| Summary: | There have been few studies in non-western countries on the relationship between low levels of daily fine particulate matter (PM2.5) exposure and morbidity or mortality, and the impact of PM2.5 concentrations below 15 μg/m3, which is the latest World Health Organization Air Quality Guideline (WHO AQG) value for the 24-h mean, is not yet clear. We assessed the associations between low-level PM2.5 exposure and cardiorespiratory admissions in Japan. We collected the daily hospital admission count data, air pollutant data, and meteorological condition data recorded from April 2016 to March 2019 in 139 Japanese cities. City-specific estimates were obtained from conditional logistic regression models in a time-stratified case-crossover design and pooled by random-effect models. We estimated that every 10-μg/m3 increase in the concurrent-day PM2.5 concentration was related to a 0.52% increase in cardiovascular admissions (95% CI: 0.13–0.92%) and a 1.74% increase in respiratory admissions (95% CI: 1.41–2.07%). These values were nearly the same when the datasets were filtered to contain only daily PM2.5 concentrations <15 μg/m3. The exposure–response curves showed approximately sublinear-to-linear curves with no indication of thresholds. These associations with cardiovascular diseases weakened after adjusting for nitrogen dioxide or sulfur dioxide, but associations with respiratory diseases were almost unchanged when additionally adjusted for other pollutants. This study demonstrated that associations between daily PM2.5 and daily cardiorespiratory hospitalizations might persist at low concentrations, including those below the latest WHO AQG value. Our findings suggest that the updated guideline value may still be insufficient from the perspective of public health.
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