The number of fires attributed to cigarettes has been on a sharp decline since the 1980s. Markowitz places the percentage of fires caused by conventional cigarettes at 2.8 in 2011 but points out that cigarette fires are considered the deadliest fires second only to arson fires (Markowitz, 2). The number of forest fires attributed to smoking is now about 10 percent of where they stood in 1980 (Butry, Prestemon and Thomas 2014, 790). This essay examines the reasons for the drop in cigarette attributed fires and examines why the estimates have been so much higher in past years.
Cigarettes and CombustionCigarettes burns by smoldering combustion which occurs where the fuel, heat and oxygen occur in insufficient proportions to cause a flame but enough to cause decomposition of the fuel. Cigarettes in particular combust by way of diffusion burning meaning that the oxygen and the fuel are not already mixed at the onset. The smoldering capacity of cigarettes rests on two factors; firstly, the tobacco particles in a cigarette allow for easy circulation of oxygen, and secondly, the tobacco particles provide a large fuel surface area.
Smoldering fires have low temperatures and will not convert into a flame unless the proportions of the three elements are changed. In the case of a cigarette, altering the elements could involve placing it next to a flammable material. Cigarettes burns in two phases. During the Solid-phase the combustion of the tobacco are 790 centigrade during smouldering and 900 centigrade when puffed. When measured from the outside the temperatures are at 675 centigrade during smoldering and 930 centigrade when puffed. At the gas phase stage, temperatures are at 790 centigrade during smoldering and 860 centigrade when puffed (Liu and Woodcock 2014, 258).
Experiments to measure the temperature of burning cigarettes measured by infrared equipment recorded that temperatures fluctuated heavily within a 2 second window of the puff and achieved temperatures of up 1200 centigrade. Liu and Woodcock describe that 2 second period as comprising three stages. First the increased oxygen flow caused an increase in temperature; secondly the cigarette hits peak temperatures and finally a decrease in temperatures reflecting the decrease in the rate of oxygen flow (Liu and Woodcock 2014, 260).
This infrared method also allowed researchers to observe that there were distinct burning events during the second stage linked to the combustion of tobacco subjected to the highest oxygen flow around the perimeter of the cigarette (Liu and Woodcock 2014, 264). A smoldering cigarette can cause another material to smolder or it can provide the conditions that lead to a flame. Transition from smoldering to flaming is influenced by oxygen flow which is enhanced if a window is open nearby. If conditions are met, it tends to occur between 20 minutes and a few hours of the smouldering.
Grass material has been observed to burst in to flame when the speed surpasses 0.83m/s point. Once the transition occurs, the flame is identical to one started by other means. Ignition of a solid such as a mattress by a smoldering cigarette causes pyrolysis that creates char. It is this char, which contains more carbon than the original mattress that then undergoes the exothermic process of smoldering. A smouldering item will combust slower than a flaming one, for instance a smoldering chair smolders at 0.1g/s and a flaming one at 100 g/s (Babrauskas and Krasny 1985, 5).