12.2 Measuring cigarette smoke constituents
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Cigarette smoke is a complex mixture of thousands of chemicals. It has long been understood as having particulate and vapour phases.5 The particulate phase can be understood technically as combusted particles that are large enough to be captured on the filter of a smoking machine, which samples mainstream smoke and is used to measure the 'tar', nicotine and carbon monoxide 'yields' (or 'emissions') of cigarettes. The vapour phase consists of smaller particles and gases. 'Tar' is the collected mass of particles captured on the filter when a cigarette is machine smoked, with the word also evoking the brown stains seen on the end of cigarette filters and on smokers' fingers.
Measuring the 'yield' of 'tar' captured by a smoking machine filter has been one of the most long standing ways of comparing the cigarette smoke of different brands. Comparing the 'tar' yields of different brands has also long been assumed to be a useful means for comparing their relative harmfulness. In the 1950s it was found that when 'tar' dissolved in acetone was painted on mouse skin, tumours developed. Further, there was a dose-response relationship between the amount of 'tar' to which mice were exposed and the frequency with which tumours developed.5, 6 It appeared to follow, then, that cigarettes yielding less 'tar' would be less harmful.
Carbon monoxide yields have also been widely used for making comparisons between brands, because carbon monoxide is believed to be a particularly important cause of the cardiovascular damage caused by smoking. Nicotine yields have also long been used for making comparisons between brands on the basis that nicotine is the primary addictive ingredient of cigarette smoke and that reducing smokers' nicotine exposures will facilitate quitting.
The tar, nicotine and carbon monoxide figures that were printed on the side of Australian cigarette packs until March 2006 are derived from the standard ISO (International Standards Organization) cigarette yield test, in which a cigarette is machine smoked with a 35ml puff of two seconds duration, once per minute, until it has been smoked down to a 30mm butt. All cigarettes are smoked in the same manner for this test, regardless of whether they are 'full strength', 'light' or 'ultra-light'.
Since the 1990s, a number of other smoking machine test protocols have been introduced. For present purposes, the most important of these is the ISO Intensive Condition test (also frequently referred to as the Canadian Intensive Condition test), in which the machine takes a 55ml puff of two seconds duration, once every 30 seconds (and any perforations in the side of the filter—known as filter ventilation—are taped over).
Other more recent developments in measuring smoke constituents have included smoking machines which quantify sidestream smoke yields as well as mainstream smoke yields and testing for a much larger number of specific smoke constituents than 'tar', nicotine and carbon monoxide. Where both mainstream and sidestream smoke measurements and smoke constituents other than 'tar', nicotine and carbon monoxide are involved, it has become more usual to refer to 'emissions' than 'yields' but the two terms remain largely interchangeable.
A number of assumptions underlie the presentation of standard ISO tar, nicotine and carbon monoxide yield figures as risk information that is useful to either regulators or to consumers in making informed choices. One assumption is that 'tar' does not vary significantly in composition. Thus, a milligram of tar from one cigarette will contain roughly the same amounts of carcinogens and cardiovascular/respiratory toxicants as a milligram of tar from any other cigarette. This is a questionable assumption, particularly when comparing cigarettes of different blend types but also when comparing cigarettes of the same blend type. However, the most seriously flawed assumption behind presenting standard ISO yield figures as a means for comparing the relative harmfulness of cigarettes is the assumption that individual smokers will always take the same volume of smoke from any cigarette. In fact, the constant for most smokers is their target nicotine intake and not the volume of smoke they take from each cigarette.10
Addicted smokers have a target nicotine intake from each cigarette, in order to receive rewarding sensations. The majority of addicted smokers appear to require somewhere between 0.9mg and 1.4mg of nicotine from each cigarette for it to be satisfying.11 Smokers unconsciously change smoking parameters such as puff size and time taken between puffs when they change brands, in order to achieve their target nicotine intakes.12, 13 These changes in smoking behaviour are known as compensatory smoking. Whether one looks at individual smokers 'down-switching' to brands with lower tar, nicotine and carbon monoxide yields or looks at entire populations of smokers, one finds that, as standard ISO tar and nicotine yields decrease, parameters such as puff size and total number of puffs taken per cigarette increase.
While standard ISO tar and nicotine yields do not provide a useful guide to smokers' intakes, they do provide a rough measure of the effort required for any particular smoker to gain her/his target nicotine dose.12 Below certain yield levels, many smokers will find that the effort required to gain their target nicotine dose has become excessive and cigarettes in these yield ranges will no longer be acceptable to them. Accordingly, many heavily addicted smokers are unable to 'down-switch.' However, the fact that certain groups of smokers will not accept brands that have tar and nicotine yields below certain levels is an entirely separate matter to smokers being able to reduce their intakes by switching to brands with lower tar, nicotine and carbon monoxide yields. Where smokers have been able to 'down-switch' successfully, they almost certainly have not reduced their intakes of nicotine or other harmful smoke constituents.
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