While environmental regulations successfully diminished tailpipe emission exposures, according to recent findings, relative disparities in the effects of traffic air pollution and other exposures linked to traffic continue to persist.¹

This data resulted from a new birth cohort study examining variations in residential exposures across sociodemographic groups, utilizing comprehensive assessments of both total and truck-specific vehicle miles traveled (VMT), as well as measurements of nitrogen dioxide (NO2) and a cancer risk index derived from emissions.

This research—authored by Mary D. Willis, PhD, MPH, from Boston University’s School of Public Health—was seen as uniquely important given that traffic-related air pollution (TRAP) is known to be a powerful source of air pollutant disparities and affect environmental justice outcomes.²

“We further quantified how the magnitude of exposure disparity changed over time between the most and least advantaged groups (defined by characteristics such as race, ethnicity, and educational attainment),” Willis and colleagues wrote. “We hypothesized that the absolute and relative disparity based on TRAP markers would be reduced among all recorded pregnancies from 1996 to 2016.”

Background and Findings

The investigators ended up employing a population-based birth cohort design to conduct a descriptive analysis of pregnant individuals in the state of Texas from 1996 - 2016. The team’s inclusion criteria encompassed pregnant participants with valid residential addresses, demographic, and socioeconomic information.

The research team included birth certificate data as it provided individual-level information on the study’s participants such as ethnicity, race, education, and maternal place of birth. Neighborhood-level elements such as household income and historical neighborhood disinvestment (redlining) were evaluated by the team through the use of residential addresses, and the team’s data analyses took place between June of 2022 and June of 2023.

The primary outcome, which was determined to be residential TRAP exposure, was examined through traffic-related metrics such as total and truck-specific VMT within 500 meters and NO2 concentrations which were derived from a spatial-temporal land use regression model (indicating vehicle tailpipe emissions).

The investigators also used the National Air Toxic Agency cancer risk index which is based upon on-road vehicle emissions. They made use of sociodemographic indicators in order to assess distinctions with regard to TRAP exposure over the timeframe between 1996 - 2016.

In the state of Texas at the time of study, the investigators ended up with 7,043,598 pregnant individuals as participants with a mean maternal age of 26.8 years. These individuals had a diverse ethnic composition: 12% as non-Hispanic Black, 48% were found to have identified as Hispanic or Latinx, 4% as non-Hispanic Asian or Pacific Islander, and 36% as non-Hispanic White.

The research team reported that substantial disparities in TRAP were shown to have been evident across all of the analyzed sociodemographic elements. Although absolute disparities decreased over the course of the years in question, relative disparities still were shown to have expanded.

The investigators noted that in 1996, non-Hispanic Black pregnant participants were found to have experienced a mean of 15.3 ppb of NO2 compared to 13.5 ppb for those in the non-Hispanic White group, whereas in 2016, these levels were 6.7 ppb for the Black group and 5.2 ppb for those in the White group.

The research team suggested that this trend of substantial traffic exposure disparities was shown to have persisted over time, with major distinctions found to have been based on sociodemographic qualities. An example of this was that non-Hispanic Black pregnant participants experienced 83% more VMT within 500 miles of their homes versus non-Hispanic White pregnant participants in 2016.

“This birth cohort study found that although TRAP exposures during pregnancy decreased through the 20-year period of our study (1996-2016), the relative disparities between groups with higher and lower socioeconomic positioning largely increased over time,” they wrote. “TRAP is an important environmental justice issue that affects pregnancy, and large disparities in traffic-related exposure levels remain, requiring renewed policy attention.”

References

1. ^{Willis MD, Hill EL, Ncube CN, et al. Changes in Socioeconomic Disparities for Traffic-Related Air Pollution Exposure During Pregnancy Over a 20-Year Period in Texas. JAMA Netw Open. 2023;6(8):e2328012. doi:10.1001/jamanetworkopen.2023.28012.}
2. ^{US Environmental Protection Agency. Environmental justice. Accessed May 23, 2022. https://www.epa.gov/environmentaljustice.}