Satellite-derived Formaldehyde-indicated Photochemical Oxidant Pollution and Under-5 Mortality in Low-income and Middle-income Countries

Methods We linked birth histories for 1·42 million children from Demographic and Health Surveys in 43 LMICs with OMI satellite-derived HCHO columns. Associations between prenatal HCHO-indicated photochemical oxidant pollution and under-5 mortality (U5M) were estimated using a fixed-effects Cox model. We derived a spline-based exposure–response function and applied it to gridded HCHO and under-5 mortality data for 100 LMICs to estimate HCHO-attributable U5M from 2005 to 2023. Robustness was assessed by additionally adjusting for ozone, biomass burning and solar radiation, and effect modification by temperature was examined.

Findings The fixed-effects Cox model showed a robust association, with each 1 × 10¹⁵ molecules/cm² increase in prenatal HCHO column associated with higher U5M risk (hazard ratio 1·030, 95% CI 1·022–1·038). The association was strongest during the early-life period spanning pregnancy and the first year of life and intensified at higher temperatures. IQR-standardised analyses showed a stronger association for HCHO columns than for ozone. The non-linear exposure–response function indicated an overall positive association across the observed HCHO distribution. Using this function, we estimated that the HCHO-attributable fraction of U5M across 100 LMICs increased from 2·46% (95% UI 1·58–3·30) in 2005 to 5·35% (3·43–7·18) in 2023. Sub-Saharan Africa carried the heaviest absolute burden and the highest HCHO-attributable U5M rate per 1000 live births, whereas Central and South Asia showed the steepest relative increase. At the country level, Nigeria remained the leading contributor to attributable deaths and India rose from seventh to second place.

Interpretation Prenatal exposure to HCHO-related photochemical oxidant pollution was associated with increased U5M in LMICs, with a growing attributable burden. These findings suggest that broader photochemical oxidant pollution beyond ozone alone may be an important child-health risk. Strengthened control of photochemical pollution and its precursors, supported by satellite-based surveillance, could help protect child survival in high-burden settings with limited ground-based monitoring.