Preacclimatization
Pre-acclimatization is the process of exposing the body to altitude or simulated altitude before a high-altitude trip. For trekkers heading to Nepal, it may help the body begin adapting before arrival, but it should be viewed as a supplement to — not a replacement for — a gradual ascent and a properly paced itinerary.
What Is Pre-Acclimatization?
At high altitude, the reduced partial pressure of oxygen places additional stress on the body. Acclimatization is the collection of physiological changes that helps the body function more effectively in this lower-oxygen environment.
Pre-acclimatization attempts to begin some of this adaptation before travel. This can involve spending time at real altitude or using a hypoxic training system that reduces the oxygen concentration of the air being breathed.
The most important point
Pre-acclimatization does not make a fast ascent safe. Even if you have used simulated altitude before your trip, you still need to ascend gradually, include appropriate acclimatization days and pay attention to symptoms once you are in the mountains.
How the Body Adapts to Altitude
Immediate Adaptations
When you first ascend, the body responds quickly by increasing ventilation and making other short-term adjustments that improve oxygen delivery.
These early changes are especially important during the first several days at altitude.
Longer-Term Adaptations
With more sustained exposure, additional changes develop over days and weeks, including changes in blood volume and eventually increased red blood cell production.
These adaptations can improve tolerance of lower oxygen levels, although maximum physical performance at altitude remains lower than at sea level.
Can Pre-Acclimatization Help?
Pre-acclimatization can be useful, particularly for people who have limited time before reaching higher elevations. Prior exposure to high altitude may reduce the physiological shock of a rapid transition from low elevation to the mountains.
Altitude Tolerance
Recent altitude exposure may help the body begin adjusting before a trek, potentially making the initial days at elevation more comfortable.
Sleep & Comfort
Acclimatization generally improves sleep, comfort and submaximal endurance at altitude, although sleep disturbance remains common at higher elevations.
Acclimatization Head Start
Pre-trip exposure may provide a head start, but the degree of benefit varies significantly between individuals and depends on the type, duration and timing of exposure.
Sleeping in a Hypoxic Tent
One form of simulated-altitude training uses a tent or enclosure supplied with hypoxic air. The system lowers the percentage of oxygen in the air while the person sleeps.
This allows several hours of exposure each night without requiring travel to actual altitude. Because sleeping exposure is prolonged, it is one of the more practical ways people attempt pre-acclimatization at home.
The effectiveness of any protocol depends on the simulated altitude, number of hours of exposure, frequency of use and the individual’s response.
Exercise in Simulated Altitude
Some systems deliver hypoxic air through a mask while a person performs light or moderate exercise. This is sometimes used alongside a normal training program.
Hypoxic exercise should not replace the cardiovascular and muscular training needed for trekking. Hiking endurance, leg strength and time spent walking on uneven terrain remain important parts of preparation.
Real Altitude Exposure
If practical, spending time at real altitude shortly before a trip can also help. Current travel-medicine guidance notes that spending at least two nights above approximately 2,750 m / 9,000 ft before a later high-altitude trip can be beneficial, particularly when that exposure occurs relatively close to departure.
This is sometimes called staged acclimatization. For many trekkers, however, geography and travel schedules make this difficult, which is why simulated-altitude systems are sometimes considered instead.
What Pre-Acclimatization Cannot Do
- It cannot guarantee that you will avoid acute mountain sickness.
- It does not make rapid ascent risk-free.
- It does not replace acclimatization days in a trekking itinerary.
- It does not mean symptoms of altitude illness can be ignored.
- It does not guarantee better performance or completion of a trek.
Acclimatization Still Happens on the Mountain
The safest approach to high-altitude trekking remains a gradual ascent. Once above approximately 3,000 m / 9,800 ft, sleeping altitude should generally increase conservatively, with additional acclimatization time built into longer ascents.
Even trekkers who arrive with some previous altitude exposure should monitor how they feel each day. Never ascend to a higher sleeping altitude while experiencing worsening symptoms of altitude illness.
Read our complete altitude acclimatization guide →
Before Using an Altitude Training System
People with heart or lung disease, sleep apnea, pre-existing low oxygen levels or other medical conditions should speak with a physician familiar with high-altitude medicine before using simulated-altitude systems or undertaking high-altitude travel.
Any pre-acclimatization plan should complement — not replace — an appropriately paced trekking itinerary.