Who is more vulnerable to effects of long-term exposure to air pollution on COVID-19 hospitalisation?

Objective: Factors that shape individuals’ vulnerability to the effects of air pollution on COVID-19 severity remain poorly understood. We evaluated whether the association between long-term exposure to ambient NO₂, PM_2.5, and PM₁₀ and COVID-19 hospitalisation differs by age, sex, individual income, area-level socioeconomic status, arterial hypertension, diabetes mellitus, and chronic obstructive pulmonary disease. Methods: We analysed a population-based cohort of 4,639,184 adults in Catalonia, Spain, during 2020. We fitted Cox proportional hazard models adjusted for several potential confounding factors and evaluated the interaction effect between vulnerability indicators and the 2019 annual average of NO₂, PM_2.5, and PM₁₀. We evaluated interaction on both additive and multiplicative scales. Results: Overall, the association was additive between air pollution and the vulnerable groups. Air pollution and vulnerability indicators had a synergistic (greater than additive) effect for males and individuals with low income or living in the most deprived neighbourhoods. The Relative Excess Risk due to Interaction (RERI) was 0.21, 95 % CI, 0.15 to 0.27 for NO₂ and 0.16, 95 % CI, 0.11 to 0.22 for PM_2.5 for males; 0.13, 95 % CI, 0.09 to 0.18 for NO₂ and 0.10, 95 % CI, 0.05 to 0.14 for PM_2.5 for lower individual income and 0.17, 95 % CI, 0.12 to 0.22 for NO₂ and 0.09, 95 % CI, 0.05 to 0.14 for PM_2.5 for lower area-level socioeconomic status. Results for PM₁₀ were similar to PM_2.5. Results on multiplicative scale were inconsistent. Conclusions: Long-term exposure to air pollution had a larger synergistic effect on COVID-19 hospitalisation for males and those with lower individual- and area-level socioeconomic status.