Hot, humid pregnancy harms child growth more than heat alone ^{CEFR B1}

The study, published in Science Advances, examined prenatal exposure to extreme heat and humidity across South Asia and used wet-bulb globe temperature (WBGT), a combined heat-stress metric. WBGT combines air temperature, humidity, radiant heat sources and airflow to measure how heat affects the body.

Humidity reduces the body’s ability to cool by slowing sweat evaporation. The researchers linked child health records from the Demographic and Health Surveys (DHS) with daily weather data from the Climate Hazards Center at UCSB to identify prenatal exposure. They set thresholds of 35° Celsius for temperature and 29° C for WBGT so hot and hot-humid categories were comparable.

Accounting for humidity roughly quadrupled the effect of extreme heat on child growth. For exposure in the third trimester, heat with humidity was about four times worse than heat alone. A one-standard-deviation increase in heat and humidity in the year before birth corresponded to children being 13% shorter for their age, while extreme heat alone gave a 1% reduction. The authors tested five alternative threshold sets and checked birthrates and infant mortality to address possible bias.