Wildfire smoke: keeping indoor air cleaner

Plan before the smoke arrives

The EPA recommends choosing a filtration option for your home before a smoke event, if possible, because the supplies may be hard to find during one. A short list worth keeping: a portable air cleaner sized to your main room, a spare set of HVAC filters rated MERV 13 or higher, tape or weatherstripping for leaky windows, and a way to check air quality. For the outdoor picture, the EPA points to the AirNow Fire and Smoke Map, your state air quality website or local news.

Step 1: Choose a portable air cleaner by smoke CADR

The EPA says to choose a unit whose clean air delivery rate for smoke is at least two-thirds of the room area in square feet. A 12 × 12 ft bedroom (144 sq ft) needs a smoke CADR of about 96 cfm; a 300 sq ft room, about 200 cfm. Our air purifier room size calculator does the math and adjusts for ceiling height. Other points from the EPA:

• Look for a CARB-certified air cleaner, which makes little to no ozone. Avoid devices that intentionally produce ozone.
• Consider the size and weight if you plan to move it between rooms.
• Replace filters on the manufacturer’s schedule; a dirty filter loses effectiveness.

Step 2: Create a clean room

You do not need to clean the whole house. Pick one room, often a bedroom, close its windows and doors when it is safe, and run the air cleaner there. The EPA says to use the air cleaner in a room with doors and windows closed. Add a second unit only if the first is too small for the room.

Step 3: Use your HVAC filter and fan

• MERV 13 filters. The EPA says to use high-efficiency filters marked MERV 13 or higher. Before you swap one in, check your system’s manual or ask an HVAC technician that the blower can handle the added resistance and that the filter slot fits.
• Fan setting. For forced-air systems, the EPA says to switch the thermostat fan from Auto to On so air runs through the filter continuously. Many smart thermostats let you schedule a minimum fan runtime or an air-quality-based fan mode.
• Fresh-air settings. If your system or window AC has a vent or fresh-air intake, check the manual for a recirculate or closed setting while smoke is outside.

Replace the filter sooner than usual after a smoke event, because it loads up quickly.

A DIY box-fan filter as a stopgap

If you cannot buy an air cleaner, the EPA describes a do-it-yourself version: a box fan with a high-efficiency filter attached. It advises using a box fan made since 2012 with a UL or ETL logo to reduce the risk of overheating, and a filter rated MERV 13 or higher. Never leave it running unattended, and treat it as temporary.

Add data: a monitor and a few automations

An indoor monitor shows whether your efforts work. Watch PM2.5 in the clean room and compare it with the outdoor reading on AirNow to see whether the room is cleaner than outside. A smart plug can start a purifier on a schedule if the purifier resumes by itself after power returns. Learn what the numbers mean in our air quality monitor guide.

If you are away, the same setup can run before you return. See the vacation smart home checklist.

Limits to remember

• Filtration reduces indoor smoke particles; it does not remove every risk. People with heart or lung conditions, children and older adults should follow advice from their doctor and local health department.
• Things that add particles indoors, such as frying, candles and smoking, work against you.
• A leaky house lets smoke in faster than an air cleaner can clean it. Seal obvious gaps around doors and windows.
• Keep carbon monoxide alarms working if you use fuel-burning appliances; see carbon monoxide detector placement.