Passive smoking is: 1) exposure to second- or third-hand smoke by breathing ambient air containing toxic substances resulting from the combustion of tobacco products after birth; or 2) exposure in utero to maternal blood contaminated with the combustion of tobacco smoking products.
Second-hand smoke (SHS) or environmental tobacco smoke (ETS) is the name given to the mixture of ‘mainstream smoke’ (smoke exhaled by a cigarette smoker), ‘sidestream smoke’ (emitted from the smouldering tobacco stick between puffs), contaminants emitted into the air during the puff and contaminants that diffuse through the cigarette paper and mouth-end between puffs. It is a complex mixture of gases and some 4000 particulate chemicals, which are generated during the burning and smoking of tobacco products. Of these, ≥250 are known to be toxic or carcinogenic.
The majority of the compounds present in mainstream smoke are formed during combustion. The constituents of mainstream and sidestream smoke are broadly similar but there are important differences in their rates of emission into the air due to physical and chemical differences in the way cigarettes burn during and between puffs. Mainstream smoke is generated during inhalation at a temperature of approximately 800–900°C, which is higher than that seen in sidestream smoke generation (600°C), due to increased levels of oxygen passing through the cigarette. Mainstream smoke also has a pH of 6.0–6.7, making it more acidic than sidestream smoke (pH 6.7–7.5). The combustion process that generates sidestream smoke also produces smoke with much smaller particulate matter (PM) size and considerably higher concentrations of many carcinogens and toxins. However, dilution, chemical reactions, deposition and other removal processes may decrease the concentration of airborne SHS constituents, alter the size distribution of suspended particles and chemically modify some of the more reactive constituents of SHS (table 1). Third-hand smoke is the name given to substances that are re-emitted from solid surfaces having been initially deposited there during smoking.