In this population-based case-control study of lung cancer in Xuanwei, China, we found that lung cancer risk was much higher among smoky coal users than smokeless coal and wood users. This was true especially for users of Laibin coal. There was substantial heterogeneity in risk estimates for the different subtypes of smoky coal that were used. To the best of our knowledge, this is the first time that lung cancer risk has been reported by specific coal subtypes in an epidemiologic study.
An increased risk of lung cancer has been observed for subjects exposed to combustion products from indoor coal burning in several previous case-control studies, with risks varying from 1.03 to 18.75.15;24–27
The reason for this variation in risk across studies has not been systematically addressed as yet, but it could plausibly be explained, at least in part, by exposure to different types or levels of carcinogens present in coals from different locations.
Incomplete coal combustion can produce respirable particles, as well as many volatile and nonvolatile organic compounds, such as BaP, benzene, and formaldehyde.28
The health threat posed by a given coal subtype depends largely on its composition.21
This in turn is a consequence of its plant precursors, the environment in which it was produced, and the chemical/physical processes that the precursor organic matter went through over time. The fact that we found a wide variation in lung cancer risk associated with the different coal subtypes strongly suggests that coal subtypes’ composition and combustion products may be different. Further, indoor BaP concentrations positively and significantly correlated with ORs of lung cancer for the different coal subtypes. This suggests that in Xuanwei at least, there is wide variation in exposure to BaP by coal type. Even though the measurements were limited, they suggested a plausible and coherent link between exposure to BaP, and quite possibly other carcinogens, and lung cancer. To fully evaluate risk in this population requires collection of detailed exposure data on coal type, usage, and personal habits in the home including cooking practices and other activities that could influence exposure intensity and duration. A new case-control study of lung cancer among non-smoking women in this population is currently in progress. This includes detailed exposure assessment, and will provide an opportunity to replicate and extend the findings reported here.
This study has several strengths. It is a population-based case-control study with very high participation rates, conducted in a rural, stable population with very little exposure to automotive or industrial air pollution. Most study subjects lived in only one or two residences over their lifetimes, and most households used only one type of fuel for cooking and heating. Thus, it is very unlikely that our results were affected appreciably by exposure misclassification. This contrasts with studies carried out in urban populations, which are far more mobile than the Xuanwei population, which often use multiple types of fuels, and which are exposed to complex, changing mixtures of ambient air pollution.
Our study also has several limitations. As this is a case-control study, recall bias is a potential concern. However, as mentioned above, this is unlikely to have occurred as most subjects only lived in 1–2 residences over their lifetime, and differential recall of coal source seems improbable. There were surrogate respondents for 17% of the cases and only 0.4% of the controls, and reporting bias could conceivably have affected the results. However, results from analyses of lung cancer risk by coal type, after excluding proxy interviews, were essentially identical to results reported for the entire study population. Further, the great majority of surrogates, 87%, were the subject’s spouse, parent, or child, who well knew where he or she purchased coal.
It is possible that lung cancer cases in districts that were distant from diagnosing hospitals were less likely to receive a lung cancer diagnosis. To explore the influence of this potential bias, we conducted a sensitivity analysis, restricting the data set to study subjects who lived relatively close to diagnosing hospitals, and found that risk estimates in the reduced data set were similar to risk estimates in the full data set.
In addition, among the 498 patients diagnosed with lung cancer, only 39% of the diagnoses had cytological or histological confirmation, and disease misclassification could have occurred. However, other diseases that might have been responsible for the clinical symptoms and abnormal chest x-rays of potential study subjects were carefully considered by diagnosing physicians. Also, the clinical course was very similar in diagnosed lung cancer patients with and without cytologic/histologic confirmation.20;25
Further, any misclassification is unlikely to be associated with type of coal, so it should – be nondifferential in nature. Restricting the analysis to cases diagnosed by cytology or histology yielded very similar results. Finally, most cases were interviewed in the hospital and almost all controls were interviewed at their homes, raising the possibility of recall bias. Again, however, this would have been unlikely, because Xuanwei residents consistently purchased coal from the mine closest to home.
We found a modest effect of smoking with lung cancer risk in this study. Our results are consistent with previous findings in this population, including relatively weak observed effects found among smokers in the highest risk coal subtypes (data not shown).9;10;20
We hypothesize that combined exposure to tobacco smoke and very high indoor air pollution levels tends to attenuate the observed effect of smoking. Analyses are underway to test this hypothesis formally. Most importantly, we note that the pattern of variation of risk by coal subtype was similar in men, almost all of whom smoke, and women, essentially none of whom smoke. In addition, the smoking frequencies among male controls for different coal types were very similar (ranging from 81–100% in the 10 smoky coal subgroups), even though lung cancer risks for these subtypes varied substantially. Finally, risk estimates for coal subtypes were essentially unchanged among men after adjustment for tobacco use. These observations indicate that the observed relationship between exposure to coal and lung cancer risk could not be due to smoking.
The sampling procedure for control selection could be a potential source of bias. Although population weighting factors were applied for communes, they were not applied for large or small farming groups. Thus, our strategy of randomly selecting farming groups implied that individuals living in larger farming groups would be underrepresented among controls. However, populations of large and small farming groups were quite similar throughout Xuanwei. The population of a large farming groups varied from about 2000 to 3000, and for small farming groups varied from several hundred to about 1000. Therefore, we seriously doubt whether our sampling strategy introduced appreciable bias into the results.
An International Agency for Research on Cancer (IARC) monograph working group has recently concluded that indoor emissions from household combustion of coal are carcinogenic to humans (Group 1) and are associated primarily with an increased risk of lung cancer.6
Our study of coal types on lung cancer risks provides further evidence on the link between coal use and lung cancer risk and suggests that the association varies dramatically between different types of coal.
The quality of coal that is used in households around the world varies dramatically across countries and regions due to differences in local coal deposits. Little attention has been paid to the types of coal that are used by households and commercial users in developing countries, with regard to health hazards to people who are in closest proximity to the coal. Our study suggests that household coal fuels should be evaluated for their carcinogenic potential and that steps should be taken to minimize exposure to hazardous coals.
We found some evidence that BaP levels in air were associated with increased risk of lung cancer. However, the analysis was based on a small number of data points. Further, coal combustion products are a complex mixture, and can contain other carcinogenic or co-carcinogenic compounds, so it is possible that more than one type of exposure has contributed to the striking lung cancer excess in this region. Additional studies are underway to identify and characterize the full range of potential carcinogens (e.g., polycyclic aromatic hydrocarbons, particulates, and heavy metals) present in coal combustion products, and to study their dose-response relationships with lung cancer in a new, larger study of lung cancer among nonsmoking women in this region of China. In addition, this study will be able to evaluate genetic modification of these relationships, which should provide new insights into the underlying mechanism of the association, as well as increase our understanding of lung cancer risk for distinct subgroups of the population.
In summary, our results show substantial variation in risk of lung cancer by different types of coals used in Xuanwei. These findings strongly suggest that the carcinogenic potential of smoky coal varies by coal type, and strongly caution against overgeneralizing results obtained in one location to other locations. Thus, these findings provide an important justification for additional research into, and comparison of, indoor air pollution health effects in multiple locations.