Humidity prevents pregnant women from cooling their bodies through sweat evaporation. This biological reality transforms extreme heat into a far deadlier threat for unborn children than previously understood.
A new study from the University of California, Santa Barbara reveals that accounting for humidity quadruples the estimated health damage from extreme heat exposure during pregnancy. The research examined nearly 200,000 children across Bangladesh, India, and Nepal.
“Exposure to hot, humid conditions in-utero is dangerous for child health, and more dangerous than just hot temperatures alone,” said Katie McMahon, the study’s lead author and a doctoral student in the Geography Department at UC Santa Barbara.
The findings appear in the journal Science Advances and challenge how scientists, doctors, and public health officials assess climate-related health risks.
How Scientists Measured Problem
The research team analyzed data from the Demographic and Health Surveys, which tracks public health and demographics across multiple countries. They combined this with daily weather records from the Climate Hazards Center at UC Santa Barbara.
The scientists focused on height-for-age ratios in children under five years old. This measurement compares a child’s height to the average for their age group and serves as a reliable indicator of chronic health problems.
Researchers linked pregnancy timing and location data with weather conditions to determine exactly when mothers experienced extreme heat and humidity. They set specific thresholds for comparison: 35 degrees Celsius for temperature alone and 29 degrees Celsius for wet bulb globe temperature.
Wet bulb globe temperature differs from simple temperature readings. The metric accounts for air temperature, humidity, radiant heat sources, and airflow, all factors that determine whether humans can effectively cool themselves.
“We needed our hot and hot-humid thresholds to be comparable,” McMahon said. “And this approach led us to two thresholds that occur with nearly equal frequency in South Asia.”
The Shocking Four-To-One Difference Found
The study revealed dramatic differences between heat exposure alone and heat combined with high humidity.
Children exposed to extreme heat and humidity during the third trimester showed health impacts approximately four times worse than those exposed only to extreme heat. A child experiencing one standard deviation increase in humid heat throughout the year before birth would be 13 percent shorter for their age than expected.
The same increase in dry heat exposure caused only a one percent reduction in height-for-age.
Kathy Baylis, a professor in the Geography Department at UC Santa Barbara and study co-author, explained that looking at the third trimester exposure clearly demonstrated this fourfold difference.
When Unborn Children Face Greatest Risk
The research identified two critical windows of vulnerability during pregnancy. Very early pregnancy poses serious risks that most people do not recognize. A woman may not even know she is pregnant during this dangerous period.
“My guess is that almost nobody appreciated these risks during the first trimester, including me, before this study,” said Chris Funk, Director of the Climate Hazards Center and study co-author.
Late pregnancy also creates heightened danger. Heat stress can trigger premature labor, resulting in babies born before full development. These children often face developmental problems and poor health that persist into childhood and beyond.
Pregnant women face multiple biological factors that increase their vulnerability. The added weight of pregnancy causes bodies to generate more heat. Hormonal changes simultaneously reduce their ability to regulate body temperature. High humidity then prevents the evaporation of sweat that would normally cool them down.
“And when evaporation can’t happen, then cooling can’t happen,” McMahon said. “All that heat builds up in our bodies, causing heat stress.”
Where Problem Concentrates Globally
Temperature-only risk assessments miss some of the world’s most densely populated areas.
Hot, humid conditions concentrate along coastlines and river valleys. These locations happen to host massive human populations. Scientists estimate that 38 percent of the global population lived within 100 kilometers of coastline as of 2018. An even higher percentage lives near rivers or lakes.
“These are literally the cradles of civilization,” Funk said. “And so they’re the most densely populated places on the planet.”
South Asia faces particularly severe threats. The region contains over 1.7 billion people and already experiences high rates of child malnutrition.
The study projects that exposure to conditions expected by 2050 under high-emissions scenarios would result in approximately 3.5 million additional stunted children in Bangladesh, India, and Nepal alone.
How Climate Change Amplifies Danger
Hot, humid conditions are becoming more frequent and intense due to climate change. Even if global warming stays limited to two degrees Celsius above pre-industrial levels, South Asia will experience deadly heat events annually.
The researchers checked their findings against multiple alternative thresholds and confounding factors. “We show versions of the results that use five different sets of alternative thresholds,” McMahon said. “No matter the threshold, our main conclusion remained the same.”
The team also examined whether prenatal heat exposure affected birth rates and infant mortality. “It does not look like early death or failed pregnancies are seriously impacting the results of our study,” Funk said.
Missing Health Impacts In Research
Most climate research focuses on deaths rather than ongoing health problems from extreme weather.
“But, extreme weather harms many more people than it kills,” McMahon said. Ignoring non-fatal health impacts leaves out how these conditions affect people’s lives. “This affects our calculus of the cost of climate change for human health and society overall.”
Poor health creates economic consequences that can span generations. These effects create cycles of poverty and declining health that compound over time.
Simple Solutions Could Save Lives
Understanding humidity’s role could enable effective interventions with relatively modest investments.
Funk and colleagues at the Climate Hazards Center are developing extreme heat forecasting and early warning systems. One project works with Kenya’s meteorological department to improve forecasts for the Kakuma refugee camp in northern Kenya. Camp administrators can take protective measures when heatwaves approach.
“The picture painted by this research is grim, but it could lead to a lot of potentially positive interventions,” Funk explained.
McMahon plans to continue studying heat and humidity effects on vulnerable populations, including California farmworkers. She will partner with health clinics in the Salinas Valley to quantify heat-related medical visits in agricultural communities.
The research suggests that education campaigns and messaging about heat risks during early pregnancy could produce significant benefits. Small interventions might generate dramatic improvements by breaking cycles of poor health and poverty.
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