In 2015, when Diana and Charles Cox were considering where to retire, they drove their R.V. across the Southwest to visit several possibilities: Santa Fe, Sedona, Phoenix, Las Vegas.

They’d lived in San Jose, Calif., for nearly 20 years, but Ms. Cox was winding down her practice as a biotech patent attorney, and her income was dropping as taxes, housing and other living costs were rising.

Her husband, 71, a contractor, had retired years earlier. “I was having more and more trouble paying the mortgage,” said Ms. Cox, who is 69.

Phoenix won out because of its lower costs, international airport and many health care providers, essential for two people with chronic medical conditions. The couple bought a house in a 55-plus community in suburban Goodyear, Ariz., in 2016. Knowing the summer heat there would be intense, they planned to spend the season back in the Bay Area in their R.V.

But the pandemic made travel feel unsafe for years. Mr. Cox underwent treatment for prostate cancer. Ms. Cox’s father moved in and needed care. So they have mostly summered in Goodyear.

The number of older Americans like the Coxes who are exposed to extreme heat is increasing, the result of an aging population, continuing migration to heat-prone places and climate change. Researchers say the trend will only get worse.

“The places that are hot now are precisely the places getting older,” said Deborah Carr, a sociologist at Boston University and lead author of a recent study of population aging and heat exposure.

Phoenix, long a retirement destination, has averaged 108 days a year of 100-plus degree temperatures since 1970. But this year has been brutal: By July 31, Phoenix had already reached 68 days this year with temperatures over 100 degrees. Temperatures hit at least 110 degrees Fahrenheit for 31 straight days, from the last day of June to the end of July, setting a record.

And hazardous heat returned to the city just this weekend.

Summer in the Phoenix suburbs has been “miserable,” Ms. Cox said, on a midmorning when the temperature in Goodyear had already reached 106. “You really can’t go out and do things. We haven’t been as sociable as I’d like.”

This year has been particularly miserable because a delayed home renovation project forced the couple to move into their R.V. for three months, starting in June. The vehicle’s two air conditioning units are struggling. So is the refrigerator, causing salads to wilt and milk to spoil.

“A couple of days ago it got up to 92 in here,” Ms. Cox said. “The cats were prostrate under the ceiling fan.” She called the inside heat “uncomfortable, but not deadly.”

Heat can indeed be deadly, though, particularly for seniors. Last year Maricopa County, which includes Phoenix, recorded 425 heat-associated deaths, a 25 percent increase from 2021. Two-thirds occurred in people over 50.

The over-65 population increased 52 percent in Arizona between 2009 and 2019; it grew 57 percent in Nevada and 47 percent in Texas. That reflects the aging of current residents, but also continuing migration to those states.

The Census Bureau reported last year that more than 600,000 older adults moved to new states annually from 2015 to 2019, with the greatest net migration to Florida, Arizona, North Carolina, South Carolina and Texas.

At the same time, climate change is driving up temperatures in typically moderate locations. “The places that are already older — the Midwest, the Northeast, New England — are having heat exposure increase at the most rapid clip,” Dr. Carr said. “And we’re less prepared for it.”

Seniors, especially those with chronic illnesses like heart disease or diabetes, are vulnerable to extreme heat because they have more trouble with thermoregulation, the body’s ability to retain its temperature.

“Older bodies are less efficient at pumping blood to the skin and less efficient at sweating,” decreasing their ability to cool themselves, said Dr. Neelu Tummala, a surgeon and co-director of the Climate Health Institute at George Washington University.

“That makes it harder for the heart to pump,” she said, adding to cardiovascular stress and illness. Commonly used medications like diuretics and beta blockers can increase the risk of unnoticed dehydration.

The risk of kidney disease or failure rises. Struggles with mobility or cognition may prevent seniors from seeking relief.

“Extreme heat is the deadliest form of weather in the United States, much more than hurricanes or tornadoes or wildfires,” said Brian Stone, Jr., who teaches environmental planning at Georgia Tech.

He is lead author of a grim recent study estimating the impact of a major blackout during a powerful heat wave in three cities: Detroit, Atlanta and Phoenix — though the probability of such blackouts is increasing everywhere, Dr. Stone said. Electrical grid failures affecting more than 50,000 residents more than doubled in the most recent six years for which data was available.

The researchers’ models assumed five days of temperatures as high as 95 degrees (in Detroit), 97 degrees (Atlanta) and 113 degrees (Phoenix), combined with blackouts of all residences for 48 hours, followed by 72 hours of power restored gradually to the populace.

Heat-related deaths, would exceed 220 in Detroit, which has fewer air-conditioned homes than many Southern cities, the study found. In Atlanta, the death toll would be six.

In Phoenix, the intense heat could kill more than 13,000 people — not a typo — and most would be older, as in virtually every natural disaster.

Yet Dr. Carr doubts even this summer’s extreme heat will dissuade moves to popular retirement spots. Apart from mild winters, “older adults want to move where the cost of living and housing costs are lower,” Dr. Carr said.

They may see summer heat as transient or aberrational, she noted, or “they may prioritize family over the possibility of heat waves.”

That’s exactly why Jean Swain Horton moved from Sacramento (itself a hot spot) to Frisco, Texas, two years ago. Her son and daughter-in-law were relocating with a new baby, Theo, and they wanted her to come along; she moved into the same apartment complex.

Ms. Horton, 67, doesn’t love staying mostly indoors for nearly five months of the year, or living in a darkened apartment with shades pulled to block the sun. But she loves being close to Theo and helping to care for him. “I would go anywhere to be near my grandson,” she said.

John Berger, 68 and newly retired, just sold his house near Long Beach, Calif., where he and his wife never installed or needed air conditioning. They’re heading to Albuquerque, where they plan to buy a house to share with their adult daughter and her roommate.

In Long Beach, he figures a multigenerational residence would cost at least $900,000, an unaffordable price for him as a retiree. In Albuquerque, he thinks he can spend half that.

True, Albuquerque will be hot, but it averages just four days a year of 100-plus temperatures (although this year the city tallied 15 such days through July).

“Perhaps it’s denial,” Mr. Berger said of the family’s decision to live with the heat. “Perhaps it’s, ‘I’ll figure out how to make it work for me.’ People learn to adapt.”

The Coxes have adapted. They have installed solar panels on their house and plan to buy a backup battery. In case of blackouts, there’s a backup generator for the R.V. Ms. Cox always takes water with her when she leaves the house.

In her overheated R.V., however, she sometimes yearns for the breezy Bay Area. San Jose’s number of days topping 100 degrees so far this year? Zero.

“If we could afford it, I’d move back to the California coast,” Ms. Cox said. “I prefer being able to open the windows.”