Prenatal and early life exposure to particulate matter, environmental tobacco smoke and respiratory symptoms in Mexican children

Background: Exposure to particulate matter <2.5 μm in diameter (PM_2.5) and environmental tobacco smoke (ETS) are associated with respiratory morbidity starting in utero. However, their potential synergistic effects have not been completely elucidated. Here, we examined the joint effects of prenatal and early life PM_2.5 and prenatal ETS exposure on respiratory outcomes in children. Material and methods: We studied 536 mother-child dyads in the Programming Research in Obesity, Growth, Environment and Social Stressors (PROGRESS) study in Mexico City. Exposure to PM_2.5 was estimated using residence in pregnancy and child's first year of life with a satellite-based spatio-temporal model. ETS exposure was assessed by caregiver's report of any smoker in the household during the second or third trimester. Outcomes included report of ever wheeze and wheeze in the past 12 months (current wheeze) assessed when children were 6–8 years old considered in separate models. Associations were modeled using distributed lag models (DLM) with daily PM_2.5 averages for pregnancy and the first year of life, adjusting for child's sex, birth weight z-score, mother's age and education at enrollment, maternal asthma, season of conception and stratified by prenatal ETS exposure (yes/no). Results: We identified a sensitive window from gestational week 14 through postnatal week 18 during which PM_2.5 was associated with higher risk of ever wheeze at age 6–8 years. We also observed a critical window of PM_2.5 exposure between postnatal weeks 6–39 and higher risk of current wheeze. We found significant associations between higher prenatal and early life PM_2.5 exposure and higher cumulative risk ratios of ever wheeze (RR:3.76, 95%CI [1.41, 10.0] per 5 μg/m³) and current wheeze in the past year (RR:7.91, 95%CI [1.5, 41.6] per 5 μg/m³) only among children born to mothers exposed to ETS in pregnancy when compared to mothers who were not exposed. Conclusions: Exposure to prenatal ETS modified the association between prenatal and early life PM_2.5 exposure and respiratory outcomes at age 6–8 years. It is important to consider concurrent chemical exposures to more comprehensively characterize children's environmental risk. Interventions aimed at decreasing passive smoking might mitigate the effects of ambient air pollution.