Researchers at the University of California, Santa Barbara have found that prenatal exposure to hot and humid conditions can significantly worsen child growth outcomes in South Asia. The study, published in Science Advances, indicates that humidity intensifies the effects of heat exposure during pregnancy because it prevents effective cooling through sweating.
"Exposure to hot, humid conditions in-utero is dangerous for child health, and more dangerous than just hot temperatures alone," said Katie McMahon, lead author and doctoral student under Professor Kathy Baylis in the Geography Department.
The research highlights that focusing solely on temperature may underestimate the true impacts of extreme weather on public health. This is especially concerning as climate change is expected to increase both the frequency and severity of such conditions. Many densely populated regions along rivers and coastlines are particularly vulnerable due to their hot and humid climates.
Humidity reduces the body’s ability to cool itself through evaporation. "And when evaporation can't happen, then cooling can't happen," McMahon explained. "All that heat builds up in our bodies, causing heat stress." The study used wet-bulb globe temperature (WBGT), a metric that incorporates air temperature, humidity, radiant heat sources, and airflow.
To assess health effects, researchers analyzed height-for-age ratios from Demographic and Health Surveys (DHS) alongside daily weather data from UCSB’s Climate Hazards Center. They identified thresholds of 35°C for temperature and 29°C for WBGT—levels commonly reached in South Asia.
Findings show that exposure to extreme heat was harmful but became four times worse when combined with high humidity during pregnancy's third trimester. According to Professor Baylis: "Heat and humidity was approximately four times worse than heat alone." Children exposed to a one-standard deviation increase in both factors before birth were found to be 13% shorter for their age compared to expectations; a similar increase in only extreme heat led to a 1% reduction.
Pregnant women are especially susceptible due to physiological changes during pregnancy. The most critical periods were early and late stages of pregnancy. Heat stress can also induce premature labor late in pregnancy, leading to developmental issues for infants.
Co-author Chris Funk noted cultural gaps: "But my guess is that almost nobody appreciated these risks during the first trimester - including me, before this study."
Limitations include lack of precise birth dates or gestational lengths in available data. Still, researchers believe their findings are robust across various thresholds considered. "We show versions of the results that use five different sets of alternative thresholds," McMahon stated. "No matter the threshold, our main conclusion remained the same."
The implications are significant since assessing risk based only on temperature overlooks many vulnerable coastal or river valley populations—areas where about 38% of people lived within 100 kilometers of a coast as of 2018.
South Asia faces particular risk; if future conditions align with high-emissions scenarios by 2050, millions more children could experience stunted growth even if warming is limited globally.
McMahon pointed out: "Extreme weather harms many more people than it kills." Focusing exclusively on mortality fails to capture broader impacts on human health and society.
Poor childhood health can have long-term economic consequences across generations—a cycle potentially reversible through targeted interventions such as education campaigns or adaptive behaviors prompted by early warning systems.
Funk’s team at UCSB’s Climate Hazards Center collaborates with organizations like Kenya’s meteorological department on forecasting tools using WBGT measurements for refugee camps. Partnerships with Microsoft’s AI for Good Lab aim to enhance machine learning models for weather prediction relevant to vulnerable populations.
Further research will examine compounding hazards such as simultaneous exposure to heat and air pollution while investigating disparities among mothers and children most at risk from these environmental threats. McMahon also plans studies focusing on farmworkers’ vulnerability in California’s Salinas Valley—a region where quantifying rates of heat-related medical visits remains an ongoing challenge.
