Joint Effects of Heatwaves and Elevated Ground-Level Ozone on Mortality Risk and Population Vulnerability

Air pollution and extreme thermal conditions present compounding risks to human health, yet their interactive effects on respiratory outcomes require further specification. This study investigated the combined impact of heatwaves and elevated ambient ozone () on pulmonary disease mortality (ICD-10 J00–J99) in Seoul, South Korea, using daily time-series data from 2001 to 2019. Daily death counts were analyzed across warm-season days (May–September) stratified by demographic subgroups. A high-ozone day was defined as an 8-hour daily maximum exceeding 0.06 ppm, while a heatwave day was defined according to NOAA criteria as a maximum daily heat index  sustained for at least two consecutive days. Differences in daily mean pulmonary mortality and lag structures (up to 5 days) were assessed using stratified descriptive statistical comparisons. Both heatwaves and high ozone were independently associated with increased pulmonary mortality, with ozone-related impacts persisting for 1 to 2 days post-exposure. Simultaneous exposure to heatwaves and elevated ozone led to a synergistic increase in mortality that exceeded additive expectations, particularly among older adults. For individuals aged   years, mortality on Lag 1 following combined exposure reached 6.38 deaths/day compared to 4.50 deaths/day under non-heatwave, low-ozone conditions—yielding a net excess mortality of 1.88 deaths/day (additive expectation: 1.62 deaths/day). Stratified analyses showed elevated vulnerability among middle-aged (41–65 years) unmarried males, older (66–75 years) males, and older females with lower educational attainment. For individuals aged  years, heightened risk was evident across all demographic subgroups, suggesting that advanced physiological frailty is the primary driver of susceptibility. Factors such as reduced self-care capacity, limited social support, and poorer access to information represent plausible interpretations for these sociodemographic disparities. These findings highlight the need for integrated heat–pollution warning systems and targeted public health strategies to protect vulnerable populations from concurrent environmental hazards.