The Centers for Disease Control updated their “Ventilation in Buildings” guidelines to five air changes per hour in May to help prevent airborne disease transmission. Fresh, filtered and recirculated air proved to be effective tools in the fight against COVID-19.
During the Canadian wildfires however, fresh air became more of a liability at certain times of day. Many offices in the U.S. ended up having better air quality than homes in the same areas, owing to a better building envelope and air exchange rates that kept indoor air circulating, selectively drawing in outside air when air quality was better.