Exploring the short-term role of particulate matter in the COVID-19 outbreak in USA cities

The role of particulate matter (PM) in the COVID-19 pandemic is currently being discussed by the scientific community. Long-term (years) exposure to PM is known to affect human health by increasing susceptibility to viral infections as well as to the development of respiratory and cardiovascular symptoms. In the short-term (days to months), PM has been suggested to assist airborne viral transmission. However, confounding factors such as urban mobility prevent causal conclusions. In this study, we explore short-term relationships between PM concentrations and the evolution of COVID-19 cases in a number of cities in the United States of America. We focus on the role of PM in facilitating viral transmission in early stages of the pandemic. We analyzed PM concentrations in two particle size ranges, < 2.5 {micro}m, and between 10 and 2.5 {micro}m (PM2.5 and PM10 respectively) as well as carbon monoxide (CO) and nitrogen dioxide (NO2). Granger causality analysis was employed to identify instantaneous and lagged effects of pollution in peaks of COVID-19 new daily cases in each location. The effect of pollution in shaping the disease spread was evaluated by correlating the logistic growth rate of accumulated cases with pollutants concentrations for a range of time lags and accumulation windows. PM2.5 shows the most significant results in Granger causality tests in comparison with the other pollutants. We found a strong and significant association between PM2.5 concentrations and the growth rate of accumulated cases between the 1st and 18th days after the report of the infection, peaking at the 8th day. By comparing results of PM2.5 with PM10, CO and NO2 we rule out confounding effects associated with mobility. We conclude that PM2.5 is not a first order effect in the cities considered; however, it plays a significant role in facilitating the COVID-19 transmission. We estimate that the growth rate of COVID-19 cases would be risen by 12.5% if PM2.5 is increased from 25 to 35 {micro}g m-3.