Extreme heat places an enormous strain on the cardiovascular system, as the heart must work harder to regulate body temperature. When the air is hot, the heart increases its pumping rate to move blood closer to the skin, allowing heat to be released through sweat and evaporation. However, this extra workload can significantly increase the risk of serious complications such as a heart attack or stroke, especially in individuals already living with heart conditions. The burden on the body intensifies further when high humidity is added to the mix, reducing the effectiveness of the body’s natural cooling system.
A recent study conducted by Tulane University has highlighted the dangers of this combination. Published in Science of the Total Environment, the research analysed more than 340,000 emergency room visits for cardiovascular issues in Dhaka, Bangladesh, between 2014 and 2019. By comparing medical data with historical weather records, researchers were able to draw clear connections between extreme weather conditions and surges in hospital visits. The results showed that while high heat alone increased risks, the danger escalated sharply on days when both temperature and humidity were at their highest.
The numbers were striking. On days of intense heat combined with humidity levels above 82 per cent, the likelihood of a person requiring emergency treatment for a heart-related issue was six times greater than on days with more moderate weather. Even when considered in isolation, heat was associated with an 8% rise in emergencies. On days that were hot but with relatively low humidity, the risk increased by about 4.4 per cent. However, when oppressive humidity was layered on top of already high temperatures, the probability increased by 26.7 per cent. This consistent pattern held across different age groups and between men and women, demonstrating the universality of the risk.
Lead author Mostafijur Rahman, an assistant professor at Tulane’s Celia Scott Weatherhead School of Public Health and Tropical Medicine, emphasised the importance of these findings. He explained that while the dangers of heat are well understood, the extent to which humidity amplifies the risks was surprising even to the research team. “We know extreme heat can have a negative health impact,” Rahman noted, “but I never expected such a dramatic increase in risk when high humidity is also factored in.” Notably, the study found that humidity alone was not responsible for the increased number of heart emergencies, but rather its combination with heat, which disrupts the body’s ability to cool itself.
In Dhaka, the stakes are exceptionally high. Air conditioning remains rare in most households, leaving millions of residents vulnerable during periods of severe weather. Bangladesh is also consistently ranked among the nations most exposed to the impacts of climate change, meaning that its citizens will face growing challenges as average temperatures continue to rise. The study highlights how climate-related health threats disproportionately affect regions with limited access to cooling technology, underscoring the need for governments and health systems to develop strategies to protect those at risk.
The implications of the Tulane research extend far beyond Bangladesh. Across large parts of Southeast Asia, Africa, and other regions prone to high heat, billions of people live with limited access to air conditioning. They are regularly exposed to extreme heat and humidity. Rahman and his colleagues hope their findings will inspire both governments and communities to take proactive steps. These include early warning systems for dangerous weather conditions, as well as public health campaigns urging individuals to adopt protective behaviours. Simple measures, such as staying hydrated, wearing breathable clothing, avoiding strenuous outdoor activities, and utilising public, air-conditioned spaces, can reduce risks. In a warming world, the message is clear: the combination of heat and humidity poses a grave danger to heart health, and ignoring their combined impact could be deadly.
More information: Mostafijur Rahman et al, Compounding effects of heat and high humidity on cardiovascular morbidity in Dhaka, Bangladesh: An implication of climate crisis, Science of The Total Environment. DOI: 10.1016/j.scitotenv.2025.180220
Journal information: Science of The Total Environment Provided by Tulane University
