New research in Science Advances shows that prenatal exposure to hot, humid conditions harms child growth far more than high temperature alone. The team used wet-bulb globe temperature (WBGT), a combined heat-stress metric that tracks four factors: air temperature, humidity, radiant heat sources and airflow. Humidity matters because it reduces the body’s ability to cool by slowing sweat evaporation.
The authors linked child health records from the Demographic and Health Surveys with daily weather data from the Climate Hazards Center at UCSB to estimate prenatal exposure across South Asia. They set comparable thresholds—35° Celsius for temperature and 29° C for WBGT—and report that accounting for humidity roughly quadrupled the effect of extreme heat on child growth. For exposure in the third trimester, heat plus humidity was about four times worse than heat alone.
Quantitatively, a one-standard-deviation increase in heat and humidity in the year before birth corresponded to children being 13% shorter for their age than expected; by contrast, a similar increase in extreme heat alone implied a 1% reduction in height-for-age. The worst exposure windows were very early and very late in pregnancy, when the fetus may be vulnerable and when maternal risks such as heat-induced premature labor rise.
The study notes limitations: researchers lacked exact birth dates and pregnancy lengths, so premature births cannot be fully accounted for. They tested five alternative threshold sets and found the main conclusion remained. They also examined birthrates and infant mortality and judged that early death or failed pregnancies do not seriously bias results. The authors highlight broad implications for densely populated coastal and river valleys—about 38% of the global population lived within 100 kilometres of the coast as of 2018—and for South Asia, a region of over 1.7 billion people. Under a high-emissions scenario by 2050, around 3.5 million children in the study region would have experienced stunting. The authors and their institutions are working on WBGT forecasting, early warning systems and further studies, including projects with the Kenyan meteorological department and Microsoft’s AI for Good Lab.
Difficult words
- prenatal — Before birth; occurring during pregnancy.
- wet-bulb globe temperature — A combined measure of heat stress factors.WBGT
- humidity — Amount of water vapor in the air.
- trimester — One of three equal pregnancy time periods.third trimester
- quadruple — To increase to four times a previous amount.quadrupled
- stunting — Reduced growth resulting in low height for age.
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Discussion questions
- What public health measures could reduce prenatal exposure to extreme heat and humidity in the densely populated regions mentioned?
- How might WBGT forecasting and early warning systems help pregnant people and health services?
- Why are coastal and river valley populations especially important to consider when planning responses to heat and humidity?
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