Final Report: Traffic Density and Human Reproductive Health

EPA Grant Number: R827352C008
Subproject: this is subproject number 008 , established and managed by the Center Director under grant R827352
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Southern California Particle Center and Supersite
Center Director: Froines, John R.
Title: Traffic Density and Human Reproductive Health
Investigators: Ritz, Beate R.
Institution: University of California - Los Angeles
EPA Project Officer: Chung, Serena
Project Period: June 1, 1999 through May 31, 2005 (Extended to May 31, 2006)
RFA: Airborne Particulate Matter (PM) Centers (1999) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Particulate Matter , Air


Topic B: Studies of Emission Sources and Related Adverse Health Effects

This project proposed two research goals. The first goal was to determine whether residential proximity to heavy traffic roadways, such as freeways and major arterials, affected the risk of low birth weight (LBW) and preterm birth in infants born to women living in Los Angeles County, California between 1994–2000. Residential proximity to heavy traffic roadways was used as a surrogate measure of exposure to motor vehicle exhaust. The second research goal of this project was to evaluate whether maternal in-vehicle air pollutant exposures during commutes affected the risk of LBW and preterm birth in infants born to women living in Los Angeles County, California during 2003, who answered to a survey within 6 months of delivery.

Summary/Accomplishments (Outputs/Outcomes):

First, we used an epidemiologic case-control study design to examine whether residential proximity to heavy-traffic roadways influenced the occurrence of LBW and/or preterm birth in Los Angeles County between 1994 and 1996 (Wilhelm and Ritz, 2003). We mapped subject home locations at birth and estimated exposure to traffic-related air pollution using a distance-weighted traffic density (DWTD) measure. The clearest exposure-response pattern was observed for preterm birth, with a risk ratio (RR) of 1.08 (95% confidence interval [CI]=1.01–1.15) for infants in the highest DWTD quintile. Although higher risks were observed for LBW infants, exposure-response relations were less consistent. Examining the influence of season, we found elevated risks primarily for women whose third trimester fell during fall/winter months (ORterm LBW = 1.39; 95% CI=1.16–1.67; ORpreterm and LBW = 1.24; 95% CI=1.03–1.48; RRall preterm = 1.15; 95% CI=1.05–1.26) and exposure-response relations were stronger for all outcomes for these women. This result is consistent with elevated pollution in proximity to sources during more stagnant air conditions present in winter months.

In a second study (Wilhelm and Ritz, 2004), we expanded our analysis to the time period 1994–2000 and incorporated available information on the number of trucks frequenting freeways in our study area. We did not observe associations between DWTD and LBW (term and preterm) for 1997–2000, but in certain subgroups we still observed associations with preterm birth: women whose third trimesters fell primarily during fall/winter months (November–April) (RR=1.07, 95% CI = 0.99–1.16, comparing the highest to lowest DWTD quintile) and women living in census block groups with a fraction of children in poverty at or above the median value (RR=1.08; 95% CI=1.00–1.18). However, we observed a 23% greater risk of a preterm-LBW birth for women with ≥13,290 freeway trucks passing within 750 feet of their residence per day (95th percentile) (OR=1.23, 95% CI=1.06–1.43) during 1997–2000.

In the work summarized above, we used electronic birth certificate data as the source of information on both health outcomes and other covariates we evaluated in our statistical models. However, the birth certificate data are missing information on some potentially important risk factors such as smoking, maternal stature and weight gain during pregnancy, and stress. We therefore conducted a nested case-control study in which we surveyed a sample of approximately 2,500 women in LA County (half of whom gave birth to a low weight or preterm infant) approximately four months after delivery to collect additional information on these factors (the survey was funded by the National Institute of Environmental Health Sciences [NIEHS]). As part of our survey, we asked women detailed information about their residential history and commuting habits during pregnancy. We calculated an average commute length and time for each woman who worked outside the home during pregnancy, weighting the distance (in miles and minutes) for each home by the time during pregnancy spent in each home. We observed a 47% increase in risk of delivering a preterm and LBW infant and a 55% increase in risk of delivering a LBW infant at term for women who commuted 45 minutes or more compared to those who commuted 5 minutes or less (OR=1.47, 95% CI=0.97–2.24 and OR=1.55, 95% CI=0.98–2.43, respectively). Adjustment for a number of important covariates did not change these estimates substantially.


In conclusion, we observed an approximately 10–20% increase in risk of preterm birth (both normal and low weight) and term LBW in infants born to women potentially exposed to high levels of traffic-related air pollution, as represented by DWTD when focusing on births during 1994–1996. These risks appeared to be strongest for women whose third trimester fell during fall/winter months, who lived in high background air pollution areas, and/or who lived in more impoverished areas according to census block-group level indicators of socioeconomic status (SES). Although residential proximity to traffic did not appear to be associated with higher risks of term LBW or preterm-LBW birth in the later time period included in this analysis (1997–2000), residential proximity to trucks on freeways did appear to be associated with greater risks of these outcomes, especially preterm-LBW, during 1997–2000. This suggests more heavily polluting vehicles within the overall cleaner motor vehicle fleet, such as trucks, may have become more important for these outcomes in the later years. Our finding of positive associations between ambient CO concentrations and term LBW, preterm-LBW birth, and preterm birth in 1997–2000 suggests, overall, air pollution may still be harmful. We feel a more refined exposure assessment approach is needed at this point to arrive at further conclusions concerning the associations between residential proximity to traffic and the adverse birth outcomes we have seen in our studies. One approach would be to develop an individual exposure model that takes into account exposure to indoor sources of air pollution and in-vehicle exposures, in addition to residential exposures to outdoor air pollution by accounting for concentrations within each of these microenvironments and time-activity patterns of pregnant women. Measurements of key traffic-related pollutants (CO, NO2, ultrafine particles [UF]) inside a sample of Los Angeles homes with varying levels of traffic in close proximity would provide needed information about typical at-home exposures and the influence of indoor pollution sources to overall exposure. Our results also indicate that in-vehicle exposures to air pollution while commuting during pregnancy may be important for low birth weight. We plan to analyze these data further and take into consideration the importance of type of commute (e.g., car versus bus, freeway versus surface streets), whether commuting distances vary by pregnancy period and if yes, the importance of this variation to the outcomes of interest, the importance of car age, and the importance of total time spent in a car including other in-vehicle activities besides commuting to work.


Wilhelm M, Ritz B. Residential proximity to traffic and adverse birth outcomes in Los Angeles County, California, 1994–1996. Environmental Health Perspectives 2003;111:207-216.

Wilhelm M, Ritz B. Motor-vehicle related air pollution and adverse birth outcomes in Los Angeles County, California, 1994–2000. Presented at the Sixteenth Conference of the International Society for Environmental Epidemiology, August 2004, New York City (abstract).

Technical Report:

Full Final Technical Report (PDF, 6pp., 92.5KB, about PDF)

Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other subproject views: All 2 publications 2 publications in selected types All 2 journal articles
Other center views: All 150 publications 149 publications in selected types All 149 journal articles
Type Citation Sub Project Document Sources
Journal Article Wilhelm M, Ritz B. Residential proximity to traffic and adverse birth outcomes in Los Angeles County, California, 1994–1996. Environmental Health Perspectives 2003;111(2):207-216. R827352 (Final)
R827352C008 (Final)
  • Full-text from PubMed
  • Abstract from PubMed
  • Associated PubMed link
  • Journal Article Wilhelm M, Ritz B. Local variations in CO and particulate air pollution and adverse birth outcomes in Los Angeles County, California, USA. Environmental Health Perspectives 2005;113(9):1212-1221. R827352 (Final)
    R827352C008 (Final)
  • Full-text from PubMed
  • Abstract from PubMed
  • Associated PubMed link
  • Full-text: EHP-Full Text PDF
  • Abstract: EHP-Abstract and Full Text HTML
  • Supplemental Keywords:

    RFA, Scientific Discipline, Air, Geographic Area, HUMAN HEALTH, particulate matter, Environmental Chemistry, Health Risk Assessment, Air Pollutants, State, mobile sources, Environmental Monitoring, Health Effects, ambient aerosol, asthma, engine exhaust, epidemiology, human health effects, motor vehicle emissions, quinones, particulate emissions, automotive emissions, air pollution, automobiles, automotive exhaust, children, PAH, human exposure, indoor air quality, California (CA), allergens, PM characteristics, aerosols, atmospheric chemistry

    Relevant Websites:

    Full Final Technical Report (PDF, 6pp., 92.5KB, about PDF) exit EPA

    Progress and Final Reports:

    Original Abstract
  • 1999
  • 2000
  • 2001 Progress Report
  • 2002 Progress Report
  • 2003 Progress Report
  • 2004 Progress Report

  • Main Center Abstract and Reports:

    R827352    Southern California Particle Center and Supersite

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R827352C001 The Chemical Toxicology of Particulate Matter
    R827352C002 Pro-inflammatory and the Pro-oxidative Effects of Diesel Exhaust Particulate in Vivo and in Vitro
    R827352C003 Measurement of the “Effective” Surface Area of Ultrafine and Accumulation Mode PM (Pilot Project)
    R827352C004 Effect of Exposure to Freeways with Heavy Diesel Traffic and Gasoline Traffic on Asthma Mouse Model
    R827352C005 Effects of Exposure to Fine and Ultrafine Concentrated Ambient Particles near a Heavily Trafficked Freeway in Geriatric Rats (Pilot Project)
    R827352C006 Relationship Between Ultrafine Particle Size Distribution and Distance From Highways
    R827352C007 Exposure to Vehicular Pollutants and Respiratory Health
    R827352C008 Traffic Density and Human Reproductive Health
    R827352C009 The Role of Quinones, Aldehydes, Polycyclic Aromatic Hydrocarbons, and other Atmospheric Transformation Products on Chronic Health Effects in Children
    R827352C010 Novel Method for Measurement of Acrolein in Aerosols
    R827352C011 Off-Line Sampling of Exhaled Nitric Oxide in Respiratory Health Surveys
    R827352C012 Controlled Human Exposure Studies with Concentrated PM
    R827352C013 Particle Size Distributions of Polycyclic Aromatic Hydrocarbons in the LAB
    R827352C014 Physical and Chemical Characteristics of PM in the LAB (Source Receptor Study)
    R827352C015 Exposure Assessment and Airshed Modeling Applications in Support of SCPC and CHS Projects
    R827352C016 Particle Dosimetry
    R827352C017 Conduct Research and Monitoring That Contributes to a Better Understanding of the Measurement, Sources, Size Distribution, Chemical Composition, Physical State, Spatial and Temporal Variability, and Health Effects of Suspended PM in the Los Angeles Basin (LAB)