Risks shoot up when virus particles accumulate in buildings, but it’s not clear how best to improve ventilation.

When Lidia Morawska leaves home, she takes with her a slick, shoe-sized device that provides some sobering insights about the restaurants and offices she visits. Outside these buildings, her carbon dioxide monitor reads just above 400 parts per million (p.p.m.). But indoors is a different story.

Even in a seemingly spacious, high-ceilinged restaurant, the number sometimes shoots up as high as 2,000 p.p.m. — a sign that the room has poor ventilation and could pose a risk for COVID-19 infection. Visual cues can be deceptive, even for Morawska, an aerosol scientist from the Queensland University of Technology in Brisbane, Australia. “The general public has no idea about this,” she says.

The situation is no different inside cafes or kindergartens around much of the world, according to researchers who have wielded similar handheld CO2 meters. And that’s bad news for hopes of defeating the coronavirus SARS-CoV-2.