In many developed countries, sizeable decreases in total consumption, sales and smoking rates have occurred. In most developed and some developing countries, about one-third of women smoke, although in some countries fewer do. Tobacco is smoked principally in cigarettes, with pipes, cigars, bidis and other forms being used either to a minor extent or only in certain regions. Combustion of tobacco products delivers mainstream and sidestream smoke which differ in physicochemical nature. In addition, sidestream smoke contains greater amounts of identified carcinogens than mainstream smoke.
Passive smoking is a universal phenomenon where smoking is common.
The uptake of smoke constituents by smokers and by passive smokers has been studied in only a few countries, although extensive analysis of smoke shows cigarette smoking to be a major source of exposure to tobacco-specific nitroso compounds, polynuclear aromatic compounds, aromatic amines and some other carcinogens. Exposure of hamsters and rats to whole smoke results in the induction of malignant respiratory-tract tumours. Cigarette smoke condensate induces skin cancers in mice and rabbits after application to the skin, and lung cancers in rats after intrapulmonary injection. No study was available that was designed specifically to investigate the carcinogenicity of passive smoking to experimental animals. Exposure to these complex mixtures results in mutagenic urine in smokers and in increased chromosomal damage in the somatic cells of smokers compared to nonsmokers.