Source: GeoHealth
Small particulate matter (PM2.5) in air pollution raises the risks of respiratory problems, cardiovascular disease, and even cognitive decline. Heat waves, which are occurring more often with climate change, can cause heatstroke and exacerbate conditions such as asthma and diabetes. When heat and pollution coincide, they can create a deadly combination.
Existing studies on hot and polluted episodes (HPEs) have often focused on local, urban settings, so their findings are not necessarily representative of HPEs around the world. To better understand premature mortality associated with pollution exposure during HPEs at multiple scales and settings, Huang et al. looked at a global record of climate and PM2.5 levels from 1990 to 2019.
The team used data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2), which included hourly concentration measurements of PM2.5 in the form of dust, sea salt, black carbon, organic carbon, and sulfate particles. Daily maximum temperatures were obtained via satellite data from ERA5 (the fifth-generation European Centre for Medium-Range Weather Forecasts atmospheric reanalysis).
The researchers also conducted a meta-analysis of health literature, identifying relevant research using the search terms “PM2.5,” “high temperature,” “heatwaves,” and “all-cause mortality” in the PubMed, Scopus, and Web of Science databases. Then, they conducted a statistical analysis to estimate PM2.5-associated premature mortality events during HPEs.
They found that both the frequency of HPEs and maximum PM2.5 levels during HPEs have increased significantly over the past 30 years. The team estimated that exposure to PM2.5 during HPEs caused 694,440 premature deaths globally between 1990 and 2019, 80% of which occurred in the Global South. With an estimated 142,765 deaths, India had the highest mortality burden by far, surpassing the combined total of China and Nigeria, which had the second- and third-highest burdens. The United States was the most vulnerable of the Global North countries, with an estimated 32,227 deaths.
The work also revealed that PM2.5 pollution during HPEs has steadily increased in the Global North, despite several years of emission control endeavors, and that the frequency of HPEs in the Global North surpassed that of the Global South in 2010. The researchers point out that the study shows the importance of global collaboration on climate change policies and pollution mitigation to address environmental inequalities. (GeoHealth, https://doi.org/10.1029/2024GH001290, 2025)
—Sarah Derouin (@sarahderouin.com), Science Writer