Effects of ambient PM₁ air pollution on daily emergency hospital visits in China: an epidemiological study

China is experiencing severe ambient air pollution. However, few studies anywhere have examined the health effects of PM₁ (particulate matter with aerodynamic diameter <1 μm), which are a major part of PM_2·5 (particulate matter with aerodynamic diameter <2·5 μm) and even potentially more harmful than PM_2·5. We aimed to estimate the effects of ambient daily PM₁ and PM_2·5 concentrations on emergency hospital visits in China.

In this epidemiological study, we collected daily counts of emergency hospital visits from the 28 largest hospitals in 26 Chinese cities from Sept 9, 2013, to Dec 31, 2014. Ground-based monitoring data for PM₁ and PM_2·5 and meteorological data were also collected. Hospital-specific emergency hospital visits associated with PM₁ or PM_2·5 were evaluated with a time-series Poisson regression. The effect estimates were then pooled at the country level using a random-effects meta-analysis.

The mean daily concentration of PM₁ in all cities was 42·5 μg/m³ (SD 34·6) and of PM_2·5 was 51·9 μg/m³ (41·5). The mean daily number of emergency hospital visits in all hospitals was 278 (SD 173). PM₁ and PM_2·5 concentrations were significantly associated with an increased risk of emergency hospital visits at lag 0-2 days (cumulative relative risk [RRs] 1·011 [95% CI 1·006-1·017] for a 10 μg/m³ increase in PM₁ and 1·010 [1·005-1·016] for a 10 μg/m³ increase in PM_2·5). Slightly higher RRs of ambient PM₁ and PM_2·5 pollution were noted among women and children than among men and adults, respectively, but without statistical significance. Given a cause-effect association, 4·47% (95% CI 2·05-6·79) and 5·05% (2·23-7·75) of daily emergency hospital visits in China could be attributed to ambient PM₁ and PM_2·5 pollution, respectively.

Exposure to both ambient PM₁ and PM_2·5 were significantly associated with increased emergency hospital visits. The results suggest that most of the health effects of PM_2·5 come from PM₁.

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