| Summary: | Background: Under-5 mortality rate is an important indicator in Millennium Development Goals and Sustainable Development Goals. To date, no nationally representative studies have examined the effects of fine particulate matter (PM2.5) air pollution on under-5 mortality. Objective: To investigate the association of short-term exposure to PM2.5 with under-5 mortality from total and specific causes in China. Methods: We used the national Maternal and Child Health Surveillance System to identify under-5 mortality cases during the study period of 2009 to 2019. We adopted a time-stratified case-crossover study design at the individual level to capture the effect of short-term exposure to daily PM2.5 on under-5 mortality, using conditional logistic regression models. Results: A total of 61,464 under-5 mortality cases were included. A 10 μg/m3 increase in concentrations of PM2.5 on lag 0–1 d was significantly associated with a 1.15% (95%confidence interval: 0.65%, 1.65%) increase in under-5 mortality. Mortality from diarrhea, pneumonia, digestive diseases, and preterm birth were significantly associated with exposure to PM2.5. The effect estimates were larger for neonatal mortality (<28 days), female children, and in warm seasons. We observed steeper slopes in lower ranges (<50 μg/m3) of the concentration-response curve between PM2.5 and under-5 mortality, and positive associations remained below the 24-h PM2.5 concentration limit recommended by WHO Air Quality Guidelines and China Air Quality Standards. Conclusions: This nationwide case-crossover study in China demonstrated that acute exposure to PM2.5 may significantly increase the risk of under-5 mortality, with larger effects for neonates, female children, and during warm seasons. Relevant control strategies are needed to remove this roadblock to achieving under-5 mortality targets in developing countries.
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