Health Consequences of Smoking
The health consequences of smoking are often assumed to be widely understood. In fact, ignorance of the magnitude of tobacco hazards is widespread in terms of both individual health and population policy. Thus, the salient aspects of tobacco epidemiology are outlined in this section.
Key Messages for the Individual Smoker
More than 50 years of epidemiology on smoking-related diseases have led to three key messages for individual smokers worldwide (Doll and others 2004; Peto and others 2003).
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The eventual risk of death from smoking is high, with about one-half to two-thirds of long-term smokers eventually being killed by their addiction.
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These deaths involve a substantial number of life years forgone. About half of all tobacco deaths occur at ages 35 to 69, resulting in the loss of about 20 to 25 years of life, compared with the life expectancy of nonsmokers.
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Cessation works: those adults who quit before middle age avoid almost all the excess hazards of continued smoking.
Worldwide, about 80 percent of deaths among the 2.7 billion adults over age 30 involve vascular, respiratory, or neoplastic disease. Smoking is associated with an increase in the frequency of many of these diseases, although important differences exist between and across populations. The following discussion focuses on the consequences of smoking on adult mortality. Detailed epidemiological reviews of worldwide mortality from smoking are found elsewhere (C. Gajalakshmi and others 2000; V. Gajalakshmi and others 2003; Gupta and Mehta 2000; Liu and others 1998; Niu and others 1998; Peto and others 1994).
Current Mortality and Disability from Smoking
Recent updates of indirect estimates of global tobacco mortality (Ezzati and Lopez 2003; M. Ezzati, personal communication,) indicate that in 2000, 5.0 million premature deaths were caused by tobacco. About half (2.6 million) of those deaths were in low-income countries. Males accounted for 3.7 million deaths, or 72 percent of all tobacco deaths. About 60 percent of male and 40 percent of female tobacco deaths were of middle-aged persons (ages 35 to 69).
In high-income countries and former socialist economies, the 1 million middle-aged male tobacco deaths were largely composed of cardiovascular disease (0.45 million) and lung cancer (0.21 million). In contrast, in low-income countries, the leading causes of death among the 1.3 million male tobacco deaths were cardiovascular disease (0.4 million), chronic obstructive pulmonary disease (0.2 million), other respiratory disease (chiefly tuberculosis, 0.2 million), and lung cancer (0.18 million). The specific numbers of deaths from tobacco and of total disability-adjusted life years (DALYs) by gender and World Bank region are shown in table 46.2. Disability estimates are not discussed here; however, disability is highly correlated with mortality in most settings.
[Table .]
Past and Future Trends in Mortality
In high-income and former socialist economies with more complete and reliable mortality statistics, one can measure the effects of increased smoking prevalence and subsequent decreases that have been observed among large numbers of adults. These changes are best documented by examining lung cancer mortality rates among young adults because lung cancer is not often misclassified with other causes of death at young ages and it is almost entirely attributable to smoking.
Age-Standardized Lung Cancer Mortality Rates
Age-standardized male lung cancer rates at ages 35 to 44 per 100,000 men in the United Kingdom had fallen from 18 in 1950 to 4 by 2000. In contrast, comparable French male lung cancer rates show the reverse pattern (Peto and others 2003; figure 46.1). In France, the increase in smoking occurred some decades later than in the United Kingdom, and declines in smoking began only after 1990. Similarly, a large increase in female lung cancer at young ages was avoided in the United Kingdom, but female lung cancer at young ages continues to rise in France.
[Figure
46.1]
Future increases in tobacco deaths worldwide are expected to arise from increased smoking by males in developing countries and by women worldwide. Such increases are a product of population growth and increased age-specific tobacco mortality rates, the latter relating to both smoking duration and the amount of tobacco smoked. Peto and others (1994) have made the following calculation: if the proportion of young people taking up smoking continues to be about half of men and one-tenth of young women, there will be about 30 million new long-term smokers each year. As previously noted, epidemiological studies in developed and developing countries suggest that half of these smokers will eventually die from smoking. However, if we conservatively assume that "only" about one-third of smokers die as a result of smoking, then smoking will eventually kill about 10 million people a year. Thus, for the 25-year period from 2000 to 2025, there would be about 150 million tobacco deaths, or about 6 million deaths per year on average; from 2025 to 2050, there would be about 300 million tobacco deaths, or about 12 million deaths per year.
Further estimations are more uncertain, but current smoking trends and projected population growth indicate that from 2050 to 2100 there will be an additional 500 million tobacco deaths. These projections for the next three to four decades are comparable to retrospective and early prospective epidemiological studies in China (Liu and others 1998; Niu and others 1998), which suggest that annual tobacco deaths will rise to 1 million before 2010 and to 2 million by 2025, when the young adult smokers of today reach old age. Similarly, results from a large retrospective study in India suggest that 1 million annual deaths can be expected from male smokers by 2025 (V. Gajalakshmi and others 2003). With other populations in Asia, Eastern Europe, Latin America, the Middle East, and (less certainly) Sub-Saharan Africa showing similar growth in population and age-specific tobacco death rates, the estimate of some 450 million tobacco deaths over the next five decades appears plausible. Almost all of these deaths will be among current smokers.
Benefits of Cessation
Current tobacco mortality statistics reflect past smoking behavior, given the long delay between the onset of smoking and the development of disease. The prevention of a substantial proportion of these tobacco deaths before 2050 requires adult cessation. For example, halving the per capita adult consumption of tobacco by 2020 (akin to the declines in adult smoking in the United Kingdom) would avert about 180 million tobacco deaths. Continuing to reduce the percentage of children who start to smoke will prevent many deaths, but its main effect will be on mortality rates in 2050 and beyond (figure 46.2; Jha and Chaloupka 2000a; Peto and Lopez 2001).
[Figure
46.2]
Substantial evidence indicates that smoking cessation reduces the risk of death from tobacco-related diseases. Among doctors in the United Kingdom, those who quit smoking before the onset of major disease avoided most of the excess hazards of smoking (Doll and others 2004). The benefits of quitting were largest in those who quit before middle age (between ages 25 and 34 years) but were still significant in those who quit later (between ages 45 and 54 years).
Cessation before middle age avoids more than 90 percent of the lung cancer risk attributable to tobacco, with quitters possessing a pattern of survival similar to that of persons who have never smoked. In the United Kingdom, among those who stopped smoking, the risk of lung cancer fell steeply with time since cessation. For men who stopped at ages 60, 50, 40, and 30, the cumulative risks of lung cancer by age 75 were 10 percent, 6 percent, 3 percent, and 2 percent, respectively (Peto and others 2000; figure 46.3). These results have been supported by a recent multicenter study of men in four European countries; for men who quit smoking at age 40, the study found that the excess lung cancer risk avoided was 85 percent, 91 percent, and 80 percent in the United Kingdom, Germany, and Italy, respectively (Crispo and others 2004). Smoking cessation is uncommon in most developing countries, but some evidence exists that, among Chinese men, quitting also reduces the risks of total and vascular mortality (Lam and others 2002).
[Figure
46.3]
