By Arjun Mehta, M.Tech. Environmental Engineering — Air Quality Monitoring & Emission Control. Last updated: September 17, 2026.
Short answer: yes, but how much they help depends on the purifier, the room, and how long you run it. Here’s what the research actually says before you buy one.
How HEPA filters work
A true HEPA filter captures at least 99.97% of particles at the hardest-to-catch size (0.3 microns) — well below PM2.5, which is 2.5 microns and larger. In lab conditions, that’s close to total removal. Real rooms aren’t lab conditions, though: air leaks in from outside, the purifier only cleans air that passes through it, and filter performance drops as the filter loads up with captured particles over a smoke event.
What the studies found
Results vary more than marketing pages suggest. Field studies of portable HEPA cleaners running for 18 to 24 hours during wildfire smoke events found indoor PM2.5 drops ranging from about 48% to 78%. Other studies found reductions as high as 63% to 88%, while some real-world residential tests found smaller gains — around 15% to 45% — depending on room size, how well the purifier was sized to the space, and whether doors and windows stayed shut. The U.S. EPA recommends portable HEPA air cleaners specifically to supplement, not replace, your home’s HVAC system during smoke events.
Why the range is so wide
Three things drive most of the difference: sizing, sealing, and time. A purifier rated for a bedroom won’t meaningfully clean a great room. An open window or a leaky door lets outdoor smoke back in as fast as the purifier removes it. And a purifier switched on for an hour during a multi-day smoke event does far less than one left running continuously. Filter condition matters too — a clogged filter loses efficiency, so during extended smoke events, check and replace filters more often than the manufacturer’s normal schedule.
What this means for you
- Match the purifier’s CADR (Clean Air Delivery Rate) to your room size — an undersized unit won’t keep up.
- Run it continuously during smoke events, not just when the air “smells” bad.
- Close windows and exterior doors; even a cracked window undoes a lot of the benefit.
- Skip ionizers and ozone generators marketed as air “purifiers” — some produce ozone, which is itself a lung irritant and regulated pollutant.
- In a pinch, a box fan with a MERV-13 filter taped to the intake (a “DIY box fan filter,” a design the EPA has studied) can meaningfully cut indoor PM2.5 at a fraction of the cost of a commercial unit.
A simple chart worth picturing: indoor vs. outdoor PM2.5 over 24 hours, one line with a HEPA purifier running continuously and one without — the gap between the two lines is the real-world benefit, and it widens the longer the purifier runs.
The takeaway
Air purifiers genuinely reduce indoor PM2.5 during smoke events, but “HEPA” on the box isn’t a guarantee of any specific percentage. Size it to your room, seal your space, and run it the whole time smoke is present — that combination matters more than which brand you buy.
Related reading: how PM2.5 and PM10 differ in health effects, and how the AQI scale translates pollutant concentrations into the number you see on your phone.
Sources: U.S. EPA — Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors, IQAir — Can air purifiers reduce indoor PM2.5 levels?, npj Clean Air — Fine particulate matter levels and HEPA filtration in Los Angeles homes during a wildland-urban-interface fire.
Researched and fact-checked using the primary sources above; reviewed by our editorial team for accuracy before publication.
