Fine particulate matter and birth outcomes: preliminary evidence for a potential threshold in the concentration-response relationship.
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Objective: A causal relationship exists between long-term fine particulate matter (PM2.5) exposure and cardiovascular disease and mortality. Studies evaluating the shape of the concentration-response (C-R) relationship generally focus on these outcomes and indicate a linear, non-threshold C-R relationship. There is less certainty in the causal nature of the relationship between PM2.5 and birth outcomes, or the C-R relationship.
Materials and Methods: We evaluated associations between PM2.5 exposure during critical gestational windows and preterm birth (PTB), birth defects, and infant mortality in a North Carolina birth cohort from 2003-2015. Daily PM2.5 concentrations from validated air quality models were estimated at the census tract or block and linked to residential address at delivery. Average PM2.5 concentrations were estimated for clinically significant exposure windows for each birth outcome. We conducted logistic or log-binomial regression analyses to estimate odds ratios (ORs) or prevalence ratios (PRs) and 95% confidence intervals (95% CIs) for PTB and PM2.5 and conducted threshold analyses restricting the dataset to participants with PM2.5 exposure <12, <10, and <8 µg/m3.
Results: We observed positive associations between PM2.5 and PTB (ORs range 1.02 – 1.03), birth defects (PRs range 1.02-1.08), and infant mortality (ORs range 1.05-1.12) in unrestricted models. In threshold analyses, effect estimates were attenuated or reversed, and confidence intervals were wider. For example, a positive OR (95% CI) reported for PTB (1.03; 1.03, 1.04) in unrestricted analyses shifted to 0.93 (0.92, 0.94) and 0.96 (0.93, 0.99) for thresholds of <10 and <8 µg/m3, respectively.
Conclusion: Preliminary results suggest the potential for a threshold in the PM2.5 C-R relationship for some birth outcomes. These results should be interpreted cautiously; additional evidence to better characterize the causal nature of the relationship between PM2.5 exposure and birth outcomes, as well as replication in other geographic areas, will be useful in further examining this question.