Changes in traffic-related air pollution exposures and associations with adverse birth outcomes over 20 years in Texas
Researchers studied over 6 million births in Texas from 1996 to 2016 to see if government regulations requiring cleaner car emissions actually improved babies' health. This is one of the first large-scale studies to examine whether these expensive regulations delivered on their promise to protect public health.
College of Health researcher(s)
Highlights
65% reduction in premature births. A massive Texas study shows cleaner cars = healthier babies 👶 Important work showing how policy changes affect human health. #Environmentalhealth #PublicHealth #Science #AcademicSky doi.org/10.1093/ije/...
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— College of Health at Oregon State University (@health.oregonstate.edu) January 10, 2025 at 12:32 PM
Abstract
Background
Billions of dollars have been spent implementing regulations to reduce traffic-related air pollution (TRAP) from exhaust pipe emissions. However, few health studies have evaluated the change in TRAP emissions and associations with infant health outcomes. We hypothesize that the magnitude of association between vehicle exposure measures and adverse birth outcomes has decreased over time, parallelling regulatory improvements in exhaust pipe emissions.
Methods
Using birth records in Texas from 1996 to 2016, we calculated residential exposure measures related to TRAP: nitrogen dioxide (NO2, a marker of the TRAP mixture), vehicle miles travelled within 500 m of homes (VMT500), a measure of traffic volume, and highway proximity. Using an accountability study framework, our analysis examined term birthweight, term low birthweight (TLBW) (<2500 g), preterm birth (PTB) (<37 weeks) and very preterm birth (VPTB) (<32 weeks). We implemented linear and logistic regression models to examine overall and time-stratified associations, including trends by race/ethnicity and socioeconomic groups.
Results
Among exposures for 6,158,518 births, NO2 exposures decreased 59% over time but VMT500 remained relatively stable. TRAP-related exposure measures were persistently associated with harmful birth outcomes [e.g. OR1996-2016 of 1.07 (95% CI: 1.04, 1.08) for TLBW comparing the highest vs lowest NO2 quintile]. The magnitude of associations decreased for total VMT500 and TLBW (-60%, OR1996: 1.08 to OR2016: 1.03 for the highest vs lowest quintile) and PTB (-65%) and VTPT (-61%), but not for term birthweight.
Conclusions
We observed evidence of small improvements in birth outcomes associated with reductions in exhaust pipe emissions over a 20-year period in Texas.
Frequently Asked Questions on Traffic-Related Air Pollution and Birth Outcomes
What is traffic-related air pollution (TRAP) and why is it a concern for infant health?
TRAP is a mixture of pollutants, including nitrogen dioxide (NO2), particulate matter, carbon monoxide, and volatile organic compounds, that come from vehicle exhaust. These pollutants are known to have a detrimental impact on health, including adverse birth outcomes such as low birth weight, preterm birth, and very preterm birth. Exposure to TRAP during pregnancy can increase the risk of these issues and subsequent health problems later in life.
How has TRAP changed in Texas over the past 20 years, and how was it measured in the study?
Over the 20-year study period (1996-2016) in Texas, nitrogen dioxide (NO2) concentrations, a marker of the TRAP mixture, decreased by 59%. However, vehicle miles traveled (VMT) within 500 meters of residential addresses remained relatively stable, decreasing by only 9%. The study measured TRAP exposure using a combination of NO2 concentrations from a land-use regression model and calculated VMT from historical roadway data. They also assessed highway proximity, which is if a birth address was within 300m of a highway.
Did the study find that TRAP is linked to adverse birth outcomes, and if so, what were those outcomes?
Yes, the study consistently found that TRAP exposure was associated with adverse birth outcomes. Specifically, higher levels of NO2, VMT, and proximity to highways were linked to lower term birth weight, an increased risk of term low birth weight (TLBW), preterm birth (PTB), and very preterm birth (VPTB). This means that babies born to mothers exposed to higher levels of TRAP were more likely to be born smaller, earlier, or both.
How did the strength of the association between TRAP and adverse birth outcomes change over time?
The study observed that the magnitude of the association between TRAP exposure and adverse birth outcomes decreased over time for some outcomes. Specifically, the negative effects of VMT on term low birth weight (TLBW), preterm birth (PTB), and very preterm birth (VPTB) were found to lessen. However, the association between TRAP and term birthweight did not show similar improvement and actually worsened slightly for some populations. This suggests that while regulations have helped improve some aspects of birth health, the issues are not fully resolved.
What does the study mean when they say they used an "accountability study framework"?
An accountability study framework is used to evaluate the effectiveness of environmental policies. This framework looks to determine if past regulatory actions or changes resulted in the improvements that were intended. In this case, it looks at whether policies to reduce vehicle emissions led to improvements in birth outcomes. By using this approach, the researchers aimed to understand if and how changes in TRAP levels were related to changes in the health of newborns.
Were there any differences in TRAP impact based on race/ethnicity or socioeconomic status?
Interestingly, the study found larger overall associations between TRAP and adverse birth outcomes for White non-Hispanic pregnant individuals and those in higher socioeconomic groups. While the effects of TRAP on preterm and very preterm birth lessened over time for most groups, these reductions weren't always seen across the board, and Black non-Hispanic pregnancies, which already have higher rates of adverse birth outcomes, did not experience the same decreases over time. This suggests that some groups may be more susceptible to TRAP and that health disparities persist.
What are the limitations of the study, and how might they affect the results?
The study has a few limitations that are important to note. First, it relies on residential addresses at the time of birth, not capturing residential mobility, daily movement or daily patterns of exposure during pregnancy. Second, the data only include live births, not miscarriages or stillbirths, which TRAP may also impact, potentially underestimating the true effect on reproductive health. Third, TRAP is analyzed at an annual level, meaning short but critical periods of exposure may not be captured. Also, there were some differences between births included in the study and excluded ones, with more data missing in the early years of the study which could attenuate effect changes over time. Finally, the research examines many combined policy efforts related to TRAP, making it impossible to attribute specific health outcomes to singular regulations.
What are the key conclusions of the study and their implications?
The study concluded that while regulations aimed at reducing vehicle exhaust emissions have had some positive impact on birth outcomes (specifically term low birth weight, preterm birth, and very preterm birth), traffic-related exposures continue to be associated with adverse birth outcomes. This means that more needs to be done to further improve air quality and protect vulnerable populations during pregnancy. The results underscore the importance of accountability studies and that addressing traffic-related air pollution is still a significant public health priority.