Tens of millions of people sweltered across the US Northeast and Midwest on Monday as record-high heat disrupted electricity supplies and train travel.

Heat warnings, which began over the weekend and are set to continue through the week, were in effect across much of the United States. East Coast officials warned that, as the heat combines with humidity, many places could see conditions that “feel like” 110 degrees Fahrenheit (43 degrees Celsius).

The National Weather Service warned that nighttime temperatures may not offer much respite from the “oppressive heat”, with nighttime forecasts of temperatures in the 70s and possibly even the 80s for East Coast cities.

Some East Coast trains were halted en route for safety precautions on Monday, and Amtrak said in a statement that its cars would operate at lower speeds due to heat restrictions.

Amtrak heat restrictions kick in when temperatures are higher than 95F (35C). The company also reduces its train speeds when the tracks themselves reach 128F (54C).

Health experts urged employers to adjust working hours for outdoor labourers, to ensure they had adequate breaks and opportunities for hydration, and to monitor for signs of heat stroke or exhaustion.

“There is a disconnect between the severity of heat waves as a public health risk and the public recognition of that risk,” said Howard Frumkin, an expert in environmental and occupational health sciences at the University of Washington.

Police departments said they were taking steps to protect officers who were stationed outdoors – urging them to stay hydrated, or to stay in the shade where possible, according to the departments in the North Carolina city of Raleigh and the national capital, Washington DC.

US electrical grid operators directed power plants to be ready to run at maximum force, with electricity demand expected to spike as people run fans and air conditioners to stay cool.

In New York City, temperatures for Tuesday were forecast to hit 97F (36C), which would break the city’s previous heat record for the date – set more than a century ago, when the mercury hit 96F on June 23, 1888.

Scientists have warned that such extremes are fast becoming the new normal – while also upending assumptions about which regions might be spared the worst of climate change.

“A lot of people ask the question, ‘where is it safe to be?’ And the answer is probably – no place,” Frumkin said. “We did not think the upper Midwest was going to be vulnerable to heat extremes.”

The heat was also being felt in Britain, while the Arctic state of Alaska registered its first ever heat advisory last week. Last month, China saw its temperatures soar.

NEW NORMAL

Of all the weather impacts linked to climate change, extreme heat poses the biggest threat to human life – more dangerous than even floods or hurricanes.

Last year marked the world’s warmest on record – and temperatures are set to continue climbing for the next few decades as climate-warming emissions also keep rising.

“It’s just getting hotter everywhere,” said Este Geraghty, chief medical officer and health solutions director at Esri, where she uses data and mapping analysis to understand how and where climate risks are developing and help is needed most.

Across England this week, the extreme heat could result in hundreds of deaths, according to a rapid analysis by a team of UK scientists. That forecast followed another report by the UK Met Office that found that extreme heat was now 10 times as likely to happen as it was decades ago.

Extreme heat can take a heavy economic toll – threatening crop yields, livestock, electrical outages and wildfires, and leading to disruptions to utility services, healthcare, or transportation systems, according to a report this month on insurance risk by Swiss Re Institute.

The global heat event this week “is sounding an alarm bell for society,” said Nina Arquint, who works as CEO UK & Ireland at Swiss Re Corporate Solutions.

“These events are more dangerous than natural catastrophes in terms of human lives lost, yet the true cost is only starting to come to light,” Arquint said.