Outdoor pollution — including wildfire smoke and traffic emissions — is a major contributor to degraded indoor air quality and exposure. Its influence depends on how pollutants enter and accumulate within buildings, making envelope tightness, ventilation, and filtration critical to reducing infiltration and protecting health.

Wildfire smoke originates from the combustion of vegetation, buildings, and other fuels, releasing fine particles and reactive gases that travel long distances and infiltrate buildings. Once indoors, smoke contributes to elevated PM_2.5, CO, NO_x, and VOCs that persist even after outdoor concentrations subside. Exposure is associated with adverse respiratory and cardiovascular effects, including asthma exacerbation, reduced lung function, and increased hospitalizations during fire events. Maintaining airtight building envelopes, using high-efficiency filtration, establishing dedicated “clean rooms,” and performing thorough post-fire remediation are key strategies for reducing indoor exposure during and after wildfires.

Traffic-related air pollution arises from fuel combustion and mechanical wear, generating PM_2.5, UFPs, BC, NO_x, CO, and VOCs that accumulate near busy roads. These pollutants readily enter nearby buildings and vehicles, exposing occupants to soot and reactive gases that contribute to asthma, cardiovascular disease, and cognitive decline. Infiltration depends on proximity, airflow, and building design, with higher exposures near major roads and in poorly ventilated spaces. High-efficiency filtration, thoughtful air intake placement, and broader efforts to reduce traffic emissions are the most effective means of reducing indoor exposure.

The following subsections examine these two outdoor-origin sources of indoor pollution in detail.

3.3.1 Wildfires

Wildfire smoke is a complex source of indoor pollution, introducing fine particles and reactive gases that pose significant health risks. Sealing the building envelope, upgrading central air filtration, and using high-efficiency portable air purifiers are essential to reducing infiltration and maintaining healthy indoor air quality.

Wildfires have become increasingly frequent and severe in recent years due to warmer, drier climate conditions, leading to extended periods of degraded air quality and elevated health risks. When vegetation, buildings, and other fuels burn, they release a complex mixture of gases and particles whose composition depends on fuel type, moisture content, and combustion conditions. PM_2.5 is typically considered the dominant pollutant, accompanied by CO, NO_x, and a wide range of VOCs and SVOCs that continue to transform as smoke ages. These pollutants can travel long distances and infiltrate buildings far from the source, making wildfires a recurring driver of indoor air contamination.

Exposure to wildfire smoke is associated with a wide range of respiratory and cardiovascular effects. PM_2.5 penetrates deep into the lungs and enters the bloodstream, triggering inflammation, oxidative stress, and impaired vascular function. Fire-associated smoke particles are notably fine and chemically reactive, making them more damaging than typical urban pollution. Epidemiological studies link smoke exposure to asthma exacerbation, reduced lung function, and higher hospital admissions for respiratory and cardiac disease, while reviews identify elevated mortality during major fire events. Long-term exposure increases risks of chronic respiratory and cardiovascular illness, and emerging evidence points to additional effects on mental health and reproduction.

Reviews and field studies show that wildfire smoke readily infiltrates buildings and often raises indoor particle levels to unhealthy concentrations even when windows remain closed. Across monitored homes, schools, and public buildings, infiltration factors typically range from about 0.2 (strong protection) to 0.8 (weak protection), with rapid penetration sometimes occurring within minutes in childcare settings. Experimental and monitoring evidence consistently identify envelope tightness, ventilation rate, and filtration efficiency as the main determinants of indoor exposure; low-income and older residences tend to experience higher infiltration due to leaky construction. Beyond PM_2.5, recent work shows that vapor-phase PAHs can exceed outdoor levels and that PAHs and trace metals persist on surfaces after fires, indicating that smoke contributes both short-term and residual indoor contamination.

Mitigation indoors depends on both building filtration and supplemental devices. Field and modeling studies show that upgrading HVAC filters from MERV 8 to MERV 13 can roughly halve indoor/outdoor smoke ratios in mechanically ventilated buildings. Controlled and residential measurements indicate that portable air purifiers with HEPA filters typically remove 50–80% of indoor PM_2.5 when properly sized and operated, while experimental evaluations of DIY fan filters using MERV 13 media demonstrate meaningful but more variable performance in homes without central systems. Keeping windows and doors closed limits smoke entry, though controlled ventilation remains necessary to prevent CO₂ and humidity buildup. Establishing a dedicated “clean room” with continuous HEPA or high-MERV filtration provides the most reliable short-term protection for sensitive occupants. After fires, studies of affected homes report persistent surface contamination from PAHs and metals, underscoring the need for validated cleaning and remediation practices to address residual indoor pollutants.

Research demonstrates that wildfire smoke readily infiltrates buildings but its impacts can be reduced through effective filtration and remediation. Using high-efficiency air purifiers, creating “clean rooms,” and performing thorough post-fire cleaning are effective strategies for limiting exposure.

3.3.2 Traffic-Related Air Pollution

Traffic-related air pollution is a pervasive urban source that infiltrates buildings and vehicles, exposing occupants to fine particles and gases. While city-wide emission controls remain essential, localized filtration and design measures are critical for protecting indoor environments near busy roads.

Traffic-related air pollution (TRAP) is a dominant outdoor source of indoor air contamination in urban environments. Motor vehicles emit PM_2.5 and UFPs, BC, NO_x, CO, and VOCs during fuel combustion, while brake and tire wear and resuspended road dust contribute additional metals and coarse particles. These emissions accumulate along congested roadways and street canyons where dispersion is limited, producing elevated levels of PM_2.5, soot, and aromatic hydrocarbons near traffic corridors. As one of the most persistent and spatially variable pollution sources in cities, TRAP degrades IAQ across nearby homes, schools, and workplaces.

Exposure to TRAP is consistently associated with a wide range of adverse health effects. Epidemiologic and toxicologic evidence links this exposure to asthma, cardiovascular and neurological disease, metabolic dysfunction, and adverse pregnancy outcomes. Population studies show that even moderate traffic pollution increases respiratory symptoms among adults, particularly women and smokers, while long-term cohort data confirm higher rates of childhood asthma near major roads. Broader assessments identify premature mortality, cognitive decline, neurodegenerative disorders, and metabolic diseases such as diabetes and obesity among the expanding list of outcomes. Modeling work also demonstrates that roadway congestion substantially elevates morbidity and mortality risks for drivers, commuters, and nearby residents.

Modeling and field investigations show that traffic emissions penetrate buildings and elevate indoor concentrations, particularly in spaces close to major roads. Homes and schools near high-traffic corridors often contain higher levels of PM_2.5, soot, and VOCs than those in quieter areas, reflecting both exhaust infiltration and resuspended road dust. Infiltration is influenced by building orientation and ventilation design: windward openings allow greater pollutant entry, while leeward air intakes or roof vents can maintain airflow while limiting ingress. Dispersion is also constrained in dense urban street canyons, where recirculating air keeps pollutant levels elevated even several stories above ground. Seasonal and meteorological factors modulate these effects, with stronger infiltration during heavy traffic and cooler months. Within vehicles, pollutant levels depend on traffic density, ventilation mode, and sunlight, with evidence of in-cabin O₃ formation under high NO₂ and VOC conditions.

Reducing exposure to TRAP requires both outdoor emission control and indoor protective measures. Urban strategies such as cleaner fuels, vehicle electrification, and traffic management can lower ambient concentrations, but non-exhaust sources like brake and tire wear remain significant. Within buildings, high-efficiency filtration and careful placement of air intakes substantially reduce infiltration near roadways. Field studies in roadside schools showed that HVAC filters rated MERV 14 or higher could cut indoor PM and BC by one-third to more than 90%, with activated carbon media also removing NO₂ and VOCs. Keeping windows and doors closed during rush hour, positioning classrooms and offices away from traffic-facing façades, and using recirculated or filtered air in vehicles further reduces exposure.

Evidence shows that traffic-related air pollution is a major contributor to indoor exposure in urban areas but can be mitigated through design and filtration. Installing high-efficiency filters, optimizing air-intake placement, and supporting city-level measures are effective strategies to limit infiltration and protect occupant health.