Wildfire smoke and the house: keeping the air inside clean

Wildfire smoke used to be a Western problem, and for a week or two most summers it is now a problem for the whole country. The fine particles in it go deep into the lungs, and the outdoor air quality index tells you when they are high. What it does not tell you is what to do about the air inside the house, which, on a bad day, can be nearly as dirty as the air outside unless the house is set up to keep it out.

Close it up

• Close the windows and doors and keep them closed. If the house has no air conditioning and it is hot, this is a real conflict; on the worst days, a single room with a window unit or a portable air cleaner, kept closed, is the answer.
• Set the HVAC system to recirculate. Most central systems do this by default; if yours has a fresh-air intake or a ventilation system, close or turn it off for the duration. Window air conditioners have a vent lever; close it.
• Do not run the bath fans, the range hood or the dryer more than you must. Each one pulls outside air in through the gaps to replace what it exhausts.
• Do not add smoke of your own: no candles, no fireplace, no frying, no vacuuming without a HEPA filter.

The furnace filter

The filter in the central system is the biggest air cleaner in the house, and most of them are not up to smoke. Filters are rated by MERV, a scale from 1 to 16 where higher catches smaller particles. The cheap fiberglass filter is a MERV 4 and catches almost none of the smoke. A MERV 13 catches most of it. Before smoke season, check that the system can take a MERV 13 without starving for air; a one-inch MERV 13 is restrictive, a four-inch one is not, and an HVAC Pro can tell you in a minute whether the blower can handle it. Then run the fan on "on" during the smoke so the air keeps moving through the filter. Replace the filter after a smoke event; it will be gray.

Portable air cleaners

A portable HEPA air cleaner in the bedroom and the main living room does more, faster, than anything else. Buy one sized for the room, by the clean air delivery rate on the box, and run it on high with the door closed. A box fan with a MERV 13 filter taped to the back, the corsi-rosenthal design, works surprisingly well for the price, and a fan with four filters taped into a cube works better. Ionizers and ozone generators do not help and the ozone ones are harmful.

Watch the numbers

An inexpensive indoor air quality monitor that reads fine particles tells you whether what you are doing is working, and when it is safe to open up again. Outdoor readings by ZIP code are free from the government's air quality site; when the outdoor index goes back to moderate, open the house wide and flush it.

After

Smoke leaves a film and a smell. Wipe the hard surfaces, wash the bedding, change the filters, and run the air cleaners for a few more days. If the smoke was heavy or long, a duct cleaning is reasonable. If a fire came close enough for ash, that is a restoration Pro's job, since ash is corrosive and the cleanup is a chemical one.

Who is at risk

Children, older people, anyone with asthma or a heart condition, and anyone pregnant. For those households a sealed clean room with a HEPA cleaner is not an option on a bad day, it is the plan.