As of late September 2026, the reflex in my house here in Riverside hasn’t changed in years: the minute smoke drifts in, somebody throws the windows open to get the air moving and flush out whatever’s been sitting inside. Stale air out, fresh air in. I did it for years in this old ranch house, and it’s the reflex most of us have here in California.
Then I read the smoke tips from the South Coast AQMD, the air district page I check here in Riverside when the sky turns brown. The first thing it says to do if you smell smoke or see ash is “Remain indoors with windows and doors closed or seek alternate shelter.” It also says that if you can stay cool without them, avoid using a whole-house fan or a swamp cooler with an outside air intake. My swamp cooler pulls straight from outside. So on a smoke day my go-to cooling trick is part of the problem.
What EPA says
EPA’s guidance on reducing smoke exposure indoors is not subtle: “Close windows and doors: Keep doors and windows closed to keep smoke out. Use fans and air conditioning to stay cool.”
Why closed beats open
EPA’s material spells out what an open window does during a smoke event: in any home, if doors and windows are left open, particle levels indoors and outdoors will be similar. The open window doesn’t trade bad air for good. It lets the living room match the air outside.
Closing up isn’t a perfect seal either. One published study of wildfire smoke indoors notes that fire smoke particles can enter indoor spaces through continuous air exchange with the outdoors even when windows and doors are closed, and AirNow says that when smoke is heavy for a prolonged period, fine particles can build up indoors even though you may not be able to see them. That’s why the agencies pair closing up with filtering…