Air pollution is largely attributed to anthropogenic aerosols, with the role of natural aerosols, including sea salt, dust, and other terrestrial emissions considered to be less important. However, natural aerosols have strong geographic gradients and this suggests that spatially invariant air quality guidelines may handicap regions close to natural sources. The impacts of changes since industrialization that have led to an Earth system heavily influenced by human emissions are well recognized in climate research. Most prior work has focused on assessing changes in anthropogenic aerosol radiative forcing over the industrial era by quantifying pre-industrial (PI) aerosol radiative properties. No effort has been attempted to assess the impact of pristine aerosols on present-day air quality. I use CMIP5/6 to construct a view of pre-industrial "pristine" air quality, including fine particulate matter with diameters less than 2.5 μm (PM2.5). 

Under pristine conditions, PM2.5 levels over regions in geographic proximity to dust sources, including parts of Africa and Asia, exceed World Health Organization air quality guidelines. We estimate that this pristine air pollution, which is unassociated with human activities, impacts up to about one billion people globally. The results show that absolute air quality thresholds may unfairly bias countries close to natural sources and that policies targeting anthropogenic emissions may not yield clean air. (Zhao, X., Allen, R. J., & Thomson, E. S. 2021. Earth's Future)