Clean-Air Room
A single room you can breathe in on a bad smoke day, built from a box fan and furnace filters in under an hour.
What this buys you
On wildfire-smoke days, indoor air can be nearly as bad as outdoor air. One well-sealed room with a filter running gives you a place to sleep, work, and recover, which matters most for kids, older adults, and anyone with asthma or heart or lung conditions. A simple box-fan filter can drop fine-particle (PM2.5) levels in a closed room dramatically within an hour.
Bill of materials
Standard track · one room
| Item | Qty | Est. cost |
|---|---|---|
| 20" box fan (newer model with thermal cut-off) | 1 | $25–$40 |
| MERV-13 furnace filters, 20×20×1 | 1–4 | $10–$45 |
| Cardboard, tape, and a fan shroud (or a ready-made bracket) | 1 set | $0–$15 |
| PM2.5 monitor (optional but recommended) | 1 | $0–$50 |
Procedure
- Pick the smallest room with the fewest windows and doors, ideally a bedroom you already sleep in.
- Build the filter. Quick version: tape one MERV-13 filter to the intake side of the fan, airflow arrow pointing into the fan. Better version: build a four-filter cube (a Corsi-Rosenthal box) for much higher airflow.
- Confirm filter direction. The airflow arrows on the filters must point toward the fan.
- Place the fan so it pulls from the center of the room, not jammed against a wall.
- Reduce leaks: close windows, put a towel along the door gap, and turn off any exhaust fans pulling outside air in.
- Run it on medium during smoke events and keep the door closed.
On a smoke day, run the unit in the sealed room with the door closed and watch a PM2.5 monitor. You should see the room reading fall well below the outdoor reading within 30 to 60 minutes. If it does not, recheck the filter direction and seal the gaps around the fan and door.
Replace filters when they look gray or roughly each smoke season, whichever comes first. Keep one spare set on hand so you are never shopping during an event. Note the install date on the filter edge with a marker.
A clean-air room handles smoke, not heat. If your concern is summer heat or winter cold during an outage, that is a separate system, covered in the energy and shelter projects.