How Wildfires Affect Air Quality

Wildfires have become the single largest source of episodic air quality emergencies in the United States. Understanding how wildfire smoke affects air quality — and what you can do about it — is increasingly important for all Americans, not just those living near fire-prone areas. ## How Wildfire Smoke Travels Wildfire smoke consists of a complex mixture of gases and fine particles produced when wood and other organic materials burn. The smallest particles (PM2.5) are the most dangerous because they penetrate deep into the lungs and can enter the bloodstream. Smoke from large wildfires can travel thousands of miles. In 2023, smoke from Canadian wildfires turned skies orange across the northeastern United States, pushing AQI to hazardous levels in New York City — a city more than 1,000 miles from the nearest fire. In 2020, smoke from West Coast fires reached the East Coast and was visible from satellite imagery across the entire continent. Smoke tends to be trapped at ground level during temperature inversions, which occur when a layer of warm air sits above cooler surface air, preventing vertical mixing. Mountain valleys are particularly susceptible to this effect, which is why cities like Missoula, Boise, and Reno can experience weeks of hazardous air quality during fire season. ## What Is in Wildfire Smoke Wildfire smoke contains hundreds of individual compounds, many of which are toxic: **Fine particulate matter (PM2.5):** The primary health threat. Concentrations can exceed 500 micrograms per cubic meter during severe smoke events — more than 40 times the EPA annual standard. **Carbon monoxide:** A colorless, odorless gas that reduces oxygen delivery to organs. While outdoor concentrations rarely reach immediately dangerous levels from wildfire smoke, indoor accumulation in poorly ventilated spaces can be hazardous. **Volatile organic compounds (VOCs):** Including formaldehyde, benzene, and acrolein. These irritate the respiratory tract and eyes. Acrolein is one of the most potent lung irritants found in wildfire smoke. **Polycyclic aromatic hydrocarbons (PAHs):** Known carcinogens produced by incomplete combustion. Long-term exposure is associated with increased cancer risk. When fires burn through developed areas, the smoke may also contain metals, plastics, asbestos, and other toxic materials from structures and vehicles. ## Health Effects Short-term exposure to wildfire smoke causes eye and throat irritation, coughing, headache, and difficulty breathing. People with asthma may experience severe attacks. Emergency room visits for respiratory and cardiovascular complaints increase dramatically during smoke events. Emerging research links wildfire smoke exposure to cardiovascular events including heart attacks and strokes, even in healthy individuals. Children, the elderly, outdoor workers, and pregnant women face elevated risks. Long-term health effects of repeated wildfire smoke exposure are still being studied, but preliminary evidence suggests increased risk of chronic respiratory disease, cognitive decline, and adverse pregnancy outcomes. ## Protecting Yourself **Indoor protection:** Close all windows and doors. Run HEPA air purifiers on the highest setting. Seal gaps around windows with damp towels if needed. Set HVAC systems to recirculate mode rather than drawing in outside air. Replace HVAC filters with MERV-13 or higher rated filters. **When you must go outside:** Wear an N95 or KN95 respirator mask properly fitted with no gaps. Surgical masks and cloth masks do not filter PM2.5 effectively. Limit time outdoors and avoid physical exertion. **Create a clean air room:** Designate one room in your home as a clean air space. Use a portable HEPA air purifier sized for the room. Keep the door closed and seal any gaps. This provides a refuge during extended smoke events. **Monitor AQI continuously:** Use AirNow.gov and the Fire and Smoke Map for real-time data. PurpleAir sensors can provide hyperlocal readings. Download an AQI alert app to receive notifications when conditions change. **Evacuate if necessary:** If you have serious respiratory or cardiovascular conditions and AQI remains in the Hazardous range for multiple days, consider temporarily relocating to an area with better air quality. Hotels in cleaner air zones fill up quickly during major smoke events, so plan early. ## The Growing Wildfire Threat Climate change is making wildfires more frequent, more intense, and longer-lasting. The western US wildfire season has lengthened by 84 days since the 1970s. The area burned annually has roughly doubled. And fires that once stayed in rural forests are increasingly threatening urban areas and their air quality. Adaptation is essential. Communities should invest in public clean air shelters, improve building air filtration standards, expand air quality monitoring networks, and develop wildfire smoke response plans. Individuals should invest in air purification, maintain a supply of N95 masks, and understand their local wildfire risk.