Hot, humid conditions prevent pregnant women from cooling down. Thus, their exposure to such conditions quadruples the effect of extreme heat on child health, according to a new study by the University of California, Santa Barbara, which carried out the research in the Indian subcontinent.
The researchers looked at the effect of prenatal exposure to extremely hot, humid conditions on the health of children in South Asia.
“Exposure to hot, humid conditions in-utero is dangerous for child health, and more dangerous than just hot temperatures alone,” said lead author Katie McMahon, a doctoral student under Kathy Baylis in the Geography Department, in a statement by the university.
The study
The authors looked at height-for-age: a ratio of a child’s height compared to the average for their age. This ratio is a commonly-used indicator of chronic health status for children under age five.
The researchers culled data on 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 produced by the Climate Hazards Center at the university.
“Much of the work involved linking demographic data and surveys with heat and humidity data to identify prenatal heat exposure. After this, the authors ran the data through their statistical models with carefully selected temperature, humidity and growth thresholds,” according to the statement.
These thresholds were 35° Celsius (95° Fahrenheit) for temperature and 29° C (84° F) for wet bulb globe temperature, which accounts for four factors that affect heat stress including air temperature, humidity, radiant heat sources, and airflow. “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 scientists found that exposure to extreme heat was bad, but humidity made outcomes even worse. For example, looking at exposure during the third trimester revealed that heat and humidity was approximately four times worse than heat alone.
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 per cent shorter for their age than expected. In contrast, a one-standard-deviation increase in extreme heat exposure translated to a one per cent reduction in height for age.
According to the authors, many researchers, doctors and public health officials may be underestimating the true impacts of extreme weather by just focusing on the effects of temperature.
This is particularly concerning given that hot, humid conditions are predicted to become more frequent and extreme due to climate change.
“What’s more, some of the most densely populated regions on Earth are hot, humid areas along rivers and coastlines. Populations are also growing rapidly in many of these locations,” noted the university statement.
Understanding the role played by humidity in increasing the damaging effects of heat can help guide interventions, the authors claim.
Their paper, Does humidity matter? Prenatal heat and child health in South Asia, has been published in the journal Science Advances.
The authors are Kathryn McMahon, Kathy Baylis, Stuart Sweeney and Chris Funk.
