The effects of indoor and outdoor air pollution on the prevalence of adults’ respiratory diseases in four Chinese cities: a comparison between 2017–2018 and 1993–1996

Background Over the past decades, both ambient and household air pollution have changed in several aspects, including the emission sources and the concentrations of pollutants, in many Chinese cities. It is unknown whether these changes are associated with changes in health conditions, especially given changes in other factors due to rapid economic growth. Methods Two cross-sectional surveys were conducted in two periods spanning more than twenty years (1993-1996 vs. 2017-2018) in four Chinese cities of Chongqing, Wuhan, Lanzhou, and Guangzhou. Data were collected regarding adults' respiratory disease, smoking status, education, occupation, and household characteristics. Ambient air pollution data were obtained for each study. We first used logistic regression models to construct the district-specific adjusted disease prevalences. In the second stage, first-difference regression models were employed to examine whether the change in respiratory diseases prevalences was associated with the change in outdoor air pollution and indoor air pollution surrogates. Results A total of 7,557 and 9,974 households were participating in Period 1 (1993-1996) and Period 2 (2017-2018), respectively. Compared to Period 1, we found substantial reductions in the ambient air pollution concentrations, and a suggestive improvement in cooking-related indoor air pollution in Period 2. We observed decreases in the district-specific covariate-adjusted prevalences of both asthma and chronic bronchitis among participants, with an average reduction of 3.6% (range: 0.0% to 24.3%). From Period 1 to Period 2, one percent decrease in the proportion of cooking with coal was associated with a 19.0% (95% CI, 0.96-37.04%) decrease in the prevalence of males' chronic bronchitis and a 1.86% (0.69-3.04%) increase in the prevalence of females' asthma. Little evidence was observed regarding the potential health benefits associated with the decreases in ambient air pollution levels. Conclusions The substantial reduction in household use of coal for cooking might be an important contributor to the decrease in adults' respiratory disease prevalence from 1993-1996 to 2017-2019 in four Chinese cities. Changes in this indoor air pollution source, along with other risk factors for respiratory diseases, may have masked respiratory health benefits associated with reductions in outdoor air pollution levels.