Beyond the Mean: Non-Linear Effects of Air Pollution on Health Outcomes

This paper uses high-frequency data on fine particulate matter air pollution (PM 2.5) to study the effects of high pollution on health outcomes in Mexico City. We combine hourly monitoring station data on air pollution and weather conditions with a rich dataset of 10 million health episodes between 2003 and 2019, including deaths, hospitalizations, and urgent care visits. We disaggregate daily mean concentrations of PM 2.5 using the daily share of hours with PM 2.5 concentration above each WHO threshold to uncover a positive non-linear and convex relationship between hourly air pollution concentrations and same-day respiratory health outcomes of all severities. Specifically, a 1% increase in the share of hours with PM 2.5 concentrations above the highest WHO interim threshold (IT1) increases the number of respiratory deaths, hospitalizations, and urgent care visits per 1 million inhabitants by 0.001, 0.0008, and 0.024, respectively. We find that hours above IT1 have effects on respiratory health outcomes that are 20 to 30 times greater than those of additional hours above the air quality guideline, the most restrictive (i.e. lowest) WHO threshold. Furthermore, one additional hour a day with PM 2.5 above IT1 has the same effects on respiratory health outcomes as does increasing the daily average concentration of PM 2.5 in Mexico City by 41 µg/m3. We find that the effects of PM 2.5 on respiratory mortality and morbidity are distributed differently across ages and that the effect of PM 2.5 on respiratory deaths is driven by individuals with lower educational attainment.