Cigarette tar

Cigarette tar yields in relation to mortality from lung cancer in the cancer prevention study II prospective cohort, 1982-8

Objective To assess the risk of lung cancer in smokers of medium tar filter cigarettes compared with smokers of low tar and very low tar filter cigarettes. Design Analysis of the association between the tar rating of the brand of cigarette smoked in 1982 and mortality from lung cancer over the next six years. Multivariate proportional hazards analyses used to assess hazard ratios, with adjustment for age at enrolment, race, educational level, marital status, blue collar employment, occupational exposure to asbestos, intake of vegetables, citrus fruits, and vitamins, and, in analyses of current and former smokers, for age when they started to smoke and number of cigarettes smoked per day.

Setting Cancer prevention study II (CPS-II).

Participants 364 239 men and 576 535 women, aged >= 30 years, who had either never smoked, were former smokers, or were currently smoking a specific brand of cigarette when they were enrolled in the cancer prevention study.

Main outcome measure Death from primary cancer of the lung among participants who had never smoked, former smokers, smokers of very low tar (<= 7 mg tar/cigarette) filter, low tar (8-14 mg) filter, high tar (>= 22 mg) non-filter brands and medium tar conventional filter brands (15-21 mg).

Results Irrespective of the tar level of their current brand, all current smokers had a far greater risk of lung cancer than people who had stopped smoking or had never smoked. Compared with smokers of medium tar (15-21 mg) filter cigarettes, risk was higher among men and women who smoked high tar (>= 22 mg) non-filter brands (hazard ratio 1.44, 95% confidence interval 1.20 to 1.73, and 1.64, 1.26 to 2.15, respectively). There was no difference in risk among men who smoked brands rated as very low tar (1.17, 0.95 to 1.45) or low tar (1.02, 0.90 to 1.16) compared with those who smoked medium tar brands. The same was seen for women (0.98, 0.80 to 1.21, and 0.95, 0.82 to 1.11, respectively).

Conclusion The increase in lung cancer risk is similar in people who smoke medium tar cigarettes (15-21 mg), low tar cigarettes (8-14 mg), or very low tar cigarettes (<= 7 mg). Men and women who smoke non-filtered cigarettes with tar ratings >= 22 mg have an even higher risk of lung cancer.
Introduction

During the past 50 years, changes in the design and manufacture of cigarettes have markedly reduced their machine measured "tar" yields. The introduction of cellulose acetate filters in the 1950s, and subsequently more porous cigarette papers, reduced the average tar rating per cigarette in the United States from about 37 mg in 1950 to 22 mg in 1967. The introduction of air ventilation holes in the filter tip in the late 1960s and expanded tobacco in the 1970s permitted manufacturers to market low tar (generally in the range of 8-14 mg per cigarette) and very low tar cigarettes (<= 7 mg per cigarette). Concomitantly, the US average tar level per cigarette, as rated by the US Federal Trade Commission (FTC), declined to about 13 mg by 1990. Similar trends in standardised tar yields have been reported in the United Kingdom and other countries.

While many case-control and cohort studies have examined risk of lung cancer in relation to type of cigarette smoked, nearly all have compared the risks of smoking high tar non-filter brands with smoking medium tar filter brands, or to the corresponding ranges of tar yield. The three case-control studies that have included participants who smoked low tar brands yielded negative or equivocal results, but the observation periods for these studies ended in 1980-1, when the combined market share of low tar and very low tar cigarettes in the United States had exceeded 10% for only five or six years. In most epidemiological studies,observation period ended before 1986, when the market share in the United States had exceeded 10% for only a decade. Thus no large, long term prospective study has specifically compared the risk of lung cancer in smokers of medium tar filter cigarettes with that in smokers of low tar and very low tar filter cigarettes.

We analysed the relation between the tar rating of the brand of cigarette smoked in 1982 and mortality from lung cancer over six years among men and women in the cancer prevention study II (CPS-II), a nationwide prospective cohort of over one million US adults aged 30 years or older. We specifically compared the risk of lung cancer among smokers of very low tar (<= 7 mg) filter, low tar (8-14 mg) filter, or high tar (>= 22 mg) non-filter brands with the risk among those who smoke conventional medium tar (15-21 mg) filter brands.
Methods

Details of the cancer prevention study, initiated by the American Cancer Society (ACS) in 1982, have been published elsewhere.From the cohort of 508 318 men and 676 270 women, we excluded those who reported a history of cancer other than non-melanoma skin cancer; men who ever smoked pipes or cigars or chewed tobacco; and men and women whose current smoking status could not be ascertained. The resulting cohort comprised 364 239 men and 576 535 women. The outcome measure was death from cancer of the trachea, bronchus, or lung as the underlying cause, coded from the death certificate. During the six year follow up, 2622 men and 1406 women died from these cancers.

On the basis of brand name reported by each current smoker at enrolment, as well as the size (regular, king size, 100 mm, 120 mm), presence or absence of menthol and of a filter, we assigned a tar rating from the Federal Trade Commission tables for December 1981. We then grouped current brand tar ratings into very low tar (<= 7 mg), low tar (8-14 mg), medium tar (15-21 mg), and high tar (>= 22 mg). Unspecified current brands, as well as those current brands that could not otherwise be classified, were considered as a separate category. All brands in the very low and low tar ranges, as well as 99% of brands in the medium range, were filter cigarettes. Those in the high tar range were exclusively non-filter cigarettes.

he American Cancer Society did not collect information on changes in smoking behaviour during follow up of the entire cancer prevention study-II cohort. We therefore restricted our mortality follow up to six years (1982-8) to reduce possible mis-classification of exposure due to quitting or brand switching during longer follow up. However, we were able to assess changes in the smoking status of 14 523 men and 15 509 women who reported current smoking at enrolment in the initial CPS-II cohort in 1982 and were also enrolled in the subsequent CPS-II nutrition cohort in 1992.For this subgroup, we computed the proportions of current smokers in each tar category in 1982 who had quit smoking 10 years later.

We used Cox proportional hazards methods to estimate hazard ratios and 95% confidence intervals of mortality from lung cancer in people who had never smoked, former smokers, and current smokers of very low, low, and high tar brands, relative to smokers of brands with tar ratings of 15-21 mg (medium tar). Former smokers were stratified into those who had quit aged <= 35 years, aged 35-54 years, and aged >= 55 years. All statistical analyses were performed separately for men and women.

In our proportional hazards analyses of mortality from lung cancer among current, former, and never smokers we adjusted for multiple covariates that reflected possible differences in participants' demographics, dietary practices, occupational exposures, or medical histories. Demographic covariates included exact age at enrolment, race, education, and marital status. Dietary covariates included intake of vegetables, citrus fruits, and vitamins A, C, and E. Occupational covariates included whether the most recent job was blue collar (such as car mechanics and construction workers) and whether the participant had been employed in an occupation with high asbestos exposure (such as pipe fitters and shipyard workers) for >= 10 years. Other indicator variables were a history of chronic bronchitis, emphysema, heart disease, stroke, and diabetes and self report of being currently sick, taking heart drugs, or pain in the legs during walking that went away with rest. All covariates except exact age at enrolment were modeled as categorical variables, where missing values were coded as separate categories.

Excluding participants who had never smoked, we further performed multivariate proportional hazards analyses of current and former smokers that adjusted not only for demographic, dietary, occupational, and medical history covariates but also for age when they began smoking and the average number of cigarettes smoked a day.

Finally, in a series of sensitivity analyses of current smokers only, we restricted our analysis to people who had smoked their current brand for a minimum of 5 or 10 years; excluded smokers with a history of emphysema; excluded participants who reported any smoking related condition (emphysema, chronic bronchitis, heart disease, use of heart drugs, stroke, diabetes, claudication, currently sick); varied the definition of the tar categories to include 8 mg tar brands in the very low tar category and 15 mg brands in the low tar category; and estimated hazard ratios without controlling for the average number of cigarettes smoked a day. The latter analysis examined the view that a study of risk of lung cancer in relation to type of cigarette smoked should exclude number of cigarettes smoked a day as a covariate because smokers of lower tar and nicotine brands may compensate by smoking more cigarettes a day.