OBJECTIVES: Although there is substantial evidence for the short-term effect of fine particulate matter (PM_2.5) on daily mortality, few epidemiological studies have explored the effect of prolonged continuous exposure to high concentrations of PM_2.5. This study investigated how the magnitude of the mortality effect of PM_2.5 exposure is modified by persistent exposure to high PM_2.5 concentrations.METHODS: We analyzed data on the daily mortality count, simulated daily PM_2.5 level, mean daily temperature, and relative humidity level from 7 metropolitan cities from 2006 to 2019. Generalized additive models (GAMs) with quasi-Poisson distribution and random-effects meta-analyses were used to pool city-specific effects. To investigate the effect modification of continuous exposure to prolonged high concentrations, we applied categorical consecutive-day variables to the GAMs as effect modification terms for PM_2.5.RESULTS: The mortality risk increased by 0.33% (95% confidence interval [CI], 0.16 to 0.50), 0.47% (95% CI, -0.09 to 1.04), and 0.26% (95% CI, -0.08 to 0.60) for all-cause, respiratory, and cardiovascular diseases, respectively, with a 10 μg/m3 increase in PM_2.5 concentration. The risk of all-cause mortality per 10 μg/m3 increase in PM_2.5 on the first and fourth consecutive days significantly increased by 0.63% (95% CI, 0.20 to 1.06) and 0.36% (95% CI, 0.01 to 0.70), respectively.CONCLUSIONS: We found increased risks of all-cause, respiratory, and cardiovascular mortality related to daily PM_2.5 exposure on the day when exposure to high PM_2.5 concentrations began and when exposure persisted for more than 4 days with concentrations of ≥35 μg/m3. Persistently high PM_2.5 exposure had a stronger effect on seniors.