If the air is hot, the body may struggle to transfer heat to the environment. If humidity is high, sweat evaporates more slowly. If someone is working outdoors, exercising or wearing heavy clothing, the body may produce and retain even more heat.
When heat gain becomes greater than heat loss, internal body temperature can rise. Dehydration may develop as fluid is lost through sweating. Blood circulation can also be placed under additional strain as the body tries to move heat toward the skin.
This is why heat illness can develop even when a person does not feel as though they are doing anything particularly strenuous.
The Difference Between Temperature and Heat Index
A forecast showing an air temperature of 95°F does not tell the entire story.
The heat index combines air temperature and relative humidity to estimate how hot conditions feel to the human body in shaded conditions. When humidity is high, the heat index can be considerably higher than the actual air temperature.
For example, a hot and humid afternoon may feel more oppressive than a hotter but much drier afternoon. This is because sweat does not evaporate as efficiently in humid air.
The heat index is useful, but it also has limitations. It is generally calculated for shaded conditions and does not fully represent the additional heat absorbed in direct sunlight. Physical activity, clothing, wind and individual health can also change the risk.
People working in direct sun may therefore face greater heat stress than a general weather-app number suggests.
Why Hot Nights Can Be Especially Harmful
A heat wave becomes more dangerous when temperatures remain high after sunset.
Normally, cooler nighttime conditions give the body and buildings an opportunity to release some of the heat accumulated during the day. People can sleep more comfortably, and homes may gradually cool.
When nights remain unusually warm, that recovery period becomes shorter or disappears.
A person who spends the day in a hot home may have difficulty sleeping, and the indoor temperature may remain uncomfortable long after outdoor temperatures begin to fall. Repeated nights of poor sleep and heat exposure can contribute to exhaustion and make it harder to cope with the following day.
Buildings that absorb heat during daylight can remain warm well into the night. This is especially problematic in apartments with poor ventilation, homes with inadequate insulation and neighborhoods with little tree cover.
For older adults, people with chronic medical conditions and those without reliable air conditioning, a sequence of hot days and warm nights can be particularly difficult.
The Urban Heat Island Effect
Cities can experience higher temperatures than nearby rural areas because buildings, asphalt, concrete and other surfaces absorb and retain heat.
During the day, roads and rooftops absorb solar energy. At night, they release some of that stored heat into the surrounding environment.
Trees, soil and vegetation can help cool an area through shade and evaporation. But neighborhoods with limited green space and extensive paved surfaces may remain much hotter.
This is known as the urban heat island effect.
The difference can be significant within the same metropolitan area. A neighborhood with mature trees, parks and shaded streets may feel noticeably cooler than a district dominated by wide roads, parking lots and buildings with dark roofs.
The impact is not distributed equally. Lower-income neighborhoods may have fewer trees, less access to air-conditioned buildings and a greater proportion of residents who work outdoors.
As a result, the danger of a heat wave is not determined only by the temperature recorded at an airport weather station. Local living conditions can make a major difference.
3. The French Mortality Figures in the Screenshot: Why They Matter
One of the most striking details in the supplied article is not the American map. It is the headline about Île-de-France, the region surrounding Paris.
The screenshot states that nearly 3,000 deaths were recorded in Île-de-France during the last week of June and describes a 122% increase in mortality. It also refers to a regional report published on July 16, 2026, and says that the heat wave occurred between June 22 and June 28.
Those figures are alarming, but they need to be interpreted carefully.
The screenshot appears to describe a comparison between the number of deaths observed during a specific period and the number expected under more typical conditions. It also refers to an excess of approximately 1,565 deaths. If the reported total was close to 3,000 and the increase was 122%, the figures suggest a substantial rise above the expected number for that period.
However, the screenshot alone does not provide the full report, its methodology or the detailed breakdown needed to independently verify every number. It would therefore be inappropriate to present the figures as a complete, independently confirmed account of the event.
There is also an important difference between deaths that occur during a heat wave and deaths directly attributed to heat as the immediate cause. Heat can contribute to a person's death by worsening an existing condition, such as cardiovascular or respiratory disease. In public-health statistics, researchers may estimate excess mortality by comparing the number of deaths observed during an event with the number that would normally be expected.
That approach can reveal the wider impact of extreme weather, including deaths that may not have been recorded on death certificates as heatstroke.
What Does a 122% Increase in Mortality Mean?
A percentage increase can sound abstract until it is translated into everyday terms.
If the expected number of deaths in a particular period were 1,000 and the observed number were 2,220, the increase would be 122%. The number of deaths would be more than double the expected level.
That does not mean every death was caused by heat, nor does it mean that every individual who died had experienced the same medical problem. It indicates that substantially more deaths occurred than would normally be expected under the comparison used by the researchers.
Excess mortality is an important tool in public health because it can capture the broad impact of an event. During a heat wave, the additional deaths may include people whose existing illnesses became more dangerous under extreme conditions.
Older adults with heart disease, people with respiratory problems and individuals who are already medically vulnerable may be particularly affected.
The figure also highlights why heat warnings should not be treated as ordinary weather announcements. A period of extreme heat can become a public-health emergency, especially when large populations are exposed and access to cooling is limited.
Why the Paris Region Can Be Vulnerable
Île-de-France includes Paris and surrounding communities. It is a densely populated region with extensive urban development, roads, buildings and paved surfaces.
Like other large metropolitan areas, it can experience urban heat island effects. Buildings and roads absorb heat during the day and release it slowly, making some neighborhoods remain warm long after sunset.