What is altitude sickness and why is it so dangerous?

Mountain climber Corey Richards reached the top of Mount Everest without the help of extra oxygen this week, but his climbing partner had to turn back just short of the summit, and at least half a dozen others died in the past week attempting the same peak.

Mount Everest, the world's highest mountain, is 29,000 feet above sea level and several of those deaths have been blamed on altitude sickness.

At five and a half miles up, the atmosphere thins and oxygen levels are significantly lower -- only about a third of what you'd find at sea level. There's a reason climbers refer to it as the "death zone."

Altitude sickness is actually a constellation of different conditions that occur when the body doesn't have enough time to adapt to the lower air pressure and lower oxygen level at high altitudes, explains Dr. Clayton Cowl, chair of the division of preventive, occupational and aerospace medicine at the Mayo Clinic.

"The body doesn't like things to be out of kilter," Cowl, who's also a pulmonologist, told CBS News.

The breathing rate increases and that pushes blood oxygen levels to abnormal levels -- forcing the body to function with less oxygen than usual. He said it can affect various systems in the body including the lungs, the kidneys and the brain.

Also called mountain illness, common early symptoms are headache, nausea, dizziness and shortness of breath. A mild case might be relieved with over-the-counter drugs such as ibuprofen. More intense symptoms can be serious or life-threatening.

If symptoms become severe -- including shortness of breath even when resting, discolored skin, coughing and mental confusion -- medical attention is needed.

Cowl said buildup of fluid could occur in the lungs and the brain and put someone at risk for death.

But he said, "Most people are not going to die from acute mountain illness unless it's experienced at very extreme conditions."

Robert Roach, director of the Altitude Research Center and associate professor of emergency medicine at the University of Colorado School of Medicine, told CBS News that in Denver -- which sits at 5,000 feet above sea level -- about 1 in every 100 people who visit from lower elevations experience some symptoms of severe altitude sickness.

He said nearby Summit County, in the Rockies, where people go to ski, climb and hike, some areas are in the range of 10,000 feet above sea level.

Visitors from lower elevations -- for example, New York City, which sits an average of only 33 feet above sea level -- can reduce their chances of getting sick by acclimating themselves to mountain altitudes more slowly.

"Twenty-five percent of people who go to summit country will get symptoms, but if they spend one night in Denver at 5,000 [feet], only 10 percent of them will get mountain sickness," said Roach.

They can also take a medication called acetazolamide, a prescription drug that increases a person's breathing rate and oxygen intake, said Cowl.

In some ski resorts and high-altitude tourist destinations, he said condos and hotels even outfit rooms with oxygen pumps.

"At Machu Picchu, some places have supplemental oxygen that comes out right at the headboard of the bed and blows around your face when you sleep," Cowl said.

But Roach pointed out that Mount Everest is a whole other ballgame from the mountains in the U.S., where death due to altitude sickness is rare.

High up the slopes of Everest, once climbers hit about 26,000 feet, the air becomes dangerously thin.

Details are still not known about what led to the recent deaths of the climbers on Everest. In such treacherous conditions, many factors could play a role, including exhaustion, lack of sleep, lack of food, dehydration and temperature drops. "I think it's very unclear what they died of. We're relying on the worst kind of information -- second hand," Roach said.

At those heights, under such weather extremes, Roach said, "People are at the end of their ropes."