Individual and contextual socioeconomic status as effect modifier in the air pollution-birth outcome association.

Background: Several studies have examined whether air pollution is associated with adverse births outcomes, but it is not clear if socioeconomic status (SES) modifies this relationship.

Objectives: We investigated if maternal education and area-level socioeconomic status modified the relationship between ozone, nitrogen dioxide and particulate matter with aerodynamic diameter <10 μm (PM) on preterm births (PTB; gestational age <37 weeks) and term low birth weight (TLBW; weight < 2500 g on term deliveries).

Methods: Analyses were based on almost 1 million singleton live births in São Paulo municipality between 2011 and 2016. The final sample included 979,306 births for PTB analysis and 888,133 for TLBW analysis. Exposure to PM, NO and O were based on date of birth and estimated for the entire gestation and for each trimester. Multilevel logistic regression models were conducted to examine the effect of air pollutants on both adverse birth outcomes and whether it was modified by individual and area-level SES.

Results: In fully adjusted models, over the entire pregnancy, a 10 μg/m increase in O and PM was associated with increased chance of PTB (odds ratio; OR = 1.14 CI 1.13, 1.16 and 1.08 CI = 1.02, 1.15 respectively) and PM with TLBW (OR = 1.08 CI 1.03, 1.14). Associations were modified by maternal educational and area-level SES for both outcomes. Mothers of lower education had an additional chance of PTB and TLBW due to PM exposure (OR = 1.04 CI 1.04, 1.05 and 1.10 CI 1.08, 1.14 respectively), while mothers living in low SES areas have an additional chance for TLBW (OR = 1.05 CI 1.03, 1.06). Similar modification effects were found for O exposure. Trimester specific associations were weaker but followed a similar pattern.

Conclusion: Socioeconomic status modifies the effect of air pollution on adverse birth outcomes. Results indicate that mothers with lower SES may be more susceptible to air pollution effects.