When you head into the mountains, it’s important to be aware of the effects that altitude can have on your body and to be able to recognize symptoms of acute mountain sickness.

Approximately 25% of people who ascend to 3,000 meters or 10,000 feet experience acute mountain sickness (AMS), which is the most common symptom of altitude exposure. The incidence increases among those who ascend quickly without allowing time for acclimatization. Symptoms include headache, nausea, vomiting, dizziness, lightheadedness, lassitude, insomnia, loss of appetite, and impaired coordination.

The higher you go, the less oxygen there is in the atmosphere. The body compensates for the lack of oxygen by increasing respiratory rate and cardiac output. This can result in episodes of hypoxia, which is lack of oxygen in the tissues; respiratory alkalosis, which is an imbalance of your body’s pH levels caused by lack of CO2 in your system; and sleep apnea. Long-term compensation includes producing more red blood cells to carry oxygen and developing more dense capillary beds in body tissues, but this process can take months to years.

Mild hypoxia is defined as a reduction in arterial oxygen saturation (SaO2) to below 90%. Symptoms include tachycardia (heart rate above 100), dyspnea (difficulty breathing) on exertion, increased respiratory rate, hyperventilation, and headache. Left untreated, mild hypoxia can progress to moderate or severe hypoxia. 

Moderate hypoxia is defined as a reduction in arterial oxygen saturation (SaO2) between 89-80%. Symptoms include tachycardia, shortness of breath at rest, confusion, amnesia, ataxia (clumsiness), and paralysis.

Severe hypoxia, where arterial oxygen saturation is below 80%, is a medical emergency that requires immediate descent and supplemental oxygen. At this stage, serious neurological deficits such as coma can occur. 

So if you’re heading into the backcountry, be sure to monitor your symptoms and descend if necessary. If left untreated, AMS can progress to more serious conditions such as high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE). 

Always remember, safety first—or third, depending on what kind of crowd you roll with.