The process of smoking a cigarette produces three different types of tobacco smoke. The first is mainstream smoke, the smoke directly inhaled through the burning cigarette by the smoker. Second is exhaled mainstream smoke, the smoke breathed out by the smoker from their lungs. The composition of mainstream and exhaled mainstream smoke is likely to differ, some of the compounds in smoke being retained by the smoker or otherwise altered by the process. Third is sidestream smoke, the smoke which drifts from the end of the lit cigarette. Environmental tobacco smoke (ETS) consists of exhaled mainstream smoke and sidestream smoke.
Mainstream and sidestream smoke both contain a large number of chemical carcinogens and other toxic substances, but undiluted sidestream smoke carries many compounds in far greater concentrations. For example, sidestream smoke contains greater amounts of ammonia, benzene, carbon monoxide, nicotine and the carcinogens 2-napthylamine, 4-aminobiphenyl, N-nitrosamine, benz[a]anthracene and benzo-pyrene per milligram of tobacco burned. The particles of sidestream smoke are smaller than those of mainstream smoke, meaning that they can be inhaled more deeply into the lungs.(6)
Although it has been estimated that 85% of the smoke present in an average room in which smoking has occurred is composed of sidestream smoke, passive smokers still have a lower exposure to the harmful components of tobacco smoke than active smokers, who draw smoke directly into their lungs. Quantification of exposure by the non-smoker is defined by a number of factors including the filtration, tar level and quantity of cigarettes smoked, room size, ventilation rates and duration of exposure.(7) Of course, active smokers are exposed to environmental tobacco smoke as well.
A wealth of scientific evidence now exists showing that the breathing of tobacco smoke polluted air by non-smokers can lead to serious harm, such as increased bronchitis, pneumonia and other chest illnesses in children, lung cancer and other lung disease, and cardiovascular disease. This is, of course, in addition to the well known 'irritant' effects of tobacco smoke to the eyes, nose, throat and airways passages.(6)
The medical evidence about passive smoking has serious ramifications for the continuation of smoking in public places and the workplace, and has been central to a number of court cases in Australia. These issues are discussed in Chapter 6.