What the AQI is actually telling you
The Air Quality Index is a US EPA construct: a way of taking several regulated pollutants, each measured in its own units, and putting them on one scale that runs the same direction every time. Higher is worse. The categories have names — Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, Hazardous — and those names are the useful part, because they describe who the air is a problem for rather than asking you to remember a number.
Two details change how you read it. The index reported for your area is usually driven by whichever pollutant is worst at that moment, so the same number can mean ozone on a hot still afternoon in July and fine particulate from smoke in September — ozone is mostly an outdoor-exertion problem that peaks in the afternoon heat, while particulate hangs around and comes indoors with you. And air quality is not uniform across a metro area: the monitor generating your number may be miles away and on the other side of a ridge, smoke pools in valleys overnight, and if the category and the view out the window disagree badly, look for a nearer monitor.
| Category | What the name is telling you |
|---|---|
| Good | Nothing to plan around. |
| Moderate | Acceptable for most people. A small number of unusually sensitive individuals may notice it during heavy outdoor exertion. |
| Unhealthy for Sensitive Groups | The category exists specifically to warn a subset of people before it affects everyone else. If you are in one of those groups, this is your cue, not the next category up. |
| Unhealthy | Everyone may be affected; sensitive groups more so. This is where general outdoor plans start getting moved. |
| Very Unhealthy | A health alert affecting the whole population. |
| Hazardous | Emergency conditions. Everyone. |
Sensitive groups means people with asthma or other lung disease, people with heart disease, pregnant people, young children, older adults, and anyone who works outdoors and therefore has no choice about exposure. If that is you, the plan your clinician gave you outranks anything here, and the conservative reading of the forecast is the right one.
Why PM2.5 is the number that matters in smoke
Wildfire smoke is a mixture, but the fraction that drives the health concern is fine particulate — particles small enough that the nose and throat do not catch them, so they travel down into the deep lung. That is what PM2.5 refers to: particulate matter under a nominal 2.5 micrometers. Coarser dust, the kind that makes your car dirty and your eyes gritty, is largely filtered by your own upper airway. The fine fraction is not.
This is also why smoke is a problem so far from fire. Coarse ash falls out within tens of miles; fine particulate stays suspended, rides a weather system across a continent, and arrives as a haze that reads high on a monitor in a place with no fire within a thousand miles. Smell is a poor gauge of it, because the odor compounds and the particles disperse at different rates — smoke can smell strong and read moderate, or barely register while the monitor reads badly. Use the measurement.
Masks, and why fit beats the rating
A cloth mask and a loose surgical mask do very little against PM2.5. They were never designed for it — the surgical mask is a splash and droplet barrier and it is not sealed to your face, so most of the air you inhale takes the path of least resistance around the edges rather than through the material. Against a particle that small, an unsealed mask is close to no mask.
A NIOSH-approved N95 does work, and so does a well-made KN95 or an elastomeric half-mask with particulate cartridges — the media in these captures fine particulate at high efficiency. But every one is rated for what the material does on a sealed test fixture, and what you get in the driveway is decided by leakage. A respirator with a gap at the nose bridge or under the chin is a rated filter with an unrated hole next to it.
| Type | Against fine particulate | The practical failure |
|---|---|---|
| Cloth or bandana | Essentially nothing you can rely on | Loose weave and no seal. It catches lint. |
| Surgical or procedure mask | Marginal | Not sealed. Air goes around the sides, which is fine for its intended job and useless for this one. |
| N95 or KN95 respirator | Effective when it seals | Beard stubble, a bent nose wire, ear loops instead of head straps on a face they do not fit, or wearing it pulled down for a phone call |
| Elastomeric half-mask with particulate cartridges | Effective, reusable, seals well | Bulky, needs the right cartridge, and you have to clean it. Worth it if you work outdoors through smoke season. |
| Any respirator with an exhalation valve | Protects the wearer normally | Vents your unfiltered breath outward, which some workplaces and settings do not allow |
Check the seal every time: cup your hands loosely over the mask and breathe out, and pressure should build inside rather than a jet escaping past your eyes. Glasses fogging is the classic sign of a bad nose seal, and facial hair along the sealing surface defeats a respirator no matter how good it is. Do not expect adult performance from one on a small child — they are not made in a size that seals a small face, and indoor air and a shortened outing are the tools you have there.
Indoors is not automatically clean air
Houses exchange air with the outside continuously, through gaps around windows and doors, through the attic hatch, through the dryer vent and the bath fan and every penetration a builder cut. On a smoke day the indoor level chases the outdoor level with a lag, and a leaky house catches up fast. The point of the indoor protocol is to slow that exchange down and clean what does get in.
| Do | Why it matters |
|---|---|
| Close windows and exterior doors; keep them closed while the smoke lasts | The single largest lever you have. Reopen when the forecast clears, not when it stops smelling. |
| Run central HVAC on the fan setting with a high-efficiency filter, if the system takes one | Turns the ductwork into a whole-house filter loop. Check the manual for the highest filter rating the blower is rated to push through. |
| Set the system to recirculate rather than pulling in outside air | Some systems have a fresh-air intake or an economizer. On a smoke day that is a smoke intake. |
| Run a portable air cleaner in the room you sleep in | One clean room beats a half-treated house. See choosing an air purifier. |
| Burn nothing indoors | Candles, incense, a fireplace, a gas stove or oven without the range hood running. Combustion adds fine particulate to a room you are trying to keep clean. |
| Skip the vacuum unless it has a sealed HEPA path | A leaky vacuum aerosolizes settled dust back into the air. Damp-mop and wipe instead. |
| Turn off bath and kitchen exhaust fans when you are not using them | Every cubic foot they push out is replaced by outdoor air pulled in somewhere else |
| Use the recirculate setting in the car | Cabin air filters are more effective when the system is not pulling fresh smoke through them. Change the cabin filter after a bad stretch. |
If you have been meaning to get around to air sealing, a smoke week makes the case better than any energy bill does — the same gaps that let heat out let smoke in, and insulation and air sealing covers where they usually are. If you want to know how much air your system actually moves and how often it turns the house over, the air changes calculator puts a number on it.
The box fan and filter build
Taping a furnace filter to the intake side of a box fan is a widely used low-cost approach to cleaning air in one room, and it has been studied enough that its rough performance is not in dispute — the versions using several filters in a cube move considerably more air than a single filter taped flat, because a single filter chokes the fan. It is not an official recommendation from anyone, and it should not be presented as one.
The honest limitations are about the fan, not the filtration. A box fan is a household appliance being asked to run continuously against added resistance. If you build one: use a fan in good condition, keep it on a hard surface where it cannot tip into bedding, do not run it unattended or overnight, replace the filter when it is visibly loaded, and unplug it if it gets warm, smells hot or slows down. A purpose-built cleaner has a motor sized for the resistance of its own filter, which is most of what you are paying for.
Ventilating, and when to reopen
Sealing a house up has its own cost. Cooking, showering, breathing and burning all add moisture and pollutants indoors, and a closed house accumulates them, so during a multi-day event watch for the ordinary signs of staleness — persistent window condensation, a musty edge to the air. If the outdoor category improves for a few hours, that is your window to flush the house with a short, deliberate cross-draft rather than leaving one window cracked continuously. If the closed-up stretch runs long enough to push indoor humidity up, a dehumidifier belongs in the plan — see dehumidifier sizing, and mold removal for what happens if you let it run.
Planning a smoke day
The decisions that make a difference are unglamorous. Move the workout indoors or skip it, because exertion multiplies how much air you pull past your airways. Shift outdoor work to whichever part of the day the forecast is better, and treat the forecast as a forecast — smoke concentration swings by the hour with wind and mixing height. If you have a child with asthma, the school office should know what the plan is before the day it matters. And smoke arrives alongside the other summer hazard: heat wave safety matters on the same days, and air conditioning on recirculate resolves the conflict between closing the house up and cooling it, where an open window does not.
The part people skip is checking again. Smoke conditions are not a state, they are a forecast that changes several times a day, and the household that reopens the windows on Wednesday morning because Tuesday afternoon was fine is the household that spends Wednesday night breathing what it spent Tuesday keeping out.
Questions people ask
Does a cloth or surgical mask help with wildfire smoke?
Not meaningfully. The particle that matters in smoke is fine particulate, small enough to bypass the filtering your nose and throat do on their own, and a cloth or loose surgical mask neither filters at that size nor seals to your face. Air takes the easy path around the edges. A NIOSH-approved N95, a well-made KN95, or an elastomeric half-mask with particulate cartridges will genuinely reduce what you inhale, but only if it seals — check by cupping your hands over it and exhaling, and expect pressure to build inside rather than a draft past your eyes. Facial hair on the sealing surface defeats any of them.
Is the air inside my house safe when the AQI is bad outside?
Better, usually, but not automatically clean. Every house exchanges air with outside continuously through gaps, vents and every penetration in the envelope, so indoor levels follow outdoor levels with a lag that depends on how leaky the building is. The protocol that actually works is closing windows and doors and keeping them closed, running central HVAC on fan with a high-efficiency filter if the blower is rated for one, running a portable cleaner in the room you sleep in, and burning nothing indoors — no candles, no fireplace, no gas cooking without the range hood running. Skip vacuuming unless the machine has a sealed HEPA path.
How far can wildfire smoke travel?
Across a continent. The coarse ash falls out within tens of miles, but the fine fraction stays suspended and rides weather systems for days, which is why cities with no fire anywhere near them now get their worst air-quality days of the year from fires in another time zone. Smell is a poor gauge of it, because the odor compounds and the particles disperse at different rates — smoke can smell strong and measure moderate, or barely register to your nose while the monitor reads high. Read the current index rather than trusting the window.
Does a box fan with a furnace filter taped to it work?
It is a widely used low-cost approach and it does move filtered air, particularly in the versions that use several filters arranged in a cube rather than one filter taped flat, since a single filter chokes the fan. It is not an official recommendation from any agency and should not be treated as one. The real caution is the fan: a household box fan running continuously against added resistance is being used outside its design, so keep it on a hard surface where it cannot tip, keep bedding and paper away from the intake, do not run it unattended or overnight, and unplug it if it gets warm, smells hot or slows down. A purpose-built cleaner has a motor sized for its own filter, which is largely what the price buys.
What does the AQI category "Unhealthy for Sensitive Groups" mean for me?
The category exists to give a specific set of people a warning one step earlier than everyone else: people with asthma or other lung disease, people with heart disease, pregnant people, young children, older adults, and people who work outdoors and cannot choose their exposure. If you are in one of those groups, that category is your cue rather than the next one up, and any plan a clinician has given you takes priority over general advice. If you are not, it is a signal to keep an eye on the forecast and reconsider heavy outdoor exertion, since exertion is what multiplies exposure.