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Humidity is key in keeping cool

By Staff Sgt. C. Todd Lopez

SOUTHWEST ASIA (Aug. 29, 2004) -- During the hottest hours of the day, technicians in the medical group here step outside to take three measurements that impact operations base-wide.

Those three measurements are used to calculate the wet bulb globe temperature, a number used to set work-rest cycles for the base. The WBGT is more relevant than a simple temperature measurement because it includes a key factor in the body's ability to cool itself, said Lt. Col. (Dr.) Brian P. Hayes, Chief of Aerospace Medicine here.

Outdoors, a man in a military uniform holds a clipboard and looks at a piece of weather-sensing equipment mounted on a tripod.
Master Sgt. Jim Kinsey, 379th Expeditionary Medical Group, checks the readings on an environmental monitor. The device measures an "in the shade" temperature, a wet bulb temperature and a radiated heat temperature. The three numbers are used together to calculate a wet bulb globe temperature. That number, weighted mostly towards the humidity reading, is used to set work-rest cycles for Airmen throughout the region. A higher number means a greater chance for heat related illnesses. Sergeant Kinsey is deployed from the Florida Air National Guard's 125th Medical Group, Jacksonville Fla.

"The WBGT is important because it takes humidity into account," he said.

Humidity or water vapor in the air is important because it affects the way the human body regulates its internal temperature, Colonel Hayes said.

"Our bodies are designed to keep us at an internal temperature of 98.6 degrees Fahrenheit," Colonel Hayes said. "One of the most effective ways to do that is to sweat. When the humidity is low, sweat evaporates and you get evaporative cooling."

As humidity rises, evaporation becomes less effective at cooling the body. In a high-humidity environment, water in a puddle stays around longer, a towel doesn't dry, a swimmer just out of the pool stays wet, and sweat simply beads up on the skin until it rolls off the body, Colonel Hayes said.

"Part of the challenge we face here is that a lot of our warriors have spent time at Prince Sultan Air Base or other bases in the AOR, and are under the impression they will experience the same conditions here as they did there," Colonel Hayes said. "If you talk to them, they say they didn't sweat at PSAB. But they did sweat there … it just evaporated quickly. They have like ten percent humidity at PSAB. Here, it is more like 85 percent. We have phenomenal humidity here."

Above a certain humidity level, the body loses its ability to cool off through perspiration, Colonel Hayes said.

"In this environment, once you get above 75 percent humidity, your body's ability to cool by evaporation drops dramatically," he said. "It becomes a very ineffective way to cool yourself. This has been a big challenge for us, to change that mindset. Folks say that you just need to drink more water because then you sweat and cool off. Here, in many ways, the only way to get the body cool is to get into an air conditioned environment."

The WGBT is released every two hours during the daytime in order to alert commanders to outside conditions. When the WGBT exceeds 90 degrees, the Heat Stress Category is set to 5, the highest level. Commanders with Airmen most susceptible to heat related illness -- those working outdoors in direct sunlight -- are advised to work them in modified shifts, so they have adequate time to cool off.

When the body cannot cool off, the result can be heat related illnesses like heat exhaustion. Symptoms of heat exhaustion are dizziness, lightheadedness, weakness, mood change, upset stomach, vomiting and fainting. More dangerous than heat exhaustion is heat stroke. Symptoms of heat stroke are similar to those of heat exhaustion, with one primary difference. With heat stroke, the body usually stops sweating.

"Heat stroke is when the body gives up on regulating its own temperature," Colonel Hayes said. "When your body gives up, your core temperature begins to climb."

As the body's temperature rises from its average 98.6 F, the potential for internal damage increases. During a typical "warm-up" routine before exercise, the body's core temperature can raise by as much as one point -- which is average too. The medical community considers a core temperature of 101 F to be fever condition, though not necessarily life threatening. At about 104 F, a little over 5 degrees above normal, the body can start to cook itself from the inside out.

"You start seeing organ damage around 104 F," Colonel Hayes said. "Your organs are made out of protein, like an egg. Imagine throwing it into a frying pan. All those organs start cooking."

Colonel Hayes said about 50 percent of those that get heat stroke will die.

"That is why it is so critical when you see somebody go down on the flightline to act quickly," he said. "You can put ice under their armpits, around the neck and near the groin -- areas where there are major circulation points and where the blood vessels are close to the surface. And you have to get them off the tarmac. Get them off that surface and ice them down. If not ice, then cold water."

One key to day-to-day survival in a high temperature environment is acclimatization. Acclimatization is the process your body goes through as it gets used to performing normal functions in a new environment. In a high temperature environment, the body takes about 10-14 days to fully adapt itself.

"One way you acclimate is that you sweat sooner," Colonel Hayes said. "When Airmen first get here, and they are not acclimated, it takes longer for the body to start sweating to cool itself. Roughly over a two-week period, you begin to sweat sooner. Also, the composition of the sweat changes."

Sweat isn't just water. Typically, the body leaks out electrolytes in sweat. Electrolytes -- often mentioned during television ads for sports drinks -- are really just different types of salt. The salts are dissolved in the blood and are contained in all cells. Some electrolytes include sodium, potassium, calcium and magnesium. The body uses electrolytes as conductors, to carry the electrical impulses needed to control the contraction of the heart and other muscles.

As the body becomes acclimated to its new, hotter environment, it learns to retain those electrolytes instead of leaking them out. A dangerous imbalance in electrolyte levels can occur when the moisture and electrolytes lost through perspiration are replaced with only water. The condition, called water toxicity poisoning, is seen most often in athletes, though one case was seen in late 1999 with the death of Air Force basic trainee Micah Schindler. The condition is easily preventable, however. Airmen need simply replace the electrolytes they lose through perspiration.

"We suggest that Airmen drink about one bottle of sports drink to every two bottles of water," Colonel Hayes said. "Also, eating right and eating often help maintain electrolyte levels in the body. Airmen here are given ample opportunity to eat well."

Over the last year, Colonel Hayes said, the medical group has worked to make sure base agencies with Airmen at the highest risk for heat related illnesses can use operational funds to purchase sports drinks for their Airmen.

"That is just one of the many efforts base leadership has taken to look out for our Airmen and to reduce the incidents of heat related illness," Colonel Hayes said. "Airmen need to work to protect themselves from heat related illness as well."

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