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Ambient air pollution, in the form of fine particulate matter (PM 2.5), poses serious population health risks. We estimate cross-national longitudinal models to test whether the negative relationship between life expectancy and PM 2.5 concentration is larger in nations with higher levels of income inequality. The dependent variable is average life expectancy at birth, and the focal predictor variables include PM 2.5 concentration, income inequality, and the two-way interaction between them. We also estimate the average marginal effects of PM 2.5 concentration from low to high values of income inequality, and the predicted values of life expectancy from low to high values of PM 2.5 concentration and income inequality. Results indicate that the negative relationship between life expectancy and PM 2.5 concentration is larger in nations with higher levels of income inequality, and the reductions in predicted life expectancy are substantial when both PM 2.5 concentration and income inequality are high. We suggest that the theoretical principles of Power, Proximity, and Physiology help explain our findings. This study underscores the importance in considering the multiplicative impacts of environmental conditions and socioeconomic factors in the modeling of population health. Unlabelled Image • PM 2.5 poses serious population health risks throughout the world. • We estimate longitudinal models of life expectancy for 136 nations. • Average life expectancy is negatively associated with PM 2.5 concentration. • Income inequality amplifies the negative association between them. [ABSTRACT FROM AUTHOR]