Setting up a clean room and box-fan filter before Puget Sound's smoke season

Smoke season around Puget Sound usually means August and September, when fires in the Cascades, eastern Washington or British Columbia send haze over King, Snohomish, Pierce and Kitsap counties. Many houses here, older ones especially, have drafts and gaps that let it in, and you cannot seal a whole house. You can make one room much cleaner.

November is a good time to set this up. Fans and filters can be hard to find during a bad smoke week, and by then you would be shopping in the smoke. Buy the parts now, test them once and put them on a shelf.

Choose the room

Pick the room that suits the plan, not the biggest one:

• Small and closable. A filter fan cannot clean a large room as well. A bedroom, ideally with a bathroom attached, works if it fits everyone in the household.
• Few windows and outside walls. Fewer gaps mean less smoke to fight.
• Cool. A shut upstairs bedroom in August can get hot. A north-facing room or a daylight basement level often stays cooler.
• Easy to leave. Nothing you do to keep smoke out should make it harder to get out.

Write down a second plan for days when the room gets too hot: a friend's house, family or a public clean-air space.

Close the gaps and stop adding smoke

Before the smoke comes, go over the room:

• Close and latch the windows, and add weatherstripping or painter's tape where you feel a draft.
• Lay a towel along the bottom of the door while you are in the room.
• Keep bath fans, the dryer and the range hood off as much as you can. Exhaust fans pull outside air in through every gap in the house.
• If you have an air conditioner with a fresh-air setting, close it and use recirculate.
• If you have a forced-air furnace, ask an HVAC technician whether it can take a MERV 13 filter.

During smoke, skip what makes indoor air worse: candles, incense, smoking, aerosol sprays, frying and broiling, and vacuuming unless the vacuum has a HEPA filter. Clean dust with a damp cloth or mop.

Step by step: the box-fan filter

Public Health - Seattle & King County publishes a one-minute version of this build. It reports that the Puget Sound Clean Air Agency tested filter fans and found they greatly reduce fine particles and black carbon. EPA's clean-room guidance adds that evidence on homemade units is limited and that they work best in a small room where you spend a lot of time, and its DIY air cleaner page shows other builds.

1. Get the right fan. A 20-by-20-inch box fan made in 2012 or later, with a UL or ETL mark. EPA says those have thermal safety fuses. If yours is older, replace it, or never leave it unattended.
2. Get the right filter. A 20-by-20-inch furnace filter rated MERV 13, or FPR 10 on the scale some stores use. Those are the ratings that catch smoke particles. Buy at least two.
3. Find the arrows on the edge of the filter.
4. Press the filter against the back of the fan with the arrows pointing toward the fan.
5. Strap it on. Use a 48-inch bungee cord, or several linked together, so it does not stretch tight and snap back. Duct tape also works.
6. Seal the edges with strong packing tape so air goes through the filter rather than around it.
7. Set it toward the middle of the room on a stable surface with the cord out of the way, close the windows and door, and run it for at least 15 minutes.
8. Let it rest. The county's instructions say to turn it off about every 30 minutes for about 15 minutes so it does not overheat, and not to leave it running while you are out or asleep. Move it into the bedroom for at least 15 minutes before bed.

Change the filter when it looks dirty or starts to smell of smoke. Keep spares wrapped and in a dry closet rather than a damp garage.

A stronger version uses five MERV 13 filters taped into a cube, with the box fan on top. It costs more, but it moves air through much more filter.

If you want something to run all night

A portable HEPA air cleaner is generally more effective than a homemade filter, especially in a bigger room, and it is the tool to use if you want filtered air while you sleep. EPA's sizing rule is a clean air delivery rate (CADR) for tobacco smoke of at least two-thirds of the room's area, so a 150-square-foot bedroom needs a CADR of 100 or more. Check that it does not produce ozone; the California Air Resources Board keeps a list of certified cleaners.

How to tell it is working

Test it now. The filter should pull flat against the fan, and you should feel steady air out of the front with no whistling at the edges.

In a smoke event, your nose is a poor test, because you get used to the smell. A home particle monitor that reads PM2.5 is the simple check:

• Note the reading with the door closed before you start.
• Run the fan for 15 to 30 minutes and read it again. It should fall noticeably and sit below the outdoor number for your area.
• If it barely moves, look for the leak: an open door or window, an exhaust fan running, or air getting around the filter.
• Look at the filter after a few days of smoke. A filter turning grey is doing its work.

Leave for somewhere else if you cannot stay cool, the power goes out or smoke keeps getting in. People with asthma, heart or lung disease, older adults and anyone pregnant should make that plan with their doctor ahead of time.

What it costs

These are typical Puget Sound ranges and vary by store and season.

• 20-inch box fan: roughly $25 to $50.
• MERV 13 filter, 20 by 20 inches: roughly $15 to $35 each.
• Bungee cord and tape: usually under $15.
• Five-filter cube, with the fan: roughly $100 to $200.
• Portable HEPA air cleaner: about $100 to $300 for a bedroom, more for larger rooms.
• Home PM2.5 monitor: roughly $50 to $250.
• Professional cleanup after a bad smoke week: roughly $1,000 to $5,000, as our smoke damage cost guide explains, and higher if the house was open or the smoke was heavy for days.

Some policies cover wildfire smoke damage and some do not, so ask your agent before assuming either way. If you file a covered claim and use a restoration contractor, it is $0 upfront on covered claims - insurer billed directly - deductible applies.

When to call a professional

A clean room protects you while the smoke is outside. It does not clean a house the smoke has already reached. Call when:

• Ash or soot is inside, on sills, floors or around vents.
• The furnace filter came out grey, or there is residue at the registers.
• The smoke smell stays for more than a day or two after the outdoor air clears. See odor removal.
• Someone in the household has breathing or heart problems and the house still smells.
• Clothes, bedding or curtains still smell after one cold wash. Contents and textile restoration handles those.
• Buildings burned nearby, so the residue is more than wood smoke. See smoke and soot cleanup.

For ash outdoors, see wildfire ash on your deck, gutters and car. Wildfire smoke cleanup explains the indoor work. Dispatch covers King, Snohomish, Pierce and Kitsap counties any hour at (425) 842-3483.