Male

5% 2.92 a 2.72 4.97 4.24 3.55 3.22 8.24 6.38

95% CI (1.71–4.90) (1.64–4.26) (3.09–7.66) (2.81–5.85) (2.11–5.82) (2.00–4.85) (5.66–11.4) (4.89–7.59)

50% 2.44 2.30 4.93 4.21 3.33 3.02 7.52 5.90

95% CI (1.43–4.12) (1.37–3.67) (3.11–7.51) (2.84–5.67) (1.99–5.43) (1.87–4.54) (5.12–10.5) (4.50–6.98)

95% 2.14 2.02 4.92 4.22 3.15 2.88 7.14 5.71

95% CI (1.25–3.62) (1.20–3.29) (3.13–7.44) (2.87–5.64) (1.89–5.15) (1.78–4.36) (4.84–10.1) (4.32–6.77)

Female

5% 2.90 2.70 4.22 3.72 3.08 2.85 8.06 6.23

95% CI (1.70–4.87) (1.63–4.20) (2.56–6.73) (2.38–5.34) (1.81–5.13) (1.73–4.40) (5.53–11.2) (4.78–7.41)

50% 2.41 2.26 4.36 3.81 2.89 2.67 7.35 5.77

95% CI (1.41–4.06) (1.35–3.60) (2.70–6.80) (2.50–5.29) (1.71–4.79) (1.63–4.12) (5.00–10.3) (4.39–6.83)

95% 2.10 1.99 4.46 3.89 2.72 2.52 7.00 5.60

95% CI (1.23–3.55) (1.18–3.23) (2.80–6.87) (2.58–5.33) (1.61–4.42) (1.53–3.93) (4.74–9.91) (4.22–6.66)

Child-B b

5% 2.76 2.60 4.22 3.73 4.21 3.72 7.73 6.20

95% CI (1.62–4.64) (1.56–4.11) (2.56–6.69) (2.39–5.38) (2.56–6.68) (2.39–5.37) (5.28–10.8) (4.65–7.52)

50% 2.24 2.13 4.27 3.75 3.99 3.54 7.05 5.63

95% CI (1.31–3.79) (1.27–3.44) (2.65–6.66) (2.46–5.20) (2.45–6.30) (2.28–5.01) (4.77–9.98) (4.25–6.74)

95% 1.89 1.80 4.25 3.71 3.91 3.45 6.56 5.21

95% CI (1.10–3.21) (1.07–2.96) (2.67–6.56) (2.47–4.99) (2.42–6.12) (2.25–4.78) (4.42–9.37) (3.96–6.15)

Child-A c

5% 3.87 3.52 5.40 4.62 5.39 4.61 7.73 6.20

95% CI (2.28–6.36) (2.17–5.27) (3.34–8.32) (3.07–6.32) (3.34–8.31) (3.06–6.31) (5.28–10.8) (4.65–7.52)

50% 3.15 2.92 5.20 4.44 4.96 4.28 7.05 5.63

95% CI (1.85–5.29) (1.77–4.49) (3.26–7.96) (3.00–5.88) (3.09–7.66) (2.86–5.75) (4.77–9.98) (4.25–6.74)

95% 2.66 2.49 5.00 4.26 4.73 4.07 6.56 5.21

95% CI (1.56–4.47) (1.50–3.91) (3.16–7.64) (2.92–5.51) (2.96–7.29) (2.74–5.39) (4.42–9.37) (3.96–6.15)

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indicator of the thoracic fraction consistent with ISO
[26,27] recommendations for occupational or non-
occupational environments. Our estimates show less pene-
tration of coarse particulate matter into the thoracic and
gas exchange regions of the respiratory tract than current
criteria. For typical breathing habits (i.e., not oral breath-
ing), we would predict less than 20% penetration of 10 μm
particles into the thorax, whereas a 50% penetration of 10
μm is currently used in both occupational and non-
occupational criteria [2,4,8,9]. Recognizing that there are
differences in the sources and chemical composition be-
tween ambient fine (nominal mean da ≤ 2.5 μm) and larger
coarse PM, our finding may, in part, explain why causal

relationships are observed between morbidity and mortality
with short and long-term exposure to fine PM but not lar-
ger coarse PM (see Chapter 2 in Ref [28]).
There are two primary reasons for the dramatic differ-

ence between our estimates and the current criteria. First,
the ACGIH [4] criteria considered all inspired air to enter
via the oral airway which increases the penetration
through the ET airways. With the exception of a labora-
tory setting, however, few individuals breathe exclusively
through the mouth. This would make the breathing habits
other than oral breathing preferable for the purposes of
estimating actual exposures. Second, the ACGIH criteria
are intentionally conservative (Figure 1) as the committee

Brown et al. Particle and Fibre Toxicology 2013, 10:12 Page 10 of 12
http://www.particleandfibretoxicology.com/content/10/1/12

chose to afford extra protection by over representing the
true penetration of particles into the lower respiratory
tract. In Figure 2, we predicted a 50% cut-point of 7.0 μm
for the conditions considered by the ACGIH, namely, an
orally breathing adult male engaged in light exercise. Add-
itionally, our predicted upper bound 95th percentile for
50% cut-points during oral breathing corrected for
inhalability in Table 7 are ~10 μm. Thus, selection of 10
μm as having 50% penetration into the thorax was consist-
ent with over representing the true penetration of particles
into the lower respiratory tract of most individuals.
Route of breathing has a dramatic affect on particle deliv-

ery to the thoracic and respiratory regions since the depos-
ition efficiency of the nasal passages greatly exceeds that of
the oral pathway. Most subjects in the Niinimaa et al. [17]
study, 87% (26 of 30), breathed through their nose until an
activity level was reached when they switched to oronasal
breathing. Thirteen percent (4 of 30) of the subjects, how-
ever, were oronasal breathers even at rest. These two sub-
ject groups are commonly referred to in the literature (e.g.,
see [12]) as “normal augmenters” and “mouth-breathers,”
respectively. Becquemin et al. [18] and Bennett et al. [19]
showed that children tend to have a greater fraction of oral
breathing than adults at rest and during exercise. Route of
breathing may also vary between races; Bennett et al. [20]
found that African-Americans and females had a greater
nasal contribution to breathing during exercise than Cauca-
sians and males. The abrupt change in route of breathing
occurring in normal augmenters has not been observed by
others. The gradual augmenter breathing habit based on
Bennett et al. [19,20] may be preferable to the normal aug-
menter in representing the general population. Chadha
et al. [29] found that the majority (11 of 12) of patients with
asthma or allergic rhinitis also breathe oronasally at rest. In
healthy individuals, a small fraction (around 5%) may
breathe solely through the mouth [21]. Our estimates for
gradual augmenters provide particle penetration fractions
most typical of healthy populations. Our estimates for
mouth-breathers may be more appropriate for patients
with mild upper respiratory disease.
The ICRP model [12] appears to underestimate the

penetration of particles through the ET airways of children.
A SFt is applied in Equations 1-3 with the presumption
that oral and nasal particle deposition increase with de-
creasing body size and increasing flow resistance.
For oral breathing on a mouthpiece, Bennett et al. [30]

showed greater ET deposition in children than adults. This
finding suggests that the application of a SFt of 1.26 in
Eq 1 is appropriate for laryngeal deposition. However, for
nasal breathing, Becquemin et al. [22] and Bennett et al.
[19] showed less nasal deposition in children than in
adults. These two studies, not considered in the ICRP
model [12], suggest that it may be inappropriate to apply a
SFt in Equations 2 and 3 for nasal deposition in children.

Lower nasal deposition of particles in children than adults
means greater penetration of particles into the lower
respiratory tract of children than adults. Accordingly,
we conducted simulations for the child with the SFt in
Equations 2 and 3 set equal to 1.0 in addition to the SFt of
1.26 recommended by ICRP [12]. The estimated nasal ET
deposition efficiency of 2 μm particles in the normal aug-
menter child during light exercise decreased from 68% to
48% when the SFt was decreased from 1.26 to 1.0. For
comparison, under the same level of activity, the estimated
ET deposition efficiency was 57-58% in the adult male and
female. Decreasing the nasal deposition efficiency of the
child relative to the ICRP model [12] increased the particle
size estimated to have 50% penetration into the thoracic
and respiratory regions (see Tables 5, 6, 7 and Figures 4
and 5). These estimates of larger 50% cut points for child-
A than adults appear consistent with studies in children
that were not incorporated into the ICRP model [12].
With the exception of Table 7, the thoracic and respir-

able fractions that we present are the amount of particles
entering a specified respiratory tract region relative to the
amount of particles entering the respiratory tract. In effect,
we assumed 100% inhalability across the range of particle
sizes (0.5-20 μm) examined. We have opted on this con-
vention since the inhalable fraction depends on factors not
considered here such as wind speed and direction relative
to the exposed individual. For recent reviews of the litera-
ture on particle inhalability, the reader is referred to
Brown [31] and Millage et al. [32]. Adjusting our data for
inhalability, shifts penetration curves to smaller particle
sizes, but mainly only where there is a substantial contri-
bution of oral breathing (see Table 7 vs. Table 5).

Conclusion
Our analyses show that occupation and non-occupational
criteria for thoracic and respirable fractions overestimate
the size of particles entering these regions. As already
noted, penetration fractions for workplace criteria were
chosen to afford extra protection by over-representing the
true penetration of particles into regions of the respiratory
tract [4]. However, accepted definitions for thoracic and
respirable fractions speak specifically to particles that pene-
trate into these regions. As such, current occupational and
non-occupational criteria may misinform practitioners
with regard to the actual size of particles expected to reach
regions of the respiratory tract during typical behavior. For
instance, the current criteria suggest that 10 μm particles
(50%) penetrate into the thorax, thus, leaving the expect-
ation that observed health effects may be modulated by
their deposition in either the upper or lower airways. How-
ever, we predict that about 20% or less of these 10 μm par-
ticles would penetrate through the ET airways and into the
lower respiratory tract. Our modifications to the ICRP
model [12] related to breathing habit and nasal deposition

Brown et al. Particle and Fibre Toxicology 2013, 10:12 Page 11 of 12
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in children reflect more recent data and provide consistent
estimates of greater particle penetration into the thoracic
and respiratory regions of children than adults. With those
modifications, for median activities, we predict 50% cut-
points for P(ET)avg at ~3 μm in adults and ~5 μm in chil-
dren. The predicted 50% cut-points for P(TB)avg are slightly
less than 3 μm in adults and slight greater than 4 μm in
children. Our estimates of particle penetration into the
thoracic and respiratory regions of the respiratory tract
should be useful in the design of experimental studies and
interpretation of PM health effects evidence.

Endnotes

aMore typically, the literature has defined this term in re-

lation to the fraction of particles entering the gas-exchange
region or the fraction penetrating through the tracheobron-
chial region, the ciliated airways, or conducting airways.

bFor completeness, other groups such as the British
Medical Research Council offered size-selective sampling
recommendations prior to the ACGIH. For a historical
perspective, the reader is referred to Lippmann [33].

cFor accuracy it should be recognized that the sampler
collection efficiency curves for EPA’s PM10 and ACGIH’s
thoracic fraction are different. The criteria are similar for
particles smaller than the 50% cut-point at 10 μm. How-
ever, the curves diverge at about 12 μm, with a dramatic
drop in collection efficiency (dictated by policy consider-
ations) for EPA’s PM10, and a more gradual decrease in
collection efficiency for the ACGIH criterion.

dThe MPPD model typically outputs estimates of regional
deposition for the entire respiratory cycle. For the purposes
of this project, the software output was modified by the de-
velopers to provide inspiratory deposition fractions for par-
ticles in the ET and TB regions. Designating the ET and TB
regions as separate compartments, the deposition efficiency
in the TB region (ηTB) during inhalation was calculated
from the MPPD output as DFTB / (1-DFET), where DFTB
and DFET are the deposition fractions of particles in the TB
and ET region during inhalation, respectively. For more in-
formation about this model, the reader is referred to: http://
www.ara.com/products/mppd_capabilities.htm.

Additional file

Additional file 1: Comparison of respiratory particle fractions
predicted by the MPPD and ICRP [12] models. In general, the ICRP
[12] model predicts less particle penetration into the respiratory region
than the MPPD model.

Abbreviations

ACGIH: American Conference of Governmental Industrial Hygienists;
CEN: European Committee for Standardization; da: Aerodynamic diameter;
DFTB: Particle deposition fraction in the TB region during inhalation;
DFET: Particle deposition fractions in the ET region during inhalation; EPA: U.
S. Environmental Protection Agency; ET: Extrathoracic; f: Breathing frequency;
Fm: Fraction of breath passing through the mouth; FRC: Functional residual

capacity; ICRP: International Commission on Radiological ProtectionISO
International Organization for Standardization; MPPD: Multiple path particle
dosimetry; NAAQS: National ambient air quality standard; ηTB: Particle
deposition efficiency in the tracheobronchial region; η(ET)larynx: Extrathoracic
particle deposition efficiency in the larynx; η(ET1)nose: Extrathoracic particle
deposition efficiency in anterior nasal region; η(ET2)nose: Extrathoracic particle
deposition efficiency in posterior nasal region; P(ET): Particle penetration past
the larynx and the thoracic fraction; P(TB): Particle penetration through the
ciliated airways and the respirable fraction; PM: Particulate matter;
PM2.5: Particles with a nominal mean aerodynamic diameter ≤ 2.5 μm;
PM10: Indicator for thoracic coarse particles; Qnose: Inspiratory flow through
the nose; Qtotal: Total inspiratory flow rate; SFt: Scaling factor, ratio of trachea
diameter in adult reference male to that of subject; t: Time spent engaged in
specific activity; TB: Tracheobronchial; TSP: Total suspended particulate;
URT: Upper respiratory tract volume; Vdaily: Total volume inspired in 24 hours;
VT: Tidal volume.

Competing interests

The authors have no competing interest.

Authors’ contributions

JB conceived the project, coordinated and drafted the manuscript. TG
contributed to defining the project’s scope and drafting the manuscript. OP
modified software for the purposes of this project to provide inspiratory
deposition fractions for particles and participated in the interpretation of
data. BA contributed to the methodology, participated in software
development and drafting of the manuscript. All authors read and approved
the final manuscript.

Acknowledgments

The authors thank Drs. Beverly Cohen (NYU), Martin Harper (NIOSH), Mort
Lippmann (NYU), and Lindsay Wichers Stanek (U.S. EPA) for their helpful
comments. TG was funded, in part, by a faculty appointment with the U.S.
EPA through a program administered by Oak Ridge Institute for Science and
Education (EPA-ORD/NCEA-RTP-2009-02). This document has been reviewed
in accordance with U.S. Environmental Protection Agency policy and
approved for publication. Mention of trade names or commercial products
does not constitute endorsement or recommendation for use. The views
expressed in this article are those of the authors and do not necessarily
reflect the views or policies of the U.S. Environmental Protection Agency.

Author details

1National Center for Environmental Assessment, U.S. Environmental
Protection Agency, MD B243-01, Research Triangle Park, Raleigh, NC 27711,
USA. 2NYU School of Medicine, 57 Old Forge Road, Tuxedo, NY 10987, USA.
3Applied Research Associates, Inc, 801 N. Quincy St., Suite 700, Arlington, VA
22203, USA. 4Applied Research Associates, Inc, 8537 Six Forks Road., Suite
600, Raleigh, NC 27615, USA.

Received: 4 May 2012 Accepted: 3 February 2013
Published: 10 April 2013

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doi:10.1186/1743-8977-10-12
Cite this article as: Brown et al.: Thoracic and respirable particle
definitions for human health risk assessment. Particle and Fibre Toxicology
2013 10:12.

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Areas of Expertise

Essay

Thesis

Presentation

Dissertation

Term Paper

Research Paper

Book Review

Assignment

Report

Case Study

Letter

Article

Coursework

Speech

Q & A

Critical Thinking

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Grammar Check Report

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Plagiarism Report

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Our Services

Academic Writing

Professional Editing

Thorough Proofreading

Thorough Proofreading

Delegate Your Challenging Writing Tasks to Experienced Professionals

Check Out Our Sample Work

It May Not Be Much, but It’s Honest Work!

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Happy Clients

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Words Written This Week

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Ongoing Orders

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Customer Satisfaction Rate

Process as Fine as Brewed Coffee

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See How We Helped 9000+ Students Achieve Success

We Analyze Your Problem and Offer Customized Writing

We Mirror Your Guidelines to Deliver Quality Services

We Handle Your Writing Tasks to Ensure Excellent Grades