Adverse Birth Outcomes Attributable to High Heat in Nigeria

Exposure to fine particulate matter (PM_2.5) has been linked with adverse birth outcomes in Nigeria. Emerging evidence suggests that high temperatures may also be associated with these outcomes. However, this association, as well as whether temperature modifies the effects of PM_2.5 on these outcomes, has not been explored in Nigeria.
Using data from the 2018 Nigerian Demographic and Health Survey, we examined the association between maternal exposure to maximum temperature (Tmax) during pregnancy and adverse birth outcomes, including Low Birth Weight (LBW), Preterm Births (PTB), and Stillbirths (SB). A daily maximum near-surface air temperature gridded dataset (2012-2018) at 1-km² resolution was obtained from Zhang et al. (2022) and linked to birth clusters based on geographic coordinates. Temperature metrics (hot days and heatwave events) were derived from the 90th percentile threshold of the daily Tmax values, based on all pregnancy periods. Logistic regression analysis was used to estimate the association between these metrics and birth outcomes. The intensity, frequency, and duration of these temperature metrics in relation to the birth outcomes were also evaluated. We then estimated the Relative Excess Risk due to Interaction (RERI) using interaction terms for each temperature metric during the corresponding PM_2.5 exposure period.
We observed a strong correlation (r=0.93) between the model temperature data and observational data (2012-2018). An increasing positive association was observed between the duration of hot days and PTB, while an increase in heatwave events was positively associated with LBW. Intensity in hot days was positively associated (1.59; 95% CI: 1.28-1.96) with LBW. At the same time, frequency in hot days showed no significant relationship with any of the birth outcomes. Positive additive interaction between high temperature and PM_2.5 was observed across exposure categories for LBW and SB. The magnitude of interaction was greater at moderate PM_2.5 levels (Q2) for LBW, while the highest levels (Q3) had a greater effect for SB. As global temperatures rise, these findings provide evidence that maximum temperature can intensify the health burden of ambient PM_2.5 during pregnancy, underscoring the need for climate-adaptive maternal health interventions.