Global heatwaves: A scoping review on the trend, characteristics, impacts on air pollution and health.

With the global climate change, heatwave events have become severe challenges to ecological system and human health. This study presents an integrated analysis and summary on the typical heatwave events since 2000, and elucidates the definitions and classifications of heatwaves, global trends in frequency and intensity, regional characteristics and their impacts on air pollution and human health. It reveals that the global frequency and intensity of heatwave events show an increasing trend. During heatwave events, temperature, humidity, solar radiation, and atmospheric circulation factors exert notable influences on ozone (O3), nitrogen-containing pollutants, and secondary particulate matter. O3 concentrations exhibit a positive correlation with temperature during heatwaves, however, the rate of increase slows down or is even suppressed under extreme high temperatures (O3 suppression). Heatwaves promoted the generation of nitrogen-containing pollutants, particularly reactive nitrogen through localized diffusion control, high temperatures, wildfires, and intense solar radiation. Secondary organic aerosol (SOA) formation relies on the photochemical oxidation and heterogeneous-phase reaction, with the contribution of isoprene weakening and that of terpenes enhancing under high temperatures. Heatwaves influence nitrate and sulfate in different ways: the former tends to exist in gas phase due to high temperature, while the latter is influenced by the combined effects of temperature and humidity. During heatwaves, high temperatures and O3 exert extensive negative effects on human health resulting in risks of cardiovascular and respiratory diseases, heatstroke and neurological abnormalities. Global society and local governments should pay more attention to the vulnerable populations such as the children, elders, pregnant women and low-income groups. More protective measures or policies related to extreme heatwaves should be formulated, such as establishing a high-temperature warning system, increasing green areas, providing high-temperature subsidies and relief funds.