The world is baking under extreme heat - with Asia, Europe, and the United States all dealing with scorching temperatures.
WHAT ARE THE HEALTH RISKS?
Heat affects health in several ways.
Heat exhaustion, which can include dizziness, headaches, shaking, and thirst, can affect anyone and is not usually serious, providing the person cools down within 30 minutes.
The more serious version is heatstroke when the body's core temperature goes above 105 degrees Fahrenheit (40.6 degrees Celsius). It is a medical emergency and can lead to long-term organ damage and death. Symptoms include rapid breathing, confusion or seizures, and nausea.
WHO IS AT RISK?
Some people are more vulnerable, including young babies and older people, as well as people who must stay active or are more exposed, such as homeless people.
Existing conditions, including respiratory and cardiovascular diseases, as well as diabetes, can also heighten risk - and be exacerbated by heat.
Globally, just under half a million deaths a year are estimated to be due to excess heat, according to a 2021 study in The Lancet, although data is lacking from many low-income countries. As many as 61,000 people may have died in Europe during heatwaves last summer, with a repetition feared this season.
"Heat waves are a silent and invisible killer. We don't often see the impact that they have had on human health until the mortality statistics are published many months later," said Professor Liz Stephens, a researcher in climate risk and resilience at Britain's University of Reading.
LESS OBVIOUS RISKS
Air pollution also poses a health risk, with serious potential effects from wildfire smoke including inflammation and tissue damage.
Heat also can lead to low birth weight and premature birth for pregnant women and babies, a number of studies have shown.
There are less obvious risks, too. Dr Vikki Thompson, Climate Scientist at, Royal Netherlands Meteorological Institute, said extreme heat often contributes to poorer mental health, as well as a rise in car crashes and drownings.
"Heatwaves are one of the most deadly natural hazards," she said.
Experts say more deaths occur earlier in the summer when people's bodies have not had a chance to acclimatize.
Location matters, too; people are at higher risk in places where they are not used to such heat, including parts of Europe.
However, there are limits, and people all around the world are at risk in extremely hot weather caused by climate change, particularly people who must continue to work in physical jobs, for example.
"It is more important than ever that we put in place measures to limit the harm on our health," said Dr Modi Mwatsama, head of capacity at Wellcome, a London-based global health charity. She said this ranged from providing shade and painting buildings white to investing in early-warning systems for climate-sensitive infectious diseases, like cholera.
WHAT YOU CAN DO
Public health agencies from Italy to the United States have issued advice on keeping cool, including avoiding exertion where possible and staying hydrated. Workers should think about having more breaks and changing their clothing too, scientists said.
It is also important to check on the vulnerable, including older and isolated people, they said.
Heatstroke is a medical emergency and requires immediate professional attention.