Traffic-Related Air Pollution Research

Roughly 45 million Americans live within 300 feet of a highway, major road, or rail line. Traffic-related air pollution (TRAP) is a distinct subfield because the mix near roadways differs from the regional average: more nitrogen dioxide, more ultrafine particles, more black carbon, more PAHs, more brake and tire wear. Forty years of research has built a strong case that living close to busy traffic is a measurable health risk on top of the regional pollution load.

Background

The TRAP exposure label usually covers PM2.5, PM10, NO2, NOx, ultrafine particles (UFP, particles below 100 nanometers), black carbon, and elemental carbon. Concentrations of most of these decay sharply within 300 to 500 meters of a roadway, which lets researchers compare people living close to traffic with people living a few blocks away. Land-use regression models, developed by Gerard Hoek (Utrecht) and others, are the standard way to estimate exposure.

What the studies found

The Health Effects Institute (HEI) commissioned a multi-year systematic review released in 2022. A 13-person expert panel evaluated more than 350 epidemiological studies published between 1980 and 2019. The panel rated the confidence in TRAP-health associations:

• High confidence: all-cause mortality, circulatory mortality, ischemic heart disease mortality, and asthma onset in children.
• Moderate to high confidence: acute lower respiratory infection in children, lung cancer mortality.
• Moderate confidence: asthma onset in adults, COPD mortality.

The 2022 HEI review updated and broadly confirmed the earlier 2010 HEI traffic monograph. The panel emphasized that TRAP effects persist after adjustment for regional PM2.5, meaning the proximity signal is not just regional pollution in disguise.

Haneen Khreis (then at Texas A&M, now at Cambridge) and colleagues published a widely cited meta-analysis in Environment International in 2017. Pooling 41 studies, they estimated that each 4 microgram per cubic meter increase in long-term NO2 exposure was associated with a 5 percent higher risk of childhood asthma onset. A follow-on burden-of-disease analysis estimated that roughly 4 million new childhood asthma cases worldwide each year are attributable to traffic NO2 exposure (Achakulwisut et al., Lancet Planetary Health 2019).

Cardiovascular evidence is also strong. Studies of the Rome and Stockholm cohorts (Cesaroni et al., BMJ 2014; ESCAPE project) linked NO2 and traffic proximity to incident myocardial infarction. The MESA Air study in the U.S. linked long-term NO2 exposure to subclinical atherosclerosis as measured by carotid artery thickness.

Why it matters

TRAP exposure correlates strongly with race and income. The American Lung Association's annual State of the Air reports consistently show Black, Latino, and low-income communities are disproportionately near major roads. EPA's 2022 NO2 review and the 2024 PM2.5 standard both cite TRAP-near-road monitoring data.

The policy implications are concrete. London's Ultra Low Emission Zone (ULEZ), the Paris Crit'Air system, and California's truck rules at the Ports of Los Angeles and Long Beach all target traffic-specific emissions. Several studies have measured before-and-after improvements: London's ULEZ cut roadside NO2 by 20 to 30 percent in its first years of operation, with corresponding reductions in childhood asthma exacerbations reported by Imperial College researchers.

Open questions

Ultrafine particle exposure is poorly captured by regulatory monitoring (which measures PM2.5 by mass; UFPs add little mass). Whether UFPs are independently harmful, and how to regulate them, is an active area of research. Brake- and tire-wear particles (which won't go away with electric vehicles) are another emerging concern, especially with heavier EV battery packs. Diesel exhaust was classified by IARC as a Group 1 human carcinogen in 2012, and gasoline exhaust as Group 2B (possibly carcinogenic).

Sources

• Systematic Review and Meta-analysis of Selected Health Effects of Long-Term Exposure to Traffic-Related Air Pollution (HEI Special Report 23, 2022)
• Traffic-Related Air Pollution: A Critical Review of the Literature on Emissions, Exposure, and Health Effects (HEI Special Report 17, 2010)
• Exposure to traffic-related air pollution and risk of development of childhood asthma: A systematic review and meta-analysis (Khreis et al., Environment International 2017)
• Global, national, and urban burdens of paediatric asthma incidence attributable to ambient NO2 pollution (Achakulwisut et al., Lancet Planetary Health 2019)
• Long term exposure to ambient air pollution and incidence of acute coronary events (Cesaroni et al., BMJ 2014)
• Diesel and gasoline engine exhausts and some nitroarenes (IARC Monograph Volume 105)
• Impacts of the London Ultra Low Emission Zone on air quality (Mudway et al., Lancet Public Health 2019)