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Clean Air at Home: Air Purifier Size Calculator for Wildfire Smoke

Health disclaimer: this page is general information, not medical advice. If you have asthma, COPD, heart disease or another health condition, are pregnant, or have symptoms from smoke, follow your doctor's advice and your local officials' instructions. Call 911 for trouble breathing or chest pain.

Find the right purifier size for your room in seconds, using the EPA's wildfire smoke rule: a tobacco-smoke CADR of at least two-thirds of the room's square footage.

Clean water is the other half

This site's main guide is the Water Freedom System, a written plan for collecting water from the air for backup drinking water.

See the Water Freedom System

Purifier size calculator

Room length (ft) Width (ft) Ceiling (ft)

Heavy or frequent smoke (size up to CADR = room area, as AHAM suggests)

How the 2/3 rule works

EPA's sizing table (for an 8-foot ceiling) asks for a CADR of 65 for 100 sq ft, 130 for 200, 195 for 300 and 260 for 400. For wildfire smoke, use the tobacco smoke CADR because it represents the smallest particles (PM2.5). The Association of Home Appliance Manufacturers (AHAM) suggests a CADR equal to the room area if you get heavy smoke.

Reading the Air Quality Index (AQI)

The AQI is EPA's index for outdoor air. At 100 or below, air is generally satisfactory. Above 100, it is unhealthy first for sensitive groups, then for everyone. Check your area at AirNow.gov, including the Fire and Smoke Map.

Set up a cleaner air room (EPA and CDC)

Pick a room you can close off from outside air, and run the purifier there continuously on high.
If you have central air, use a MERV 13 or higher filter if your system can take it, and set it to recirculate or close the outdoor intake.
Avoid burning anything indoors: candles, incense, gas, propane or wood. Avoid frying or broiling, smoking and vacuuming during smoke events.
A DIY box-fan filter is a lower-cost option. Use a box fan made since 2012 with a UL or ETL mark, and never leave it running unattended.
If you must go outside in smoke, wear a well-fitting NIOSH Approved respirator. Always follow evacuation orders.

Do not use ozone generators

Some "air purifiers" deliberately make ozone, sometimes sold as "activated oxygen" or "energized oxygen". EPA and the California Air Resources Board (CARB) advise against using them where people or pets are. Ozone irritates the lungs, can worsen asthma, does not remove particles, and can react with indoor chemicals to form formaldehyde. Ionizers and electrostatic cleaners can also release some ozone. Choose a CARB-certified air cleaner.

Purifiers and kits

Levoit HEPA purifiers

Check the smoke CADR against your calculator result and the CARB certification.

See Levoit purifiersPLACEHOLDER: LEVOIT

AirDoctor purifiers

Larger units for bigger rooms. Check the CADR and CARB listing.

See AirDoctor purifiersPLACEHOLDER: AIRDOCTOR

Printable clean air kit

Our smoke-day checklist, filter log and cleaner-air-room plan.

Get the printable kit on EtsyPLACEHOLDER: ETSY_CLEAN_AIR_KIT

Sources

EPA, Guide to Air Cleaners in the Home: epa.gov
EPA, How to Create a Cleaner Air Room at Home: epa.gov
EPA, Indoor Air Filtration (wildfire smoke): epa.gov
EPA Wildfire Smoke Course, Preparing for Smoke and Heat (2/3 rule, AHAM): epa.gov
EPA, Ozone Generators that are Sold as Air Cleaners: epa.gov
AirNow, AQI Basics: airnow.gov
CDC, Safety Guidelines: Wildfires and Wildfire Smoke: cdc.gov
California Air Resources Board, Hazardous Ozone-Generating Air Purifiers: arb.ca.gov

FAQ

What size air purifier do I need for wildfire smoke?

EPA says to choose a tobacco-smoke CADR of at least two-thirds of the room's area in square feet. A 10 x 12 ft room (120 sq ft) needs a CADR of at least 80. Size up for ceilings above 8 ft or heavy smoke.

Are ozone generators safe air purifiers?

No. EPA and the California Air Resources Board advise against using ozone generators in occupied spaces. Ozone irritates the lungs and does not remove particles. Choose a CARB-certified air cleaner.

What AQI is unhealthy?

Above 100 is unhealthy for sensitive groups, above 150 is unhealthy for everyone, and above 300 is hazardous (AirNow).

Should I run my purifier all the time during smoke?

Yes. EPA advises running it continuously on the highest fan setting in a room with doors and windows closed, when it is safe to do so.