Prevalence of smoking in 1990
Among surviving spouses interviewed in about 1990 the prevalence of smoking in men was high, and higher in rural than urban areas, but in women it was low, and lower in rural than urban areas (fig ). These prevalences are consistent with those in the 1984 and 1996 nationwide surveys of smoking prevalence.12–14
At ages 35-69 the prevalence of smoking remained fairly constant among men, but among women it was much lower at 35-39 than at 50-69. Over the past few decades young women, particularly in cities, have become much less likely to start smoking. The proportion who started to smoke before age 25 was 10% for all urban women born before 1940, but only 1% for those born in 1950-64. For rural women the prevalences were 4% for those born before 1940 and 2% for those born in 1950-64. These unexpected decreases are statistically reliable. For women born before 1920, 1920-9, 1930-9, 1940-9, 1950-9, and 1960-4 the respective proportions who began smoking before age 25 were 9.8%, 10.1%, 8.8%, 3.3%, 1.3% (91/6866), and 0.6% (6/988) for urban women and 3.9%, 4.3%, 3.9%, 2.9%, 2.2% (93/4291), and 2.5% (17/674) for rural women.
Figure 2 Smoking prevalence in 1990 among interviewees whose spouse had died in 1986-8
In general the excesses of neoplastic, respiratory, and vascular deaths among smokers exist because smoking can actually cause those diseases. Hence, the risk ratio (comparing smokers with non-smokers) for such diseases and the proportion of smokers among those dying from these diseases imply the proportion of all such deaths attributed to tobacco.
The main analyses are of deaths at ages 35-69 because medical information about the underlying cause of death for this group is likely to be more reliable than at older ages, and deaths at ages 35-69 produce a greater loss of life expectancy than deaths at older ages. The overall proportion of deaths attributed to tobacco, however, was similar for ages 35-69 and 70 and over.
Risk ratios for men
Figure shows the numbers of smokers and non-smokers among men aged 35-69 who died of lung cancer and those in the reference group (men who died from causes other than neoplastic, respiratory, or vascular disease). In the 24 cities 16
317 men died from lung cancer, 82% (13
440) of whom were reported to have smoked before 1980. In the reference group 30
709 men died, 60% (18
544) of whom were reported to have smoked. This suggests that lung cancer is about three times as common among urban smokers as among otherwise similar non-smokers (as crude division of 82/18 by 60/40 gives a risk ratio of 3), and with standardisation for age and city district this becomes 2.98 (SE 0.04, 99% confidence interval 2.9 to 3.1). This calculation does not, however, allow for any biases, the net effect of which would probably have been to decrease the risk ratio. Hence, the true value of the urban smoker:non-smoker lung cancer risk ratio may slightly exceed 3.
Figure 3 Numbers of deaths of smokers and non-smokers aged 35-69 from lung cancer and all causes other than neoplastic, respiratory, or vascular disease (reference group) together with relative risks and 99% confidence intervals. Results presented (more ...)
The findings were similar for rural men (fig ). Among the 3882 who died of lung cancer, 83% (3219) were reported to have smoked compared with 64% (14
208) of the 22
046 who died in the reference group. After age and county were standardised for, the risk ratio for lung cancer in rural men was 2.57 (0.07, 2.4 to 2.8). Again, however, the true risk ratio probably exceeds this.
The number of lung cancer deaths in men aged 35-69 ranged from only 80 in the large city of Xi’an up to 2961 in Shanghai (fig ). In general there is a remarkable consistency between the lung cancer risk ratios in different cities: all (and those of the two rural areas) are statistically compatible with the range 2.0 to 4.0.
Risk ratios for women
The age adjusted lung cancer risk ratios for women (right side of fig ) are similar to those for men. In the 24 cities 7300 women died from lung cancer, 42% (3080) of whom smoked; in the reference group 21
171 women died, 15% (3124) of whom smoked. After the bias caused by the strong correlation of smoking with age among women was allowed for the risk ratio was 3.24 (0.07, 3.1 to 3.5). Thus, for urban women (as for urban men) lung cancer was at least three times as common in smokers as in non-smokers. Among rural women, 21% (325) of the lung cancer deaths and 9% (1191) of the reference group deaths were in smokers. After age was adjusted for the risk ratio was 1.98 (0.13, 1.7 to 2.3). This twofold risk ratio for rural women was seen in both coastal and inland provinces.
Absolute rates in smokers and non-smokers
The age standardised (35-69) death rate from lung cancer for the whole population of each geographical area is known. This local lung cancer rate, combined with the local prevalence of smoking (in the reference group) and the smoker:non-smoker risk ratio can be used to calculate the local lung cancer rates for smokers and non-smokers. These are plotted against each other in figure . Even among non-smokers the lung cancer rates differ enormously between cities. For both sexes, for example, the rate in non-smokers at ages 35-69 in Xi’an was only 0.1 per 1000 (about the same as in American non-smokers5,6
), whereas in Harbin it was nearly 1 per 1000. Thus, although the risk ratio was about the same in both cities, the absolute excess of lung cancer produced by smoking was much greater in Harbin than in Xi’an. This was true for both men and women. The very high lung cancer death rate of 3 per 1000 female smokers in Harbin, which is based on large enough numbers to be statistically reliable, is three times the similarly age standardised rate of 1.1 per 1000 female smokers in a large US prospective study during the 1980s,5,6
and greatly exceeds the US national lung cancer death rates at these ages (0.6 per 1000 women, 1.4 per 1000 men) in 1990.
Figure 4 Death rates at ages 35-69 from lung cancer in smokers versus non-smokers, 1986-8. For women (right) the rates for six of the smaller cities have been averaged (dark square)
Although the rates of lung cancer in non-smokers varied tenfold between cities, the lung cancer mortality in smokers was consistently about three times that in non-smokers, producing the striking pattern in fig . Since most of these cities are large the general pattern of absolute risks is reliable. Apart from the consistently high rates in the cold northeast (Harbin, Jilinshi, Changchun, Shenyang), where there is lifelong heavy indoor exposure to domestic coal smoke and cooking fumes, the geographical pattern was irregular, with cities that are not far apart often having disparate disease rates. For example, although the large cities of Chongqing and Chengdu were in the same southwestern province, the rates of lung cancer in them differed fourfold.
Taking all cities together, the lung cancer mortality was 0.5 per 1000 non-smokers (0.52 men, 0.42 women) and 1.5 per 1000 smokers (1.54 men, 1.35 women, representing 12% of all deaths of urban smokers at these ages). For rural China, however, the absolute lung cancer rates were only about half as great, partly, we suspect, because some rural deaths from lung cancer in later middle age were not recognised as such.
Relative risks for specific diseases among men
Table gives risk ratios for men aged 35-69 in urban China, rural China, and, by a weighted combination of these, all China. About 70% of all deaths from cancer in China were from just four diseases: cancer of the lung, oesophagus, stomach, or liver. Each was significantly more common among smokers. For lung cancer the all China risk ratio was 2.72 (SE 0.05) for men aged 35-69, indicating that at non-smoker death rates about half (52.3%) of all deaths from lung cancer in middle aged men would have been avoided. The other risk ratios were 1.61 (SE 0.04) for oesophageal cancer, 1.35 (0.03) for stomach cancer, 1.40 (0.03) for liver cancer, and 1.51 (0.05) for an aggregate of five “minor” cancer sites that studies in other countries have associated with smoking (mouth, pharynx, larynx, pancreas, and bladder, each of which was significantly related to smoking in this study). Finally, the aggregate of all other neoplastic diseases was also somewhat related to smoking (risk ratio 1.24 (0.03)). Hence, taking all neoplastic diseases together, the overall risk ratio was 1.51 (0.02) and the attributable fraction 24.4% (and 18.7% at older ages). Thus, about a quarter of all deaths from cancer in middle aged men would have been avoided at non-smoker death rates.
Table 1 Numbers of deaths and smoker to non-smoker mortality ratios (standard errors) for urban, rural, and all China at ages 35-69 with summary calculations for ages 70
Chronic obstructive pulmonary disease (risk ratio 1.43 (0.03)) and respiratory tuberculosis (risk ratio 1.20 (0.04)) were also significantly related to smoking. For overall respiratory mortality (mainly from chronic obstructive pulmonary disease and tuberculosis) the risk ratio was 1.48 (0.03) in urban and 1.28 (0.03) in rural China. For all China the proportion of respiratory deaths attributable to smoking was 17.2% at ages 35-69 (and 24.6% at older ages). Respiratory diseases remain a common cause of death in men, particularly in rural China, so although smoking accounts for only about a sixth of such deaths in middle age and a quarter in old age, the absolute hazard is substantial.
Vascular mortality in China mainly involves stroke, which is slightly more common among smokers (risk ratio 1.17 (0.02)), or ischaemic heart disease (risk ratio 1.28 (0.03)). Rheumatic heart disease is also important, particularly in rural areas, and those dying from it were less likely to have smoked. But rheumatic heart disease severe enough to cause death in middle age may be associated with chronic symptoms starting in childhood that might discourage smoking, so the risk ratios among men aged 35-69 (0.77 (0.04) urban, 0.79 (0.05) rural) may reflect such discouragement rather than protection. (Rheumatic heart disease is, however, retained in the analyses of all vascular deaths in table , slightly weakening the apparent effects of tobacco.)
Daily number of cigarettes
Table relates the daily cigarette consumption when men last smoked to the risk ratios for death at ages 35-69. It is restricted to male smokers whose most recent habits involved only cigarettes (75% of the urban and 46% of the rural male smokers in table ) as other tobacco use is less easily quantified. Most were reported to have smoked exactly 10, 20, or 30 a day. In both urban and rural areas the strongest dose-response relations were for lung cancer (risk ratios for about 10, 20, or 30 cigarettes per day: urban 2.08, 3.59, and 6.92; rural 2.23, 3.65, and 7.26; both trends P<0.0001). In places where mortality from lung cancer was substantial even among non-smokers these sevenfold risk ratios would imply large absolute hazards among smokers.
Dose-response relations among men aged 35-69. Values are risk ratios (standard errors) comparing non-smokers with smokers whose most recent habits involved only cigarettes
Several of the other neoplastic and respiratory diseases also show strong (and significant) dose relations. This illustrates the causal nature of the associations of smoking with not only lung cancer but also cancer of the oesophagus, cancer of five “minor” sites, chronic obstructive pulmonary disease, and respiratory tuberculosis—indeed, men in urban China smoking more than 20 cigarettes a day had double the death rate from tuberculosis of non-smokers. For stomach cancer, liver cancer, stroke, and ischaemic heart disease the dose relations were weaker (though still significant). For the aggregate of all other vascular diseases, however, they were non-significantly reversed, perhaps because rheumatic heart disease discourages heavy smoking. But taking all neoplastic, respiratory, and vascular diseases together in men aged 35-69 (subtotal A-C in table ), the urban risk ratios of 1.28, 1.48, and 1.93 (with negligibly small standard errors) for about 10, 20, and 30 cigarettes per day show a strong trend, as do the corresponding rural risk ratios of 1.26, 1.42, and 1.87.
Age smoking began
Table relates the age when men reportedly began smoking to mortality at ages 35-69. It includes all male smokers, irrespective of the type or amount of tobacco, except for the 0.6% for whom the age smoking began was not reported. The lung cancer risk ratios were greatest for those reportedly starting before age 20, intermediate for those starting at ages 20-24, and least for those starting later. The trends were highly significant both in urban areas (respective risk ratios 4.11, 2.94, and 2.45; trend P<0.0001) and in rural areas (risk ratios 3.07, 2.62, 2.26; trend P<0.0001). The trends must, however, have been weakened by misreporting of the age smoking began because this information relates to habits several decades earlier, perhaps before the informant knew the dead person. Moreover, the great fluctuations in Chinese social circumstances during the decades before 1980, with large changes in cigarette sales, mean that few middle aged cigarette smokers who died in 1986-8 will have had consistent tobacco consumption since early adult life. In urban China, however, table shows that for those reportedly starting before age 20 (mean starting age 17) the excess of lung cancer associated with smoking was almost twice as great as for those starting later (age 20-24 or
25: mean starting age 24). The tendency for those who start early to smoke more accounts for only a small part of this difference.
Age at starting smoking: effect on death rates in men aged 35-69. Values are risk ratios (standard errors) comparing non-smokers with smokers
Except perhaps for liver cancer, the excesses of the other neoplastic and respiratory diseases among smokers tended to be greatest for those who started before age 20: for all respiratory disease the respective risk ratios were 1.91, 1.50, and 1.29 in urban and 1.44, 1.28, and 1.20 in rural China; both of these trends were highly significant (P<0.0001). But for vascular disease there was no apparent relation with the age smoking began. Taking all neoplastic, respiratory, and vascular deaths together, however, men reported to have started smoking before age 20 still had significantly greater risks than those reported to have started later.
Absolute risks among men
Figure shows the absolute risks in each area for neoplastic, respiratory, and vascular mortality and for all causes of death. For cancer as a whole the death rates among non-smokers varied twofold, but in most areas the overall cancer death rates among smokers were about 50% greater than those of non-smokers. Deaths from lung cancer account for about half of this excess cancer mortality among smokers.
Figure 5 Death rates of male smokers and non-smokers aged 35-69 in 1986-8 from four main categories of cause
Throughout China, respiratory disease is a major cause of adult death, even among non-smokers. The rural rates tend to be higher than the urban rates, and the rural respiratory mortality of 3.3 per 1000 non-smokers and 4.2 per 1000 smokers (fig ) implies a greater absolute tobacco hazard than the corresponding urban rates of 1.4 and 2.1. Values vary greatly among counties and cities, however, with particularly high respiratory death rates in the inland southwest.20,22
The small southwestern city of Zigong had extraordinarily high respiratory mortality among both non-smokers and smokers: at 1986-8 rates, one man in six would die before age 70 from chronic obstructive pulmonary disease (which includes respiratory heart disease) or some other respiratory condition. (As Zigong is relatively small, the overall urban rates would not have been much changed by its exclusion.) Respiratory death rates were comparably high in some of the rural counties within a few hundred kilometres of Zigong. Although vascular mortality in non-smokers varied widely between areas, the small absolute excess among smokers did not.
For non-smokers and smokers respectively the overall male mortality at ages 35-69 was 9.7 and 12.8 in urban China, 14.3 and 17.7 in rural China, and 12.9 and 16.2 in all China. At a constant death rate of 16.2 per 1000 smokers, 43.3% of those aged 35 would die before 70. If the fraction of these deaths attributed to smoking is 3.3/16.2 then at 1986-8 death rates 9% of male smokers would be killed by tobacco in middle age and at the death rates of the late 1990s the risk must already be at least 10%. But the death rates of smokers and of non-smokers are likely to diverge substantially in future so that for young men who now become persistent smokers the risk of being killed by tobacco in middle age will be much greater.
Results among women
Only 15% of the smokers who died at 35-69 were women. The results for women in some cities are therefore based on small numbers, and we combined the results for the six cities with fewer than 50 female smokers in the reference group as “other urban.” The risk ratios were similar for women and men (tables and ), but because fewer women smoked only 2.7% of the deaths of women aged 35-69 were attributed to smoking compared with 13.0% of those of men. The only substantial difference was that no significant association of stroke with smoking was seen in rural women, although the association with ischaemic heart disease remained. Likewise, the general pattern of absolute risks was similar among women (fig ) and men (fig ), except that in rural China there was no significant association of smoking with female vascular mortality. In urban areas the absolute hazards of tobacco were about as great for women as for men, but in rural areas they were only about half as big, chiefly because in rural areas women smoked less and started later (table ).
Table 4 Risk ratios of death from all causes for smokers compared with non-smokers among women aged 35-69 according to number of cigarettes smoked per day and age at starting smoking
Figure 6 Death rates of female smokers and non-smokers aged 35-69 in 1986-8 from four main categories of cause
Deaths attributed to tobacco
We applied the percentages of various diseases attributed to smoking in table to independent estimates28
of the numbers of deaths from these diseases at ages 30-69 and
70 (table ). This suggests that in 1990 there were about 0.6 million deaths from tobacco in China. (The difference between the age range 35-69 and 30-69 is unimportant, as the relevant diseases cause few deaths at ages 30-34.) The calculation can be done in several ways, but all give similar results. For example, it could have been calculated for the three broad catergories of neoplastic, respiratory, and vascular disease or, even more crudely, for the 13% of deaths in men and 3% in women in table . Alternatively, the cause specific death rates from the study could have been used (indicating fewer deaths from heart disase but more from cancer of the stomach or oesophagus). In each case, however, 0.6 million of the deaths in 1990 would be attributed to tobacco (0.5 million in men and 0.1 million in women and 0.3 million at ages 35-69 and 0.3 million at older ages). These deaths attributed to tobacco were due to respiratory disease in 0.3 million cases, neoplastic disease in 0.2 million, and vascular disease in 0.1 million. The estimate of 0.1 million vascular deaths due to tobacco is less reliable than that of deaths from neoplastic and respiratory diseases, but in other populations cigarettes have been shown to cause some vascular hazard. The main uncertainties lie not in the number of deaths at ages 35-69 but in the number of deaths at older ages, particularly from respiratory disease. Our estimate of 0.6 million deaths from tobacco in 1990 is based more directly on local epidemiological evidence than the previous WHO estimate of 0.8 million for 1990.18
Deaths attributed to tobacco/total deaths (thousands) for all China, 1990
The annual number of adult deaths in China is projected to rise from 7 million in 1990 (table ) to 9 million in 2000, mainly because of population growth.28
Hence even without any further growth in the percentage of deaths attributed to tobacco the absolute number of such deaths will rise to 0.8 million in 2000. The expected increase in the percentage attributable to tobacco will, however, further increase this figure so that during the first decade of the next century Chinese mortality from tobacco will probably be about 1 million a year.