Air pollution health inequalities in a low-carbon future

Understanding the health co-benefits of climate change mitigation and how they manifest across different socioeconomic groups is crucial to justify and prioritise future decarbonisation pathways to achieve net zero. In this work, we quantify future worldwide air quality and public health co-benefits of decarbonisation to limit end-of-century warming to either 2ºC (scenario SSP1-2.6) or 1.5ºC (scenario SSP1-1.9), relative to the middle-of-the-road pathway with a medium long-term radiative forcing target of 4.5 W m^-2 (scenario SSP2-4.5). We use simulated ambient fine particulate matter (PM_2.5) concentrations for the period 2015-2100 from the Coupled Model Intercomparison Project (CMIP6) experiments. We estimate the mortality burden attributable to exposure to ambient PM_2.5 using population attributable fractions of relative risk, incorporating projected changes in population demographics and per-capita GDP. We find that following a future decarbonisation pathway could produce substantial global reductions in population exposure to PM_2.5 pollution and associated premature mortality across all socioeconomic groups, with maximum health benefits achieved for middle-income populations (predominantly in Asia) around mid-century. Overall, the more moderate 2ºC-compliant mitigation scenario (SSP1-2.6) could reduce the global PM_2.5-attributable mortality burden by 24% in 2050 relative to SSP2-4.5, averting ~2.5M (95% uncertainty interval (UI): 2.1-2.8M) annual deaths worldwide. The more stringent 1.5ºC scenario (SSP1-1.9) could reduce the PM_2.5 mortality burden by 29% in 2050, averting ~2.9M (UI: 2.5-3.4M) annual deaths. The magnitude of the air quality and health benefits of reduced PM_2.5 pollution through decarbonisation vary with the socioeconomic status of the exposed population, with greater reductions in the PM_2.5 mortality burden in middle- and high-income regions (22%) than in the low-income region (15%). Overall, the disparity in PM_2.5 exposure between low- and high-income populations is predicted to reduce by 2100 under all three future scenarios. However, the global PM_2.5 exposure disparity is projected to increase up to mid-century under the SSP2-4.5 scenario, thus, immediate reduction in the disparity in the near term, is only achieved under a decarbonisation scenario. Despite overall reductions in global PM_2.5 exposure inequalities by the end of the century, the disparity in PM_2.5 exposure remains around 30%, with the low- and lower-middle-income populations continuing to experience PM_2.5 exposures that are over three times the WHO Air Quality Guideline.