UC Santa Barbara

Extreme heat under climate change has already begun to threaten health, particularly for mothers and babies in the hottest parts of the world. When exposure occurs in utero, extreme heat can undermine child growth and development, leading to devastating later-life consequences for both health and socio-economic stability. Previous research, however, has often overlooked the role of humidity, which, when paired with extreme heat, can result in deadly heat stress and associated health complications. Understanding the relative effects of heat versus humid heat is important for understanding the magnitude and location of the effects of climate change, and for targeting interventions. I compare the impact of prenatal exposure to heat versus humid heat extremes on child height attainment in three South Asian countries (India, Bangladesh, and Nepal) using fine-scale climate records, trimester-level exposure identification, comprehensive data on 200,000 children from the Demographic and Health Surveys, and a rigorous fixed effects design. I find that extreme humid heat in the third trimester is five times more detrimental to height attainment than heat alone, and that maternal heat exposure in the period preceding pregnancy may have lasting negative impacts on growth trajectories after birth—a critical exposure period that has received little-to-no attention thus far with respect to child health. Specifically, the average child’s height-for-age Z-score declines by 0.002 units for every additional day in the third trimester that wet-bulb globe temperatures exceed 29°C. Combining these effects with new projections of wet-bulb globe temperature, I estimate that by 2050, climate change could increase the mean number of hot-humid days in the third trimester by 56.5% in my study region, pushing more than 930,000 additional children under 5 into stunting even before accounting for future population growth. This estimate shrinks to 315,000 children when I consider future exposure to heat alone, implying that failing to account for humidity may lead to significant underestimates of the true effects of extreme heat on child health. I further find that children without adequate sanitation access and whose mothers lack formal education or belong to systematically marginalized castes are more vulnerable to the adverse health effects of prenatal exposure to humid heat. My results provide new insight into which heat events are most dangerous for early life health and when, in a region where near-annual heat waves already affect millions and are projected to worsen under further climate change.