The heat that harms children most isn't the heat you feel. It's the heat you sweat in. A study of nearly 200,000 South Asian children finds that hot, humid days in pregnancy set back child growth far more than heat measured by temperature alone, and that climate models ignoring humidity hide millions of future cases of stunting. This is a critical finding, as it highlights the importance of considering humidity in climate projections, especially for vulnerable populations like children in the womb. (Personally, I think this study is a wake-up call for climate scientists and policymakers to take humidity seriously.)
The study, led by researchers at the University of California, Santa Barbara, linked household survey data on nearly 200,000 children in India, Bangladesh, and Nepal to daily records of temperature and humidity. The results were striking: a standard increase in humid-heat days during the third trimester was associated with a 5.1% fall in a child's height-for-age score, compared to a 1.3% fall when measured by temperature alone. (What makes this particularly fascinating is that the study shows how humidity can have a much greater impact on child growth than temperature alone.)
The distinction between humid heat and dry heat is crucial, as it affects different parts of India. Dry, scorching heat concentrates over the western desert and the arid interior, while humid heat, the kind that leaves the body unable to cool itself, settles over the eastern Gangetic plain, through Bihar, eastern Uttar Pradesh, and Bengal, among the most densely populated places on earth and home to some of India's highest birth rates. (From my perspective, this highlights the need for targeted heat action plans that consider the specific risks of humid heat in these regions.)
The study also found that the damage from humid heat clustered at two moments: the earliest weeks of pregnancy, when the fetus is most fragile, and the final trimester, when the strain on the mother peaks. (This raises a deeper question: why are pregnant women not more explicitly recognized as a vulnerable group in heat action plans?)
One thing that immediately stands out is that spells of extreme humid-heat were followed by fewer births six to 12 months later, a pattern consistent with lost or disrupted pregnancies. Spells of extreme temperature, by contrast, were followed by a rise in births within three months, the signature of labor brought on early. (What this really suggests is that humid heat can have long-term effects on fertility and pregnancy outcomes, which should be a major concern for policymakers.)
The study's findings have significant implications for the future, as the researchers project that humid heat could leave between 3 million and 3.7 million additional children across South Asia stunted by 2050, depending on emissions scenarios. (If you take a step back and think about it, this highlights the urgent need for climate adaptation measures that consider the impact of humidity on child health.)
However, the study also has its limitations. It is a statistical association drawn from a single snapshot of children, not a group followed through time, and stunting has many causes, including nutrition, sanitation, and income, that no model fully strips out. (In my opinion, this underscores the need for more comprehensive research that considers the interplay of various factors that contribute to stunting.)
India does not enter that calculation as a bystander. More than a third of the country's children under five, 35.5%, were already stunted at the last count, according to the National Family Health Survey of 2019–21. The states carrying the humid-heat burden, Bihar, eastern Uttar Pradesh, and West Bengal, are among those where that rate is highest, and adaptation money is thinnest. (This is a surprising angle, as it highlights the need for targeted adaptation measures in regions that are already facing significant health challenges due to stunting.)
In conclusion, the study highlights the critical need to consider humidity in climate projections and heat action plans, especially for vulnerable populations like children in the womb. (Personally, I think this study should be a wake-up call for policymakers to take action to protect the health of children in South Asia.) The thermometer has been telling India only half the story, and it's time to fill in the missing pieces.
