With the summer only half over and Baltimore settling into a mental fog as thick as the haze shrouding its buildings, health authorities are issuing a warning that they hope will be taken seriously: Heat kills.

It killed Haden Skinner, an 86-year-old man who lived alone and was inclined to sit in his Brooklyn apartment with the windows shut and an electric fan recycling hot air. When he was discovered dead in his recliner July 5, the indoor temperature measured 95 degrees.

It killed Gloria Turner, 67, who became ill after waking recently in a 104-degree rowhouse in lower Charles Village. Her companion called paramedics after she started trembling so fiercely she could barely let go of the bathroom sink, but she was close to death by the time the ambulance arrived.

"I was here and I saw it, but I still don't believe it," said her friend, Edward Davis Eades, remarking how vibrant she had seemed the day before.

The two are among the 25 people who have died in Maryland of heat-related illnesses since steamy, polluted air descended in late June. The majority were in Baltimore.

Though most of the deaths occurred during the first week of this month, when temperatures exceeded 90 degrees for five straight days and reached 100 on Independence Day, health officials are concerned the current hot spell and others that will likely follow could take a further toll.

Particularly at risk, they say, are the elderly and people with heart disease, respiratory ailments and other conditions that make their bodies less able to cope. Most of the recent victims had underlying illnesses - Skinner had heart disease and Turner had diabetes and hypertension - but medical authorities found that heat was a contributing factor in their deaths.

Dr. David Fowler, the state's medical examiner, noted that in hot weather the body begins shunting blood to the extremities in an effort to cool itself. This aids in the evaporation of sweat but places undue pressure on a heart that might already be diseased.

"They wouldn't have died of the heat had it not been for this pre-existing disease," said Fowler. "But they get additional stress from the heat, and that pushes them over." A few recent victims had Alzheimer's or schizophrenia, which can make a person unaware of the heat or what to do about it.

Dr. Peter L. Beilenson, the city health commissioner, said his agency has taken steps to protect vulnerable citizens.

Volunteers who regularly visit elderly residents are opening windows, running fans and taking seniors to air-conditioned shopping malls, he said. The city has opened its clinics to people who just want to cool off. Case workers who visit pregnant women and the elderly are distributing information about how to protect oneself from the heat.

But, in frequent public appeals since heat first gripped the city, Beilenson has asked people to check their elderly neighbors and take them to air-conditioned places if necessary.

"I don't think there is a city in the country that can systematically check on every person with a chronic condition to make sure everyone is OK," he said.

Recently, experts have cast doubt on the ability of fans to protect people on the most stifling days. Dr. Michael McGeehin, an environmental hazards expert with the U.S. Centers for Disease Control and Prevention, said fans are incapable of evaporating a person's perspiration when the temperature rises above the low 90s and the air thickens with humidity.

Spending even an hour in an air-conditioned mall will probably help more than sitting by a fan that blows hot air all day long. So will taking a cool shower or bath, he said.

It wasn't until the 1990s that public health authorities began widely to pay attention to the role that heat plays in death rates. The trend started in 1993 when a heat wave was blamed for 118 deaths in Philadelphia. In 1995, Chicago experienced a weather disaster when 465 people died of heat-related illnesses during two weeks in July.

Though some experts argued at the time that the numbers were inflated - many of the people, they said, were old and infirm and would have died anyway - the CDC now says that reports of heat-related deaths are probably low.

The case for heat as a contributing cause of death is often a circumstantial one, said Fowler. Besides conducting a physical exam and sometimes an autopsy, medical examiners also check temperatures in the deceased's home and see whether windows were open or fans or air conditioning running.

Only in rare instances will the medical examiner find that a person died solely of classic hyperthermia - the effects of an elevated body temperature brought on by the heat.

Such was the case with a 23-year-old firefighter who died two weeks ago after completing a 3-mile run during a training exercise near Frederick. When paramedics arrived, his temperature was 107.2 degrees.

Each year, about 400 deaths nationally are attributed to excessive heat, according to the CDC. Though it might seem counterintuitive, the deaths appear to be less common in the South than in the more temperate mid-Atlantic and Midwest.

Larry Kalkstein, a climatologist with the University of Delaware, said Southerners are more acclimated to the heat. Also, their bodies do not have to adjust to the temperature fluctuations common in such places as Baltimore, Philadelphia or Chicago.

What's more, the housing stock here is particularly unsuited to hot weather. "You've got rowhomes with brick walls and black tar roofs," he said. "They are like brick ovens."

Haden Skinner lived in the downstairs apartment of a two-story, flat-roof rowhouse in Brooklyn. Though air conditioners hung from many of his neighbors' windows, the former railroad worker insisted on living without one, according to maintenance man Lee McGee.

"He was a tough old man, very stubborn," said McGee. "I'd say, 'Mr. Skinner, you can get one for only $120,' but he wouldn't say anything."

Skinner was often seen dressed nattily in alligator boots, a large hat and a wide belt with an oversized buckle. He drove a car until the end of his life. But he was showing his age, too, having become forgetful and frail.

Frequently, he parked his car out front with the engine running all night long. He had lost considerable weight, and had recently fallen and broken his arm.

McGee said he checked on the man daily, opening windows that Skinner would predictably slam shut. McGee didn't see him July 4 because he had the day off.

The next morning, two days after anyone had seen him, a concerned friend called police, who entered the apartment and found Skinner dead.

"He was in that old, large chair, looking like he had gone to sleep," said McGee, who went in with them. "That's where we found him."

Though Skinner's case points to the dangers faced by people who live alone, Turner's shows that living with a caring friend doesn't guarantee safety when the summer turns brutal.

Turner, a retired counselor for delinquent youths, and Eades had lived together for 27 years in an immaculate rowhouse where they kept fans running and windows open in hot weather.

Eades said she religiously took insulin for her diabetes and seemed healthy on July 4, when they entertained friends.

"She was sitting here laughing and joking," Eades said Wednesday in the kitchen, where the thermometer read 94.

The next morning, Eades found her shaking and talking incoherently as she gripped the bathroom sink. He called 911, then helped her to the floor and held her head while they waited for the ambulance. By the time paramedics arrived, she was no longer responsive.

The medical examiner's office said she died of the combined effects of heat, hypertension and diabetes. Tearfully, Eades said they had been planning to get an air conditioner and had friends lined up to help install it.

"It's unbelievable," said Eades, struggling to cope with the death of his friend. "She had a heart of gold and she'd give you her last if you needed it. I wish everyone could have known her."