When the Air Quality Index (AQI) crosses 151, the air is officially classified as “unhealthy” for everyone, not just people with lung conditions. At that level, fine particulate matter (PM2.5) in the air reaches 55.5 micrograms per cubic meter or higher, and the priority shifts to reducing how much of it you breathe. Here’s what to do, starting with the steps that matter most.
Stay Indoors and Seal Your Home
The single most effective thing you can do is get inside and keep outdoor air from coming in. Close all windows and doors, and seal any noticeable cracks or gaps around exterior walls, window frames, and door sweeps. If you have a fireplace, close the damper. Avoid using exhaust fans in kitchens and bathrooms if possible, since they pull outdoor air in through gaps elsewhere in the house.
Set your HVAC system to recirculate rather than pulling in fresh air from outside. The filter in your system matters enormously here. A MERV-13 rated filter or higher is what you need to capture wildfire smoke and fine particulate matter. The average particle size in wildfire smoke is around 0.3 micrometers, which is the hardest size for filters to catch, and lower-rated filters let most of it pass through. If you don’t have a MERV-13 filter on hand, even upgrading from a basic fiberglass filter to whatever higher-rated option your local hardware store carries will help.
Set Up a Clean Air Room
If you don’t have central air or a good HVAC filter, create one room in your home with the cleanest air possible. Pick a bedroom or smaller space you can seal off, and run a portable air purifier with a true HEPA filter inside it. HEPA filters capture 99.97% of particles at 0.3 micrometers, making them highly effective against smoke and fine pollution.
When shopping for or choosing a portable air purifier, look at the Clean Air Delivery Rate (CADR) for smoke, not dust or pollen. The smoke rating covers particles in the 0.09 to 1.0 micrometer range, which matches the fine particulate matter that causes health problems. As a general rule, the CADR number should be at least two-thirds of the room’s square footage. A 150-square-foot bedroom needs a purifier with a smoke CADR of at least 100.
If you don’t own a purifier, you can build a basic one by taping a MERV-13 furnace filter to the back of a box fan. It won’t match a commercial HEPA unit, but it meaningfully reduces particle levels in a small room.
Wear the Right Mask Outside
If you have to go outdoors, an N95 or KN95 respirator is the only type of mask that provides real protection. Both filter at least 95% of fine particles, and tested KN95 respirators perform as well as N95s. Surgical masks fall about 15% below that, and cloth masks made of cotton or polyester perform roughly 70% worse than an N95. A bandana or scarf does almost nothing against PM2.5.
Fit matters as much as the filter material. An N95 that gaps at the sides or nose bridge lets unfiltered air in with every breath. Press the metal nose clip firmly against your face and check that the edges sit flush against your skin. Facial hair significantly reduces the seal. If you can smell smoke through the mask, it isn’t fitting properly.
Move Exercise Indoors
Physical activity dramatically increases how much air you pull into your lungs. During a jog or bike ride, your breathing rate can increase tenfold, which means tenfold the exposure to whatever is in the air. The American College of Sports Medicine notes that the benefits of outdoor exercise generally outweigh the pollution risks up to an AQI of about 150, but once the AQI crosses into the “unhealthy” range, the math flips. For people in vulnerable groups, including children, older adults, and anyone with lung or heart disease, outdoor exercise risks may start at AQI levels as low as 50.
Move workouts indoors and keep windows closed while you exercise. If you’re exercising in a room without air filtration, you’re still breathing indoor air that may contain elevated particle levels from outdoor infiltration, so running a purifier in your workout space helps.
Know Who Is Most at Risk
Unhealthy air affects everyone, but certain groups face outsized danger. People with asthma, COPD, coronary artery disease, or heart failure are the most vulnerable. Children breathe faster relative to their body size and have developing lungs. Older adults are more likely to have underlying heart or lung conditions, even undiagnosed ones.
The warning signs differ depending on the underlying condition. People with heart disease may notice chest pain, palpitations, unusual fatigue, or shortness of breath. Those with asthma or COPD typically experience coughing, chest tightness, wheezing, or a feeling that they can’t breathe as deeply as normal. If you’re pregnant, particle pollution also poses risks because the inflammatory response it triggers is systemic, not limited to the lungs.
Why Fine Particles Are Dangerous
PM2.5 particles are small enough to pass through the lungs and enter the bloodstream directly. Once in circulation, they trigger oxidative stress and inflammation throughout the body. This isn’t just a respiratory problem. Exposure raises blood pressure, disrupts heart rhythm, accelerates plaque buildup in arteries, and increases the tendency for blood clots to form. Short-term spikes in PM2.5 are linked to heart attacks and strokes, even in people who were previously healthy.
This is why the advice goes beyond “protect your lungs.” You’re protecting your cardiovascular system, too.
Eat Foods Rich in Vitamins C and E
Because oxidative stress is one of the primary ways air pollution damages the body, antioxidant-rich foods can offer a modest buffer. A large cohort study found that sufficient dietary intake of vitamins C and E reduced some adverse health effects associated with air pollution exposure, with these two vitamins showing stronger protective effects than other antioxidants studied. This isn’t a substitute for reducing exposure, but during prolonged poor air quality events, loading up on citrus fruits, bell peppers, nuts, seeds, and leafy greens is a low-effort way to support your body’s defenses.
Track Air Quality in Real Time
The EPA’s AirNow.gov website and app provide AQI readings from regulatory-grade monitors across the country. For more localized data, PurpleAir operates a network of low-cost sensors that report readings block by block. PurpleAir sensors tend to read higher than government monitors, especially during smoke events, so the EPA applies a correction formula that accounts for humidity and sensor characteristics. The EPA’s Fire and Smoke Map integrates both data sources with this correction already applied, making it the most practical single tool during wildfire season.
Check conditions before opening windows in the morning or heading outside. AQI can shift significantly over just a few hours, particularly as wind patterns change or temperature inversions trap pollution close to the ground overnight. Many air quality apps let you set alerts for specific AQI thresholds so you don’t have to keep checking manually.
Understanding AQI Levels
The “unhealthy” label covers AQI 151 to 200, but conditions can be far worse. “Very unhealthy” runs from 201 to 300, with PM2.5 concentrations reaching 150 to 250 micrograms per cubic meter. “Hazardous” starts at 301, where PM2.5 exceeds 250 micrograms per cubic meter. At hazardous levels, everyone should avoid all outdoor activity and take every indoor air quality measure available.
Even the “unhealthy for sensitive groups” range (AQI 101 to 150) warrants precautions if you have asthma, heart disease, or young children. The EPA recently tightened the annual PM2.5 standard to 9.0 micrograms per cubic meter, down from the previous 12.0, reflecting growing evidence that health effects occur at lower concentrations than previously recognized. There is no truly safe level of PM2.5 exposure, only degrees of risk.