  Objectives: The primary purpose of this study is to analyze the relationship
between the first occurrence of hospitalization for cardiovascular disease
(CVD) and particulate matter less than 2.5 micrometre in diameter (PM2.5)
exposure, considering average PM2.5 concentration and the frequency of high
PM2.5 concentration simultaneously. Methods: We used large-scale cohort data
from seven metropolitan cities in South Korea. We estimated hazard ratios (HRs)
and 95% confidence intervals (CIs) using the Cox proportional-hazards model,
including annual average PM2.5 and annual hours of PM2.5 concentration
exceeding 55.5 microgram/m3 (FH55). Results: We found that the risk was
elevated by 11.6% (95% CI, 9.7-13.6) for all CVD per 2.9 microgram/m3 increase
of average PM2.5. In addition, a 94-h increase in FH55 increased the risk of
all CVD by 3.8% (95% CI, 2.8-4.7). Regarding stroke, we found that people who
were older and had a history of hypertension were more vulnerable to PM2.5
exposure. Conclusion: Based on the findings, we conclude that accurate
forecasting, information dissemination, and timely warning of high
concentrations of PM2.5 at the national level may reduce the risk of CVD
occurrence.
