Same environment, stratified impacts? Air pollution, extreme temperatures, and birth weight in Southeast China

Ambient air pollution and extreme temperatures have been associated in a number of settings with adverse birth outcomes. However, some newborns may be more vulnerable than others due to inequalities in at least two dimensions of their endowments. First, the pathway from ambient conditions to adverse birth outcomes may vary due to inequalities in child endowments related to socioeconomic status such as maternal education. For example, children in utero with less-educated mothers may be more vulnerable than those of more-educated mothers if their mothers have less access to living, work, transportation, and leisure spaces with indoor air filtration and temperature regulation, or if they have less knowledge of or resources for mitigation strategies. Second, babies’ unequal underlying innate health endowments may relate to unequal outcomes in the same environmental context. Protective effects of maternal education may be more pronounced for the most physically vulnerable babies. These effects of unequal endowments may be particularly large in environments with greater air pollution. We examine, for the first time to our knowledge, the effects of these two dimensions of endowment inequalities on birth outcomes in a fairly polluted environment.
We use unusual data that permits linking 54,828 singleton live birth records from a district in Guangzhou, China, a relatively polluted environment, to ambient air pollution (PM10 and a composite measure) and extreme temperature data, we test whether, overall, maternal education is an “effect modifier” in the relationships between ambient air pollution, extreme temperature, and birth weights. Via conditional quantile regressions, we then test for effect heterogeneity according to the underlying innate physical vulnerability of babies—those further to the left in the conditional distribution of birth weight—after conditioning on other confounders. Results show that the protection associated with a college-educated mother with respect to pollution and extreme heat is substantial: up to 0.31 standard deviations of birth weight. Importantly, this protection is amplified under more extreme ambient conditions and for physically innately more vulnerable infants, after conditioning on other confounders.

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