Long-term solid fuel use and risks of major eye diseases in China: A population-based cohort study of 486,532 adults

Background Over 3.5 billion individuals worldwide are exposed to household air pollution from solid fuel use. There is limited evidence from cohort studies on associations of solid fuel use with risks of major eye diseases, which cause substantial disease and economic burden globally. Methods and findings The China Kadoorie Biobank recruited 512,715 adults aged 30 to 79 years from 10 areas across China during 2004 to 2008. Cooking frequency and primary fuel types in the 3 most recent residences were assessed by a questionnaire. During median (IQR) 10.1 (9.2 to 11.1) years of follow-up, electronic linkages to national health insurance databases identified 4,877 incident conjunctiva disorders, 13,408 cataracts, 1,583 disorders of sclera, cornea, iris, and ciliary body (DSCIC), and 1,534 cases of glaucoma. Logistic regression yielded odds ratios (ORs) for each disease associated with long-term use of solid fuels (i.e., coal or wood) compared to clean fuels (i.e., gas or electricity) for cooking, with adjustment for age at baseline, birth cohort, sex, study area, education, occupation, alcohol intake, smoking, environmental tobacco smoke, cookstove ventilation, heating fuel exposure, body mass index, prevalent diabetes, self-reported general health, and length of recall period. After excluding participants with missing or unreliable exposure data, 486,532 participants (mean baseline age 52.0 [SD 10.7] years; 59.1% women) were analysed. Overall, 71% of participants cooked regularly throughout the recall period, of whom 48% used solid fuels consistently. Compared with clean fuel users, solid fuel users had adjusted ORs of 1.32 (1.07 to 1.37, p < 0.001) for conjunctiva disorders, 1.17 (1.08 to 1.26, p < 0.001) for cataracts, 1.35 (1.10 to 1.66, p = 0.0046) for DSCIC, and 0.95 (0.76 to 1.18, p = 0.62) for glaucoma. Switching from solid to clean fuels was associated with smaller elevated risks (over long-term clean fuel users) than nonswitching, with adjusted ORs of 1.21 (1.07 to 1.37, p < 0.001), 1.05 (0.98 to 1.12, p = 0.17), and 1.21 (0.97 to 1.50, p = 0.088) for conjunctiva disorders, cataracts, and DSCIC, respectively. The adjusted ORs for the eye diseases were broadly similar in solid fuel users regardless of ventilation status. The main limitations of this study include the lack of baseline eye disease assessment, the use of self-reported cooking frequency and fuel types for exposure assessment, the risk of bias from delayed diagnosis (particularly for cataracts), and potential residual confounding from unmeasured factors (e.g., sunlight exposure). Conclusions Among Chinese adults, long-term solid fuel use for cooking was associated with higher risks of not only conjunctiva disorders but also cataracts and other more severe eye diseases. Switching to clean fuels appeared to mitigate the risks, underscoring the global health importance of promoting universal access to clean fuels. Author summary Why was this study done? Household air pollution from solid fuel use has been linked to higher risks of cataracts and a range of acute eye symptoms, but most previous studies used relatively crude exposure assessment methods and cross-sectional or case-control designs and were relatively small. The relationships of long-term solid fuel use with common eye diseases other than cataracts, including conjunctiva disorders, keratitis, and glaucoma, are poorly understood. What did the researchers do and find? We analysed data from 486,532 adults aged 30 to 79 years recruited from 10 areas of China into the China Kadoorie Biobank during 2004 to 2008 to assess the associations of self-reported long-term solid fuel use for cooking with risks of conjunctivitis, cataracts, disorders of sclera, cornea, iris and ciliary body (DSCIC), and glaucoma during approximately 10-year follow-up. Multivariable logistic regression was used to estimate odds ratios comparing long-term clean fuels and solid fuels users, as well as those who had switched from solid to clean fuels prior to the initial baseline survey. Long-term solid fuel use was associated with 32%, 17%, and 35% higher risks of conjunctiva disorders, cataracts, and DSCIC, respectively, but not associated with glaucoma. Individuals who had switched from solid to clean fuels appeared to have smaller risks than those who used solid fuels persistently. What do these findings mean? To our knowledge, this is one of the first cohort studies on the relationships between long-term solid fuel use and risks of multiple common eye diseases. Our findings support a significant association between solid fuel use and cataracts, but the strength of association appeared to be considerably weaker compared to that observed in previous studies; the associations with conjunctiva disorders and DSCIC indicate that solid fuel use may have more extensive harm on eye health, which should be further investigated.

Automated summary

Long-term solid fuel use for cooking in Chinese adults was associated with higher risks of conjunctiva disorders, cataracts, and other severe eye diseases, compared to clean fuel users. Switching to clean fuels appeared to mitigate these risks.