Physiological studies show that the human body's ability to dissipate heat is already limited in milder conditions than previously assumed. Another Israeli study found that during the summer, an increase from 27 to 32 degrees was associated with a 32% increase in the risk of stroke. Prof. Maya Negev of the University of Haifa calls for making heat preparedness part of health, urban planning and welfare policies
We are used to treating the Israeli summer as a familiar nuisance that can be dealt with with the help of drinks, air conditioning, and a little patience. However, studies from recent years show that the human body's ability to adapt to heat andhumidity There is a physiological limit, and that this limit is lower than was commonly assumed.
This does not mean that an air temperature of 26 or 31 degrees in itself poses a risk to every person. The numbers discussed in studies usually refer to "Humid desert temperature" — A measure that combines air temperature with humidity and the ability of water, and in our case sweat, to evaporate. The higher the humidity, the slower the sweat evaporates and the more difficult it is for the body to emit the heat it produces.
This distinction is especially important in Israel. In dry areas, the body can use sweat evaporation even when the temperature is very high. In coastal areas, however, the combination of high temperature and heavy humidity can overload the body's cooling mechanisms even when the number on the thermometer does not seem particularly extreme.
The physiological limit is lower than 35 degrees.
For years, a humidified bulb temperature of 35 degrees Celsius was cited as the upper limit at which a healthy person could maintain a stable body temperature. This was largely a theoretical limit. Physiological experiments conducted in recent years have indicated that the practical limit may be lower and depend, among other things, on age, exertion, duration of exposure, and the particular combination of heat and humidity.
A study published in 2025 in the journal PNAS involved 12 volunteers exposed to controlled conditions in a climate chamber. The researchers wanted to see if a protocol in which the heat or humidity was gradually increased actually identified the point at which the body could no longer compensate for the ambient heat.
After each participant had identified the change point, they were exposed for up to nine hours to conditions just above and just below it. In conditions above the limit, where the average wet bulb temperature was about 33.7 degrees, the body's core temperature continued to rise. The researchers calculated that if exposure had continued, it could have reached the temperature associated with heat stroke within about ten hours.
Even below the change point, at a humidified temperature of about 30.9 degrees, the body had difficulty reaching full equilibrium, but its rate of warming was much slower. It is important to emphasize that this was a small experiment and under unusual laboratory conditions, and not a determination that every person would be affected at the same temperature and for the same period of time. However, the results reinforce the conclusion that the old theoretical limit of 35 degrees should not be relied upon.
Heat and humidity have been linked to an increased risk of stroke in Israel
The danger doesn't begin until the body completely loses its ability to regulate heat. Exposure to heat puts a strain on the cardiovascular system, increases blood flow to the skin and sweating, and can cause dehydration, changes in blood concentration, and salt imbalances.
In a study we conducted inHaifa University With data from the National Registry ofStroke The relationship between ambient temperature, humidity, and heat index and the risk of stroke and transient ischemic attack during the hot seasons in Israel from 2014 to 2019 was examined.
The final analysis included 22,269 people hospitalized for a first stroke and 8,728 people hospitalized for a transient ischemic attack. Temperature data were estimated at a one-kilometer resolution by residential address, and humidity data were taken from about 100 weather service stations. The researchers also took into account air pollution.
When the average daily temperature was 32 degrees, the chance of having a stroke that day was 32% higher than on a day when the temperature was 27 degrees. Relative humidity of 90%, compared to 70%, was associated with a 9% increase in risk, with the strongest association occurring two days after exposure.
A heat index of 100 degrees Fahrenheit, or 37.8 degrees on the combined index, was associated with a 40% increase in stroke risk compared with a heat index of 80 degrees Fahrenheit, or 26.7 degrees. The association was strongest on the day of the event.
These are statistical relationships and do not prove that every stroke is caused by heat. However, the design of the study, in which each person was essentially compared to themselves on parallel days, reinforces the possibility that heat stress acts as an immediate factor that increases risk in some of the population.
The final findings reinforce an earlier report of the study, which was based on a preliminary analysis and indicated a strong effect about a week after the heat stress. The peer-reviewed paper found that the strongest association with temperature and heat index appears on the day of the stroke, while the effect of humidity may be delayed by several days.
Not everyone can just turn on the air conditioner.
Exposure to heat is not evenly distributed. Workers who spend hours outdoors, the elderly who live alone, families who cannot afford constant cooling, and people living in dense, treeless neighborhoods are more exposed—and have fewer means to protect themselves.
Climate change does not alone create social and health disparities, but it deepens them. A general recommendation to stay in an air-conditioned home is not helpful for those who do not have proper air conditioning in their homes, for those who have to work outside, or for those who have to walk to a bus stop that is not shaded.
Relying solely on air conditioning is also not enough. Air conditioning may save lives and provide essential protection inside a building, but it consumes electricity and emits heat into the urban environment. During peak hours, it also increases the load on the electrical grid, precisely when a power outage could be particularly dangerous.
Preparing for heat is a health policy
The fight cannot be reduced to a recommendation to drink water. Israel needs a multi-systemic plan that combines public health, urban planning, transportation, welfare, education, labor and energy.
Shading sidewalks and bus stops, planting trees that are appropriate for the urban environment, reducing dark asphalt surfaces, creating shaded walking paths, and establishing public cooling centers are not “green additions.” These are measures to reduce morbidity and mortality.
The health system needs to receive targeted alerts before heatwaves, identify vulnerable patients, and direct medical and community teams to contact them. Local authorities need to map not only temperatures, but also populations that lack adequate access to cooling, shade, and accessible transportation.
Clear protection for outdoor workers is also needed: adjusting working hours, taking proactive breaks, accessing water and shade, and limiting exertion on days when the combination of heat and humidity endangers health.
Warming is not a future event that can be prepared for later. Some of the conditions studied are already occurring in the Israeli summer. As the climate warms, the number of hours during which outdoor activity is dangerous will increase, and the gap between those who can isolate themselves in an air-conditioned space and those who cannot will widen.
The response required is not a choice between adaptation and emissions reduction. We need to do both: reduce greenhouse gas emissions to prevent further deterioration, while transforming cities, workplaces and health systems now so that they can protect the public in a warmer world.
For the original publication: Opening the original publication
More on the subject on the science website
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