Research: Air Quality and Environment

Transportation behaviors and trends have a significant environmental impact.  Transportation contributes 27 percent of the nation’s greenhouse gas emissions, and saw a greater absolute increase during the period from 1990 to 2013 than any other sector (e.g. industry, commercial, electricity, etc.) (EPA, 2015). Exposure to air pollution has been connected to elevated risk of asthma, lower lung function, higher blood pressure, higher cardiac-related mortality, and lower overall life expectancy (National Institute for Transportation and Communities, 2014; Pope, 2009). However, increased physical activity through active travel generates long-term health benefits that outweigh the health risks of air pollution (Tainio et al., 2016). Still, air pollution exposure is also an environmental justice concern, with disparities in exposure by demographic and socioeconomic characteristics (Jones et al., 2014; Mejia et al., 2011).By shifting shorter trips to walking and biking, Safe Routes to School programs and efforts to promote active travel can improve air quality by reducing vehicle trips and miles travelled.

The literature in this section explores the relationship between transportation mode choice and vehicle emissions, air pollution, and health impacts. This section also includes increasingly emerging research examining the effects of climate change on active travel. 


  • Motor vehicles are responsible for up to 30% of total greenhouse gas emissions (Solomon et al., 2009)
  • In one study, a 5% increase in neighborhood walkability (as measured using an index based on residential density, street connectivity, land use mix, and retail floor area ratio) was associated with:
    • 6.5% fewer vehicle miles traveled (VMT) per capita
    • 5.6% fewer grams of nitrogen dioxide per capita
    • 5.5% fewer grams of volatile organic compound (VOC) emitted per capita (Frank, et al., 2006)
  • Living in majority white neighborhoods has been associated with lower air pollution exposure, and living in majority Hispanic neighborhoods has been associated with higher pollution exposures (Jones et al., 2014)
  • Lower socioeconomic status has been connected with closer school proximity to pollution sources (Mejia et al., 2011)
  • Rates of asthma have been associated with closer residential distance to a freeway among children (Gauderman et al., 2005); exposure to roads with high vehicle traffic (representing near-road traffic-related pollution) accounted for 14% of all asthma cases across 10 European cities (Perez et al., 2013)
  • In a study model, substitution of cycling for short vehicle trips had the potential to reduce gasoline demand about 35% and CO2 emissions by 12% (Higgins et al., 2005)
  • Health benefits of shifting from car to bicycle was associated with greater benefits from increased physical activity (3-14 months of life gained) compared with potential effects of inhaled air pollution (0.8-40 days of life lost) (de Hartog et al., 2010)
  • Improved air quality accounted for as much as 15% of the overall increase in life expectancy in one study (Pope et al., 2009)
  • In another study, improved air quality—resulting from an increase in cycling modal share—reduced disability-adjusted life years for cardiopulmonary disease caused by poor air quality (Hitchcock et al., 2014)
  • Exposure to traffic-related air pollution has been connected to an increased risk of asthma, lower lung functioning, higher blood pressure, and cardiac mortality (National Institute for Transportation and Communities, 2014). However, the long-term health benefits of increased physical activity through active travel outweigh the health risks of air pollution (Tainio et al., 2016)
  • Bicyclists’ respiration rates are two to five times faster than motorists, thereby resulting in more exposure to traffic-related air pollution and risk of related health problems, like asthma, lower lung function, higher blood pressure, and cardiac mortality. To reduce bicyclists’ exposure to traffic-related air pollution, researchers recommend separated bicycle facilities, low-volume routes, and off-peak travel (National Institute for Transportation and Communities, 2014)
  • Evidence over the past twenty years strongly indicates that climate change is associated with negative health outcomes, including heat stress, respiratory disorders, infectious diseases, food insecurity, mental health disorders, and significant economic consequences of reduced work capacity (Patz et al., 2014)
  • A recent UK report on traffic-related air pollution recommends that authorities establish 20 mph speed limits and make changes in the built environment to make walking and cycling safer to encourage more active transport (National Institute for Health and Care Excellence, 2017)
Cycling and Urban Air Quality: A Study of European Experiences

Key takeaway: Cycling measures can improve urban air quality levels as part of a multifaceted approach toward reducing road traffic.

Evaluation of Bicyclists Exposure to Traffic-Related Air Pollution along Distinct Facility Types

Key takeaway: Bicyclists’ exposure to air pollution can vary with roadway and travel characteristics, and transportation-related strategies can reduce exposure.

Residential Traffic Exposure and Childhood Leukemia: A Systematic Review and Meta-analysis

CONTEXT:  Exposure to elevated concentrations of traffic-related air pollutants in the near-road environment is associated with numerous adverse human health effects, including childhood cancer, which has been increasing since 1975.

Race/Ethnicity, Residential Segregation, and Exposure to Ambient Air Pollution: The Multi-Ethnic Study of Atherosclerosis (MESA)

Objectives. We described the associations of ambient air pollution exposure with race/ethnicity and racial residential segregation.

Critical Environmental Factors for Transportation Cycling in Children: A Qualitative Study Using Bike-Along Interviews

Key takeaway:

  • Through interviews conducted while cycling, this study identified environmental factors and perceptions of road safety influencing children ages 10-12 to bicycle.
Cobenefits of Replacing Car Trips with Alternative Transportation: A Review of Evidence and Methodological Issues

It has been reported that motor vehicle emissions contribute nearly a quarter of world energy-related greenhouse gases and cause non-negligible air pollution primarily in urban areas.

Health Co-Benefits and Transportation-Related Reductions in Greenhouse Gas Emissions in the San Francisco Bay Area

Background:  Greenhouse gas emissions (GHGE) linked to climate change is the biggest threat to public health worldwide. 

Chronic burden of near-roadway traffic pollution in 10 European cities (APHEKOM network)

Recent epidemiological research suggests that near road traffic-related pollution may cause chronic disease.

Co-Benefits of Replacing Car Trips with Alternative Transportation: A Review of Evidence and Methodological Issues

In this paper, the links between vehicle emissions and air quality, as well as the health and economic benefits from alternative transport use, are considered, and methodological issues relating to the modelling of these co-benefits are discussed.

Air Quality and Exercise-Related Health Benefits from Reduced Car Travel in the Midwestern United States

Automobile exhaust contains precursors to ozone and fine particulate matter (PM ≤ 2.5 µm in aerodynamic diameter; PM2.5), posing health risks. Dependency on car commuting also reduces physical fitness opportunities.