Residents told not to venture outside. Salt trucks spraying sand on roads to stop them from melting. Doctors pausing surgeries. Rail authorities warning tracks could buckle. Experts saying hundreds or thousands could die in the heat.

The Meteorological Office, the United Kingdom’s national weather service, has issued its first “red warning” for extreme heat, urging people to avoid exercise, travel or even going outside if possible.

For those venturing outside into the heat, the Met Office urged them to adhere to basic safety by drinking lots of water, applying sunblock and finding shade whenever possible.

There was a 50% chance the U.K. could record its hottest temperature to date, currently 101.6 Fahrenheit in July 2019, it said.

If it does hit 105, that would make it hotter than all but 2% of the world, including some areas of the American Plains, the Middle East and China, according to NBC News meteorologist Kathryn Prociv.

Meanwhile, the U.K. Health Security Agency announced its first “level 4” heat warning — the highest possible — which it describes as a “national emergency.” Government ministers discussed contingency plans at a special interdepartmental meeting last week.

“The extreme heat we are forecasting right now is absolutely unprecedented,” Met Office boss Penny Endersby said in a rare public service broadcast last week. “Here in the U.K., we are used to treating a hot spell as a chance to go and play in the sun — this is not that sort of weather. Our lifestyles and infrastructure are not adapted to what is coming.”