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Smoke-ready home
Wildfire smoke indoors is an air-filtration problem. It is not R327 and not a listed vent. EPA’s wildfire pages prefer a commercial portable cleaner with a known clean-air delivery rate, or a MERV 13 HVAC filter if the furnace can take it. A DIY box-fan cleaner is the backup when those are not available or affordable. Use a 2012-or-newer fan with a UL or ETL mark. Hardening the soffit does not filter PM2.5.
You are in a bedroom with the windows shut, the sky the color of ash, and a box fan still in its cardboard. The fan plus a furnace filter is EPA’s DIY cleaner. The attic vent is an ember path. They are not the same object.
This page is indoor smoke, not a vent listing. Do not photograph a neighbor’s house. Do not treat this as R327.3.2.
Why it matters
Smoke events outrank most detailing searches in late summer. People tape furnace filters to a 2008 box fan and leave it running overnight. EPA’s research page says not to use pre-2012 fans for that job. People also buy a listed eave vent and expect the living room to clear. The vent keeps embers out of the attic. It does not scrub the air you breathe.
How it works
Fine particles in smoke affect heart, lungs, and other organs. Filtering indoor air removes particles. EPA names three tools:
- A high-efficiency HVAC filter, run often.
- A portable air cleaner sized to the room.
- A DIY box-fan-and-filter unit when the first two are missing.
Commercial units with a measured CADR are typically preferred because the number tells you the room size. DIY CADR is usually unknown and varies by fan, filter, and how you tape it.
EPA does not recommend electronic air cleaners (ionizers, electrostatic precipitators) for smoke. They may produce ozone. For store-bought portables, EPA points to California Air Resources Board–certified units because they produce little or no ozone.
A clean room is a bedroom you can close off, without cooking or smoking, with a cleaner running. Evacuate if local authorities say so. Filtration is not a shelter-in-place order.
The paths
| Option | When EPA says use it | Safety / size rule |
|---|---|---|
| HVAC MERV 13+ | Forced-air system that can take the filter | Confirm with the manufacturer or a technician. Run the fan “On.” |
| Portable cleaner | Most homes | Smoke CADR at least two-thirds of the room’s square footage. CARB-certified. |
| DIY box-fan cleaner | Portables not available or affordable | 2012-or-newer fan, UL or ETL mark, MERV 13 filter. Shroud helps. |
| Listed / mesh vents | Ember entry at the eave | Not a smoke filter. See mesh versus listed. |
1. HVAC first if you have it. EPA: choose a filter marked MERV 13 or higher. You may need a technician to confirm the highest filter the system will accept. Fit it snugly. Switch the thermostat fan from Auto to On. Change the filter when it is dirty or smells.
2. Portable cleaner for the room you live in. CADR for smoke at least two-thirds of the floor area in square feet. Run it continuously on the highest setting you can stand. Keep doors and windows closed when it is safe. Do not buy an ozone gadget for smoke.
3. DIY only with a new-enough fan. Attach one or more furnace filters to a box fan with tape, clamps, or a bungee. EPA suggests MERV 13. Use a 20-inch fan if that is the filter size. 2012 or later. UL or ETL mark. EPA does not recommend pre-2012 box fans for this. Newer fans have fused plugs and thermal cutoffs. A cardboard shroud on the front, a 4-inch filter instead of 1-inch, and two-to-five-filter designs improve the basic unit. Keep spare filters. Do not block airflow.
4. Make a clean room. EPA: pick a room large enough for the household, often a bedroom with a bath. Close it off. No cooking, no smoking. Put the cleaner there. Watch AirNow. If you cannot stay cool, power is out, or smoke still pours in, leave.
5. Do not confuse this with R327. Ember vents and mesh are exterior. This page is interior air. Do both jobs. Do not substitute.
What people mix up
- Treating a listed vent as a HEPA.
- Taping a filter to a pre-2012 fan and leaving it unattended.
- Buying an ionizer for smoke.
- Opening windows “for a minute” during a smoke event and wondering why the CADR lost.
- Skipping HVAC MERV 13 because the DIY video was faster.
How to check
- Read the fan housing for 2012+ and a UL or ETL mark before you tape a filter to it.
- Read the filter for MERV 13 or higher.
- For a portable, read the smoke CADR and the room’s square footage. Two-thirds is EPA’s sizing line.
- For HVAC, ask whether MERV 13 will starve the blower.
- Open AirNow Fire and Smoke Map.
- Leave the eave-vent catalog. That is mesh versus listed.
Limits
This page is not medical advice. People with heart or lung disease should ask their clinician. EPA did not make FireApproved a listing body. Power shutoffs are a different page. This page does not reprint EPA’s infographics. Do not run a DIY cleaner as a heat source.
Related records
Questions
Do listed ember vents clean indoor smoke?
No. R327.3.2 and ember- and flame-resistant vents limit embers entering the attic and underfloor. They are not a MERV 13 filter. Smoke that is already indoors is an indoor-air problem. See mesh versus listed vents.
What filter should I buy?
EPA’s indoor-air-filtration page says choose a HVAC filter marked MERV 13 or higher if the system can take it, and a portable cleaner with a smoke CADR at least two-thirds of the room area in square feet. For a DIY cleaner, EPA suggests MERV 13 and a box fan made since 2012 with a UL or ETL logo.
Can I use a pre-2012 box fan?
EPA does not recommend DIY air cleaners built with older model box fans (before 2012). Newer fans have fused plugs and thermal cutoffs. EPA’s research page says to look for a UL or ETL safety marking.
Is a DIY cleaner as good as a store-bought one?
EPA prefers commercial cleaners with a measured CADR because the rating tells you the room size. Well-built DIY units can perform similarly, but CADR is typically not known and units vary. Add a cardboard shroud, a thicker filter, or more filters to improve the basic design.