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TL;DR: Millions of people in the U.S. live within a few hundred feet of freeways, exposing them to harmful air pollutants such as black carbon, nitrogen dioxide, and fine particulate matter. These pollutants are linked to health issues, including asthma, heart disease, and respiratory problems, especially for children, the elderly, and those with preexisting conditions. Pollution can travel more than a mile from the freeway and can also seep indoors. Communities of color and lower-income groups are disproportionately affected due to historic urban planning decisions. To avoid negative health impacts, it is best to live at least 500 feet, and ideally at least 1,000 feet, away from a freeway.
In 2014, 45 million people in the United States lived, worked, or attended school within 300 feet of a major road, airport, or railroad. In 2017, 1.2 million people in Southern California alone already lived within 500 feet of a freeway. With so many people living within proximity to these air pollution hotspots, it is worth examining the potential dangers of freeway-related air pollution and how far it can spread.
Freeway traffic and air pollution
Freeway traffic, as with other vehicle traffic, emits a variety of ambient air pollutants, including particulate matter (PM10 and PM2.5), nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs). Freeways are hotspots for air pollution emissions because of the scale at which these air pollutants are emitted; however, similar hotspots can be found on busy roadways and intersections.
Freeway vehicles produce air pollutants through not only tailpipe emissions, but also tire, brake, and road wear. Resuspended roadway dust can also contribute to local air pollution. Diesel vehicles, especially large trucks, are associated with a particularly harmful form of particulate matter known as black carbon (BC).
Black carbon exposure is significant as its health impacts are even more severe than regular particulate matter. For example, a 2021 study found 50% greater odds of asthma prevalence associated with per-mass concentrations of black carbon than for concentrations of PM2.5.
Freeway air pollution health effects
Living near sources of traffic-related air pollution is associated with many adverse health effects, including impaired lung function, cardiovascular disease morbidity and mortality, and the worsening of asthma. Traffic-related air pollutants like fine particulate matter have been linked to bronchitis, respiratory infection, and even dementia and infertility.
Certain populations are more vulnerable to the effects of air pollution. Children, the elderly, and those with preexisting conditions are particularly susceptible to the adverse health outcomes of breathing air pollution. This means that extra care should be taken concerning the proximity of schools, playgrounds, hospitals, and retirement homes to freeways and other major roads.
Commuting on the freeway itself also comes with air quality risks. Being in a car on the freeway can expose you to five to ten times higher air pollution levels than the surrounding areas. Even with the windows closed, you can still breathe significant amounts of ambient freeway air pollution. When driving on highways, make sure to close car windows and set your ventilation system to recirculate. Less time in traffic is the best way to reduce on-the-road air pollution exposure.
How far do freeway air pollutants spread?
In 2019, the Los Angeles County Department of Public Health (DPH) recommended that schools and houses should not be built within at least 500 feet of a freeway. In 2012, the city of Los Angeles issued a similar advisory establishing that air pollution is harmful to sensitive populations within 1,000 feet of a freeway. Although freeway air pollution is generally highest in those ranges, adverse health effects from highway traffic can extend far beyond even 1,000 feet.
Air pollution can extend more than a mile away from the freeway. Factors such as wind patterns, freeway design, and surrounding topography influence the extent to which freeway air pollution can spread. In Los Angeles, for instance, early morning weather conditions allow air pollutants to stretch especially far. In fact, a study in 2013 found that roughly a quarter of Los Angeles residents are exposed to noxious freeway traffic plumes every single morning in this freeway-reliant city.
Can freeway air pollution seep indoors?
Closing windows and doors can help keep air pollution from seeping inside, but they cannot completely stop ambient air pollution from getting in. Indoor pollution levels still reach 50-70% of outdoor levels. Older homes in particular let in a significant amount of outside air, even with filtration systems in place.
Living near traffic can cause a dark layer of soot and grime to settle on indoor surfaces. This form of black carbon typically indicates that there are many diesel vehicles passing by, emitting especially harmful air pollution. It also indicates that there is most likely ultra-fine particulate matter finding its way inside. While it is best to clean this grimy layer up, it is the pollutants that can’t be seen that cause the most harm.
Inequality in exposure
People of color are more likely to be exposed to vehicular air pollution. Across the country, the populations that are living closest to major roadways are generally non-White. In LA, for instance, urban planning caused communities of color to be stuck near major roadways, including freeways, and other polluting infrastructure.
In California, African Americans are exposed to 43% more fine particulate matter from vehicular traffic than White people on average. Latino people are exposed to 39% more than White Californians, and Asians to 21% more. Economic differences similarly lead to a disparity in exposure.
Looking forward: What we can do about freeway-related air pollution
Ideally, live and work far from any freeways or major roadways. There should be at least 500 feet of distance between your living space and a freeway, if not 1,000 feet, or even a mile, especially if you live downwind from the road. Unfortunately, this is not always possible due to financial and other reasons.
Soundwalls and other barriers can help protect residents from air pollution. Living at a different level, far above or below a freeway, is better than living at the same level as it. Avoid street canyons where tall buildings trap air pollution below. Living near freeway interchanges, or even street intersections, can magnify the air quality risk.
If you do live near a freeway, employ the best practices for mitigating the impacts of ambient air pollution. Use MERV 13 or higher high-efficiency air filters and maintain and replace them every few months. Notably, these usually do not filter out all air pollutants, such as toxic exhaust gases, yet they do help protect residents from other harmful pollutants. A standalone air filter can help keep a single room clean. Keep windows and doors closed as often as possible.
Improved regulation, conscious city planning, increased awareness, and better public transportation can all help decrease citizen exposure to freeway air pollution. Learn how you can monitor ambient air quality and find a solution with Clarity.