Prenatal exposure to hot and humid conditions worsens the child's growth outcomes

Prenatal exposure to hot and humid conditions worsens the child's growth outcomes

The dangers of heat and humidity are so well known that mentioning them has become a cliché. But the effects may be more far-reaching than even scientists and doctors have suspected.

In an article published inScientific advancesUC Santa Barbara researchers examined the effects of prenatal exposure to extremely hot, humid conditions on the health of children in South Asia. They found that humidity worsens the effects of heat exposure, in part by preventing pregnant women from cooling down. Taking humidity into account quadrupled the impact of extreme heat on children's health.

“Exposure to hot, humid conditions in the womb is dangerous to children’s health and more dangerous than just hot temperatures alone,” said lead author Katie McMahon, a doctoral student with Professor Kathy Baylis in the Geography Department.

By focusing only on the effects of temperature, many researchers, doctors and public health officials may be underestimating the true impact of extreme weather. This is particularly concerning as these conditions are expected to become more common and extreme due to climate change. In addition, hot, humid areas along rivers and coastlines are among the most densely populated regions on earth. The population is also growing rapidly in many of these locations.

Fortunately, understanding this phenomenon can aid in interventions. Even simple measures could have significant benefits, the authors claim.

How we measure matters

The predicted humidity doesn't just make the heat worse; The “perceived” temperature has an actual basis in our biology. People cool down by sweating. However, evaporation slows when the air is humid. “And if evaporation cannot occur, cooling cannot occur,” McMahon said. “All this heat builds up in our bodies and causes heat stress.” And this can happen at much lower temperatures and humid conditions.

This is why wet bulb temperature (WBGT) was invented in the 1950s. Unlike a simple temperature measurement, this metric takes into account four factors that influence heat stress: air temperature, humidity, radiant heat sources, and airflow. That's exactly what McMahon and her co-authors found in their study.

As for health effects, the authors examined height-for-age: a ratio of a child's height to the average for his or her age. This ratio is a commonly used indicator of chronic health status in children under five years of age.

Researchers collected child health data from the Demographic and Health Surveys (DHS), a large-scale and comprehensive household survey of public health and demographics. Their daily weather data was created by UCSB's Climate Hazards Center.

Much of the work involved linking demographic and survey data with heat and humidity data to determine prenatal heat exposure. The authors then ran the data through their statistical models with carefully chosen temperature, humidity and growth thresholds.

These thresholds were 35° Celsius (95° Fahrenheit) for temperature and 29° C (84° F) for WBGT. “We needed comparability of our hot and hot-humid thresholds,” McMahon said, “and this approach led us to two thresholds that occur at nearly equal frequency in South Asia.”

The researchers found that exposure to extreme heat was harmful, but humidity made the results worse. For example, looking at exposure during the third trimester found that heat and humidity were about four times worse than heat alone, Baylis explained. The results suggest that a child who experienced a one standard deviation increase in heat and humidity in the year before birth would be 13% smaller than expected for their age. In contrast, a one standard deviation increase in extreme heat exposure resulted in a 1% age-related reduction in height.

The pregnancy factor

Pregnant women are particularly vulnerable to heat stress for several reasons. The extra weight causes them to produce more heat, and hormonal changes also make them more susceptible to overheating. Combined with the effects of high humidity, this can cause real problems for a woman and her future child.

The worst times were very early and very late in pregnancy, the team found. “Early in the pregnancy, the fetus is at great risk,” McMahon explained, “while at the end of the pregnancy the mother is even more at risk.” For example, heat stress late in pregnancy can trigger premature labor, resulting in infants not yet fully developed. This setback can then continue into childhood, leading to developmental problems and poor health that are never fully overcome.

A particularly risky time for heat exposure, however, was the beginning of pregnancy, possibly even shortly before conception. A woman herself may not know that she is at this critical time for her future child or that she is pregnant at all.

Across cultures, women are often treated with extra care late in pregnancy, "but I suspect that almost no one recognized these risks in the first trimester - including me before this study," said co-author Chris Funk, director of the Climate Hazards Center.

Robust results

The authors acknowledge that the paper has several limitations. For example, they were unable to access accurate birth dates and pregnancy lengths and therefore cannot fully explain the effects of preterm birth. For example, if a baby were born a month early, he or she would actually be around 0.9 years old on his or her first birthday. The researchers said they would like to see better, more detailed health data from more regions.

Still, the team also says the study's results are strong enough to make causal claims. They tested their results using various alternative thresholds and considered other factors that may have influenced their results. “We show versions of the results that use five different sets of alternative thresholds,” McMahon said. “Regardless of the threshold, our main conclusion remained the same.”

They even examined the impact of prenatal WBGT on birth rates and infant mortality.

It does not appear that early death or failed pregnancies would seriously affect the results of our study.”

Chris Funk, director of the Climate Hazards Center

Far-reaching consequences

The results have a number of implications. Risk assessment based solely on temperature does not take wet coasts and river valleys into account. “These are literally the cradles of civilization,” Funk said. “And that makes them the most densely populated places on the planet.” Scientists have estimated that in 2018, about 38% of the world's population lived within 100 kilometers of the coast, and an even higher percentage lived in close proximity to a river or lake.

South Asia – a region of over 1.7 billion people – could be hit particularly hard in the coming years. If exposed to the conditions expected in a high-emissions scenario by 2050, around 3.5 million children in the study region alone would have experienced stunting. “Even if societies manage to limit warming to 2°C above pre-industrial levels, South Asia is expected to suffer deadly heat events every year,” the authors write.

Additionally, much research into the effects of extreme weather focuses on deaths rather than health effects. “But extreme weather harms many more people than it kills,” McMahon said. Focusing only on mortality misses much of the impact of these diseases on people's lives. “This affects our calculation of the costs of climate change to human health and society as a whole.”

Poor health also has large economic impacts that can occur across multiple generations, creating a cycle of further poverty and poor health, Funk explained. “The picture this research paints is bleak, but it could lead to many potentially positive interventions.”

It's important to note that many vicious cycles run the other way, meaning small interventions can have dramatic, far-reaching effects. Relatively simple education and messaging campaigns could lead to significant increases in resilience and adaptive behavior, the researchers said.

Funk and his colleagues at the Climate Hazards Center are working on extreme heat forecasts and early warning systems, such as a project with the Kenya Meteorological Agency to improve forecasts for the Kakuma refugee camp in northern Kenya. By tracking the WBGT, the organization running the refugee camp can take adaptation measures if a heat wave is imminent.

The center also studies the conditions that trigger these heat waves to understand their connection to regional weather systems. The team is working with Microsoft's AI for Good Lab to use machine learning for weather modeling to predict conditions in refugee camps.

Baylis' group studies how hazards reinforce each other, such as the effects of heat and air pollution. McMahon will examine some of the nuances of heat exposure, such as which mothers, babies and children are most at risk and how these differences might evolve in the future.

McMahon also plans to further study the effects of heat and humidity on vulnerable populations, including farmworkers in California. She will work with a group of health clinics in the Salinas Valley to understand the extreme impact these conditions have on the frequency of heat-related medical visits in agricultural communities. Heat stress in farmworkers has been well documented in the media and in a number of qualitative studies, she said, but this work will help quantify this serious health problem in one of California's most important agricultural regions.

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