You wake up and the light coming through the window is the wrong color. It’s a brownish gold, and the house smells faintly of a campfire that moved in overnight. The nearest wildfire is in another state, hundreds of miles away.
Your first instinct is probably to open a window and air the place out. Hold that thought. On a smoky day, that instinct is backwards.
Smoke seasons are no longer a West Coast problem. Fine particles ride the wind for days and settle over cities that have never seen a flame, in houses whose owners have never thought about any of this. What you do in the first few hours matters, and almost all of it is free.
What wildfire smoke is made of
The Environmental Protection Agency describes wildfire smoke as a complex mixture rather than a single substance. It carries water vapor, gases including carbon monoxide and nitrogen oxides, organic compounds such as volatile organic compounds and polycyclic aromatic hydrocarbons, and particulate matter, meaning tiny solid particles and liquid droplets suspended in air.
Those particles get sorted by size. PM10 means particles 10 micrometers across or smaller. PM2.5 means 2.5 micrometers or smaller, which EPA illustrates as roughly thirty times smaller than the width of a human hair.
PM2.5 is the part that matters. Particles that small do not get caught in your nose and throat. They travel deep into the lungs, and EPA calls fine particle pollution the component of greatest health concern in smoke.
Ozone catches people out. It is not emitted by the fire. It forms when nitrogen oxides and volatile organic compounds react in sunlight, so it can climb downwind of a fire even as the visible haze thins.
What the evidence shows, and how strongly
Start with what is well established. EPA points to a large body of research tying short-term PM2.5 exposure to respiratory harm. Smoke irritates eyes and airways, sets off asthma attacks, and causes coughing, wheezing and difficulty breathing. Healthy adults usually recover within days, though some get temporary lung inflammation and a measurable dip in lung function.
Cardiovascular effects are real but less thoroughly studied. EPA describes the findings as generally consistent while noting there are fewer of them.
Two recent reviews help put numbers on it. A 2024 systematic review and meta-analysis by Lei and colleagues in Environmental Science and Technology found the strongest associations between wildfire smoke and cardiovascular mortality, asthma hospitalization and asthma emergency department visits, with relative risks running from about 1.02 to about 1.12 depending on the outcome. A 2023 meta-analysis by Gould and colleagues in the Annual Review of Medicine found consistent effects on all-cause mortality and respiratory hospitalizations, but less consistent evidence on cardiovascular illness.
Those two do not line up perfectly, which is worth saying out loud. The respiratory signal is the one every review agrees on. The cardiovascular signal is probably real and harder to measure, partly because smoke exposure is messy to estimate and partly because heart events have long lists of other causes.
A few findings are genuinely preliminary. EPA notes initial evidence that short-term smoke exposure may be linked to preterm birth, and a single study tying smoke to lower scores on a brain training game. One study is a reason to keep looking, not a conclusion to act on.
Smoke you cannot smell is still smoke
Your nose is not a measuring instrument. It adapts within minutes, which is why a house that reeked at breakfast smells normal by lunch while the particle count sits unchanged. Individual fine particles are far too small to see, so clearing visibility does not mean clean air either.
The exposure does not end when the sky looks normal. A 2023 study by Laguerre and Gall in Environmental Science and Technology tracked polycyclic aromatic hydrocarbons left on glass, cotton and filter media after smoke exposure, and found surface levels stayed above background for about 40 days. Cleaning worked: wiping glass removed 62 to 71 percent, and laundering and machine drying cotton removed roughly half.
So there is a reason to wipe hard surfaces and wash the bedding after a smoky stretch. And a reason to check a number rather than sniff the air.
Reading the air quality index without guessing
AirNow is the government air quality site EPA runs with other federal, state, tribal and local agencies. It publishes the Air Quality Index, a scale from 0 to 500. An AQI of 100 generally corresponds to the short-term national air quality standard for the pollutant being reported.
Six categories divide that scale:
- Good, 0 to 50, green
- Moderate, 51 to 100, yellow
- Unhealthy for Sensitive Groups, 101 to 150, orange
- Unhealthy, 151 to 200, red
- Very Unhealthy, 201 to 300, purple
- Hazardous, 301 and higher, maroon
Here is how to use them. Below 51, AirNow describes the health risk as little to none. From 51 to 100 the air is generally acceptable, with some risk for unusually sensitive people.
Orange is the band people misread. It does not mean the air is fine for everyone else. It means people in sensitive groups may feel effects while most of the public probably will not. If you have asthma or heart disease, or there is a young child or an older parent in the house, orange is your cue to change the day’s plan.
Red means some of the general public may start noticing effects while sensitive groups get hit harder. Purple is a health alert, with everyone’s risk raised. Maroon is an emergency-level warning covering everybody.
Two different numbers show up on AirNow, and confusing them leads to bad decisions. The daily forecast predicts a 24-hour average for PM2.5, which is what you want for planning tomorrow. The Fire and Smoke Map shows current conditions using NowCast, calculated from the past several hours of readings, which is what you want before stepping outside right now. Smoke levels swing hard inside a single day.
Who gets hurt first
EPA sorts higher risk into three groups, and the third one surprises people.
The first is existing disease. Asthma and other lung diseases lead the list, because smoke can trigger severe breathing problems. Cardiovascular disease follows, including people whose risk is raised by conditions such as diabetes or obesity.
The second is life stage. Children up to age 18 take more in because their lungs are still growing, they spend more time outdoors, and they breathe more air for each pound of body weight than adults do. Adults 65 and older are more likely to already have heart or lung disease, and the body handles stress less well with age. In pregnancy, changes including faster breathing may raise sensitivity, and the fetus may be more sensitive as well.
The third group is not about bodies at all. It is about who can close a door. EPA names outdoor workers, who face high concentrations for long stretches with no option to step inside. It names people with lower incomes, who may have less access to care, poorer housing, longer commutes and no money for an air cleaner, and it includes people who are unhoused. It names some racial and ethnic minority groups, where economic, social and environmental factors limit the ability to reduce exposure. “Stay indoors” is advice with a price tag attached.
The clean room is the biggest win most households get
Here is the number that reframes the whole subject.
Researchers at the University of Washington measured seven Seattle homes during a 2020 wildfire smoke episode, windows closed. Writing in Science of the Total Environment in 2021, Xiang and colleagues reported mean infiltration factors from 0.33 to 0.76. In plain terms, between a third and three quarters of the outdoor fine particles were getting inside anyway. Running a HEPA portable air cleaner on auto mode cut indoor PM2.5 by 48 to 78 percent once outdoor levels and indoor sources were accounted for.
Closing the windows is step one, and by itself it is not enough. Filtering one room is what moves the number.
A clean room is not a sealed chamber or a construction project. It is one ordinary room you set up in advance and then spend your time in.
Pick the room before you need it. It should hold everyone who lives there and be somewhere you can stand to sit for hours. EPA suggests a bedroom with an attached bathroom, because you can shut it off and stay a long while. Choose a room with no fireplace and as few windows and doors as you can manage.
Keep the door closed. Exhaust fans and range hoods pull replacement air in from the rest of the house, so run them briefly or not at all while you are in there.
Run a portable air cleaner continuously. Size it for the room and leave it on the highest fan speed you can tolerate. Pick a model that does not generate ozone, and change the filter when it looks dirty. EPA treats a do-it-yourself box fan and filter as a fallback for when a commercial unit is unavailable or unaffordable, not as the first choice.
Stay cool without importing smoke. Fans, window units and central air are all fine. Close any fresh air intake or set the system to recirculate. Evaporative coolers and single-hose portable air conditioners pull outside air by design, so use them sparingly, or cover the intake with a MERV 13 filter.
Do not exercise in there, and do not run anything in that room that makes particles. Once outdoor air improves, air the room out and run the kitchen and bathroom exhaust fans.
Which cleaner to buy, what the ratings mean and how filter media traps a particle are a different subject. This library handles devices in its article on indoor air filtration, and that is where those questions get settled.
The day the usual ventilation advice flips
If you read this library’s article on home ventilation, you came away with a clear message: get outdoor air moving through the house, because indoor air is often the dirtier of the two.
On a smoky day, reverse it. The outdoor air is the problem. You want less of it coming in, not more.
In practice that means windows and doors closed, with the room still easy to leave in a hurry. If you have central heating and cooling, EPA suggests a filter rated MERV 13 or higher and switching the thermostat fan from “Auto” to “On” so it keeps pushing air through that filter. Close the fresh air intake if your system has one, or set the system to recirculate.
Do the same in the car. Windows up, air conditioning on recirculate. A commute is a small sealed box with a fan in it, and which air that fan draws from is your choice.
Then stop adding to the problem indoors. On a smoky day EPA says to skip smoking and vaping, candles and incense, gas, propane and wood stoves and fireplaces, and aerosol sprays. Go easy on frying and broiling. Skip vacuuming unless the vacuum has a HEPA filter, and use a damp cloth instead of dry dusting, which mostly relaunches what has already settled.
There is an honest trade-off here, and glossing over it gets people hurt. A closed house in summer gets hot, and heat is a serious hazard in its own right. EPA’s guidance is direct: if you have no air conditioning and it is too hot to keep the windows shut, go somewhere else. A library, a mall or a designated cleaner air shelter all count. Early heat illness looks like headache, nausea and muscle cramps. Fainting, severe weakness or trouble speaking needs immediate medical attention.
Masks: fit beats the label every time
Mask advice during smoke events is full of confident nonsense, so here is the short version, sourced.
What works. EPA recommends a NIOSH-approved respirator when outdoor PM2.5 is in unhealthy ranges, or when you have symptoms. Look for “NIOSH” and a filter class such as N95 or P100 printed on the respirator itself. EPA says NIOSH-approved N95s are more than 95 percent efficient against the fine particles in wildfire smoke and, worn correctly, will significantly reduce exposure. Note the wording. Significant reduction, not a force field.
NIOSH approval is a government certification with laboratory testing behind it. The characters “N95” printed on a package with no NIOSH approval attached are marketing, and mean nothing defined.
What does not work. EPA states it without hedging: “Paint masks, dust masks or surgical masks do not prevent smoke inhalation.” Homemade coverings offer less protection than a KN95-type respirator, and a one-strap paper mask is not a respirator at all. These were designed for droplets and visible debris, not for particles a fraction of a hair’s width across.
Fit is the whole game. Choose a size that sits snugly over the nose and under the chin, with two straps, one above the ears and one below. Supervised fit testing is the real check, and most members of the public cannot get one. The practical substitute is a user seal check every time you put the respirator on, following the package instructions. If you cannot get a seal, try another size or model. EPA’s warning is blunt: a poorly fitting respirator can leave you roughly as exposed as wearing none at all.
Facial hair breaks the seal. This is not a style preference. NIOSH, the federal institute that approves respirators, reports that facial hair crossing the sealing surface “causes 20 to 1000 times more leakage compared to clean-shaven individuals.” Some studies have found that even a day or two of stubble starts to cut protection. Individual hairs are far too large and too loosely spaced to filter anything, and they do not stop gases either. Facial hair is one of the most common reasons a person fails a fit test.
The trade-offs nobody mentions. Respirators make breathing work harder. EPA notes they can cause facial discomfort and headaches, and that people with heart or lung disease should get a clinician’s guidance before relying on one. They also raise the risk of heat illness, so take breaks, drink water, and take the respirator off in cleaner air if you feel dizzy. They are not made in sizes for very young children. Children aged 2 and up can wear one if it is comfortable and they can use it correctly.
Build the plan before the sky turns orange
Smoke arrives faster than shopping does, and the week you need an air cleaner is the week nobody has one. Work through this list on an ordinary day.
- Which room is your clean room? Name it out loud, now, to the people you live with.
- Do you own an air cleaner sized for that room, plus a spare filter?
- Does your central system take a MERV 13 or better filter, and is there one in the closet?
- Do you have NIOSH-approved respirators that fit the adults in the house, and have you done one calm seal check so you know what a good fit feels like?
- If anyone uses a rescue inhaler or a daily controller medicine, is there enough for two weeks?
- Where would you go if the house got too hot to keep closed? Name the building.
- Who in the household is in a higher-risk group, and who is checking on them?
When to stop managing and get help
People with asthma, COPD, heart disease or another chronic lung condition should agree a smoke plan with their own clinician ahead of time, not in the middle of an event. Ask three questions. What symptoms mean I step up my medication? What symptoms mean I call you? What symptoms mean I go in? Write the answers down, because nobody reasons well in a brown haze.
EPA’s guidance to clinicians treats coughing, wheezing or shortness of breath as a sign to cut smoke exposure immediately. Act on that first. Then get medical care if breathlessness is getting worse rather than holding steady, if you have chest pain, or if wheezing does not settle after a rescue inhaler. Call 911 for severe symptoms: serious difficulty breathing, chest pain that will not ease, confusion, or fainting. Do not wait for the air outside to clear before you call.
What to carry away from this
Start with the clean room: one room, door closed, air cleaner running, windows shut, for as many hours as you can manage. That beats everything else available to an ordinary household, and the indoor measurements back it up.
The rest is secondary. A respirator helps when you have to be outside, and only if it seals. Recirculate mode in the car and in the HVAC system costs nothing, and neither does leaving the candle unlit.
And use the number. Check the Fire and Smoke Map before you decide whether today is a running day. Your nose went quiet twenty minutes after the smoke arrived. The particles did not.
Sources
- AirNow (U.S. EPA and partner agencies). “AQI Basics.” airnow.gov
- U.S. Environmental Protection Agency. “Wildfire Smoke: A Complex Mixture.” epa.gov
- U.S. Environmental Protection Agency. “Health Effects Attributed to Wildfire Smoke.” epa.gov
- U.S. Environmental Protection Agency. “Who is at Increased Risk of Health Effects from Wildfire Smoke Exposure.” epa.gov
- U.S. Environmental Protection Agency. “Strategies to Reduce Exposure Outdoors.” epa.gov
- U.S. Environmental Protection Agency. “Strategies to Reduce Exposure Indoors.” epa.gov
- U.S. Environmental Protection Agency. “Using the Air Quality Index (AQI) to Plan Daily Activities.” epa.gov
- U.S. Environmental Protection Agency. “Create a Clean Room to Protect Indoor Air Quality During a Wildfire.” epa.gov
- U.S. Environmental Protection Agency. “Wildfire Smoke Course Resources.” epa.gov
- National Institute for Occupational Safety and Health, CDC. “To Beard or not to Beard? That's a good Question! (NIOSH Science Blog).”
- Centers for Disease Control and Prevention. “How Wildfire Smoke Affects Your Body.” cdc.gov
- Centers for Disease Control and Prevention. “Preparing for Wildfires.” cdc.gov
- Annual Review of Medicine (PubMed). “Gould CF et al. "Health Effects of Wildfire Smoke Exposure." Annual Review of Medicine, 2023. PubMed. DOI: 10.1146/annurev-med-052422-020909.” doi.org
- Environmental Science & Technology (PubMed). “Lei Y et al. "Wildfire Smoke: Health Effects, Mechanisms, and Mitigation." Environmental Science & Technology, 2024. PubMed. DOI: 10.1021/acs.est.4c06653.” doi.org
- Science of the Total Environment (PubMed). “Xiang J et al. "Field measurements of PM2.5 infiltration factor and portable air cleaner effectiveness during wildfire episodes in US residences." Science of the Total Environment, 2021. PubMed. DOI: 10.1016/j.scitotenv.2021.145642.” doi.org
- Environmental Science & Technology (PubMed). “Laguerre A, Gall ET. "Polycyclic Aromatic Hydrocarbons (PAHs) in Wildfire Smoke Accumulate on Indoor Materials and Create Postsmoke Event Exposure Pathways." Environmental Science & Technology, 2023. PubMed. DOI: 10.1021/acs.est.3c05547.” doi.org