Chapter 4 of the Health Effects of Climate Change (HECC) in the UK: 2023 report. The results presented in the chapter have several implications for public health. Although air pollutant emission controls will reduce concentrations of some air pollutants (such as PM2.5 and NO2), there may be local increases in O3, which may be exacerbated during heatwaves. Therefore, provision of localised alerting and monitoring will become particularly important. Ensuring that public health professionals and other stakeholders have accessible and high- quality information to provide health advice and raise awareness will continue to be important.