As altitude increases, the number of molecules of air decreases, which results in a reduction in the pressure that the gases exert. This fact is very important since gases (oxygen) diffuse based upon pressure gradients. The lower pressure of oxygen in the air results in less oxygen in the blood and the muscle tissues. At altitude, the following conditions are present:

1. Air temperature decreases about 2°F for every 500 feet.
2. Colder air holds less moisture, so the air is drier.
3. Increased solar radiation.

The sum of these changes can result in dehydration, especially during exercise, and sunburn, despite the fact that temperatures are low.

When exposed to moderate altitudes (greater than 7,000 feet), many individuals experience symptoms that have been referred to as "mountain sickness," including any or all of the following: headache, nausea, labored breathing, insomnia and a decreased appetite. These symptoms may occur within a few hours of exposure to altitude and usually dissipate within 24 to 48 hours.

Some people may experience these symptoms at altitudes as low as 6,000 feet; but most will not feel the effects until they reach altitudes of 14,000 feet and higher. There is no treatment for this condition, other than a more gradual exposure to altitude.

Due to the decrease in oxygen in the atmosphere, there is less oxygen saturation in the blood, and thus the body has to compensate for this decrease in oxygen concentration. The lower levels of oxygen in the blood (hypoxia) result in an increased breathing rate. The first few weeks of altitude exposure cause a decrease in the plasma volume and an increase in heart rate at both rest and submaximal exercise.

The major change that occurs with acute altitude exposure is a reduction in cardiovascular capacity. There appears to be little change in cardiovascular capacity below 5,000 feet; however, there is approximately a 10 to 11 percent reduction in cardiovascular capacity for every 3,000 feet beyond this altitude.

Therefore, at an altitude of approximately 7,000 to 8,000 feet, an individual can expect to be at only 90 percent of their sea level capacity. Assume someone is running at sea level at an eight minute/mile pace amounting to an intensity level of 70 percent of their maximal. The same pace, run under the same conditions, at an altitude of 7,000 to 8,000 feet would now be at an intensity of 80 percent. People should decrease the workload, resistance or pace when exercising at altitude locations, especially after acute (immediate) exposure.

Within 24 to 48 hours after exposure to altitude, the hormone erythropoietin is secreted from the kidneys. This hormone stimulates the bones to increase red blood cell production, thereby enabling the blood to carry more oxygen. At moderate altitudes of 7,000 feet, this acclimatization may take about two weeks. For higher altitudes, acclimatization will take several weeks.

The increased red blood cells, resulting in higher hemoglobin values, result in better working capacity.

After several weeks of acclimatization, an individual can perform cardiovascular activities much better than after acute exposure, but still the ability to perform at altitude is considerably lower than at sea level. Athletes who must compete at altitude should arrive at the altitude in sufficient time for acclimatization to occur.

Altitude presents a unique challenge to the cardiovascular system. As long as individuals understand their limitations, they can learn to enjoy exercises such as mountain climbing, cross-country skiing and hiking at altitude.

Robert Girandola, Ph.D., is a professor in the Department of Kinesiology at the University of Southern California in Los Angeles, Calif