The advice during smoke events is always the same: stay indoors, close your windows. Good advice if you control the building and can afford a filter. Most people breathing wildfire smoke have neither.
Han et al. (2025) found over a billion people globally face substantial indoor wildfire-PM2.5 days each year. Air purifiers reduce concentrations. Meeting WHO guidelines would cost an estimated $69 billion annually. But in rental units, aging schools, and care facilities, the person breathing the air almost never decides what happens to it. A teacher can smell something wrong in the classroom. Whether the filters get changed is somebody else's call, in some other office, on some other budget line.
The engineering exists. The question of who pays, and who decides, doesn't show up in any filtration study.
Closed windows aren't enough: Indoor PM2.5 stays at 55–60% of outdoor levels during smoke events with doors and windows shut. Older buildings leak more.
36,000 U.S. schools need HVAC updates nationally. Funding depends largely on local property taxes.
Evidence gap: Most filtration research covers portable air cleaners. Evidence on HVAC performance during smoke events is nearly nonexistent. Institutional buildings rely on HVAC.
No mandate: Wildfire PM2.5 isn't regulated like industrial emissions. ASHRAE's 2024 building smoke guideline is voluntary.
24,100 deaths per year in the U.S. linked to wildfire smoke PM2.5 (2006–2020 average). None listed as wildfire deaths on any death certificate.