After nearly two weeks of suffering through some of the worst air quality in the world, fresh winds blew the wildfire smoke out of Seattle, Portland, and other West Coast cities last weekend. About 1 in 7 Americans lived through at least one day of dangerous air quality this year, making this the worst smoke season on record.

It’s not just the air outside that’s bad when it gets smoky — it’s the air inside, too. People tend to think of air pollution as an “outside” problem created by wildfire smoke, car exhaust, and pollution from power plants, unaware of the hazards of household air pollution, which kills almost 4 million people worldwide prematurely every year.

Even if your windows and doors are shut, outdoor air seeps into your home through cracks, crawlspaces, and unfinished seams, and it brings pollution with it, said Delphine Farmer, a professor of atmospheric chemistry at Colorado State University.

The COVID-19 pandemic presents a complicated picture for indoor air. This spring, the lockdowns brought a period of clear skies and cleaner air as cars disappeared from the roads. This stretch of time, now called the “Anthropause,” improved outdoor air quality around the world.

The irony is that the pandemic has made some indoor air quality problems worse. More time at home means more time around mold spores, harsh household cleaners, and fumes from the gas-burning stove.

Researchers are just beginning to understand some of the complexities of indoor air, and they’re finding that its effects can linger and interact in unexpected ways. The emissions from what you cooked an hour or two ago can interact with the mopping you’re doing now “and do some interesting chemistry — but not one chemistry that one wants to actually breathe,” Farmer said.

Read the full article about indoor air quality by Kate Yoder at Grist.