As countries around the world have struggled to contain COVID-19, scientists have wondered whether air pollution could increase death rates among those with the disease. Fine particulate air pollution — from traffic emissions, industrial pollution and forest fires — is the most commonly studied pollutant. These particles are very small — roughly three per cent as wide as a human hair. They are carcinogens, and exposure increases the risk of developing and dying from cardiovascular and respiratory disease. Patients with COVID who have these diseases are more likely to die than patients who don’t. So, it is possible that fine particulates may increase the risk of death among those with COVID-19.