Geriatrician Mary Ann Forciea calls it an "absolutely stereotypical" picture: a family in summer sundresses, shirts, and shorts surrounding a little old lady wearing a sweater.
This time of year, as the temperature frequently climbs to 90 and above, that woman may be more than a source of amusement - or frustration - for the younger, hotter-blooded members of the family.
She may be in danger.
Doctors say a group of physical changes makes older people sense heat differently. Not only are they comfortable at higher temperatures than younger folks, but their bodies don't shed heat in the same way. In addition, they may not realize they are overheating. Put it all together, and older people are at greater risk for serious health problems and death at the height of summer.
Dementia and certain medications, including some used for allergies and psychiatric problems, can make things worse. It doesn't help that confusion - an early sign of heat stroke - further impairs judgment.
"Older people can get into trouble without realizing it, but it's hard to give an exact number at which that's likely to happen," said Forciea, who works for the University of Pennsylvania Health System.
Forciea, who is in her 60s, hastens to add that by "older," she generally means people 85 and up, although there is great variation in how people age.
A report last year from the National Center for Health Statistics found that heat-related death rates climbed significantly after age 65 and even more steeply after 75.
Michael Segal, who has a largely geriatric practice with Temple University Health System in Northeast Philadelphia, lays much of the blame on the hypothalamus, a part of the brain that responds to environmental changes. In older people, he said, "their hypothalamus doesn't regulate as it should. . . . The first thing we notice is they have a poor biofeedback loop."
Danielle Snyderman, a Thomas Jefferson University geriatrician, said that sensitivity to both heat and cold decreases with age, but that the decreased sensitivity to warmth is more pronounced.
The brain signaling changes are compounded by changes in skin, which becomes thinner with age. In addition, Snyderman said, elderly people lose subcutaneous fat - and insulation.
At any age, Forciea said, heavy people are likely to prefer a lower temperature than their thin friends and coworkers, which may explain many a fight over thermostat settings.
In the elderly, though, the thinner skin is coupled with fewer pores for sweating, she said. Evaporation of sweat is how the body sheds heat.
She worries most about her patients when high humidity - 75 percent and up - stymies skin evaporation or the outside temperature begins to approach body temperature.
If younger people sweat in the room, that's a sign the elder should be checked for thyroid or other problems.
Forciea, who regularly makes house calls as part of her job, frequently encounters patients who say they are comfortable at temperatures she finds stifling. She used to think that was all about money, but now sees that many keep the temperature high even when family members pay the utility bills.
Sometimes they tell her they think it's bad for their health to expose their bodies to cold air and then go outside. She says there is no evidence that's true. Still, many just seem to like it hot and resist efforts to cool things off.
Amanda Lathia, a Penn-trained geriatrician who works at the Cleveland Clinic, said homes are not the only problem. "When I go to nursing facilities, the rooms are unbearably hot, even in summertime," she said. She says patients should have access to plenty of water - she suggests six to eight 8-ounce glasses a day - and should be encouraged to go into the hall or to activities. The air outside their rooms is often cooler.
Segal also encourages his patients to drink lots of water to lower the risk of dehydration. Many don't feel the need because elders are also less likely than younger people to feel thirsty. He tells them they should be urinating every hour or two.
Forciea said many elderly people resist that idea, too. They may be weak or in pain. They try to limit the need to walk to the bathroom. Lathia said they also worry about incontinence.
Forciea says it does help to open windows, even when it's hot. She suggests using a fan and a mister to cool off.
While many elderly people don't like sleeping with the air-conditioning on, she said, some can be persuaded to run it for an hour or two to cool off the house before they go to bed.