Long-term exposure to fine particulate matter and natural-cause and cause-specific mortality in Japan

Background: A number of studies have linked long-term exposure to particulate matter with aerodynamic diameter <2.5 m (PM_2.5) with mortality, but most of these studies were conducted in Europe and North America. Studies in Asian countries had been conducted at relatively high exposures. We evaluated the association of long-term exposure to PM_2.5 and natural-cause and cause-specific mortality in Japan, where PM_2.5 levels are relatively low compared with levels in other Asian countries. Methods: A cohort of 75,531 participants underwent basic health checkups in Okayama City in 2006 or 2007. We followed the participants until the end of 2016. Average PM_2.5 levels from 2006 to 2010 were obtained and assigned to the participants by geographical location. We used the Cox proportional hazard models to estimate hazard ratios for a 5-μg/m³ increase in PM_2.5 levels for natural-cause or cause-specific mortality, adjusting for potential confounders. Results: PM_2.5 exposure was associated with increased risk of mortality; the hazard ratios were 1.29 (95% confidence interval = 1.18, 1.41) for mortality from natural causes, 1.16 (1.02, 1.32) for cardiorespiratory mortality, and 1.63 (1.13, 2.34) for lung cancer mortality. PM_2.5 exposure was more strongly associated with cardiorespiratory mortality from hypertension, pneumonia and influenza, and chronic obstructive pulmonary disease than with ischemic heart disease or cerebrovascular disease. Elderly participants and smokers tended to have higher effect estimates. Conclusion: Long-term exposure to PM_2.5 can increase the risk of natural-cause, cardiorespiratory, and lung cancer mortality in Japan.