In an analysis based on more than 2000 deaths from lung cancer in Xuanwei, China, we found that incidence of and mortality from lung cancer were substantially higher among users of smoky coal in their household stoves than among users of smokeless coal. We also found a positive association between the average number of hours that a smoky coal user spent at home and lung cancer mortality. An inverse association between the age at which participants started cooking and lung cancer mortality was also observed.
Absolute risks of death from lung cancer of 18% and 20% were found among men and women using smoky coal. These risks are almost as high as those reported for heavy smokers in Western countries, ranging between 20% and 26%.20
In Cox regression models, lifelong use of smoky coal compared with smokeless coal was associated with a 36-fold increase in lung cancer mortality in men and a 99-fold increase in women. This difference of effect between men and women is mainly due to the sex difference in lung cancer mortality among smokeless coal users (see table 2). Lung cancer mortality among smoky coal users was similar regardless of sex and smoking status (table 2). This observation is consistent with the hypothesis that, with exposure to high levels of airborne carcinogens such as those produced by the combustion of smoky coal, smoking could exert only a weak additional influence on lung cancer risk.22
The low rates of lung cancer observed in the smokeless coal group were consistent with the low rates of lung cancer in Yunnan Province as compared with the national average.8
The reasons why Yunnan Province has low rates of lung cancer are not clear but could be in part related to relatively low levels of cigarette smoking in the Yunnan population.23
The smoking patterns in our cohort (8.4 and 7.5 cigarettes smoked per day among smokers in the smoky coal and smokeless coal groups, respectively) are consistent with those of the Yunnan population as a whole.
We found a positive association between time spent indoors at home and risk of lung cancer, which is consistent with a previous observation in a report that included part of this study population.11
Although this association is relatively clear (fig 2) its interpretation is not straightforward. The extent to which the average number of hours per day spent indoors at home can be considered a good proxy for the average intensity of exposure to emissions from smoky coal depends on the assumption that the levels of exposures in the houses are similar after adjusting for some characteristics of the dwelling (type of stove used and number of rooms in the house). Possible violations of this assumption would probably introduce non-differential misclassification of the exposure, resulting in an attenuation of the association. It could be also questioned whether the time spent indoors is associated with the time spent cooking. However, it should be noted that that the shape of the association was almost identical for men and women, and only a small proportion of men cooked (fig 2). For this reason, the observed association between time spent indoor at home and risk of lung cancer seems to be at least partially independent from being engaged in cooking.
We used the age at which participants started cooking as a proxy for the start of exposure to emissions from smoky coal during cooking. As all the models used in the present analyses were inherently adjusted by attained age, we could not include age at starting cooking and duration of cooking together in the models because of collinearity. Thus, the inverse association observed (see fig 3) could be due to a positive association between duration of cooking and risk of lung cancer or a higher susceptibility to exposures during cooking at a younger age, or a combination of both.
Strengths and limitations of the study
Our analysis has several strengths. Firstly, we were able to compare individuals exposed to a single type of coal for their entire lifetime. The detailed information obtained through the questionnaire also allowed us to account for the roles of several possible confounders in the analysis, such as smoking, occupation, education, family history of lung cancer, a previous diagnosis of chronic respiratory diseases, and type of stove used in the household.
One possible limitation of the study is potential recall bias related to surrogate respondents. As most of the participants with lung cancer were dead at the time of the interview, their information was gathered through surrogate responders, which could have introduced recall bias. However, analyses using data only from participants who were alive at the time of the interview were consistent with the results of the primary analysis, suggesting a small effect, if any, from recall bias. Moreover, since this is a rural area, the population is stable. Most participants lived in one to two residences over their lifetime, so differential recall of coal source seems improbable.
There is some evidence that during the 1970s lung cancer may have been under-diagnosed in rural China.7
For example, it is possible that some cases of lung cancer could have been misdiagnosed as other types of respiratory disease. As such, it is possible that the absolute risks from lung cancer in the cohort are underestimated (see supplementary figures in data supplement on bmj.com).
Comparisons with other studies
Recently, a meta-analysis and a pooled analysis summarised the risk of lung cancer associated with household coal burning for heating and cooking, and highlighted the importance of geographical variation.4
The results of the present study provide additional evidence that different coal types are associated with different carcinogenicity. Carcinogenic polycyclic aromatic hydrocarbons (PAHs), methylated PAHs, and nitrogen-containing heterocyclic aromatic compounds were found in abundance in the particles emitted from smoky coal combustion.6
During combustion, these contaminants are potentially released into the air in their original or oxidised forms. The quality of coal that is used in households around the world varies markedly because of differences in local coal deposits.4
The results of our study underline the importance of evaluating the carcinogenic potential of the different types of coal and taking actions to minimise exposure to the most hazardous ones.
Conclusions and policy implications
The results of this study, which was carried out in a large population with a long period of observation, show that the domestic use of smoky coal is associated with a substantial increase of the lifetime risk of developing lung cancer. This finding has important implications for public health. The use of less carcinogenic types of coal or other fuels can translate into a substantial reduction of lung cancer risk. Additional studies are warranted to better characterise the carcinogenic potentials of various coal types.
What this already known on this topic
- Coal and biomass fuels are used for household cooking and heating by about 3 billion people worldwide
- The risk of lung cancer associated with household coal burning shows a substantial heterogeneity by geographical location because of the use of different coal types
What this study adds
- The domestic use of smoky coal compared with the use of smokeless coal was associated with a more than 30-fold increase in the risk of developing lung cancer in Xuanwei County in China and is likely to represent one of the strongest effects of environmental pollution reported for cancer risk in any population
- Use of less carcinogenic types of coal or alternative fuel sources would translate to a substantial reduction of lung cancer risk