Cigarettes seem like a probable ignition source for flammable liquids. Cigarettes have been blamed for the cause of many fires, far more than they should. But Cigarettes are more than tobacco wrapped in paper. They are in fact carefully engineered to burn in a certain way Cigarettes vary considerably from brand to brand in their burning characteristics. Modern cigarettes contain an engineered chemistry of additives to control the burning rates and moisture content of the tobacco and paper and anyone who has been a smoker knows the fact that manufactured Cigarettes go on burning when not being puffed. This spares smokers the trouble of lighting up again unlike with rolled up cigarettes and Cigars where the smoker has to keep smoking for the combustion of the tobacco to remain lit. This is good for the cigarette companies and pays off in higher sales from cigarettes. So yes this does mean that a cigarette rolling off onto a mattress or into the crack of a sofa can smoulder undetected for 30 to 40 minutes before bursting into flames. Smouldering is a form of flameless combustion which can occur in materials capable of charring. Smouldering can occur at very low oxygen concentrations then proceeds at a very slow rate before flaming occurs. This makes people understand the real hazard of manufactured cigarettes. But that is found out through the visual fact of witnessing a cigarette stay lit and burn. So people will believe most of what they see as fact, but when many people sit down in their local theatre to watch the latest Hollywood movie they usual see the spectacular visual effects of a character throwing a lit cigarette on to a puddle of fuel and watching a large fire ball appear before their eyes which causes an amazing explosion. Igniting puddles of petrol for example with cigarettes in movies is a common device. The character takes a few puffs and tosses the glowing cigarette in the puddle. Immediately the petrol ignites.
But Experiments with flammable gases and vapours have shown that many of the most commonly-encountered substances, including methane and petrol vapour, were not ignited by a lighted cigarette[1]
So Petrol vapour cannot normally be ignited by glowing cigarettes, a fact which has been verified by many experiments.
Cigarettes are rather uncommon as the source of ignition for a successful structure fire. If they are set to ignite liquid flammables, they will almost certainly fail[2]
This paper looks to answer why a lighted cigarette when tested against the ignition of common flammable vapours such as petrol fail but other more volatile liquids tested are capable of ignition. Previous work carried out on cigarette ignition, related research in this field of combustion and ignition of flammable gases and liquids will be studied and researched to give the author a better understanding of the fallacy that a lit cigarette can be a danger that can readily ignite flammable liquids or gases. But not to underestimate the potential danger of such an ignition source in circumstances where many factors can come together to form self sustained combustion.
Flammable substances and liquids are used for a wide variety of purposes and are commonly found in the home. Petrol is the most common, but there are other flammable and combustible liquids and gases used
Table shows other flammable liquids commonly found around the home
Lighter fluid
Oil
Propane
Butane
Aerosol Cans
Diesel fuel
Wood preservatives
Kerosene
White spirits
Flammable materials found in the gaseous form will burn whenever mixed with the proper amount of air and properly ignited. A flammable liquid in its liquid state will not burn. It will only ignite when the vapours from the liquid evaporate in air.
All flammable liquids give off vapours that can ignite and burn when an ignition source is introduced. Flammable liquids require an initial energy input to produce an air/vapour mixture within the limits of flammability (niamh mc daid)
Ignition is the transition from a nonreactive to a reactive state in which external actions lead to thermo chemical runaway followed by rapid transition to self sustained combustion
The usual conditions for ignition are given by a 3t rule of thumb. The three T’s stand for:
Temperature. Must be high enough to cause significant chemical reactions and/or pyrolysis
Time. Must be long enough to allow the heat input to be absorbed by the reactants so that a runaway thermo chemical process can occur
Turbulence. Must be high enough so that there is good mixing between fuel and oxidizer and heat can be transferred from the reacted media to the unreacted media (cite the book)
Ignition will occur when the process of a rapid exothermic reaction is started, which then gains momentum and causes the fuel to undergo change. When a flammable liquid is poured or spilled on a surface it is the vapours that are actually ignited. Vapours from liquids are what directly support the flame. To understand how volatile certain flammable liquids are it is useful to be familiar with the terms used to describe their chemical properties.
Vapour density
This is a property of a vapour that predicts its behaviour when released in air. when calculating the vapour density of a liquid fuel the molecular weight of gas of the vapour by that of air in normal conditions. (approx 29) (niamh mc daid)
Flash point
The flash point is the lowest temperature at which a liquid fuel will produce a flammable vapour. A liquid fuel must be able to generate a vapour in sufficient quantity to reach that lower limit in air before it can burn. This flash point is measured in two different ways, first being a closed cup measurement and the latter being an open cup.
Flammable ranges
Petrol has a narrow flammable range. Thus petrol vapour mixed with air has a lower flammability limit of just over 1% and an upper limit of 6% by volume petrol vapour in air, at normally encountered temperatures. (hollyhead) Flammable range refers to the percentage of a flammable liquid in its gaseous state to air to create an explosive mixture. Mixtures of flammable vapours with air will combust only when they are within particular ranges of vapour/air concentration. Outside of these limits the fuel-air mixture is either too lean or too rich to ignite (niamh mc daid) This varies with different flammable liquids. Gasoline has a flammability range of 1.4 to 7.6 percent. This means it will ignite when there is 1.4 parts of gasoline mixed with 100 parts air
Ignition temperatures
This is the temperature at which a particular flammable liquid gives off vapours and evaporates in air and therefore can ignite. Gasoline will ignite when a heat source or electrical spark of at least 853 degrees comes in contact with it. Natural gas (methane) needs an ignition temperature of around 1000 degrees
Auto-ignition temperature (AIT)
This is the temperature at which a fuel will ignite on its own without any additional source of ignition. (Niamh mcdaid) Fires will occur because there is high temperature introduced to an area in which there is a fuel-air mixture within its flammability range. There are so atmospheric factors that affect the Auto-ignition temperature e.g. Oxygen levels in the area of the mixture. But so long as the heat energy can be transferred from the source to the fuel ignition fire may result
Flammability
In Hollyhead’s paper he mentions the flammability of certain liquids e.g., kerosene, white spirit and diesel oil, which are flammable liquids with flash point temperatures above normal ambient. So therefore, an ignition source has to ignite not only the flammable mixture of fuel vapour but to generate this mixture in the first place by heating the bulk liquid. (hollyhead)
Cigarette Components
Modern manufactured cigarette comprise of different components. The components combined can affect the performance of the cigarette including the emissions of toxicants. In British manufactured cigarettes the tobacco used is Virginia. There are two other main types used in commercial cigarettes which are Burley and Oriental.  Each has different characteristics. These tobacco types will also vary depending on their variety, in what environment they are cultivated and how that cultivation is carried out. The paper around the tobacco of commercial cigarettes in modified in different brands. It can be more porous in some brands so that the amount of air passing through the paper affects the yield of a puff. The more air that can pass through the paper the more the smoke constituents passing through the cigarette are weakened which then result in lower yields of various smoke products. Just as Baker describes when a cigarette burns, thousands of products are formed. They are distributed between the gas phase and aerosol particles which make up smoke. (baker) Modern commercial cigarettes are made with a filter. The filter is different on most brands. The way the filter is designed can affect the amount of filtration. Different cigarette brands can regulate the taste and smoking experience. Introducing vents in the paper surrounding the filter end can allow for dilution of smoke in lighter branding cigarettes. The bigger the length results in longer fibres and also the materials used for the fibres all play a part in making a certain cigarette different from other brands in the market.
Figure shows the different components of a manufactured cigarette, how smoke moves through the tobacco and filter and how air moves through the paper (bat)
Cigarette Combustion
Many of the components identified in tobacco have also been identified in its smoke because they transfer in part from tobacco to its smoke during the smoking process. Many other identified tobacco components are not found in smoke because they decompose during the smoking process. (cite tobacco book)
There has been work carried out to measure the burning temperature of a cigarette particularly those by Dr Richard R Baker have provided fundamental knowledge about the combustion/pyrolysis processes during smouldering and puffing of a cigarette. Dr Richard R. Baker has established the distributions of combustion temperature, gas velocity and key smoke constituents inside a burning cigarette. These experiments have become the foundation for the study of cigarette combustion. Dr Barker’s paper studied the process of thermo physical concurrencies inside and around a burning cigarette (barker)

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Figure 2 shows combustion of a cigarette when being smoked (bat)
The formation of smoke from a burning cigarette depends on a series of mechanisms, including generation of products by pyrolysis and combustion, aerosol formation, and physical mass transfer and filtration processes. Each mechanism, and their interactions, has a profound effect on the levels of chemical constituents in tobacco smoke. An enormous amount of research has been done on these subjects over the last 50 years. (barker) Figure 2 describes the two types of burning that take place when the cigarette is puffed. There is also a natural smoulder occurring between the puffs by the smoker. There are two main regions at the tip on the ‘coal’, namely the combustion zone A and the pyrolysis and distillation zone B. Combustible vapours are produced in zone B prior to ignition in zone A. During puffing, air is drawn into the cigarette through the paper and at the paper bum line. When a smoker draws on a lit cigarette, the temperature of the cigarette coal rises rapidly from its resting smouldering temperature of around 600 °C.  Peak puff temperatures at the periphery of the coal can exceed 900 °C during a 35 mL, 2-sec puff. The high temperature inside the coal causes an increase in the viscosity of the air flowing through and a concomitant increase in the resistance to the draw of air through the coal.  This effect forces air to be drawn primarily into the periphery of the coal around the paper burn line, which causes more complete combustion in this peripheral region. 
The depletion of oxygen due to combustion results in the formation of a region immediately behind the coal where the temperatures remain high enough for thermal decomposition of tobacco (the pyrolysis/distillation zone).  Large amounts of volatile and semi-volatile smoke constituents are produced in this region. A small amount of air is drawn in along the tobacco rod through permeable cigarette paper and smoke temperature decreases rapidly to produce a supersaturated aerosol. The smoke thus formed during a puff is subjected to filtration by the remaining tobacco rod and cigarette filter, as well as dilution by any filter ventilation holes. Some proportion of the light gases (such as CO) will diffuse out of the highly permeable cigarette paper. The smoke that leaves the mouth end of the cigarette is called mainstream smoke.  Between puffs, hot smoke escapes from the top of the cigarette and forms the sidestream smoke.
In the author’s research for this paper the general opinion that cigarettes are the cause of fires is truthful, as there is no question that many fires are started by cigarettes and the careless smoker is usually the reason for the high number of fires originating from smoking cigarettes. Despite advances in the fire retardant foams in furnishings and mattresses, smoking in bed remains a threat to the safety of people. But it is a common occurrence to hear theories about how smoking caused a fire when the alleged ignition would not have been successful under certain circumstances.
No one combustion parameter alone can be used to explain why gasoline (petrol) vapour and methane are not ignited by cigarettes whereas hydrogen and carbon disulphide are.
In fact, the parameters themselves are inter-related and taken as a whole can express the propensity of a substance to react with oxygen to form combustion products. This is directly related to the ease with which chemical bonds are broken to lead to the formation of the more stable products of carbon dioxide and water, which result from the combustion of most of these fuels. hollyhead
Recent tests by the ATF Fire research Laboratory involved contact of burning tobacco cigarettes of different brands with petrol vapour from a pool at room temperature. A total of 137 attempts were made using both smouldering and actively drawn puffs with no ignitions observed. De haan
Tests have shown the oxygen levels in cigarettes in the vicinity of combustion to be very low and carbon dioxide levels to be very high, both factors reducing the chances of vapour ignition
The conditions in the combustion zone of a cigarette, which is deficient in oxygen and rich in hydrogen and carbon dioxide, together with the rapid and efficient tobacco-oxy- preparation of the oxygen reaction conspire to allow only the most reactive of substances to be ignited, and result in flame propagation to the vapour air mixtures outside the cigarette coal.
Repeated attempts to cause explosions by inserting a lit cigarette into an explosive fuel vapour-air mixture have resulted in failure over many experiments. Hollyhead
The residence time of airborne vapours in the cigarette being puffed is so short that there is not enough time for any but the most reactive species to ignite. The fuel elements in a commercially manufactured cigarette is such that quenching distance of all but most reactive gases is not exceeded, suppressing any sustained ignition.
Experimental evidence and consideration of the cigarette combustion process, alongside ignition parameters of substances, show that mixtures in air of petrol and methane are not ignited by a lighted cigarette. It is very likely, therefore, that many fires have been wrongly attributed to a lighted cigarette; such a cause fire can often be the refuge of the uninitiated
‘No Smoking’ regimes at chemical plants, oil gas installations and indeed petrol filling stations is, therefore, not in vain. Although a cigarette may not be an immediate problem, the lighted match or cigarette lighter flame used to light it, certainly could
Occasionally contaminates or faults from the manufacture of the cigarettes, may it be in the tobacco filling or paper can cause some brief tiny flames. Given the right conditions and such an atmosphere where there is a fault and a perfect fuel-air mixture. Such flames would be a suitable ignition source. So even though many tests have proved that it is virtually impossible for a lit cigarette or cigar to ignite most flammable vapours, this could ensure the ignition of a fire, explosion or both.
In study for this paper the author came across a message board where a guy was irate at the fact he seen another person smoke a cigarette on the fore court of a filling station. To his dismay when confronting the person in question about the stupidity of his action he promptly got the reply that cigarettes don’t lit petrol, I seen it on myth busters. Goes to show that what some people see on Television is believed as fact, just like in the Movies. Some things will never change[1]
1. www.wikilaw3k.org/forum. Cars-Transportation-Safety/Smoking-at-the-gas-pump. [cited 2010 4/11]; Available from: http://www.wikilaw3k.org/forum/Cars-Transportation-Safety/Smoking-at-the-gas-pump-345109.htm.