Merck Manuals: What Travelers Need to Know About Altitude Illness

RAHWAY, N.J., Sept. 10, 2025 /PRNewswire/ -- Anyone planning a trip to higher elevations needs to think about altitude illness. The lack of oxygen at high altitudes can cause a range of symptoms in people who are not used to these conditions.

In a new editorial from MerckManuals.com, Andrew M. Luks, MD, University of Washington, details what travelers should know about altitude illness and how best to prepare for travel that includes elevations above 8,000 feet.

1. Altitude illness falls into three categories

Generally, there are three conditions related to altitude illness, says Luks.

• Acute mountain sickness (AMS) is the mildest and most common of the three. Symptoms usually develop within six to 10 hours of ascent (increase in altitude) and often include headache and one or more additional symptoms, such as light-headedness, loss of appetite, nausea, vomiting, fatigue, weakness, or irritability.
• High-altitude cerebral edema (HACE) is a rare but potentially fatal condition in which the brain swells with fluid. People with HACE often have headaches, confusion, and unsteady and uncoordinated walking (ataxia).
• High-altitude pulmonary edema (HAPE) is a life-threatening condition where fluid builds up in the lungs. This can develop 24 to 96 hours after a rapid ascent to 8,000 feet or more. This causes difficulty breathing and low blood oxygen levels.

2. Don't overlook AMS as a cause of symptoms

Many travelers may experience a headache, irritability, or fatigue after a long flight or car ride. But if you're at a higher elevation, remember AMS could be the cause, and watch for more serious symptoms, says Luks.

If you don't feel better with rest, water, and over-the-counter headache medicine, it may be time to talk to a medical professional and get to a lower elevation, says Luks.

3. The altitude always wins

One of the biggest misconceptions about altitude illness is that being in good shape reduces your risk, says Luks. That's not true. What's more, other medical conditions like asthma or high blood pressure (hypertension) don't make people more likely to suffer from AMS. Altitude tolerance can vary from person to person, and you likely won't know how you handle different altitudes until you're exposed. If you're traveling at higher altitudes, Luks recommends operating under this assumption: the altitude always wins.

4. Staying safe starts with smart planning

Ascending too quickly is the primary risk factor for altitude illness. Build time to acclimate to higher altitudes into your trip planning, says Luks. Give yourself and your group time to get used to the new conditions. You may be anxious to start skiing or hiking, but it's often better to take it easy on that first day and make sure everyone is comfortable with the higher elevation.

To read more of Luks' insights on altitude illness, visit this editorial on MerckManuals.com.

About The Merck Manuals and MSD Manuals

First published in 1899 as a small reference book for physicians and pharmacists, The Merck Manual grew in size and scope to become one of the world's most widely used comprehensive medical resources for professionals and consumers. As The Manual evolved, it continually expanded the reach and depth of its offerings to reflect the mission of providing the best medical information to a wide cross-section of users, including medical professionals and students, veterinarians and veterinary students, and consumers. In 2015, The Manuals kicked off Global Medical Knowledge 2020, a global initiative with the goal to make the best current medical information accessible by up to three billion professionals and patients around the world by 2020. The Manuals achieved that goal, and today its medical information is available in nearly 250 countries and in 14 languages. It's continuing its ambitious mission through outreach, education and creating new reliable medical resources. For access to thousands of medical topics with images, videos and a constantly expanding set of resources, visit MerckManuals.com or MSDManuals.com and connect with us on social media: For Consumers in the U.S. and its territories: X (formerly Twitter) and Facebook. For Professionals in the U.S. and its territories: X (formerly Twitter) and Facebook.

About Merck

At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than 130 years, we have brought hope to humanity through the development of important medicines and vaccines. We aspire to be the premier research-intensive biopharmaceutical company in the world – and today, we are at the forefront of research to deliver innovative health solutions that advance the prevention and treatment of diseases in people and animals. We foster a diverse and inclusive global workforce and operate responsibly every day to enable a safe, sustainable and healthy future for all people and communities. For more information, visit www.merck.com and connect with us on X (formerly Twitter), Facebook, Instagram, YouTube and LinkedIn.

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