One of the things we’ve learned over the past year is to be wary of the air around us — especially indoors. If other people are around, exhaling, they’re filling the space with their breath. If one of those people has COVID-19, they could be filling the space with infectious breath.

There’s a way to make indoor spaces safer, though: improving the ventilation to make sure the air doesn’t stay trapped. That way, any potentially infectious particles quickly gust away, instead of lingering for someone else to breathe in.

Experts say one way to measure how well-ventilated certain spaces are is by checking how much carbon dioxide is in the air. People exhale carbon dioxide, so the amount of it in a room gives you an idea of how much of the air is made up of other people’s breath. It’s not a perfect measure of danger — it won’t tell you if there is actually virus around — but it’s a pretty good proxy for how risky a room could be. It’s easy to check a room’s carbon dioxide levels: all you need is a small, portable monitor.

To test this idea, the Verge Science team took one of those monitors all around Brooklyn, New York, to check out the ventilation at local grocery stores, bagel shops, and bars. Watch our latest video to see what we found.