In 2001, a major study of human activity patterns found that people in the US spend roughly 90 percent of their time indoors. It is safe to say that, in the age of Covid-19, that number is even higher. (Here in the Roberts household, it feels like we’ve hit 105 percent.) We also do most of our breathing inside. So it’s a little odd that we don’t think more about indoor air quality. Outdoor air is the subject of titanic legal and regulatory battles going back decades. The six common air pollutants covered by the Clean Air Act — ground-level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide (SO2), and nitrogen dioxide (NO2) — have fallen an average of 74 percent since the Act was passed in 1970.