Humid heat and cardiovascular disease: What a new study shows

A new study from Tulane University shows that when extreme heat is combined with high humidity, the risk of emergency room visits for cardiovascular problems increases dramatically. An analysis of >340,000 emergency room visits in Dhaka, Bangladesh from 2014 to 2019 found that in very humid heat, the risk was six times higher than in heat with low humidity. The study was published in Science of the Total Environment (DOI: 10.1016/j.scitotenv.2025.180220).

Background

Why is "damp heat" more dangerous than usual?
Cooling of the body in the heat relies on the evaporation of sweat and the flow of blood to the skin. High humidity sharply worsens evaporation, so heat transfer drops, the heart has to pump more blood, the load on the vessels and the risk of overheating increases. This mechanism has long been confirmed by physiological and biophysical studies.

How it is measured: from Heat Index to "wet bulb".
In practical warnings, the Heat Index is often used - the "feeling" temperature taking into account humidity. At the same temperature, an increase in relative humidity moves conditions into a more dangerous risk zone. To assess the maximum loads, the wet bulb temperature (Tw) is also used: the classic work of Sherwood & Huber showed that Tw≈35 °C is the theoretical limit of survival with long-term exposure, and more recent studies indicate that clinical risks occur even at lower Tw.

What is known about heart health in heat and humidity.
Heat has been shown to be associated with increased cardiovascular events in many regions, but the role of humidity itself as an “amplifier” has long been underestimated and studied inconsistently. New clinical and population data add to the puzzle: extreme humidity increases the risk of arrhythmia and other cardiac outcomes, especially in vulnerable groups and in cities with little green space. This is biologically expected due to the breakdown of evaporative cooling.

Why tropical megacities are important (Dhaka case).
In the tropics, high humidity is the norm, and urbanization increases overheating through the urban heat island effect and the deficit of cooled spaces. For Bangladesh and Dhaka, the contribution of thermal conditions to excess mortality and increased vulnerability to heat waves has already been shown. Therefore, it is here that the combined risk of "heat + humidity" is urgently needed in healthcare and urban planning.

Policy and alerts: not just “degrees”.
Thermal safety recommendations (CDC/NIOSH, NWS) emphasize: at high humidity, danger thresholds shift to lower temperatures; protective measures (cooled spaces, ventilation, water, reducing loads) should be included earlier. New health risk indices also integrate humidity and local climate standards.

What was missing until recently.
Many assessments have focused on the “pure” effect of temperature; the multiplicative contribution of humidity in real urban settings is less well understood, especially outside countries with extensive air conditioning. This is why the large analyses from Dhaka are valuable: they quantify how much humidity increases cardiovascular risk on hot days, bridging the gap between physiology and epidemiology and providing a basis for adjusting early warning systems.

What exactly did they find?

• Scientists compared daily temperature and humidity data with cardiovascular visits to emergency departments in the country's largest metropolis. Heat itself increased the risk (by 4.4% on the "driest" days), but at peak humidity (RH ≥ 82%) it jumped to +26.7% - in fact, ≈6× stronger than in heat with low humidity.
• The threshold for "high" heat in the analysis was above 84°F (≈29°C). Humidity alone without heat was not associated with an increase in cardiac emergency calls; the "increase" occurred specifically in the pair "heat + high humidity." The effect was observed across all age and gender groups.

Why is this important?

Our bodies cool down through the evaporation of sweat. High humidity slows down evaporation, and the heart has to work harder to transfer heat to the skin. This makes the combination of heat and humidity particularly dangerous physiologically – and it is this “synergistic” risk that has often been underestimated in population studies. The new work quantitatively demonstrates the multiplier effect of humidity, closing a long-standing gap between biophysics and epidemiology.

Context: not only degrees, but also the “stickiness” of the air

Evidence that humidity increases heat stress is mounting, from revisions to heat wave “ratings” based on humidity to reviews linking humid heat to increased mortality and cardiovascular risk. The new paper adds a major urban case study from the tropics, where air conditioning is poor and populations are particularly vulnerable.

Practical conclusions

• Early warning systems should take into account not only temperature, but also threshold humidity (≈ 80% and above) - and warn of “compound risks” (heat + humidity).
• Urban solutions: shade, green corridors, cooled public spaces, access to drinking water - are critical for megacities with a humid tropical climate.
• Personal protection in the "sticky" heat: stay cool, drink water, reduce physical activity during peak hours, wear breathable clothing; for the elderly and people with cardiovascular disease - think through "heat plans" in advance.

Limitations and what's next

This is an observational study of one city; the authors and independent groups will need to test whether the “six-fold” factor is replicated in other climates and with different cooling availability (air conditioning, etc.). It will also be important to incorporate air pollution and other factors that can increase heat stress into the models. But the message is clear: policymakers and health professionals should talk about heat and humidity together, not separately.

Source: Tulane University press release and media coverage of the publication; Science of the Total Environment (Articles in Press), DOI 10.1016/j.scitotenv.2025.180220.