What You Need to Know to Avoid Altitude Illness

According to a description from 2,000 years ago, one of the ancient routes between China and Afghanistan crossed two peaks known as the Great Headache Mountain and the Little Headache Mountain, so named for the splitting headaches and vomiting they inflicted on passing travelers and their donkeys. We now know that this was acute mountain sickness caused by the scarcity of oxygen in thin, high-altitude air. But that doesn’t necessarily mean that modern travelers know how to avoid suffering the same fate when they head to the mountains.

The Wilderness Medical Society has just published a 2019 update on its guidelines for the prevention and treatment of acute altitude illness. There are, of course, about a billion websites that offer advice and theories of varying reliability about how to stay healthy in the mountains. The WMS guidelines, assembled by a team of ten experts from across the country and published in Wilderness & Environmental Medicine, offer a no-nonsense look at what we actually know, what we think we know, and how strong the evidence is for each statement.

The Basics

The guidelines deal with acute altitude illness, which is what can happen when you ascend rapidly to an elevation that you’re not used to, as opposed to the chronic mountain sickness that can afflict people who live permanently at high elevations. For the most part, problems start above about 8,200 feet (2,500 meters), though “susceptible individuals” can develop symptoms as low as 6,500 feet (2,000 meters). The initial symptoms are pretty non-specific: mild headache, fatigue, nausea, dizziness, and so on, so it’s important to also consider other possible causes like dehydration and hyponatremia.

There are three basic flavors of acute altitude illness. The first and most common is acute mountain sickness (AMS), whose symptoms are a headaches plus at least one of the following: nausea/vomiting, fatigue, lack of energy, and dizziness. At the extreme end of the spectrum, AMS can progress to high-altitude cerebral edema (HACE), in which the brain becomes dangerously swollen with fluids. One way to think about the difference between AMS and HACE is that AMS produces a bunch of symptoms (which, in medical parlance, are feelings reported by a patient), while HACE also produces signs (which are manifestations that can be observed independently by a doctor). Feeling dizzy is a symptom of AMS, but if the dizziness is so bad that you flunk the type of balance test you’d get in a sobriety check, that indicates ataxia, a possible sign of HACE.

The third variation, which is distinct from the other two, is high-altitude pulmonary edema (HAPE), in which capillaries damaged by altered pressure leak fluid into the lungs. AMS is very common and generally dissipates after a couple of days; HACE and HAPE are potential killers that require urgent treatment and generally signal the end of your trip.

How to Prevent Altitude Illness

The most important and effective way of avoiding all forms of altitude illness is to ascend gradually. Even before you get to potentially dangerous elevations, it may help to spend a night at moderate elevation—for example, stopping overnight in mile-high Denver before driving higher into the mountains. Once you get above 10,000 feet (3,000 meters), WMS guidelines recommend that you aim to increase your sleeping elevation by no more than about 1,500 feet (500 meters) per day, and include an extra acclimatization day once every three to four days. If logistics force you to ascend more than that in one day, try to add an extra acclimatization day to keep the average ascent below 1,500 feet per day.

I should note here that many sources suggest a more conservative ascent rate of 1,000 feet (300 meters) per day. The WMS team apparently didn’t find any science supporting this rule of thumb—but, as someone who prefers to err on the side of caution when my precious vacation days and dollars are at stake, that’s the rule I’ve tried to follow.

Drugs are another option, depending on the risk profile of both the traveler and the trip. If you have a prior history of altitude illness, that’s the best predictor of your future susceptibility. And even if you don’t have a prior history, you might want to consider preventive drugs if you’ll be in a remote area where help is far away. The rate of ascent matters too: for example, any ascent of Kilimanjaro that takes less than seven days is considered high-risk for altitude illness.

For AMS and HACE, the WMS says that the first-choice preventative drug is acetazolamide (Diamox), started the day before you begin ascending and continuing until two days after you reach your highest altitude or you start descending, whichever comes first. The typical adult dose is 125 mg every 12 hours. If you’re allergic to acetazolamide, the second-line drug is dexamethasone. You’d only take both drugs simultaneously in rare circumstances, like military or rescue teams that are ascending rapidly to higher than 11,500 feet (3,500 meters) with no acclimatization.

For HAPE, you should only take preventive drugs if you have a history of the condition. In that case, the first-line drug is nifedipine, which you start the day before ascent and continue for four to seven days after reaching the highest elevation, or until you start descending.

How to Treat Altitude Illness

The best treatment is also the simplest: head down the mountain. Usually heading 1,000 to 3,000 feet (300 to 1,000 meters) downhill will clear up the symptoms. If you’ve just got AMS, you don’t necessarily need to descend, though you should at the very least stop ascending. You can take acetaminophen (Tylenol) or ibuprofen for the headache, and an antiemetic like Gravol for nausea. But if the symptoms worsen, or if they haven’t cleared up after a day or two, it’s time to head down.

If you’ve got HACE or HAPE, there are some more complex countermeasures like supplemental oxygen and portable hyperbaric chambers. For HACE, dexamethasone is a recommended treatment (as opposed to prevention, where acetazolamide is preferred). For HAPE, nifedipine (the same drug used for prevention) might be useful for treatment if you can’t descend and don’t have access to supplemental oxygen, but the evidence on its effectiveness is weak. The decision matrix gets much more complicated for these more serious conditions, and the real bottom line is that you’ve got to get expert help and/or get down the mountain immediately if you think you have either one.

There are a bunch of other drugs, herbs, and harebrained schemes considered in the review, which have either negative, conflicting, or nonexistent evidence. Don’t bother with ginkgo biloba, skip the chewed coca leaves and the mini-cans of oxygen, and try Viagra for HAPE only when all the other options (including descent) are unavailable. 

The Next Big Thing?

One relatively new option does get serious consideration: personal altitude tents for pre-acclimatization. The idea has been tested only once in a placebo-controlled study, which found a lower incidence of AMS in people who prepared for a simulated trip to altitude by sleeping in reduced oxygen compared to those at normal oxygen levels. The overall evidence gets a rating of 2B (meaning a weak recommendation based on moderate-quality evidence where the benefits are closely balanced with the risks and burdens), but in practice it’s getting a lot more popular in the real world. It’s the approach Kilian Jornet used for his double Everest ascent in 2017 (as I described here); it’s how Roxanne Vogel tackled Everest in a GU-sponsored mission, going from door-to-summit-to-door from Berkeley, California, in just 14 days; and prominent Everest guides like Adrian Ballinger and Lukas Furtenbach have adopted the method.

While there’s not much science available to tell us what the best tent protocol is, the WMS guidelines suggest that short or infrequent altitude exposure, including exercising in the tent, isn’t likely much help. Instead, you need long exposures of at least eight hours per day for at least several weeks leading up to the trip. And you need to ensure that you don’t mess up your sleep so badly that you wipe out any potential gains.

In the end, probably the most important point to take away from all this—one that the WMS hammers over and over—is that there are no guarantees. You can follow all this advice to the letter, ascend gradually, dose yourself perfectly, and even spend your nights in an altitude tent—and you still might end up with a crushing headache on day two of your trip. People respond very differently to altitude and adjust to it at different rates. But as far as science is concerned, these are the best ways to minimize your risk.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.