As relentless summer heatwaves grip the region, a silent and potent health hazard lurks in the air: ground-level ozone (O3). While stratospheric ozone shields the Earth from harmful UV radiation, its presence near the surface makes it a significant air pollutant. High temperatures and intense sunlight, combined with vehicle emissions and industrial discharge, trigger photochemical reactions that rapidly elevate ozone concentrations.
Ozone’s Health Impacts During Summer
Extensive research has linked ozone exposure to a range of serious health issues, including heart disease, bronchitis, emphysema, asthma exacerbation, and reduced lung function. Even at low concentrations, ozone can cause immediate health problems such as chest pain, coughing, nausea, throat irritation, and wheezing.
The increasing frequency and intensity of heatwaves due to climate change are directly contributing to more days with high ozone levels. A joint study by Seoul National University and Ewha Womans University, published in the journal ‘Environmental Research,’ highlights the heightened mortality risk associated with simultaneous exposure to heatwaves and ozone.
Study Reveals Combined Risks
The research team analyzed mortality data from five to September in seven major South Korean cities (Seoul, Busan, Daegu, Incheon, Gwangju, Daejeon, and Ulsan) between 2014 and 2023. During this period, a total of 474,369 deaths were recorded, with cardiovascular diseases accounting for 94,749 and respiratory diseases for 48,406.
For the study, a heatwave was defined as a day with a maximum temperature exceeding 33°C (91.4°F). High-concentration ozone was determined by an 8-hour average exceeding 0.06 parts per million (ppm). Typically, an hourly average ozone concentration of 0.12 ppm or higher triggers an ozone advisory, 0.30 ppm prompts an ozone warning, and 0.50 ppm leads to a severe ozone warning.
The analysis revealed that mortality risk was generally higher on days with both heatwaves and high ozone levels compared to exposure to either factor alone. Specifically, during extreme heatwave conditions (when the maximum temperature was in the top 5% for the region) and simultaneous exposure to both heatwaves and ozone for three consecutive days, the overall mortality risk was estimated to be 11.2% higher than on days without either condition. For two consecutive days of exposure, the risk increased by 9.4%.
These risks were particularly pronounced in men and individuals over 65. The study also observed similar patterns for specific diseases. On the second day of simultaneous heatwave and high ozone exposure, the risk of death from cardiovascular disease increased by 5.3% to 9.0%, and on the third day, by 6.4% to 12.3%. In contrast, the impact of heatwaves on respiratory and emphysema-related deaths was comparatively less pronounced than that of ozone. A significant increase in mortality for these conditions was only observed when high ozone levels persisted for three or more consecutive days.
Understanding the Danger
Interestingly, the most dangerous situations were not always associated with the longest heatwaves. The research indicated that the overall health burden was greatest from two consecutive days of exposure. When the heatwave threshold was set at above 33°C, approximately 2,375 excess deaths were attributed to two consecutive days of exposure, and 1,772 to three consecutive days.
This suggests that heatwave response strategies should not solely focus on extreme, prolonged events. Repeated, even if shorter, periods of combined heatwave and ozone exposure can significantly strain public health resources. Researchers explained that the combined effect of these two environmental factors may lead to increased inflammation and oxidative stress within the body, thereby elevating the risk of cardiovascular and respiratory issues.
However, the study did not find a dramatic, exponential increase in risk – a clear synergistic effect – when heatwaves and ozone occurred together. The researchers concluded that both factors appear to increase mortality risk independently.
Recommendations for Public Health
Experts emphasize that with climate change increasing the frequency of heatwaves and the conditions favoring ozone formation (sunlight and high temperatures), current public health systems need to evolve. Early warning systems and integrated response strategies that consider both temperature and ozone levels are crucial, moving beyond solely focusing on temperature.
To mitigate ozone exposure, individuals are advised to limit outdoor activities and strenuous exercise during peak daylight hours. It is essential to check ozone forecasts and air quality information before going out. Given that masks are ineffective against ozone, children, the elderly, and individuals with heart or lung conditions should remain indoors when ozone levels are high.
