Prolonged short-term exposure to a high concentration of ambient fine particulate matter and effect modification on daily mortality

Background: Although the evidence for the effect of short-term mortality is very high, epidemiologic studies on the effect of prolonged continuous exposure to high concentrations are rare. This study aimed to investigate how the size of the mortality effect of PM2.5 would be modified when a high concentration period persisted.Methods: We used daily mortality counts (ICD-10, non-traumatic all-cause: A00-R99, respiratory: J00-J98, and cardiovascular: I00-I99), simulated levels of daily PM2.5, measured daily mean temperature, and relative humidity data in seven metropolitan cities from 2006 to 2019. Generalized additive models (GAMs) with quasi-Poisson distribution in seven cities and random-effects meta-analyses to pool city-specific effects were used to examine the short-term effects of PM2.5. To investigate effect modification by continuous exposure to prolonged high concentrations, we applied categorical consecutive day variables to the GAMs as effect modification terms of PM2.5. Subgroup analyses were performed to examine the variation in effect modification according to the age group.Results: The mortality risk according to increase of 10 μg/m3 of PM2.5 concentration was increased by 0.30% (95%CI, 0.13–0.49), 0.48% (95%CI, -0.09–1.05), and 0.23% (95%CI, -0.10–0.57) based on the model with 0-1 lag day, for all-cause, respiratory, and cardiovascular diseases, respectively. The risk for all-cause mortality per 10 μg/m3 increase of PM2.5 in the first and fourth consecutive days were significantly increased by 0.61% (95%CI, 0.17–1.06) and 0.34% (95%CI, 0.03–0.65). The effect modification was prominent in the ³65 years old group.Conclusions: We found a significantly increased risk of all-cause, respiratory, and cardiovascular mortality for daily PM2.5 exposure when the day of high concentration began and the days lasted for more than 4 days with high concentration of ≥35 μg/m3. The elderly showed a stronger effect of persistently high PM2.5.