A mixtures approach to assessing relationships between environmental and social determinants of health and neonatal size and body composition in the Healthy Start cohort

Abstract

Background: Previous studies have identified neighborhood-level environmental and social exposures associated with birth weight, but no studies have quantified the effects of simultaneous exposures across these domains. Here we used Bayesian Kernel Machine Regression (BKMR) to estimate the independent and joint effects of environmental and social exposures on birth weight in the Healthy Start cohort in Denver, CO.Methods: The initial selection of exposures was guided by an environmental justice screening tool developed for California. Data on neighborhood-level environmental (e.g., fine particulate matter, traffic counts, impervious surfaces) and social (e.g., hospitalization rates, educational attainment, and crime rates) exposures were assigned to pregnant mothers based on their census tract at enrollment. Birth weight was abstracted from medical records. We used a two-step approach to fit our model. First, we used a Bayesian nonparametric variable selection model to reduce the number of candidate exposures. Second, we fit a BKMR model with the exposures, interaction terms for all exposure pairs, and individual-level covariates.Results: We had complete data on birth weight, exposures, and covariates for 792 infants. Out of 19 exposures originally considered, the non-parametric Bayesian model selected 9 (3 environmental and 6 social). BKMR results showed a mixture with all exposures above the median was associated with decreased birth weight. The mixture components selected most frequently by the model were respiratory hospitalization rate and the percentage of households in poverty. Respiratory hospitalization rates had the greatest effect on birth weight (β = -0.03, SE = 0.06) when all other exposures were held at the 75th percentile. For some exposures (e.g., fine particulate matter), relationships with birth weight appeared non-linear.Conclusions: We identified joint effects of environmental and social exposures on birth weight. Results suggest that analyses within one domain may limit the identification of important interactions that are associated with neonatal outcomes.