Surviving Smoke Season: Indoor Air Quality
Wildfire smoke reaches cities hundreds of kilometres from any fire. Here's how to keep the air inside your home breathable for weeks at a time.
The problem is PM2.5
Wildfire smoke is dominated by fine particulate under 2.5 microns, which passes deep into the lungs and bloodstream. Health agencies in both Canada and the US now treat multi-week smoke events as routine. Indoor air is only cleaner than outdoor air if you make it so.
Seal, then filter
Close windows, seal obvious gaps, switch HVAC to recirculate, and upgrade the furnace filter to MERV 13. Then add HEPA purifiers, sized by CADR, to the rooms you sleep and spend the day in.
HEPA Air Purifier (large room)
A true-HEPA purifier with a high smoke CADR rating, sized to give 4–5 air changes an hour in your bedroom. Run continuously through smoke events and keep spare filters on hand.
Corsi-Rosenthal Box (DIY: box fan + 4 MERV-13 filters)
A box fan taped to a cube of four MERV-13 furnace filters. Peer-reviewed testing shows it rivals commercial purifiers costing several times more. Build one per room.
Measure it
A PM2.5 monitor tells you whether your purifiers are keeping up and when it's actually safe to open the windows. Outdoor AQI maps lag your street by hours.
Outside the house
NIOSH N95 or P100 respirators, properly fitted, for any time outdoors during a smoke event. Cloth and surgical masks do nothing for PM2.5.