4.3 What is in secondhand smoke?
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Unless otherwise noted, the following section is compiled from recent reviews published by the Californian Environmental Protection Agency (2005)19 and the Office of the US Surgeon General (2006).20
Included among the thousands of chemical compounds found in secondhand smoke are more than 50 known or suspected carcinogens and a range of respiratory irritants and other toxic chemicals. As well as carbon monoxide and nitrogen oxides, gaseous components of secondhand smoke known to damage health include the carcinogen benzene, a number of other probable carcinogens, and substances known to affect the central nervous system, or cause eye, skin or respiratory problems. Particulate matter found in secondhand smoke includes the carcinogens 2-naphthylamine, 4-aminobiphenyl, arsenic, chromium VI, nickel, the polycyclic aromatic hydrocarbon benzo[a]pyrene, and the tobacco specific nitrosamines N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino-)1-(3,pyridyl)-1-butone (NNK). The metabolites of NNK are detectable in the urine of nonsmokers exposed to secondhand smoke.27 Most particulate matter in secondhand smoke is in the submicron range, meaning that it is classified as fine particulate matter. Short- and long-term exposure to fine particulate matter is also known to contribute to asthma and other respiratory diseases.
Secondhand smoke can be detected in the indoor environment well after it has been generated, research showing that it may remain at concentrations well above background levels after five hours. Over time, secondhand smoke changes in nature, with particles coagulating, expanding, condensing or evaporating, depending on concentration, ventilation, humidity, sunlight and other conditions. However even after undergoing complex reactions, the majority of particulate matter generated by secondhand smoke remains in the submicron range, and is therefore of toxicological concern on this basis alone.
Some compounds, such as nicotine and some polycyclic aromatic hydrocarbons exist in both the gaseous and the particulate phase of secondhand smoke. Classified as 'semi-volatile,' their ability to change form according to environmental conditions means that they remain detectable in the indoor environment for longer periods after active smoking has ceased. Nicotine may react with hydroxyl radicals in ambient air, giving it a half-life of approximately one day.
An analysis of experiments funded by a tobacco company during the 1980s has shown that particulate matter in inhaled fresh sidestream smoke is three to four times as toxic per gram compared with mainstream cigarette smoke. Further analysis of the same tobacco industry data has shown that toxicity of sidestream smoke increases by a further two to four times as it ages.28 Therefore, the authors of this study conclude, if aged sidestream smoke is about three times more toxic than fresh sidestream smoke, and fresh sidestream smoke is about four times more toxic than mainstream smoke, then aged sidestream smoke is approximately 12 times more toxic than mainstream smoke.28 Although they inhale a lesser volume of smoke than do smokers, the smoke to which non-smokers is exposed is substantially more toxic than the smoke inhaled by smokers, which helps explain the relatively large biological effects of secondhand smoke.28
The constituents of tobacco smoke are discussed further in Chapter 12.
4.3.1 Measuring exposure in non-smokers
Individual exposure to secondhand smoke is highly variable, depending on personal circumstances. Non-smokers who live and work in a smokefree environment and experience only brief exposure to smoke are likely to be exposed to less than 0.01 micrograms of secondhand smoke per cubic metre (24-hour time-weighted average nicotine air concentration). Conversely those exposed in the home and in vehicles may have an average exposure concentration of up to 7.4 micrograms per cubic metre, which is classified as high exposure by the California Environmental Protection Agency. These levels are of particular concern for small children, among whom exposure may occur on a daily basis. A national health survey from the USA published in 1996 suggested that nearly 90% of US non-smokers had detectable amounts of nicotine by-products in their blood, including those who did not report habitual exposure to secondhand smoke in the home or work setting.29 While reductions in opportunities for smoking in most workplaces and public settings might be expected to result in far lower levels of exposure among today's non-smokers, this finding underscores both the ubiquity and potentially unwitting nature of exposure to secondhand smoke.
Ventilation, air conditioning and heating systems alone do not reliably remove secondhand smoke from the indoor environment, and may instead distribute toxins throughout buildings. Providing separate areas where smoking is allowed also fails to eliminate exposure to secondhand smoke.
4.3.2 Mechanisms of disease
The similarities between secondhand smoke and mainstream smoke make it plausible that inhalation of secondhand smoke is a cause of disease in those who are exposed to it, by similar pathways of disease causation as in active smokers. Metabolites from nicotine, carbon monoxide and three major classes of carcinogens found in tobacco smoke—polynuclear aromatic hydrocarbons, nitrosamines and aromatic amines—are found in the urine or blood of non-smokers exposed to secondhand smoke, just as in active smokers. As noted above, both fresh and stale sidestream smoke are more toxic than mainstream smoke,28 but because secondhand smoke is diluted by ambient air, exposed non-smokers do not receive as high an exposure to the toxic chemicals in tobacco smoke as do active smokers, and their risk of developing tobacco-caused illness is generally lower. Nonetheless, secondhand smoke is regarded as a Group 1 carcinogen (known to cause cancer in humans18), for which there is no evidence of a safe level of exposure. As the following sections show, secondhand smoke is now known to cause a number of diseases and poorer health outcomes for non-smokers. Even where elevation of risk is comparatively small, because of the high number of individuals who may be exposed, secondhand smoke represents a substantial preventable public health risk.
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