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.
Difficult words
- prenatal — Before birth, during the mother's pregnancy
- humidity — Amount of water in the air
- exposure — Being in contact with a condition or factor
- threshold — A limit value where conditions change categorythresholds
- evaporation — When liquid becomes gas and leaves a surface
- wet-bulb globe temperature — A combined measure of heat, humidity, radiant heat and airflowWBGT
Tip: hover, focus or tap highlighted words in the article to see quick definitions while you read or listen.
Discussion questions
- How could knowing that humidity makes heat worse change health advice for pregnant women in your area?
- Should local weather warnings use WBGT instead of just temperature? Why or why not?
- What practical steps can families take to protect pregnant women during hot, humid weather?
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