Oxygen at Altitude is a Major Consideration in Wilderness Medicine

Wilderness medicine covers an enormous area of study and practice. From anaphylaxis to zoonotic diseases, it covers a lot of glossary. From deep-water diving to high-altitude mountaineering, it covers a lot of territory. 

In this post, we focus on four terms in that glossary to which every outdoor educator and backcountry/wilderness guide should be aware:

Oxygen is a gas comprised of two oxygen molecules bound together (O2), that’s essential for the survival of most earthly lifeforms. You can live without food for several weeks, without water for several days, but only a few minutes without oxygen.

Atmosphere is the gaseous mass that surrounds a celestial body like Earth. Earth’s atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.93 percent argon, and 0.04 percent carbon dioxide, along with trace amounts of other gases and variable water vapor. This gaseous “envelope” is divided into layers ranging from the troposphere to the thermosphere, with the percentage of oxygen remaining the same in each of those layers.

Altitude is the vertical distance of an object above a reference point — traditionally sea level. At higher altitudes, air pressure decreases, thus oxygen is less concentrated. As a result, you’re taking in less oxygen with each breath. For example, consider the difference between a tablespoon of cream and a tablespoon of whipped cream. With whipped cream, you’re getting less cream per tablespoon. Likewise, when you breathe air that’s less pressurized (at higher altitudes), you’re getting less oxygen per breath.

Pressure is the force applied perpendicular to the surface of an object. Atmospheric pressure factors in the amount of atmosphere above a certain point exerting a downward force on that point. Generally, the lower the altitude the higher the atmospheric pressure. Likewise, the deeper you dive beneath the surface of a lake or ocean, the greater the pressure you’ll feel.

Specifically, we’re going to focus on changes in oxygen availability at altitude, and the potential health consequences of oxygen deprivation.

Recognizing the Health Consequences of Insufficient Oxygen

You need more than “just enough” oxygen to stay alive. You need enough to stay healthy, which is why the nurse at your doctor’s office clips an oximeter to the tip of your finger at the start of every appointment. The nurse wants to know whether your body is sufficiently saturated with oxygen.

Ideally, oxygen content in the blood, measured as saturation of peripheral oxygen (SpO2), should be between 95 and 100 percent consistently. When it drops below 95 percent, you start to experience hypoxemia (oxygen deficiency), and your health can start to suffer:

• Mild hypoxemia (90–94 percent SpO2) may cause increased heart rate, shortness of breath, fatigue, and lightheadedness.
• Moderate hypoxemia (80–89 percent SpO2) may result in confusion or disorientation, increased respiratory rate, and cyanosis (skin, lips, or nails turn blue).
• Severe hypoxemia (below 80 percent SpO2) can cause labored breathing, loss of consciousness, organ damage, and respiratory failure. Without sufficient oxygen, cells die, tissue dies, organs die, and organ systems fail, followed shortly by the death of the organism. This process of inadequate tissue perfusion of oxygen at the cellular level of is known as shock.

Understanding the Relationship Between Pressure and Oxygen

Air in general, and oxygen as a component of air, are at the beck and call of pressure. Gas molecules move from areas of high pressure to areas of low pressure, which is what causes wind. Heat also impacts atmospheric pressure. Generally, atmospheric pressure is higher in hotter locations at lower altitudes. Conversely, pressure is lower in cooler locations at higher altitudes. I’m thinking maybe that’s why the song “Under Pressure,” by the British rock band Queen and singer David Bowie, is on an album called Hot Space?

The lower the pressure, the less concentrated the gas molecules are — including oxygen — and the less oxygen you inhale with each breath. In other words, the higher you climb, the less oxygen you’re getting with each breath, which is why your respiratory rate and heart rate increase, which leads to fatigue.

Dalton’s Law (aka The Law of Pressures) helps explain how the concentration of oxygen (and other gases) changes at higher altitudes. The law states that the “total pressure of a mixture of gases is the sum of the partial pressures of the individual gases” in the mixture. In simpler terms, at higher altitudes, air pressure decreases, because there is less air pushing down from above. Although the percentage of oxygen remains the same (21 percent), the total number of air molecules (including oxygen molecules) decreases due to the lower atmospheric pressure.

Keep in mind that Dalton’s Law is easily confused with Cole’s Law, which has to do with the amount of dressing covering thinly sliced cabbage. (Coleslaw, get it? Just checking to see if you’re still awake!)

Gauging the Danger at High Altitudes

The fact that oxygen is less concentrated at higher altitudes introduces an important question — How high is too high? At what altitude do you start feeling sick?

The highest mountain, as measured from sea level, is Qomolangma, otherwise known as Mount Everest. It’s located in the Himalayas, on the border between Nepal and Tibet (China). It’s just over 29,000 feet, which is just under 9,000 meters. 

The altitude necessary to cause harm is complicated by the fact that according to some definitions of “altitude,” it doesn’t start until 5,000 feet above sea level, with some sources claiming that no harm is possible until you reach 6,000 feet. According to most experts, altitude sickness can start to affect travelers at about 8,000 feet. Symptoms of altitude sickness include the following:

• Headache
• Nausea
• Dizziness
• Fatigue
• Loss of appetite
• Trouble sleeping
• Shortness of breath
• Confusion
• Loss of consciousness

However, a couple other factors determine the point at which climbers begin to experience symptoms:

• Starting elevation: If you’re starting in Seattle, at sea level, climbing to 8,000 feet on Mount Rainier (a mountain that reaches about 14,410 feet in elevation), you’re going to feel more of an impact than if were to start from Denver (at 5,000 feet) and climb 3,000 feet higher.
• Latitude: The atmosphere is warmer at the equator and colder at the poles, so atmospheric pressure is higher the nearer you are to the equator. For instance, if Mount Everest were further north or south than where it is, it couldn’t be climbed without supplemental oxygen.

The various types of altitude sickness is fodder for another blog. Just remember the obvious: The cure for feeling poorly at higher altitudes is to head downhill.

The cure for feeling poorly at higher altitudes is to head downhill.

Although only a small portion of Earth’s surface is high enough in elevation to cause low-pressure induced oxygen deprivation (hypobaric hypoxia), those of us who love wilderness adventure and practicing wilderness medicine have a special place in our hearts for locations with drastic ups and downs. An understanding of oxygen availability in these locations enables us to provide better guidance and care for our fellow wilderness adventurers.

If you’re interested in learning more about the role of oxygen at altitude in the context of wilderness medicine, we recommend checking out one of our Wilderness Medicine courses.

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About the Author: Todd Mullenix is the Director of Wilderness Medicine Education at The National Center for Outdoor & Adventure Education in Wilmington, North Carolina.

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