The heat wave of 2013 continues. This week it's coming to the East Coast, where temperatures are expected to soar into the mid-to-high 90s (degrees Farenheit). And there's no relief at night: Lows are expected to hover in the low 80s. The West will see temperatures in the 90s.

That's relatively bearable compared with the triple-digit sizzlers of late June, when temperatures hit 119 in Phoenix. And this week, the 108-degree temperature in Las Vegas will still be high enough to cause second-degree burns for anyone attempting a barefoot walk across a parking lot.

These brutally high temperatures have spread beyond the nation's well-known summer ovens, with the heat wave—technically defined as three days in a row with temperatures topping 90 degrees—hitting traditionally cooler parts of the country, like the Northwest, and baking the northeast well into Canada.

From Arizona to Montana, from the Great Lakes to Maine, people are hearing heat advisories and warnings.

They do well to heed those warnings, says Claude Piantadosi, director of the Duke Center for Hyperbaric Medicine and Environmental Physiology in Durham, North Carolina. Human beings aren't built to spend long periods of time in temperatures that top the body's own approximate 98.6 degrees.

Cooling Mechanisms

Normally, people stay cool when the body sheds unused energy in the form of heat flowing from the body into the environment, first by conduction, or the transfer of heat energy to the skin's surface; then by convection, the transfer of heat energy to the air. The hotter it gets, the more difficult it becomes to shed that heat. At temperatures topping 100 degrees, the system reverses and heat flows from the environment into the body, says Piantadosi.

At that point, humans depend on a second cooling mechanism: perspiration. As the liquid sweat heats up, molecules become more active until they transform into water vapor and break free, removing heat from the body and reducing our internal core temperature. But high humidity defeats the system, because sweat won't evaporate when the air is already saturated with humidity. "The combination of heat and high humidity is really quite deadly," Piantadosi says. "It defeats our heat dissipation mechanism."

Dry desert heat can be more forgiving because low humidity allows for quick evaporation of sweat—but only if people drink enough water to make up for the loss of body fluids. "The only reason you can survive at 119 degrees is that high heat makes people sweat more," Piantadosi says. But people also lose salt when they sweat. And it's not always easy to tell how much your individual body needs to replace the loss. "My simple formula is to drink enough that you pee like you normally do," says Mark Morocco, a professor of emergency medicine at the Ronald Reagan UCLA Medical Center. That means that you urinate as often as usual, and the color of urine is normal—a dark hue is a sign of dehydration.

"Eat some salty snacks to augment the water. Or put a dash of salt and a teaspoon of sugar in a liter of water," Morocco says. The salt replaces salt lost through sweat, and sugar helps transport salt through the intestinal walls.

Worst-Case Scenario

When people can't drink enough water, dehydration sets in. Blood flow to the skin decreases, along with the ability to sweat. Body heat builds up. A body temperature of 104 degrees indicates danger; 105 degrees is the definition of heat stroke; and a temperature of 107 degrees could result in irreversible organ damage or even death.

A normal, healthy person who is not used to the heat can, in heat wave conditions, sweat as much as 1.5 quarts of liquid in an hour. Someone acclimated to the hot weather, say a Phoenix telephone lineman, develops the ability to sweat (and thus cool off) at a more intense rate, losing up to two quarts of sweat in an hour. "So he'd have to drink two quarts of water an hour just to stay even," says Piantidosi.

High heat and lack of water are prime conditions for multiple organ damage. Internal temperature soars, heart rate goes up, blood flow slows down, and organs begin to shut down. "The body begins to parcel out where the water should go," says Morocco. "There's a contraction of blood flow to the gut, the liver, and the kidneys. People begin to feel really bad." The kidneys shut down, and the heart has to work harder to pump a lower volume of blood through the body. Other organs begin to shut down, then fail.

Under extreme heat conditions, it can all happen very quickly—in an hour or even less. The brain, too, is affected by reduced blood flow. That's why people in the throes of heat illness begin to make poor, often life-threatening, decisions.

Beware the Heat

Younger, healthy people can get in trouble mostly from a foolish disregard of conditions, like hiking into a shaded canyon in the morning, only to find the heat unbearable for the hike out.

Unfortunately, more and more people—largely people crossing the border from Mexico to the United States in the Arizona desert—are taking a huge risk because of desperation. "The most common individual to die a heat death in Arizona is young and healthy—not frail and elderly," says Sam Keim, head of the University of Arizona's department of emergency medicine, who has published research on heat deaths among border crossers. "They just happen to be undocumented border crossers. Heat waves are not new to Arizona. We consistently see temperatures over 100 for 30 days in a row in midsummer. People are dying because they're putting themselves at increased risk and exposure. Heat deaths in undocumented border crossers reach into the hundreds every year." (Related: "Documenting the Undocumented.")

Healthy people in big cities can also be at increased risk from heat illness. On June 30, when temperatures reached 97 degrees, six runners in the Pasadena half-marathon were hospitalized for heat-related illness.