Extreme heat and humidity sharply raise heart risks

Extreme heat pushes the heart to work harder. On humid days, that strain can quickly shift from tolerable to dangerous – and a new analysis shows just how steeply the risk of weather-related heart problems rises.

Researchers in Dhaka, Bangladesh report that the chance of a cardiovascular emergency visit rises sixfold on days that are both extremely hot and very humid, compared with hot days when the air is much drier.

How humidity makes heat worse

First author Mostafijur Rahman, an assistant professor of environmental health sciences at the Celia Scott Weatherhead School of Public Health (SPH) and Tropical Medicine at Tulane University, led the research with collaborators in Bangladesh and the United States.

Humidity matters because sweat must evaporate to cool the body, and water-laden air stalls evaporation. The resulting strain raises body temperature, increases skin blood flow, and can load the heart with extra work that some people cannot sustain.

As humidity climbs, the same thermometer reading feels much hotter to the body. That mismatch helps explain why ordinary heat advisories sometimes fail to capture the real risk for people with heart disease.

“These findings show we need to consider heat and humidity together when we discuss any kind of climate change policy,” said Rahman, who noted that the size of the effect surprised the team.

Heart risks intensified by high humidity

The researchers counted 340,758 emergency department visits for heart conditions during the hot months from 2014 to 2019 and matched those days to weather records.

Heat alone raised risk on low humidity days, but risk jumped far higher on the muggiest days when relative humidity exceeded roughly 82 percent.

High heat in the analysis was defined as above 84 degrees Fahrenheit, which raised heart related emergency visits even without intense humidity. There was no separate link between humidity alone and heart emergencies, yet humidity amplified heat effects across age and sex groups.

“Hopefully governments will be spurred to develop systems to warn cities of dangerous heat and humidity. We know extreme heat can have a negative health impact, but I never expected such a dramatic increase in risk when high humidity is also factored in,” said Rahman.

The pattern is consistent with clinical experience. When sweat fails to evaporate, the heart must move more blood toward the skin for cooling, while dehydration and electrolyte loss further stress the system.

Humidity and public health warnings

Hot extremes are becoming more frequent and more intense as the planet warms, and that trend is evident with every additional 0.9 degrees Fahrenheit of global warming, according to the Intergovernmental Panel on Climate Change (IPCC).

Independent research also links moist summer air to higher rates of heart, coronary, and stroke hospitalizations in older adults in the United States, even after accounting for temperature.

A long-term analysis of about 63 million Medicare beneficiaries found higher cardiovascular hospitalization risks with higher summer-specific humidity.

Laboratory and field studies add biological context. They show that humid heat undermines sweat evaporation and raises the physiologic load on the heart, validating the Dhaka pattern.

These threads point to one conclusion. Health systems and public warnings need to treat humidity as a co-equal driver of heat danger, not just a background variable.

Access to cooling matters

Dhaka’s results are particularly relevant in places where home air conditioning is rare. Recent national survey reporting in Bangladesh shows that only about 2.28 percent of households owned an air conditioner in 2023, a small rise from 1.6 percent in 2021.

Global analyses identify Bangladesh among countries with large populations at high risk due to a lack of access to affordable, sustainable cooling. Those gaps create exposure that individuals cannot easily avoid during multi day hot spells.

“There are billions around the world, from Southeast Asia to Africa, who are directly impacted by rising temperatures but have little access to air conditioning,” said Rahman.

Equity is central here. The people least able to afford cooling often live and work in buildings and neighborhoods that trap heat, and they may have the most to lose from heart strain.

Adapting to heat and humidity

Clinicians already flag combined heat and humidity as a risk for people with heart disease, heart failure, high blood pressure, prior stroke, and older age.

Public guidance now acknowledges that humid heat can drive blood pressure swings and raise hospitalizations for those with cardiovascular conditions.

City agencies should tune heat warnings to humidity, not just temperature thresholds. Emergency managers could extend hours for cooling centers, stagger outdoor work schedules, and coordinate hydration and medication checks with community groups.

Individuals should plan around the weather forecast on humid hot days. Small steps such as shortening exertion, spacing outdoor tasks, and favoring shaded or air conditioned spaces can spare the heart avoidable stress.

Health systems can prepare for surges. Hospitals and ambulance services can adjust staffing based on heat and humidity forecasts, which now provide advance warning of the most extreme events.

Heart risks of humidity are clear

The Dhaka study is observational, and it uses outdoor weather to represent exposure, which cannot capture every indoor environment or personal behavior.

Even so, the same day link between humid heat and heart emergencies aligns with physiology and persisted across sensitivity tests in the analysis.

Public agencies should incorporate humidity explicitly in heat alert systems. Common composites such as the heat index were not designed for extreme combinations of temperature and humidity, and updated methods could improve their performance.

The Dhaka data highlight a simple truth: on hot, muggy days, the heart works overtime, and treating humidity as a key hazard in planning can save lives.

The study is published in the journal Science of The Total Environment.

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