Deadly heatwaves: How high temperatures affect the body

Extreme heat kills thousands of people each year, but what is it that makes hot weather so deadly?

  • 17 June 2022
  • 5 min read
  • by Linda Geddes
Photo by Ketut Subiyanto from Pexels
Photo by Ketut Subiyanto from Pexels


From Nice to New Delhi, record-breaking heatwaves have struck multiple countries in recent months, with further high temperatures predicted in the coming days. Such extreme temperature events are predicted to occur more frequently as a result of climate change and can be extremely deadly. In 2010, for instance, a 44-day heatwave in the Russian Federation resulted in 56,000 excess deaths.

Heat can kill in several different ways. The first is dehydration. If you don’t drink enough water to replace that lost through sweating and urination, the blood starts to thicken, making it more prone to clotting, which increases the risk of heart attack and stroke.

Exposure to high temperatures has wide-ranging effects on the body, often amplifying existing health problems, making older people and those with chronic conditions at particularly high risk – especially those in poor and marginalised communities who lack air conditioning and may have labour-intensive jobs that require them to work outside.

Babies and young children are also highly vulnerable, both because they sweat less than adults, which makes it harder for them to cool down, and their large surface area to volume ratio means their core heats up more quickly. Children also tend to run around outside more than adults and may lack the judgment to limit their exertion and drink more fluids during spells of hot weather. Athletes too, can be at risk if they over-exert themselves in hot weather and fail to stay hydrated.

Heat loss mechanisms

The body has several mechanisms to cool itself down in response to high temperatures. As the skin heats up, blood temperature also rises. This change is detected by an area of the brain called the hypothalamus, which also receives messages from temperature receptors in the skin. It responds by instructing tiny blood vessels in the skin to dilate (get wider), bringing more blood to the body’s surface and allowing heat to radiate out. It also triggers sweating, which causes some of that excess body heat to dissipate when the sweat evaporates, cooling people down.

If it is hot and dry, sweat evaporates easily, but the more humid it is, the harder this becomes. People sometimes refer to the “wet bulb temperature” – a measure of not just how hot the air is, but how well water evaporates from a wet surface such as our skin.

Imagine wrapping the bulb of a thermometer in a wet cloth; it would cool down as the water evaporated, making the bulb cooler than the surrounding air. The wet bulb temperature is the point at which this cooling no longer happens, because the air is already saturated with too much water.

Deadly limits

If humidity is low, humans can tolerate fairly extreme temperatures, such as those found in California’s Death Valley – provided they have access to shade and plenty of water to drink. But in humid regions, such as parts of India, people struggle to survive above wet bulb temperatures of about 35°C (95°F), unless they can find other ways of cooling themselves down, such as shutting themselves in an air-conditioned room, or drinking cold water (assuming the water is clean enough to drink).

Today, wet bulb temperatures are usually calculated using measurements from electronic instruments at weather stations, but they are only a rough guide to the danger presented by extreme heat and humidity. People can, and do, die at lower temperatures – particularly if they fall into one of those high-risk groups.

How heat kills

Heat can kill in several different ways. The first is dehydration. If you don’t drink enough water to replace that lost through sweating and urination, the blood starts to thicken, making it more prone to clotting, which increases the risk of heart attack and stroke. It also means the heart must pump harder to ensure enough oxygen reaches other organs and tissues, placing it under extra strain. Both of these things are more dangerous in people with pre-existing conditions affecting the heart and blood vessels.

Excessive sweating also alters the balance of electrolytes in body fluids, which can affect nerve and muscle function. In extreme cases, this can result in seizures, breathing difficulties, or heart muscle spasms, which can be fatal.

As severe dehydration kicks in, the body tries to conserve water, and people start to sweat less. Normal core body temperature is around 37°C, but if sweating ceases, or it becomes too hot and humid for its cooling function to work, body temperature starts to rise. Above a threshold of about 42°C, proteins start to denature, resulting in organ failure and further nerve cell impairment.

Heatwaves can also have indirect effects on people’s health. For instance, drownings are a major cause of death during hot weather, as many people head to beaches, lakes, pools or rivers to cool themselves down. Hot weather may disrupt crucial infrastructure such as water or energy supply, with knock-on consequences for the provision of clean drinking water or electricity for air-conditioning units.

It also has a detrimental impact on air quality, for example increasing levels of ground-level ozone, which can cause throat and eye irritation, and exacerbate existing respiratory or cardiovascular diseases.

Staying cool

Given that population exposure to heat is increasing as a result of climate change, it is important that people know how to keep themselves cool. Many of the factors leading to death are preventable. For example, children being locked in vehicles is a major killer during heatwaves. Even though they and elderly individuals are at higher risk overall, there is plenty that can be done to reduce their risks. The following resources provide some useful tips and strategies for coping with the heat: