As students go back to school this fall and workers return to offices after summer vacations, we will need healthy indoor air to keep us all safe. In May, the US Centers for Disease Control (CDC) reiterated its advice to ventilate indoor air to reduce the spread of Covid-19, this time recommending a firm target of five exchanges of air per hour. This advice seems to be so clearly common sense that you wouldn’t dare to question it. But constantly bringing in fresh outdoor air in winter will make it very hard to keep indoor air humid, with potentially disastrous consequences.
