Heat kills. As many as 70,000 Europeans died during a heat wave four summers ago. Chicago, London, Athens and Tokyo have had epidemics of fatal heat stroke. Scientists predict more frequent heat waves as the world gets warmer.

But global warming isn't the only factor. Phoenix, the hottest city in the United States, is a classic illustration of another reason experts expect more killer heat waves: Big cities create their own heat.

It's called the "urban heat island" effect.

As cities grow, more of the landscape gets covered by pavement and buildings. They soak up the sun's heat all day long and radiate it during the night. Not only do people get no respite from the heat, it means the next day gets off to a hotter start. So temperatures are even higher in the afternoon.

Protecting the Vulnerable

That was the case on one recent July day, the 22nd day this summer when the mercury in Phoenix hit 110 degrees or more. That's double the number of super-hot days Phoenix usually has.

Before 7 a.m., it was already 92 degrees. And that was the day's low temperature reading.

By 9 a.m., the mercury had climbed to 99 degrees. Social workers Esther Trevino and L.J. Reynolds were already out on the streets, distributing hats, sunscreen, and ice-cold bottles of water to the city's homeless people.

Phoenix takes heat danger seriously – all the more because last year saw a rash of fatal heat strokes among homeless people.

"We lost 40 people that one year, that died out here in the heat," Trevino says. "Right out on the sidewalks of Phoenix." Some think it's partly because the city passed an ordinance prohibiting "camping" in the shade of public parks.

Trevino and Reynolds distribute water and sunscreen to dozens of street people, such as Jay, a 30-something man with sandy hair and an open face who lost his legs to a construction accident. They find him in his wheelchair on a street corner, sitting under the blazing sun.

"Let me go ahead and give you another water, 'cause you're right out here in the middle of it," Reynolds tells him. "And do put some sunscreen on. It'll make a difference." Reynolds gives Jay a free bus ticket and urges him to go to the homeless shelter where she works, which is air-conditioned.

"Thanks, guys, I appreciate it," Jay says, as he wheels off in search of some shade.

The 'Urban Heat Island'

Phoenix is enduring one of its hottest summers on record. Experts think the "urban heat island" effect is one reason.

"If we look at trends over the last several decades, there's absolutely no question the minimum temperatures have been rising significantly as the city has expanded," says Tony Haffer, chief meteorologist at the National Weather Service's regional office in Phoenix.

It's only going to get worse. All over town, construction workers are putting up more buildings. Bill Okland of Okland Construction Company says the Phoenix construction boom is creating a labor shortage. But he can't pay people to work extra shifts.

"The guys don't want to work overtime," Okland says. "It's really hard to ask for much overtime when you've got these kinds of weather conditions. Just a full week is tough enough on them."

To beat the heat, Okland's employees start work as early as 2 a.m. and knock off before 3 p.m., the hottest part of the day.

On this particular day, Phoenix recorded 113 degrees at 3 p.m., the day's high temperature.

How Extreme Heat Affects the Human Body

Amazingly, humans can adapt to this kind of blast-furnace heat – within limits. If you live in heat like this for just a few weeks, your body undergoes some remarkable changes.

New research shows one change involves chemicals called heat-shock proteins. They stop cells from committing suicide– their usual response to injury.

But if you live in a place where heat waves are uncommon – or spend a lot of time in air-conditioned rooms – scientists think your cells don't make as much heat-shock protein when you are exposed to high temperatures. That's one reason people in cities that aren't as hot as Phoenix can actually be at greater risk of heat stroke.

Dr. Sam Keim of the University of Arizona says chronic heat exposure touches off other changes in the body. Sweat increases, and the composition of sweat changes. It gets saltier.

Conserve salt and you prevent dehydration. That's important. In hot weather, your entire body acts like one big car radiator. Blood vessels near the skin open up to get rid of heat. Sweat cools the skin.

But if you sweat too much, your blood volume drops. Even a 1 percent drop forces the heart to pump faster and harder. That's bad news for people with heart disease.

And if your body's radiator can't shed heat fast enough, Keim says that's very bad news for the brain.

"People begin to have bizarre behavior," he says. "They're not thinking straight. So there's a direct effect of heat on brain function."

People edging into heat stroke often don't do what they should to protect themselves, such as drink water or get out of the sun. People with dementia and mental illness are at higher risk.

And after the body's core temperature reaches 104, everything begins to fall apart. The heart begins to fail, kidneys shut down, blood clots can form all over the body, and the brain slides into coma.

Victims of Heat Stroke

That's happening to hundreds of people this summer in southern Arizona. Most of them are Mexicans who've slipped across the border and tried to cross miles of desert. Keim says on days when the temperature hits 105, there's a 50-50 chance that one or more over these border-crossers will die of heat stroke.

"This is the most lethal disease occurring in Arizona today," Keim says. He says the death rate is far higher than cancer, heart attack or traffic accidents.

Most of the heat stroke victims end up in a morgue in Tucson, 120 miles southeast of Phoenix.

"There is where people are brought in, logged in, and brought into the exam room," says Dr. Bruce Parks, the chief medical examiner for the Tucson area. On a tour of the facility, he points to white erase-boards on the wall next to the cooler's massive stainless steel door.

"Numbers with orange writing are the border crossers," Parks says. "You can see that a few of these boxes have multiple numbers in the box. That tells us the individuals in those spaces are skeletal."

Parks' coolers contain dozens more heat victims than they did at the same time last summer. He says in the previous 24 hours, six more bodies were brought in.

"We're actually in the midst of perhaps the worst year we've ever had," Parks says.

But really, nobody knows just how often heat kills. Bodies aren't found until days later, when they're badly decomposed, or even weeks after death, when there's nothing left but bones. No one can tell what the temperature was at the time of death. Heat stroke is often an inference.

"I think we probably under-call heat-related deaths," Parks says, "and it's happening more frequently than we know."

One thing experts do know: Heat deaths are preventable. Health officials just have to identify who's vulnerable, make sure they get plenty of water, and get them to cooling centers when severe heat waves happen.

Being ready to do that will become more important as the planet warms and cities continue to grow. Because experts say we can expect more deadly heat waves – even in northern cities.

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