How to Keep Wildfire Smoke Out of Your House

It is the second week of August, around seven in the evening, and the light through your kitchen window has been the color of weak tea all day. Nobody has been outside since lunch. Your youngest has been coughing since dinner and you cannot tell whether it is the smoke or the start of a cold.

The nearest fire is four hundred miles away. You are not evacuating. There is no order, no alert, nothing on the news about your county at all. And you still have a problem in your house tonight.

This is the version of wildfire that most families actually experience, and it gets a fraction of the attention that evacuation does. If you are packing to leave, our wildfire evacuation guide is the one you want. This article is about the other situation, the one where you stay put and the air comes to you.

Why does smoke from a fire hundreds of miles away matter?

Because fine particles travel, and they do not thin out as fast as you would expect.

In a bad week it arrives in Ohio and Georgia and Maine.

The particles are the problem. PM2.5 is small enough to get past the defenses in your nose and throat and settle deep in the lungs, which is why the people who feel it first are children, older adults, and anyone with asthma or a heart condition. A scratchy throat in a healthy adult is the mild version.

How do I keep smoke out of the house?

You cannot seal a house completely. What you can do is slow the rate at which outside air gets in, and clean the air that is already inside.

• Close windows and doors and keep them closed. If a door is drafty, a rolled towel along the bottom does more than it looks like it should.
• Set your HVAC to recirculate. This is the one people miss. A system pulling fresh air from outside is a system pumping smoke into your house on purpose. On a window unit or a portable air conditioner, look for the fresh air or vent setting and close it.
• Upgrade the furnace filter to MERV 13 if your system will take one, and run the fan continuously rather than only when it is heating or cooling.
• Do not add smoke of your own. No candles, no fireplace, no gas stove if you can avoid it, no vacuuming. Vacuuming lifts settled particles back into the air.
• Wipe surfaces with a damp cloth rather than dusting, for the same reason.

What is a clean room, and how do I make one?

A clean room is one room in your house that you keep cleaner than the rest. It is not a sealed chamber. It is usually a bedroom, and it is where everyone sleeps while the smoke lasts.

Pick a room with few windows, on the side of the house away from the prevailing wind if you know which that is. Close it up, put a towel at the door, and run a filter in it. Keep the door shut and stop treating the whole house as the thing you are trying to fix.

One room you can actually clean beats five rooms you cannot.

Do I need an air purifier, or will a box fan do?

A HEPA purifier is the straightforward answer, and the number to look at is the room size it is rated for. A unit sized for a 200 square foot bedroom will not do much in an open-plan living area.

If the purifiers have sold out, which happens fast once a smoke event is in the news, the fallback works. Tape a MERV 13 furnace filter to the back of a box fan with the airflow arrows pointing into the fan, and run it. EPA research has found these do meaningfully reduce smoke particles indoors. It looks like a school science project and it costs about thirty dollars.

Two things worth knowing. Filters loaded with smoke stop working, so plan on replacing them more often than the box says. And nothing here removes gases, only particles. If you can still smell smoke strongly inside with everything running, that is information rather than failure.

When is it safe to go outside again?

Check AirNow.gov rather than looking at the sky. Smoke thins visually well before the particle count drops, and the color of the light is a poor guide.

AQI	What it means for your household
0-50	Good. Normal day.
51-100	Moderate. Unusually sensitive people may notice it.
101-150	Unhealthy for sensitive groups. Keep children, older adults, and anyone with asthma or heart conditions inside.
151-200	Unhealthy. Everyone limits time outdoors. No outdoor sports.
201-300	Very unhealthy. Stay in. Run the clean room.
301+	Hazardous. Everyone stays inside.

The practical line for most families is 100. Below it, an ordinary day with an eye on anyone who has asthma. Above it, the kids stay in and soccer practice is not happening.

What about masks?

Only a proper respirator does anything. A cloth mask, a surgical mask, and a bandanna are all equally useless against fine particles, and wearing one mostly produces a feeling of having done something.

What works is a NIOSH-marked N95 or P100, with two straps that go around your head rather than loops over your ears. Ear loops cannot pull a seal tight enough. Counterfeits are common enough that it is worth two minutes to check, and we wrote up how to tell whether a mask is genuine.

Two limits even on a real one. A respirator filters particles and not gases, so if you can still smell smoke through a sealed mask, that is the signal to shorten your time outside. And respirators are not made in children's sizes. An adult small often seals on a child around seven or older. Below that, the honest answer is that the mask is not the plan. The clean room is.

What if someone in the house has asthma?

Move the threshold down. A household with an asthmatic child treats 100 the way other households treat 150.

Keep the rescue inhaler somewhere findable rather than somewhere sensible, make sure the asthma action plan is current, and if a nebulizer is part of your routine, check you have enough medication for a week of bad air rather than a normal week. Smoke events run longer than people plan for.

A question worth sitting with

If the air quality index hit 175 tomorrow afternoon, which room in your house would you put everyone in, and is there a filter in it?

Most families have never picked the room. Picking it costs nothing and takes about a minute, and it is the whole plan.

The honest part

An emergency kit does not solve smoke.

Our wildfire kits cover the situation where you leave: 72 hours of food, first aid, light, shelter, and a weather radio, in a bag by the door. They do not contain an air purifier, and no kit sold anywhere does.

What a kit does buy you on a smoke day is one less thing to think about. If the wind shifts and the situation changes from breathing bad air to leaving, that decision is already made and the bag is already packed. That is the part worth having in place before August.

The smoke part is a box fan, a furnace filter, and knowing which room. Twenty minutes and about thirty dollars, and it is the highest-value preparedness work most families will ever do that involves no shopping list at all.

Frequently asked questions

Do air purifiers actually help with wildfire smoke?
Yes, for particles. A HEPA purifier sized to the room removes the fine particulate that makes smoke harmful. It does not remove gases, so if you can still smell smoke with it running, that is expected rather than a sign it has failed.

Does a box fan with a furnace filter work?
It does. EPA research has found do-it-yourself air cleaners built from a box fan and a MERV 13 filter meaningfully reduce smoke particles indoors. Tape the filter to the intake side with the airflow arrows pointing into the fan.

What AQI is unsafe for children?
Above 100, keep children, older adults, and anyone with asthma or a heart condition indoors. Above 150 everyone limits outdoor time. Check AirNow.gov rather than judging by how the sky looks, because visibility improves before particle counts do.

Will an N95 protect my child from wildfire smoke?
Not a young one. Respirators are not manufactured in children's sizes, and a mask that does not seal does not filter. An adult small often fits a child of around seven or older. For younger children the plan is filtered indoor air rather than a mask.

Should I run my air conditioning during a smoke event?
Yes, but on recirculate. Any setting that pulls in fresh air is pulling in smoke. On central systems, close the fresh air intake and run the fan continuously with a MERV 13 filter if the system accepts one.

Peace of mind, packed and ready.

Sources: U.S. Environmental Protection Agency, wildland fire air quality fact sheet and research on do-it-yourself air cleaners; AirNow.gov air quality index categories; CDC guidance on wildfire smoke and vulnerable groups.