We estimate the causal impact of air pollution on the incidence of sick leaves in a representative panel of employees affiliated to the Spanish social security system. Using over 100 million worker-by-week observations from the period 2005-2014, we estimate the relationship between the share of days an individual is on sick leave in a given week and exposure to particulate matter (PM10) at the place of residence, controlling for weather, individual effects, and a wide range of time-by-location controls. We exploit quasi-experimental variation in PM10 that is due to Sahara dust advection in order to instrument for local PM10 concentrations. We estimate that the causal effect of PM10 on sick leaves is positive and varies with respect to worker and job characteristics. The effect is stronger for workers with pre-existing medical conditions, and weaker for workers with low job security. Our estimates are instrumental for quantifying air pollution damages due to changes in labor supply. We estimate that improved ambient air quality in urban Spain between 2005 and 2014 saved at least €503 million in foregone production by reducing worker absence by more than 5.55 million days.