Exposure to hot and humid conditions during pregnancy can significantly stunt a child's growth in later years, with new research showing the effect is up to four times worse than from dry heat alone.
The Critical Role of Humidity
While high temperatures are known to impact growth, scientists have discovered that adding humidity compounds the problem considerably. The body relies on sweat evaporation to cool itself. In humid air, sweat cannot evaporate effectively, preventing the body from cooling down and drastically increasing the risk of dangerous heat stress.
The latest study indicates that exposure to hot-humid days during any trimester of pregnancy reduces a child's height-for-age score up to four times more than exposure to high dry heat. This is because core body temperature is most affected in muggy conditions, compromising infant health at birth through issues like prematurity or low birth weight.
Pregnancy Vulnerability and Lasting Effects
Pregnant women are particularly sensitive to heat. Hormonal and metabolic shifts disrupt their natural cooling systems, making them more prone to heat-induced dehydration, which can trigger early labour. This combination of factors elevates the risks of heat stress, pregnancy loss, stillbirth, preterm birth, and low birth weight.
The research, employing a detailed quasi-experimental design, analysed prenatal heat exposure for roughly 200,000 children across South Asia. It linked health data to precise, location-specific weather metrics, tracking days exceeding two key thresholds: 35°C (95°F) in maximum temperature and, more critically, 29°C (84.2°F) in maximum wet-bulb globe temperature (WBGTmax), which combines heat and humidity.
The analysis found that a one-standard-deviation increase in hot-humid days during the third trimester was linked to a 5.1% decrease in a child's height-for-age score. An equivalent increase in exposure to dry heat days above 35°C resulted in only a 1.3% decrease.
Future Climate Risks and Global Impact
Under future climate scenarios, the researchers estimate that extreme humid heat could push an additional 3 to 3.7 million children into stunted growth. This burden is 2.7 to 3.3 million greater than if only dry heat were considered, underscoring the unique danger of humidity.
A warming planet holds more moisture, leading to more humid conditions globally. Regions previously considered temperate, including cities like New York, are increasingly facing subtropical climates. In the US, areas like the Gulf states and South Atlantic already experience the warm, humid summers characteristic of this shift.
A recent Washington Post analysis found over 120 million people across 1,500 US counties endured one of their three most humid summers on record since 1979, with the Mid-Atlantic, Southeast, and Plains most affected.
The prenatal damage extends beyond stunted growth. Previous studies connect heat and humidity exposure to increased risks of severe birth defects, including heart conditions. Projections suggest that by the 2025-2035 period, specific heart abnormalities could rise by about 34% in the Southern US, and atrial septal defects could jump nearly 39% in the Northeast. Defects of the brain and spine, which can cause conditions like spina bifida, are also linked to outside temperature.
