For tens of millions of Americans this week, summer is starting with a multiday blast of fierce heat. Temperatures in parts of the country are likely to be well into the 90s through the weekend. The National Weather Service has warned that the heat wave could be the longest one some places have experienced in decades.

Weather this hot creates plenty of health risks, even for people who sweat through it for just a few hours. But researchers have found that the dangers of heat can be compounded when conditions are sweltering for day after grinding day, night after sleepless night. With so much of the United States facing such hazards this week, it’s a good time to linger a bit on what we know about heat that lingers.

In general, human-caused global warming is making heat waves even hotter, more frequent and longer lasting. Already this year, India has experienced what its top weather official described as the country’s longest hot spell on record, spanning 24 days in April and May. Greece is in its second week of deadly heat, less than a year after experiencing a 16-day heat wave, the longest it has measured.

For the planet as a whole, 2023 was the warmest year in human history and global temperatures have continued breaking monthly records well into 2024.

When it’s very hot, the human body has to work harder to keep organs and tissues at their normal healthy temperatures. The longer your body is forced to do this, the greater the strain on your cardiovascular system, and the higher the risk of negative health effects, which, in extreme cases, can include heart and kidney failure.

How much more risk? Two researchers, G. Brooke Anderson and Michelle L. Bell, crunched the numbers for 43 U.S. communities between 1987 and 2005. They found that for every additional day that a heat wave dragged on, the risk of nonaccidental death rose by 0.4 percent. For every extra degree Fahrenheit above normal temperatures, the risk increased by 2.5 percent. Both effects were more pronounced in the Northeast than in the Midwest or South, the researchers found, possibly because people there were less accustomed to stifling weather.

Other researchers have found a small additional increase in mortality risk that kicks in after four consecutive hot days. Another study looked at heat waves in 400 communities across 18 countries and regions in Asia, Europe and North and South America. That report didn’t find that longer heat waves were deadlier, though this might have been because of technical differences in the way it captured the timing of deaths.

Dr. Anderson and Dr. Bell also found that heat waves earlier in summer were more dangerous than those that happened later. This, they wrote, might be because people physically acclimatized to hot weather or changed their behaviors, buying an air-conditioner, say, or spending more time indoors.

Another grim possibility, according to researchers: Early-summer heat waves might kill off older people and those with chronic conditions, leaving a smaller pool of vulnerable people later in the season.

Long heat waves also bring hot nights, which give the body less time to recover and make it harder to sleep.

Forecasters have warned of record nighttime heat this week, with temperatures in some places dropping only into the 70s. The human body lowers its core temperature to trigger sleep, so if temperature regulation is disturbed, our sleep can be as well. This is more likely to be a problem in big cities, which generally cool down less at night compared with rural environments because of all the pavement and other surfaces that absorb and radiate heat.

A 2017 study combined nighttime temperature data with survey results from 765,000 Americans who were asked about their sleep habits by the Centers for Disease Control and Prevention. It found robust links between hot nights and troubled sleep, particularly among older and poorer people. Other studies have found that higher overnight temperatures are related to higher risk of heat-related death as well.

Such findings are already shaping the way public agencies discuss heat dangers. The new HeatRisk tool from the National Weather Service and the Centers for Disease Control and Prevention accounts for the duration of daytime and nighttime temperatures in its threat assessments.

The National Weather Service has been blunt about why the concern this week isn’t just about the mercury: “The early arrival of this magnitude of heat, the duration, abundant sunshine and lack of relief overnight will increase the danger of this heat wave beyond what the exact temperature values would suggest.”

Should FEMA disaster funds go to fight extreme heat?

Dozens of environmental, labor and health care groups banded together on Monday to file a petition pushing the Federal Emergency Management Agency to declare extreme heat and wildfire smoke “major disasters” like floods and tornadoes.

The petition is a major push to get the federal government to help states and local communities that are straining under the growing costs of climate change.

If accepted, the petition could unlock FEMA funds to help localities prepare for heat waves and wildfire smoke by building cooling centers or installing air filtration systems in schools. The agency could also help during emergencies by paying for water distribution, health screenings for vulnerable people and increased electricity use.

The support of major labor groups like the A.F.L.-C.I.O. and the Service Employees International Union is part of a broader strategy of unions to improve protections for the tens of millions of people working outside or without air-conditioning during heat waves. — Manuela Andreoni

Read about the push to use FEMA funds to fight extreme heat.

Why recycling is broken

Recycling can have big environmental benefits. For one thing, it keeps things out of landfills and incinerators, where they can produce potent greenhouse gasses and potentially hazardous pollutants.

Even more important, recycling allows us to extract fewer resources.

But recycling rates in the United States have remained stubbornly flat for years. And, in some cases, they’re dismal. Just 10 percent of plastics are actually recycled. Meanwhile, hundreds of thousands of tons of recyclable waste are exported, often to developing countries.

It’s no wonder a lot of readers have asked us whether individual efforts make any difference at all. So, we took a closer look at that question in the latest edition of our Ask Climate column. — Winston Choi-Schagrin.

Read more about why recycling is broken and what you can do it about it.

More climate news:

Correction: An essay in the Thursday newsletter about a lawsuit in Hawaii described incorrectly a point of contention in the case. Lawyers for oil companies are asserting that the matter should fall under federal law, not state law. They are not arguing that the case should be heard in federal court rather than state court.

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