Wildfire Smoke Health Effects & How to Protect Yourself

Wildfire smoke contains PM2.5 particles, benzene, and formaldehyde that penetrate deep into the lungs and bloodstream. Here's what the science says about the risks, who is most vulnerable, and what actually works to protect you.

What makes wildfire smoke dangerous

Wildfire smoke is not a single substance — it is a complex mixture of gases and fine particles produced as vegetation, structures, and vehicles burn. The primary health threat is fine particulate matter (PM2.5): particles smaller than 2.5 micrometers in diameter — about 1/30th the width of a human hair. These particles are small enough to bypass the nose and upper airway filtration system, penetrate deep into the alveoli of the lungs, and cross into the bloodstream, where they cause systemic inflammation.

Beyond PM2.5, wildfire smoke contains a range of toxic gases and compounds. Benzene (a known carcinogen), formaldehyde, and acrolein are consistently detected in high concentrations downwind of active fires. When structures burn — homes, vehicles, power lines — the smoke also carries heavy metals, flame retardants, and polycyclic aromatic hydrocarbons (PAHs). California Air Resources Board (CARB) data have shown wildfire PM2.5 to be up to three times more damaging per microgram than typical urban PM2.5, because of this more toxic chemical composition.

Wildfire smoke is also not a local problem. Fine particles stay aloft in the upper atmosphere and travel thousands of miles with prevailing winds — a dynamic confirmed by EPA monitoring data showing Canadian wildfire smoke regularly pushing AQI above 200 in the eastern United States. This long-range transport means you can experience serious health impacts from fires you never see or smell.

Who is most at risk from wildfire smoke

These groups should begin protective measures at lower AQI thresholds than the general population.

Children under 18

Any AQI > 100

Lungs are still developing through the late teen years. PM2.5 exposure during development is linked to reduced lung function and higher asthma rates in adulthood.

Adults over 65

AQI > 100

Reduced respiratory reserve and more underlying cardiovascular disease make older adults more likely to experience cardiac events during smoke events.

Pregnant people

AQI > 100

PM2.5 crosses the placenta. Smoke exposure is associated with preterm birth, low birth weight, and gestational hypertension.

People with asthma

AQI > 50

Even moderate smoke can trigger bronchoconstriction. Keep rescue inhalers accessible; consider staying home at AQI > 100.

Heart disease patients

AQI > 100

Fine particles enter the bloodstream and promote inflammation, raising risk of arrhythmia, heart attack, and stroke. People with prior cardiac events should be extremely cautious.

COPD / chronic lung disease

AQI > 50

Smoke worsens airway inflammation and can cause acute exacerbations requiring hospitalization. Limit any outdoor exposure above moderate AQI.

Outdoor workers

AQI > 100

Agricultural, construction, and utility workers face extended outdoor exposure. Cal/OSHA and federal OSHA require employer protection plans when wildfire smoke exceeds AQI 151.

AQI levels and what they mean for wildfire smoke

The Air Quality Index (AQI) translates pollutant concentrations into a 0–500 scale. During a wildfire smoke event, AQI is dominated by PM2.5. Monitor the current AQI at AirNow.gov.

0–50 · Good

No special precautions needed. Air quality poses little or no risk.

51–100 · Moderate

Unusually sensitive people — those with rare lung hypersensitivity — should consider reducing prolonged outdoor exertion.

101–150 · Unhealthy for Sensitive Groups

Children, elderly, pregnant people, and anyone with heart or lung disease should limit prolonged outdoor activity. General public is not likely to be affected.

151–200 · Unhealthy

Everyone may begin to experience health effects. Sensitive groups may experience more serious effects. Limit outdoor exertion; run HEPA purifier indoors.

201–300 · Very Unhealthy

Health alert: the entire population may experience significant effects. Stay indoors, avoid outdoor exercise, wear N95 if you must go out.

301+ · Hazardous

Emergency conditions. Everyone is at risk of serious health effects. Remain indoors with HEPA-filtered air; vulnerable groups should consider evacuation.

How to protect yourself from wildfire smoke

Monitor AQI at AirNow.gov

Check the official EPA AirNow site (airnow.gov) or the AirNow app for current and forecast AQI in your zip code. Also follow your local air district — readings can vary significantly across a metro area during a smoke event.

Stay indoors with windows and doors closed

Your home provides real protection — indoor PM2.5 is typically 30–50% lower than outdoors when you keep openings sealed. Close fireplace dampers, which are a major smoke entry point.

Run a true-HEPA air purifier

HEPA filters remove 99.97% of particles 0.3 microns and larger, which includes nearly all PM2.5. Size it for the room: a unit rated for 200 sq ft does not adequately clean 800 sq ft. Run it continuously during smoke events.

Set your HVAC to recirculate

Switch your thermostat fan to "recirculate" or "on" (not "auto") so it pulls from indoor air rather than outside. Replace your filter with a MERV-13 or better — standard MERV-8 fiberglass filters pass PM2.5 straight through.

Wear a properly fitted N95 when outdoors

An N95 respirator filters 95% of airborne particles including PM2.5 — but only when it seals to the face. Press the nose wire, cup the bottom, and do a seal check before going out. KN95s provide similar protection. Surgical and cloth masks do not protect against PM2.5.

Avoid strenuous outdoor activity

Exercise sharply increases breathing rate, pulling far more particles into the lungs. Postpone runs, yard work, and sports when AQI is elevated. Even a brief high-intensity session at AQI 150 delivers more PM2.5 to the lungs than hours of rest indoors.

Do N95 masks work against wildfire smoke?

Yes — N95 respirators filter at least 95% of airborne particles including PM2.5, making them effective protection against the primary health threat from wildfire smoke. However, protection depends almost entirely on achieving a proper face seal. An N95 worn loosely or with gaps at the nose or cheeks provides dramatically less filtration than the rated 95%.

To fit-check an N95: cup both hands over the mask and exhale sharply. If you feel air escaping around the edges rather than through the filter material, adjust the nose wire and reposition the straps. Men with beards cannot achieve a proper seal with a flat-fold or cup-style N95 — either shave or use a loose-fitting powered air-purifying respirator (PAPR) if working outdoors during smoke.

Mask comparison

N95 (properly fitted)

Filters ≥95% of PM2.5 particles

Effective

KN95 (Chinese standard)

Filters ≥95% by Chinese testing standard; fit can vary

Effective if sealed

Surgical / procedure mask

Filters large droplets; gaps around sides pass PM2.5

Not effective for smoke

Cloth mask / bandana

Minimal particle filtration; no seal

Not effective for smoke

Protecting your home's indoor air during wildfire smoke events

Your home is your best defense — but only if you actively filter the air inside it.

Size your HEPA purifier correctly

Match the unit’s Clean Air Delivery Rate (CADR) to your room: CADR in cfm ÷ 1.55 = sq ft it covers at five air changes per hour. During heavy smoke, run it on high continuously.

Upgrade to MERV-13 HVAC filters

MERV-13 captures roughly 85% of particles in the PM2.5 size range. Check your system’s airflow rating first — some older units can’t push enough air through a dense filter, which strains the motor.

Seal gaps around doors and windows

Use draft snakes or rolled towels at thresholds and window sills. Gaps around electrical outlets, plumbing penetrations, and recessed lights can let significant smoke in. Foam weatherstripping is a quick fix.

Build a DIY Corsi-Rosenthal box fan filter

Tape four MERV-13 furnace filters (20×20 in) around a 20-inch box fan, filters facing outward, add a cardboard top, and point the fan up. Costs ~$60–80, removes roughly 75% of PM2.5 in a room, and outperforms many commercial units. Instructions from the EPA validate the design.

Create a clean room for sleeping

If you can’t filter every room, concentrate protection in a single bedroom where you spend the most time. Close all vents except one with a HEPA unit nearby, and keep that door closed during the event.

When to evacuate due to smoke alone

While most wildfire evacuations are triggered by fire proximity and ember risk, smoke alone can necessitate evacuation or voluntary relocation for vulnerable individuals. At AQI above 300 (Hazardous), health authorities including the California Department of Public Health recommend that all sensitive groups — children, elderly, pregnant people, and anyone with heart, lung, or immune conditions — leave the area if they cannot adequately filter their indoor air.

Certain medical conditions lower the effective threshold for voluntary relocation. People dependent on home oxygen, those with severe COPD or end-stage heart failure, and individuals who have experienced a cardiac event in the past year face disproportionate risk even at AQI 150–200. Consult your physician before a smoke event to establish your personal action thresholds.

Evacuation maps for active fires are available on our fire evacuation map. If you receive an official evacuation order — for any reason — leave immediately.

Key threshold: AQI 301+ (Hazardous) triggers voluntary relocation recommendations for all sensitive groups. AQI 201–300 (Very Unhealthy) warrants relocation if you cannot filter indoor air to acceptable levels.

Children and wildfire smoke

Children are not simply small adults when it comes to wildfire smoke exposure. Their lungs are still developing through adolescence, and they breathe more air per unit of body weight than adults — meaning proportionally higher particle doses for the same ambient concentration. Research following major wildfire events has found measurable reductions in lung growth trajectories in children repeatedly exposed to heavy smoke events during critical developmental windows.

School districts in smoke-affected regions follow guidance from state health departments on outdoor recess. California guidance, for example, recommends canceling outdoor recess when AQI exceeds 100, moving all outdoor physical education indoors, and considering full indoor-only days above 150. Schools with older HVAC systems and poor filtration may need to close entirely at higher AQI levels.

Watch for these symptoms in children during or after smoke events, and seek medical care if they occur:

Persistent cough that worsens or does not resolve after smoke clears
Wheezing or audible breathing difficulty
Shortness of breath with routine activities (play, walking up stairs)
Chest tightness or pain
Nausea, headache, or unusual fatigue during smoke events
Eye redness and tearing that is severe or does not improve indoors
Worsening of existing asthma symptoms despite medication

Stay ahead of smoke events

Get real-time wildfire smoke maps, fire alerts for your area, and a complete preparedness plan.

Live Smoke Map →Get Fire Alerts →Preparedness Guide →

Wildfire smoke health FAQ

How far does wildfire smoke travel?

Wildfire smoke can travel thousands of miles from the source fire. Smoke from major Western wildfires regularly drifts across the entire continental U.S. to affect air quality on the East Coast. In 2023, smoke from Canadian wildfires pushed AQI into the hazardous range in New York City — more than 1,500 miles from the fires. The fine particles (PM2.5) that cause health effects are small enough to stay lofted in the upper atmosphere for days before descending.

Can wildfire smoke make me sick even if I’m not near the fire?

Yes. The health hazard from wildfire smoke is driven by fine particulate matter (PM2.5), not proximity to flames. PM2.5 particles lodge deep in the lungs and enter the bloodstream, causing inflammation that affects both the respiratory and cardiovascular systems. You can experience eye irritation, coughing, headaches, shortness of breath, and aggravated heart or lung conditions from smoke that traveled hundreds of miles. Sensitive groups have experienced emergency department visits from smoke events with no local fire at all.

What AQI is dangerous for wildfire smoke?

For healthy adults, AQI above 150 (Unhealthy) is when most people begin to feel health effects and should reduce outdoor exertion. For sensitive groups — children, elderly, pregnant people, and those with heart or lung disease — AQI above 100 (Unhealthy for Sensitive Groups) warrants precautions. AQI above 300 (Hazardous) is a health emergency for everyone. Wildfire smoke PM2.5 is particularly toxic compared to urban PM2.5 because it contains additional combustion byproducts including benzene, acrolein, and formaldehyde.

Does staying indoors protect you from wildfire smoke?

Staying indoors significantly reduces your exposure, but it does not eliminate it — outdoor air continuously infiltrates buildings. Studies show that without HEPA filtration, indoor PM2.5 during a smoke event typically reaches 30–60% of outdoor levels. With a properly sized HEPA air purifier running on high, indoor PM2.5 can be reduced to 10–20% of outdoor levels. The combination of closed windows, recirculating HVAC with MERV-13 filter, and a HEPA purifier provides the strongest indoor protection.

How long do wildfire smoke health effects last?

For healthy individuals, most acute symptoms — cough, eye and throat irritation, headache — resolve within hours to days of smoke clearing. However, research from major wildfire events including the 2018 Camp Fire found measurable reductions in lung function in exposed populations that persisted for months. Children exposed during lung development and people with pre-existing heart or lung conditions may experience longer-lasting effects. Anyone with persistent symptoms after a smoke event should consult a physician.

What should I do if I have asthma during wildfire smoke?

Keep your rescue inhaler with you at all times and make sure your controller medication is current and filled. Begin protective measures at AQI > 50 rather than waiting for the higher thresholds that apply to healthy adults. Avoid all outdoor exercise during smoke events. Run a HEPA air purifier in your bedroom. If you experience increased frequency of rescue inhaler use, nighttime symptoms, or any wheezing at rest, contact your doctor immediately — these are signs of an acute exacerbation. Have an asthma action plan and follow the yellow/red zone steps before symptoms escalate.

Is wildfire smoke worse than city smog?

Wildfire smoke is generally considered more toxic than urban smog at equivalent PM2.5 concentrations. Urban smog is primarily ozone and vehicle exhaust particles; wildfire smoke contains a more complex mixture including carcinogenic polycyclic aromatic hydrocarbons (PAHs), benzene, formaldehyde, acrolein, and heavy metals released as structures burn. California Air Resources Board (CARB) studies have found wildfire PM2.5 to be up to three times more harmful per unit than typical background PM2.5 in terms of cardiovascular impacts. This means an AQI of 150 from wildfire smoke poses greater health risk than the same AQI reading from urban sources.

The information on this page is for general awareness and educational purposes only and does not constitute medical advice. If you have a heart or lung condition, are pregnant, or are caring for children with health conditions, consult your physician about your personal action plan for wildfire smoke events. For official air-quality readings, use AirNow.gov; for emergencies, follow local authorities and call 911.