Wildfire Smoke Filtration at Home

How do you keep wildfire smoke out of a house?

Keep wildfire smoke out by closing the house, stopping outdoor air from being pulled in, and filtering what is already inside. The EPA's wildfires and indoor air page lists the steps: close windows and doors, set any system with a fresh-air intake to recirculate or shut its outdoor damper, and run filtration as much as you can.

Colorado's health department gives the same order. CDPHE's wildfire smoke page says to close the outdoor air intake or use recirculate mode, run the system fan continuously, and set up a clean room: one room where people spend most of their time, kept shut and filtered. In a valley that has lived through fires such as the 2020 Grizzly Creek Fire in Glenwood Canyon, it pays to pick that room before the smoke arrives.

When the air outside improves, even for a few hours, EPA suggests airing the house out by opening windows or the fresh-air intake and running the kitchen and bath fans. Then close up again before the smoke returns.

Does a MERV 13 filter help with wildfire smoke?

Yes: EPA and CDPHE both name MERV 13 as the filter to use for smoke in a forced-air system. EPA's wording is a MERV 13 filter "or as high a rating as your system fan and filter slot can accommodate." CDPHE adds that MERV 13 or higher is best for the fine particles in smoke.

Fit matters as much as the rating. EPA says the filter should sit snugly so air cannot slip around its edges, and it should never be bent or crushed to fit. If you are not sure your blower can push air through a denser filter, EPA suggests asking an HVAC technician which rating the system can handle.

A few points come straight from furnace manuals. Carrier's 59SC5A installation manual says the airflow arrow on the filter points toward the blower, and that the furnace must not operate filterless or with its filter access panel open; Carrier files that warning under carbon monoxide and fire hazards. The same manual limits untrained owners to basic tasks such as cleaning and replacing filters. When in doubt at the furnace, stop and have a qualified technician do it.

During heavy smoke, EPA says to keep spare filters on hand and change them more often than the maker's schedule. A filter that looks gray and loaded after a smoky week is doing its job; replace it rather than waiting for the usual interval.

PlumbingSupplyAndMore's indoor air quality section (part of Upvalley Heat's ownership group) lists pleated furnace filters from MERV 8 up to MERV 13 in common 1-inch and 2-inch sizes (category read October 2026). Measure the filter now in your furnace, which prints its size on the frame, and match length, width and depth exactly.

How big a portable air cleaner do you need?

Size a portable air cleaner by its clean air delivery rate (CADR) for smoke against the floor area of the room. EPA's page on air cleaners in the home gives a minimum CADR for each room size, based on 8-foot ceilings.

The label on most cleaners shows three CADR numbers: tobacco smoke, dust and pollen. EPA says to use the tobacco smoke figure, because it stands for the smallest particles. Mountain homes often have vaulted living rooms; EPA notes that ceilings above 8 feet call for a higher CADR than the table shows.

Skip anything sold as an ozone generator. EPA says to avoid air cleaners and HVAC filters that intentionally make ozone, a lung irritant, and notes that some ionizers, electrostatic precipitators and plasma units may also give off ozone. Place the cleaner where people spend the most time and run it on its highest speed as much as you can.

Do DIY box-fan filters work?

Yes, within limits. EPA's DIY air cleaner research found that a box fan with a MERV 13 filter attached is a cost-effective way to cut smoke indoors, and that a cardboard shroud, a 4-inch filter instead of a 1-inch one, or two to five filters raise the delivery rate further.

Safety testing with Underwriters Laboratories found no fire hazard in the box fans tested, even with loaded filters and both sides blocked for seven hours. EPA still says to use only newer certified box fans (2012 models or later), which have fused plugs and thermal cutoffs, and to keep them clear of obstructions. EPA also notes that commercial cleaners with a known CADR are usually preferred, and that any air cleaner is one part of a plan, not a substitute for ventilation.

How do you read the AQI during wildfire smoke?

Read the AQI for particle pollution, which covers PM2.5, the fine particles in smoke. AirNow's AQI basics splits the index into six bands, and a value of 100 or below is generally thought of as satisfactory.

Smoke thresholds and filter numbers

• 101AQI where "Unhealthy for Sensitive Groups" beginsAirNow
• 151AQI where "Unhealthy" begins for everyoneAirNow
• MERV 13Filter rating EPA names for smoke in a central systemEPA
• 65 cfmMinimum smoke CADR for a 100 sq ft roomEPA
• 2012Oldest box fan model year EPA advises for DIY cleanersEPA

The six AirNow bands run Good (0 to 50), Moderate (51 to 100), Unhealthy for Sensitive Groups (101 to 150), Unhealthy (151 to 200), Very Unhealthy (201 to 300) and Hazardous (301 and higher). Smoke in a narrow valley can change by the hour, so check the current reading before opening up the house, and follow CDPHE's health advice for your group.

What about HRVs, ERVs and fresh air during smoke?

An HRV or ERV pulls outdoor air in on purpose, so during smoke it counts as one of the fresh-air intakes EPA and CDPHE say to close. Learn from the owner's manual how your unit's controls lower or pause ventilation, and turn it back up once the AQI drops; the HRV and ERV ventilation page covers how these units run.

Check the manual before switching anything off for days. Broan's owner's manual for its HRV150S, HRV190S and ERV180S units says that where ventilation ducts run through an unheated space such as an attic, the unit must keep operating except during maintenance or repair. In that case, ask a qualified technician how to cut outdoor air safely during smoke.

Smoke season also overlaps with other indoor-air jobs. A shut house in winter dries out fast, which the humidifier page covers, and any home with gas appliances needs working alarms, as set out on the carbon monoxide safety page. The indoor air overview ties the three together.

Lift-line questions

Next runs

• Folder: Dry air, smoke, CO and ventilation in one place
• Indoor air: Carbon monoxide alarms and Colorado's alarm law
• Indoor air: HRV and ERV ventilation for tight homes
• Cold snaps: Safe checks when the heat stops