Traffic-Related Air Pollution and Birth Weight: The Roles of Noise, Placental Function, Green Space, Physical Activity, and Socioeconomic Status (FRONTIER).

INTRODUCTION: FRONTIER aimed to provide a robust and comprehensive evaluation of the impact of maternal exposure to traffic-related air pollution (TRAP) on fetal growth. Toward this aim it (1) disentangled the effects of noise as a co-exposure; (2) identified the relevant window(s) of vulnerability for this impact; (3) evaluated its modification by household and neighborhood level socioeconomic status (SES), stress, physical activity, and the timing of conception and delivery in relation to the COVID-19 pandemic lockdown; (4) elucidated the role of placental function as an underlying mechanism; and (5) explored the potential of urban tree canopies and green spaces to mitigate it.

METHODS: FRONTIER established a new pregnancy cohort of 1,080 pregnant women in Barcelona, Spain - Barcelona Life Study Cohort (BiSC). Fetal growth was characterized by anthropometric measures at birth, together with ultrasound-based trajectories of fetal development. We developed an innovative exposure assessment framework integrating objective data on time-activity patterns with dispersion, land use regression, and hybrid models, with campaigns of personal and home-outdoor air pollution monitoring to estimate maternal exposure levels as well as inhaled dose of black carbon (BC), nitrogen dioxide (NO2), fine particulate matter (PM2.5), and PM2.5 copper, iron, and zinc in the main microenvironments for pregnant women (home, workplace, and commuting routes). We also assessed maternal exposure to noise by integrating measurements at participants' homes and outdoors using noise monitors with modeled microenvironmental noise levels, data on noise sensitivity, annoyance, and protections against noise. We developed single- and multipollutant models to evaluate the impact of TRAP exposure and inhaled dose on fetal growth while also correcting for the exposure measurement error. We further evaluated the modification of associations by SES, stress (cortisol levels and perceived stress), physical activity (objective and subjective measures), their mitigation by urban greenness and canopy volume, and their mediation by Doppler ultrasound measures of placental function.

RESULTS: We found that higher pregnancy exposure to NO2, BC, PM2.5, and PM2.5 copper and iron contents, particularly at home and all microenvironments combined, were generally associated with lower birth weight, higher risk of small for gestational age (SGA), and a decelerated trajectory of fetal growth, although some of these associations were not statistically significant. These associations appeared to be stronger for mothers with higher SES and those with higher objective measures of psychological stress. For the COVID-19 pandemic and physical activity, as effect modifiers, and urban greenness and canopy cover, as effect mitigators, we observed mixed patterns. In multipollutant models that include different measures of exposure to noise in addition to TRAP, the associations between TRAP and fetal growth generally remained consistent with those that we observed in our main analyses. We found two potential windows of vulnerability for the association of TRAP with fetal growth: one at the end of the first trimester and the beginning of the second trimester, and another at the end of the third trimester. Finally, we observed that a small proportion of the associations between PM2.5 and fetal growth could be mediated through the impact of these pollutants on placental function (i.e., umbilical artery pulsatility index).

CONCLUSIONS: Exposure to TRAP is associated with impaired fetal growth.