Hot, humid conditions during pregnancy pose a significantly greater threat to a child’s health than heat alone, new research reveals.
Humidity’s Hidden Impact on Developing Children
Understanding how humidity exacerbates heat’s effects is crucial for protecting vulnerable populations, especially as climate change intensifies.
- Exposure to hot, humid conditions during pregnancy is more dangerous for child health than exposure to heat alone.
- Humidity hinders the body’s ability to cool down through sweating, leading to increased heat stress.
- The first and last trimesters of pregnancy are particularly vulnerable periods for heat and humidity exposure.
- Researchers found that accounting for humidity quadrupled the effect of extreme heat on child health.
- Simple interventions, like education and early warning systems, could significantly improve outcomes.
The dangers of heat and humidity are well-known, but their impact can extend far beyond what scientists and doctors previously realized. A study published in Science Advances examined the effects of prenatal exposure to extremely hot, humid conditions on the health of children in South Asia, revealing a concerning link.
Why does humidity matter so much? Because it interferes with our natural cooling system. Humans regulate body temperature by sweating, but evaporation—and therefore cooling—slows dramatically when the air is already saturated with moisture. “And when evaporation can’t happen, then cooling can’t happen,” explains Katie McMahon, a doctoral student at UC Santa Barbara and lead author of the study. “All that heat builds up in our bodies, causing heat stress.” This can occur at much lower temperatures than typically considered dangerous.
Researchers discovered that accounting for humidity quadrupled the effect of extreme heat on child health. This finding underscores the importance of using more comprehensive metrics like the wet-bulb globe temperature (WBGT), developed in the 1950s, which considers air temperature, humidity, radiant heat, and airflow to assess heat stress.
The team assessed child health by examining height-for-age, a standard indicator of chronic health status for children under five. Data on child health came from the Demographic and Health Surveys (DHS), a large-scale public health and demographic survey, while daily weather data was provided by the Climate Hazards Center at UCSB.
The researchers established thresholds of 35° Celsius (95° Fahrenheit) for temperature and 29° C (84° F) for WBGT, ensuring comparability within the South Asian context. Their analysis revealed that exposure to extreme heat was detrimental, but humidity significantly worsened outcomes. For instance, exposure during the third trimester showed that the combined effect of heat and humidity was approximately four times worse than heat alone, according to Professor Kathy Baylis.
A child exposed to a one-standard deviation increase in heat and humidity in the year before birth would be 13% shorter for their age than expected. In contrast, a similar increase in extreme heat exposure resulted in only a 1% reduction in height for age.
Pregnant women are particularly vulnerable to heat stress due to increased metabolic heat production and hormonal changes. Combining these factors with high humidity can create serious risks for both mother and child. The most critical periods appear to be early and late in pregnancy.
“At the beginning of pregnancy, the fetus is very vulnerable,” McMahon explained, “while at the end of pregnancy, the mother is more vulnerable.” Heat stress late in pregnancy can induce premature labor, leading to infants who are not fully developed, potentially causing long-term health issues. Early in pregnancy, exposure may impact development even before a woman knows she is pregnant.
Chris Funk, Director of the Climate Hazards Center, noted that women often receive increased care late in pregnancy, but the risks during the first trimester are often overlooked. “My guess is that almost nobody appreciated these risks during the first trimester —including me, before this study,” he said.
The study acknowledges limitations, including the lack of precise birth dates and pregnancy lengths, which could affect the accuracy of assessing premature birth impacts. However, the researchers maintain that their findings are robust, having tested various thresholds and considered other influencing factors. “We show versions of the results that use five different sets of alternative thresholds,” McMahon says. “No matter the threshold, our main conclusion remained the same.”
The implications are far-reaching. Focusing solely on temperature overlooks the risks in humid coastal and river valley regions—”literally the cradles of civilization,” Funk says—which are also among the most densely populated areas globally. Approximately 38% of the world’s population lived within 100 kilometers of the coast as of 2018, with an even higher percentage near rivers and lakes.
South Asia, home to over 1.7 billion people, is particularly vulnerable. Under a high-emissions scenario projected for 2050, around 3.5 million children in the region could experience stunting. Even limiting warming to 2°C above pre-industrial levels won’t prevent deadly heat events in South Asia, the authors warn.
Furthermore, much research focuses on mortality rates rather than the broader health impacts of extreme weather. “But, extreme weather harms many more people than it kills,” McMahon emphasizes. Ignoring these non-fatal effects underestimates the true cost of climate change.
Poor health generates significant economic consequences, potentially creating a cycle of poverty and ill health, Funk explains. “The picture painted by this research is grim, but it could lead to a lot of potentially positive interventions.”
Simple educational campaigns and early warning systems could yield substantial benefits. The Climate Hazards Center is already working on extreme heat forecasting and early warning systems, including a project with the Kenyan meteorological department to improve forecasts for the Kakuma refugee camp in northern Kenya. By tracking WBGT, the camp can implement adaptive measures during heatwaves.
Researchers are also investigating the interplay between heat and air pollution, and McMahon plans to study the specific vulnerabilities of different populations, including farmworkers in California, examining heat-related medical visits in agricultural communities.
Source: UC Santa Barbara
