YOU'RE getting headaches. You're dizzy and nauseous. You haven't been sleeping or eating well.

The fight on the mountainside hasn't even begun yet, and you're already at a significant disadvantage: You probably have altitude sickness, and the more you exert, the sicker you'll get.

When it comes to Soldiers reaching their peak performances, little on today's battlefield stands in the way quite like the imposing mountains of Afghanistan.

The country's vertical terrain and high altitudes pose extreme challenges. Soldiers must carry heavy loads at steep angles that threaten their normal tactical advantages. Simply put, the higher they go, the more difficult everything becomes. Fortunately, scientists at the U.S. Army Research Institute of Environmental Medicine at Natick Soldier Systems Center, Mass., are applying their expertise to the problems.

"One of the main issues with altitude illness is that it varies dramatically between individuals," said Dr. Stephen Muza, Mountain Medicine Group team leader of USARIEM's Thermal and Mountain Medicine Division. "We can take fit Soldiers, they can look almost identical in terms of their typical Army characteristics. . .yet one may get sick and one may not at 10,000 feet. Why is that? We don't know. There's a lot of work being done to try to understand what is causing this individual susceptibility or risk."

Using USARIEM facilities such as the hypobaric chamber, scientists can study this and other issues by simulating conditions experienced by Soldiers in Afghanistan and elsewhere. The chamber is capable of creating barometric pressures found from sea level to 9,000 meters, temperatures from minus 32 to 43 degrees Celsius, and relative humidity from 20 to 80 percent. Soldiers can be tested for days at a time in the chamber, which includes access to a toilet, shower and running water.

"So we can not only simulate true altitude. . .but also we can control the temperature and the humidity to reflect real environmental conditions that you find out in the field," Muza said. "It can be cold. It can be hot. It can be humid. It can be dry. We can go up to altitudes of 30,000 feet with our chamber, and that gives us great ability to study rapid responses to a range of altitudes."

Regardless of their levels of physical conditioning, Soldiers can experience increased fatigue or even acute mountain sickness in this environment. USARIEM's chamber allows measurements to be made repeatedly on small groups of volunteers before Soldiers experience symptoms in combat. Researchers study human research volunteers in simulated high- altitude environments to learn more about performance, nutrition and how to avoid mountain sickness.

Studies not appropriate for the hypobaric chamber can be done in USARIEM's hypoxia room, in its lab atop Colorado's Pike's Peak or in the field. "We do not hesitate to go out into the actual field environment," Muza said. "We carry our equipment with us."

According to Muza, sickness in Soldiers working at high altitudes can be predicted statistically. His studies indicate that at 10,000 feet, 25 to 30 percent will become sick; at 1 1,500 feet, 50 to 60 percent; at 13,200 feet, 80 to 90 percent and, finally, at 14,800 feet, 90 to 100 percent.

"I should point out that if you go high enough," said Muza, "everybody will get sick, pretty much."

The numbers point to the reason that Muza hopes to one day accurately predict which Soldiers are most likely to become sick at different altitudes.

"We've been working on models that estimate what the likelihood of developing altitude sickness is, and if you do develop the altitude sickness, whether it's going to be a mild form, a moderate-severe or severe form, because severe would require, for example, medical evacuation," Muza said. "You look at the costs of doing a medevac in the mountains. You have to use a heavy-lift helicopter. You're running the risk of that helicopter getting shot at, and potentially losing that asset and crew. It becomes a big cost.

"I think we have a chance in the next five years or so of identifying that high-risk group. If we can identify that high-risk group, then we can probably aid them in adapting. You can focus your resources, then, on that individual, rather than treating an entire company."

Though Soldiers might avoid becoming ill, their war-fighting performances could still suffer in the mountains through fatigue, decreased stamina, compromised decision-making and impaired vision, all of which can lead to dangerous outcomes. "Even if we're not talking about altitude sickness, if you look at physical performance, a unit only goes as fast as the slowest person in the unit," Muza said. "Everyone's only going to move as fast as that one individual."

Muza will use everything in his scientific arsenal to keep Soldiers from falling victim to altitude sickness or experiencing decreased performance in the mountains, including acclimatization, diet and medication.

"We're looking for the most efficient and effective approaches to altitude acclimatization," Muza said. "With acclimatization, you function better. You're less likely to develop illness once you're acclimatized, and you're going to have better work performance."

According to Muza, eating a carbohydrate-laden diet can be beneficial at high altitudes. When he goes into the mountains, for example, Muza snacks on crackers.

"We know that with time and altitude, the body uses more carbohydrates for its fuel, and carbohydrates are the most effective fuel at altitude, in fact," said Muza. He added that study subjects who increased carbohydrates "improved their physical performance by nearly 25 percent. . .so (there's) a very clear benefit of snacking or sipping on carbohydrates while you're on the move. We don't know exactly why."

According to Muza, those findings led to the development of the Modular Operational Ration Enhancement at Natick's Department of Defense Combat Feeding Directorate. MORE is meant for consumption by troops in high altitudes in cold and hot weather.

Muza said that drugs can be used to treat altitude sickness, but they have possible side effects. He added that USARIEM is now focused on drug development at the molecular level. "Possibly, someday you could be sitting at some location, like Fort Bragg, N. C, at 600 feet above sea level, and you can give an entire battalion a pill - maybe not just one, maybe over several days - and they will, in essence, adapt to high altitude prior to deploying to high altitude," Muza said.

Altitude isn't the only concern for Soldiers operating in Afghanistan's mountainous terrain, however. They also must deal with the impact of heat and cold and the possibility of dehydration.

"When you think about mountains, you think about it becoming colder as you go up. But in Afghanistan, you can be fairly high and still warm," said John Castellani, a USARIEM research physiologist. "You could be at 10,000 feet and still have an ambient air temperature in the 80s.

"So now we've got true environmental stressors on the Soldiers. Then you throw on top of that the load they're carrying and everything else."

Castellani did a dehydration study at USARIEM that produced surprising results. "Everybody knows that when you go to altitude, your performance stinks," Castellani said. "Everyone gets it. But when we tell people dehydration affects you the same way, people don't get it."

While an altitude of 10,000 feet degraded Soldier performance by 12 percent, dehydration of 4 percent resulted in an 18-percent performance decline at sea level. "When we combined the two of them, there was a 33-percent decline in performance, so a third of your performance," Castellani said. "That's such a huge impact."

According to Castellani, dehydration also made acute mountain sickness worse. "It gives us more reasons why the guidance is out there for making sure (Soldiers) try to stay hydrated," said Castellani, adding how important it was for Soldiers to carry enough water. "Soldiers tend to not carry the stuff that deals. . .with their health more. They will choose. . .the ammo and other things."

Muza pointed out that commanders now know that their Soldiers should hydrate immediately after helicopter insertions in the mountains. "And we need to réhydrate very substantially," Muza said. "What should you réhydrate with? Water is obviously No. 1 , but would there be a benefit to réhydrate with some composition of fluid? Those are the unknowns."

Castellani noted that drawing blood is the only current way to measure hydration. He added, however, that development of a "non-invasive hydration status monitor" is under way.

Cold poses yet another challenge for Soldiers in the mountains. Castellani and others are looking at that, as well. "One of the big (points of) emphasis we're going to have over the next several years is trying to keep the hands functional in the cold," he said. "When it's cold out, you have a hard time doing things. Once you put gloves on, you lose dexterity."

The goal is a system that maintains dexterity but provides warmth. "That's somewhere we're heading," Castellani said.

Muza and Castellani will continue to move forward with their work to make Soldiers more comfortable and effective in the mountains.

"A lot of our work over the last nine years has been much more applied physiology, much more nuts and bolts issues," Muza said. "What can we do for the Soldier now, not 20, 30 years from now?"