Those without homes are especially vulnerable to high temperatures, struggling to access shelter, cold water and suncream.
Temperatures are sizzling across much of Europe.
Millions in France, Spain, Italy, Poland and Germany are struggling amid what experts are calling an intense and prolonged heatwave.
But for some the impact of high temperatures is serious - and dangerous.
Charities have sounded the alarm that extreme heat is endangering homeless people, putting them at risk of life-threatening illnesses.
"Exposure to this extreme heat without a safe place to shelter will be a danger to life for some."
"It’s as stark as that," he added.
Homeless people are particularly vulnerable to high temperatures, struggling to access shelter and the means to protect themselves, such as cold water and suncream.
This puts them at higher risk of sunburn, dehydration and deadly sunstroke during freak weather - something which is only set to worsen as the climate emergency continues.
"Ultimately, we need people off the streets and into homes of their own to provide long-term protection from extreme weather events that are becoming all too common,” said Downie in a statement sent to Euronews.
Research by Shelter from January shows 2,400 people are sleeping rough in England on a given night. However, calculating the exact number is notoriously difficult due to the transient nature of homelessness and varying definitions of the phenomenon.
At least 271,000 people were recorded by the charity as homeless in January, including those in hostels or temporary accommodations.
Crisis urged members of the public to be vigilant during hot weather and help people who "have nowhere to turn".
It suggested they could ask if there was anything rough sleepers needed, such as a cold drink, food or money.
Sunday was the hottest day of the year so far, with temperatures of 32°C hitting southeast England.
Crisis Chief Downie said it was "vital that all local authorities trigger their arrangements to offer emergency shelter so that people sleeping rough in their areas are protected."
Councils typically have 'Severe Weather Emergency Protocols' to give extra help to people who don't have a safe place to stay.
Many councils should have now activated this, with temperatures soaring across the UK.