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The Bad Air Day Playbook

What to do before, during, and after the air outside turns bad — whether you have central heating and cooling or not. Print it. Put it on the fridge. The next bad air day is a lot less stressful when the decisions are already made.

If you only read one section

The whole playbook, compressed.

Close the windows and doors. Keep them closed.
Run whatever filtration you have, continuously. If that's central HVAC, set the thermostat fan to ON. If it's portable air purifiers, leave them running and don't move them room to room.
Make sure the filter doing that work is clean and current — a MERV 13 in the HVAC slot, or a fresh purifier filter in the unit.
Don't add particles indoors — no candles, no fireplace, no frying, and hold off on vacuuming.
Check your local AQI before you make outdoor plans, not after.

The setting almost everyone gets wrong

With central HVAC: your filter only does something when air is moving through it. On AUTO, the blower only runs when the system is actively heating or cooling — so on a mild, hazy day, it might barely run at all. You can own an excellent filter and get almost nothing out of it. Switch the thermostat fan to ON and air keeps cycling regardless of temperature. It's free, it takes one button, and it's the difference between owning filtration and using it.

With purifiers: the same principle, different mistake. Resist running them on the lowest setting for quiet, and resist carrying one unit around the house. A purifier needs to run continuously in one room to keep up. Turning it on when the air already smells like smoke is starting from behind.

Which setup do you have?

Almost everything in this playbook applies to everyone. The one big difference is where your filtration lives, so find yourself here first.

If you have central HVAC or a ducted furnace

Your system already moves the air in your whole house through one filter. That filter is your main line of filtration, and it's the cheapest thing on this list to upgrade.

Move up to MERV 13 if your system takes it. MERV describes how much of the air's particle load a filter captures as air passes through it. A MERV 13 captures a higher share of fine particles than a MERV 8 or a basic fiberglass filter. Two things to check before you order: that your filter slot fits the thickness you're buying, and — if your system has ever had airflow trouble — that an HVAC pro is comfortable with the change.

Keep two spares in the house. During a wildfire event, filters are exactly what everyone else is also trying to buy. A filter in transit is not a filter in your system.

If you don't — window units, ductless mini splits, baseboard, radiators, or nothing at all

Plenty of homes and apartments have no central system to filter through, and window units and mini splits use thin washable screens meant to protect the equipment rather than capture fine particles. That's normal — it just means your filtration has to come from portable air purifiers instead.

You want a true HEPA purifier, one per room you actually use. A purifier only cleans the air in the room it's standing in, and a closed door stops it at the threshold. One good unit in the living room does nothing for the bedroom you sleep in eight hours later. Prioritize in this order: the bedroom first, then wherever you spend your days.

Three things to look for:

"True HEPA," not "HEPA-type" or "HEPA-like." Those last two aren't the same thing and the labeling is deliberately close.
Rated for more square footage than the room, not less. A unit at the edge of its rating has to run flat out; one with headroom can run quieter and still turn the air over.
A carbon stage if smoke is your concern. HEPA media handles particles; activated carbon is what addresses odors and gases.

Keep spare purifier filters on hand too. Purifier filters are consumable in exactly the same way HVAC filters are, they're specific to your model, and they're harder to find in a hurry. Two per unit, bought before the season, is the version of this that works.

Read the number, then act

The AQI is a 0–500 scale. The category names below are the EPA's. The actions are what to do inside your home. You can find your current reading on the live AQI map for the USA.

AQI	EPA category	What to do at home
0–50	Good	Nothing special. This is the day to change your filters, open the windows, and restock your spares.
51–100	Moderate	Carry on as normal. If there's a fire burning anywhere in your region, start watching the trend line rather than the single number.
101–150	Unhealthy for Sensitive Groups	Close the windows. Start your filtration running continuously — fan to ON, or purifiers on in the rooms you're using. No candles, no fireplace. Set the car to recirculate.
151–200	Unhealthy	Everything above, and keep it closed. Reschedule outdoor plans. Close interior doors to the rooms with purifiers running — a smaller space is an easier space to keep clean.
201–300	Very Unhealthy	Hold the line and check your filters — HVAC and purifier both. A loaded filter moves less air per hour, and this is the stretch where that shows. Swap in a spare if it looks grey.
301+	Hazardous	Sealed up, filtration running continuously, one room as your base. Follow local emergency and public health guidance first.

This covers what to do with your home. For guidance about your own circumstances, talk to a qualified professional.

Before — the setup that only happens once

Do this on a clear day. It's the whole reason a bad air day stays boring.

Know what filter you need before you need it. With central HVAC, pull the filter out and read the size printed on the cardboard frame, and measure the slot depth if the old one looks squashed. With purifiers, find the replacement filter model number on the unit or in the manual — it's rarely guessable later.
Write it down and put it somewhere you'll find it. A photo on your phone works better than a note you'll lose.
Buy the spares now. Two, for each thing that takes a filter — MERV 13 air filters for the HVAC slot, purifier filters for the units. This is the only item on the list that runs out.
Pick a spot for them. Hall closet, laundry room, under the bed. Filters get bought and then lost.
Bookmark the live AQI map on your phone, not your laptop. You'll want it while you're standing outside wondering.
Find the fan setting on your thermostat now, if you have one, while it isn't urgent. Know where ON lives.
Look for the gaps. Window frames that don't quite seal, a door with daylight under it, a window AC with a gappy accordion panel. A rolled towel and a roll of weatherstrip go a long way, and you'll notice these on a calm day and never on a bad one.

During — the day itself, in order

Roughly the sequence to work through once the number starts climbing.

Close up early. Before the peak, not during. Once outdoor air is inside, you have to work back through it — which takes far longer than keeping it out did.
Start your filtration and leave it running. Thermostat fan to ON if you have central HVAC. Purifiers on, in the rooms you're using, doors closed.
Check the filters. If one is grey, loaded, or you can't remember installing it, swap in a spare. Now is when a fresh MERV 13 or a new purifier filter matters most.
Turn off anything that pulls outside air in. Whole-house fans, attic fans, evaporative and swamp coolers, bathroom exhaust fans left running, and window units switched to vent or fresh-air mode. They're all doing the opposite of what you want today.
Set up one room if it's a bad one. See below.
Recirculate in the car. Press the button with the arrow curving back on itself. And if your cabin air filter is old, that's a five-minute job worth doing.
Check the number again in the evening. Smoke moves. It often gets worse overnight as air settles, which is exactly when people are tempted to open a window for the cool.

Don't do these

Every one of these is something reasonable people do on a smoky day that makes their indoor air worse.

Don't "air out" the house. The instinct is strong and it's backwards. You'd be replacing filtered indoor air with the thing you're trying to avoid.
Don't open windows overnight to cool off. This is the single most common mistake. Overnight is frequently when levels are highest.
Don't light candles or use the fireplace. Both add particles directly to the room you're sitting in.
Don't fry, sear, or broil. High-heat cooking generates a lot of fine particles. Today is a slow cooker day, and use the range hood regardless — if it vents outside.
Don't vacuum unless your vacuum has a sealed HEPA filter. Otherwise you're mostly stirring settled dust back into the air.
Don't run a swamp cooler or whole-house fan. Their entire job is pulling outdoor air in.
Don't carry one purifier from room to room. It starts over every time you move it. Leave it where you sleep.
Don't assume a dust mask is doing much. A loose cloth or paper mask isn't built for fine particles.
Don't trust your nose after a few hours. You stop noticing smoke long before it stops being there. Trust the number.

The one-room fallback

When it's really bad, don't fight for the whole house. Pick one room and win there. This works whether or not you have central heating and cooling.

Choose a room you'd want to sleep in, with the fewest windows and no exterior door.
Keep the door closed. Put a rolled towel at the base if there's a gap.
Run a true HEPA purifier rated for that room's square footage or more — headroom is better than being right at the limit.
Check the purifier's own filter first. An old filter in a purifier is a fan with extra steps.
Leave it running continuously. Don't cycle it on and off to save the filter life.
Skip the humidifier. It doesn't remove particles and adds a variable you don't need.

Wildfire season: the head start

Smoke usually arrives a day or more after the news does. That gap is the most useful window you get.

When you hear about a fire in your region — before you can smell anything — put fresh filters in and confirm you still have spares. Ordering is easy now and harder in three days.
Watch the trend, not the reading. A number climbing steadily from 60 tells you more than a single reading of 90.
Close up before you can smell it. If you can smell smoke, you're already behind.
Expect it to last. Smoke events run days, not hours, and can drift a long way from the fire itself. Plan for a week of closed windows, not an afternoon.
At the start of each season, buy the season's filters at once — MERV 13, purifier filters, or both. It's the same money either way, and it removes the decision from the worst possible moment to be making it.

How often to change them

HVAC filters. Thicker filters have more surface area, so they generally go longer between changes. Pets, more people, and a system that runs constantly all shorten the interval.

Purifier filters. Follow the interval in your unit's manual, and treat it as a ceiling rather than a target. HEPA media and carbon stages often have different lifespans in the same unit — worth checking, because people replace one and forget the other.

Look at it monthly regardless of what interval you're on. Grey, matted, or light-blocking means change it, whatever the calendar says.
Smoke season counts double. A filter working through a week of heavy smoke loads up faster than one doing an ordinary month. Check it during the event, not after.
Write the date on the frame in marker when you install it. Your memory will not do this job.
Replace the spare the day you install it, not the next time you're standing at an empty slot.

Whatever your setup, spare filters in a closet are the cheapest preparedness you can buy — and they're the one thing on this list that runs out. Everything else here is a habit.

Breathe easy.

Keep this where you'll see it. The next bad air day is a lot less stressful when the decisions are already made.

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