Hiking and Trekking at Different Elevations

Elevation is the general term used when referencing points on the earth's surface above or below mean sea level.  Though the terms "altitude" and "elevation" are often used interchangeably, "altitude" refers more to the distance of an object (such as an airplane) above the earth's surface.  Conditions are much different at higher elevations on the earth's surface when compared to sea level.  The air is thinner, temperatures are nearly always much colder, and the vegetation looks different – provided there is any vegetation at all.  In places near the earth's equator where it is typically warm, there is perpetual snow on some of the mountain peaks of Africa and South America due to the high elevation.  The city of Cerro de Pasco in Peru lies well within the southern tropical zone, yet because it sits at 14,200 feet elevation, the average annual temperature is only 42° F.  For hiking and trekking, the elevation of a particular area determines what type of clothing will be worn, how long food will be cooked (on overnight treks), and how intense the solar radiation will be.

Humans are affected greatly by high altitude and high elevation.  As the altitude above sea level increases, the atmospheric pressure decreases, and every breath of air contains fewer and fewer molecules of oxygen.  One must therefore work harder to obtain oxygen by breathing faster and deeper.  As a matter of interest, the boiling point of human blood occurs at approximately 62,000 feet (18,900 meters) elevation.  The International Society for Mountain Medicine (ISMM) breaks high altitude on the ground into three categories:

• High Altitude:  5,000 to 11,500 feet (1,500 to 3,500 meters)
• Very High Altitude:  11,500 to 18,000 feet (3,500 to 5,500 meters)
• Extreme Altitude:  Above 18,000 feet (5,500 meters)

Since altitude-related illnesses don't typically occur below 8,000 feet, there is not much concern for activities in this range.  Higher up, however, normal physiologic changes occur in every person who attains a certain "threshold altitude."  This "threshold altitude" varies greatly depending on the person, their health condition, and the elevation at which they live at.  For a person who's lived most of his life at 4,500 feet elevation, 12,000 feet seems to be my threshold for noticing the altitude when hiking.  Below is a table I have compiled that indicates the varying average pressures and temperatures at different altitudes from sea level.

Note:  While sources I researched put the start of the "Death Zone" anywhere from 7,000 to 8,000 meters, I went with an average of 7,500 meters (24,606 feet).

The most common type of altitude-related illness is Acute Mountain Sickness (AMS), which is a mild form of altitude sickness.  It occurs when the body hasn't had sufficient time to acclimatize to higher altitude, and may not become noticeable until you have been at the higher altitude for several hours.  The faster you climb to high altitudes, the more likely you are to get AMS.  Symptoms may include the following:

• Difficulty sleeping
• Dizziness or light-headedness
• Fatigue or weakness
• Headache
• Loss of appetite, nausea, or vomiting
• Rapid pulse
• Shortness of breath with exertion

AMS can eventually progress to High Altitude Pulmonary Edema (HAPE) and High Altitude Cerebral Edema (HACE), which are both life-threatening.  The best prevention of any altitude-related illness is to ascend to higher altitudes slowly (no more than 2,000 feet of gain per day above 8,000 feet) and to descend to lower altitudes whenever altitude sickness symptoms appear.