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Although numerous studies have shown increased risk of mortality from elevated temperatures for adults, limited studies have examined temperature's effect on mortality for infants. Our study investigated the city-specific and overall effects of ambient temperature on infant mortality in seven major cities in Korea, 2004–2007.Birth cohort using a linked birth and death records included 777,570 births with 557 all-cause deaths. We estimated city-specific hazard ratios for each city using an extended Cox proportional hazards model with time-dependent covariates. Then we combined city-specific hazard ratios to generate overall hazard ratio across the seven cities using a Bayesian hierarchical model. Stratified analyses were conducted by cause of death (total and SIDS), exposure period (whole gestation, each trimester, lifetime, 1 month before death, and 2 weeks before death), sex, and maternal characteristics.Overall across the cities, we found significantly positive associations between ambient temperature during 1 month before death or 2 weeks before death and infant mortality from total or SIDS. The overall hazard ratio of infant mortality from total deaths and SIDS for a 1 °C increase during 1 month before death was 1.52 (95% CI, 1.46–1.57) and 1.50 (95% CI, 1.35–1.66), respectively. We also found suggestive evidence that some factors such as mother's age may modify the association.Our findings have implications for establishment of policy to reduce the risk of infant mortality from high ambient temperature under climate change.We found positive association between temperature and risk of infant mortality for total deaths or SIDS in Korea.We found associations between heat exposure during 1 month or 2 weeks before death and infant mortality.Some factors such as mother's age may modify the association.