Wildfires Can Impact Your Indoor Air Quality and Health

A vivid orange sunrise or sunset can be beautiful, but during wildfire season it may also be a sign that smoke particles are suspended in the air. Those particles scatter sunlight, creating dramatic colors in the sky. They can also travel long distances, enter homes, and affect the air your family breathes indoors.

During wildfire smoke events, outdoor air quality can change quickly. Checking trusted sources such as AirNow.gov in the United States or Canada’s Air Quality Health Index can help you decide when to limit outdoor activity and take extra steps to protect indoor air.

What Is Wildfire Smoke Made Of?

Wildfire smoke is a complex mixture of gases and particles produced when trees, plants, buildings, and other materials burn. According to the EPA’s wildfire smoke guide, smoke contains carbon monoxide, carbon dioxide, particulate matter, and other compounds that can vary depending on what is burning and how hot the fire is. Read the EPA wildfire smoke guide.

The New York State Department of Health notes that smoke may also contain chemicals such as aldehydes, acid gases, sulfur dioxide, nitrogen oxides, polycyclic aromatic hydrocarbons, benzene, toluene, styrene, metals, and dioxins. Learn more from the New York State Department of Health.

That mix is one reason wildfire smoke can be more than a temporary nuisance. It can affect outdoor air quality across entire regions and, when it enters a home, can also affect indoor air quality.

Why Particle Size Matters in Wildfire Smoke

When discussing air quality, particle size matters. Smaller particles can travel deeper into the respiratory system and are harder for the body to filter out before they reach the lungs.

Particle Type	Approximate Size	Why It Matters
PM10	10 microns or smaller	Includes larger dust, pollen, and mold particles that can irritate the eyes, nose, throat, and upper airways.
PM2.5	2.5 microns or smaller	A major component of wildfire smoke. These fine particles can reach deep into the lungs and are closely monitored during smoke events.
PM1	1 micron or smaller	Very small particles that may move even deeper into the lungs and may be more difficult to capture without high-quality filtration.

Wildfire smoke is especially concerning because it contains large amounts of PM2.5. These fine particles are small enough to remain airborne for long periods and can be carried by wind far from the fire itself. That is why wildfire smoke alerts may affect communities hundreds or even thousands of miles from the flames.

Understanding Air Quality Index During Wildfire Smoke Events

The Air Quality Index, or AQI, is a scale used to communicate how polluted the outdoor air is and what health precautions people may need to take. During wildfire smoke events, AQI levels often rise because of fine particulate matter, especially PM2.5.

AQI Category	What It Means	General Guidance
Good	Air quality is generally satisfactory.	Outdoor activities are usually acceptable for most people.
Moderate	Air quality is acceptable, but some sensitive people may notice symptoms.	People who are unusually sensitive to air pollution may consider limiting prolonged outdoor exertion.
Unhealthy for Sensitive Groups	People with higher sensitivity may experience health effects.	Children, older adults, pregnant people, and people with heart or lung conditions should reduce prolonged outdoor activity.
Unhealthy	Everyone may begin to experience health effects.	Limit outdoor activity and take steps to keep indoor air cleaner.
Very Unhealthy	Health alert conditions are present.	Avoid outdoor exertion and use cleaner indoor air strategies.
Hazardous	Emergency conditions are possible.	Stay indoors as much as possible and follow local public health guidance.

Because smoke conditions can shift quickly with wind and weather, it is helpful to check local AQI updates throughout the day during active wildfire smoke events.

How Does Wildfire Smoke Get Inside Your Home?

Wildfire smoke can affect indoor air even when windows and doors are closed. Smoke particles are small enough to enter through tiny openings in the building envelope and through normal air exchange between indoors and outdoors.

Common entry points include:

• Open windows and doors
• Gaps around windows, doors, vents, and utility penetrations
• Attics, crawlspaces, and other leakage points
• Heating and cooling systems that bring in outdoor air
• Fresh air ventilation systems without adequate filtration during smoke events

A tighter home may reduce smoke infiltration, but it does not eliminate the risk. Once smoke particles are indoors, they can linger in the air and settle on surfaces. Activities such as vacuuming, walking through rooms, or running fans can stir some particles back into the air.

Even after outdoor air improves, smoke odors may linger indoors because fine particles and volatile compounds can settle on fabrics, carpets, furniture, and other surfaces.

How Can Wildfire Smoke Affect Your Health?

Wildfire smoke can cause short-term symptoms and may worsen existing health conditions. The EPA notes that exposure to fine particulate matter can cause eye and respiratory tract irritation, reduced lung function, bronchitis, asthma exacerbation, heart failure, and premature death in some cases.

Common symptoms during smoke events include:

• Burning or irritated eyes
• Runny nose or sinus irritation
• Sore throat
• Coughing
• Wheezing
• Headaches
• Fatigue
• Shortness of breath

For many people, symptoms are temporary and improve when smoke exposure decreases. For others, especially people with asthma, COPD, heart disease, or other chronic conditions, wildfire smoke can trigger more serious reactions.

Long-term exposure to fine particulate matter has also been associated with increased risk of respiratory and cardiovascular health problems. Research continues to examine how repeated smoke exposure may affect long-term health, especially as wildfire seasons become more severe in many regions.

Who Is Most Vulnerable to Wildfire Smoke?

Wildfire smoke can affect anyone, but some people are more likely to experience health problems from smoke exposure.

• Children, because their lungs are still developing and they breathe more air relative to their body size
• Older adults
• Pregnant people
• People with asthma, COPD, bronchitis, or other respiratory conditions
• People with heart disease or high blood pressure
• People who work or exercise outdoors
• People without access to clean indoor air during smoke events

Healthy adults can also experience symptoms during heavy or prolonged smoke exposure. Physical activity can increase the amount of smoke inhaled, so outdoor exercise during poor air quality days can raise exposure even for people who are otherwise healthy.

How to Reduce Wildfire Smoke Indoors

This article explains why wildfire smoke matters for indoor air quality and health. The next step is understanding how to reduce exposure inside your home.

During wildfire smoke events, it helps to monitor local air quality alerts, limit outdoor activity when smoke levels are high, keep windows and doors closed, and use appropriate filtration when possible.

• Check local AQI levels. Use trusted sources such as AirNow, local public health alerts, or Canada’s Air Quality Health Index before opening windows or spending extended time outdoors.
• Keep outdoor smoke out when possible. Close windows and doors and reduce unnecessary outdoor air entry during heavy smoke conditions.
• Create a cleaner-air room. During severe smoke events, public health agencies often recommend setting up one room with closed windows and doors and using a properly sized portable air cleaner with a True HEPA filter.
• Use HVAC filtration appropriately. If your system can safely handle a higher-efficiency filter, a properly installed MERV-rated filter may help reduce fine particles. Always follow the HVAC manufacturer’s recommendations so airflow is not restricted.
• Use a portable HEPA air purifier. Portable air purifiers with True HEPA filtration may help reduce fine particles in a room when properly sized and maintained.
• Avoid ozone-generating air cleaners. Ozone can irritate the lungs and create additional indoor air quality concerns.
• Reduce indoor pollution sources. Avoid smoking, burning candles, frying foods, vacuuming without a HEPA filter, or using products that add particles or fumes indoors during smoke events.
• Check filters after prolonged smoke exposure. Filters may load more quickly during heavy smoke events and may need inspection or replacement.

Normally, fresh air ventilation supports healthy indoor air quality by bringing outdoor air into the home and exhausting stale indoor air. During heavy wildfire smoke, however, homeowners may need to adjust ventilation system operation, increase filtration, or follow manufacturer and public health guidance to reduce smoke entry. The right approach depends on the system design, filter compatibility, and local outdoor air quality conditions.

For practical steps on preparing your home, improving filtration, and reducing smoke exposure indoors, read How to Protect Your Home from Wildfire Smoke.

Research Highlights: Wildfire Smoke and Indoor Air Quality

• The EPA wildfire smoke guide identifies fine particulate matter, especially PM2.5, as a major public health concern during wildfire smoke events.
• AirNow.gov provides real-time air quality information that can help people understand when smoke levels may be unhealthy.
• The CDC recommends paying attention to local air quality reports, staying indoors when advised, and using cleaner indoor air strategies during wildfire smoke events.
• Health Canada recommends reducing exposure to wildfire smoke by limiting outdoor activities, keeping indoor air clean, and following local air quality guidance.
• The California Air Resources Board recommends using high-efficiency filtration when appropriate and avoiding indoor sources of pollution during smoke events.

Frequently Asked Questions

Can wildfire smoke affect indoor air quality?

Yes. Wildfire smoke can enter through windows, doors, gaps in the building envelope, HVAC systems, and ventilation openings. Once indoors, fine smoke particles can remain suspended in the air and affect indoor air quality.

Why is PM2.5 from wildfire smoke dangerous?

PM2.5 particles are small enough to travel deep into the lungs. Exposure can irritate the respiratory system, worsen asthma or heart conditions, and contribute to unhealthy air during wildfire smoke events.

Can wildfire smoke make you sick even if you are indoors?

Yes. If outdoor smoke enters the home, indoor particle levels can rise. People may experience irritated eyes, coughing, sore throat, headaches, fatigue, wheezing, or shortness of breath.

Who is most sensitive to wildfire smoke?

Children, older adults, pregnant people, people with asthma or COPD, pregnant people, people with heart disease, and outdoor workers are among the groups most vulnerable to wildfire smoke exposure.

Does closing windows keep all wildfire smoke out?

No. Closing windows and doors helps, but smoke can still enter through small leaks, vents, and air exchange with the outdoors. Filtration and reduced infiltration are both important during smoke events.

Should I use an air purifier during wildfire smoke?

A portable air purifier with a True HEPA filter can help reduce fine particles in a room when it is properly sized and used according to the manufacturer’s instructions. Avoid air cleaners that intentionally generate ozone.

What MERV filter should I use for wildfire smoke?

A higher-efficiency MERV-rated filter may help reduce fine particles from wildfire smoke if it is compatible with your HVAC system. Always check the manufacturer’s recommendations before upgrading because filters that are too restrictive can reduce airflow or strain the system.

Should I turn off my fresh air ventilation system during wildfire smoke?

It depends on the system, outdoor smoke levels, and available filtration. Fresh air ventilation is beneficial under normal conditions, but during heavy wildfire smoke events, you may need to follow manufacturer guidance, local public health recommendations, or temporarily adjust outdoor air intake to reduce smoke entry.

Can wildfire smoke come through vents?

Yes. Smoke can enter through outdoor air intakes, ventilation openings, gaps in ductwork, and other leakage points. Systems that bring in outdoor air should be properly filtered and maintained, especially during wildfire smoke events.

How long does wildfire smoke stay indoors?

It depends on how much smoke entered, how airtight the home is, how much filtration is used, and whether particles have settled on surfaces. Fine particles can remain suspended for hours, while smoke odors may linger longer on fabrics, carpets, and furnishings.

Where can I learn how to protect my home from wildfire smoke?

For step-by-step guidance, read How to Protect Your Home from Wildfire Smoke.