Altitude sickness is the most significant health risk on Nepal's high-altitude trekking routes. It is also the most misunderstood. Many first-time trekkers believe that being physically fit protects them from altitude illness — it does not. Altitude sickness is a physiological response to reduced atmospheric oxygen, not a reflection of fitness, age, or determination. The world's strongest athletes have been evacuated from Nepal's mountains. Understanding how altitude sickness works, recognising its symptoms, and following the correct acclimatisation protocol are the most important preparations any trekker can make.

How Altitude Affects the Body

At sea level, each breath delivers approximately 21% oxygen at a normal atmospheric pressure. At 5,000 metres — roughly the altitude of Everest Base Camp — that same percentage of oxygen is delivered at about half the atmospheric pressure, meaning each breath carries approximately half the oxygen molecules available at sea level. The body compensates by breathing faster, increasing heart rate, and over days and weeks producing more red blood cells to carry the available oxygen more efficiently. This process of physiological adaptation is called acclimatisation, and it takes time — time that cannot be compressed by fitness training, willpower, or medication alone.

The Three Forms of Altitude Illness

Acute Mountain Sickness (AMS)

AMS is the most common form of altitude illness and affects a significant proportion of trekkers who ascend above 3,000 metres without adequate acclimatisation. It typically develops within six to twelve hours of arrival at a new altitude. Symptoms include a persistent headache (the most reliable early indicator), nausea and loss of appetite, fatigue disproportionate to the day's effort, dizziness, and poor or restless sleep. Mild AMS is very common and not dangerous — the critical factor is monitoring the trajectory of symptoms.

If mild AMS symptoms improve within twelve to twenty-four hours of rest at the same altitude, the body is acclimatising successfully and ascent can resume. If symptoms worsen despite rest, or do not improve after twenty-four hours, descent of at least 300-500 metres is required. The golden rule is absolute: never ascend with AMS symptoms.

High Altitude Cerebral Edema (HACE)

HACE is a severe and life-threatening progression of AMS in which fluid accumulates in the brain. It manifests as loss of muscular coordination (ataxia — the inability to walk a straight line heel-to-toe), severe confusion or altered mental state, extreme fatigue, and eventually loss of consciousness. HACE requires immediate descent of at least 500 metres as the primary treatment. Dexamethasone (8 mg initial dose, then 4 mg every six hours) can stabilise the patient for descent. Supplemental oxygen is administered where available. Helicopter evacuation is appropriate if descent is not immediately possible.

High Altitude Pulmonary Edema (HAPE)

HAPE involves fluid accumulation in the lungs and is statistically the most deadly form of altitude illness. It can develop without significant preceding AMS symptoms, making it particularly dangerous. Symptoms include extreme breathlessness at rest or with minimal exertion, a productive cough (initially dry, progressing to pink or frothy sputum), extreme fatigue, and a crackling sound in the lungs detectable by stethoscope or even by placing an ear against the patient's back during breathing. Resting heart rate and breathing rate are markedly elevated. Like HACE, HAPE requires immediate descent as the primary treatment. Nifedipine (30 mg slow-release) can assist if available. Supplemental oxygen significantly reduces symptoms. Delay is dangerous — HAPE deaths occur because descent is postponed while waiting to see if the patient improves.

The Acclimatisation Schedule

Proper acclimatisation is straightforward in principle: ascend gradually, rest regularly, and follow the "climb high, sleep low" principle above 3,000 metres. In practice, this means not gaining more than 300-500 metres of sleeping altitude per night above 3,000 metres, and taking a full rest day for every 1,000 metres of sleeping altitude gained.

EBC Acclimatisation Timeline

The standard EBC itinerary builds two dedicated acclimatisation days into the schedule: one at Namche Bazaar (3,440 m) on Day 4, and one at Dingboche (4,360 m) on Day 7. The Namche acclimatisation day typically involves a hike to the Everest View Hotel at 3,880 metres — higher than the sleeping altitude, stimulating additional red blood cell production, then descending to sleep at 3,440 m. This "climb high, sleep low" cycle is the single most effective acclimatisation technique available to trekkers without medical intervention.

Diamox: What It Does and Whether You Need It

Acetazolamide (Diamox) is a carbonic anhydrase inhibitor that accelerates acclimatisation by stimulating faster and deeper breathing, which increases blood oxygen saturation. It does not eliminate the risk of altitude sickness; it reduces it by helping the body adapt more quickly to high altitude.

The standard prophylactic dose is 125-250 mg twice daily, beginning one to two days before ascending above 2,500 metres. Common side effects include increased urination, tingling in the fingers and toes (paresthesia), and occasionally a metallic taste in carbonated drinks. Diamox is contraindicated in people with sulfa drug allergies.

Whether to take Diamox prophylactically is a personal medical decision best made in consultation with your doctor before travel. Many experienced high-altitude trekkers do not take it; others swear by it. What is universally agreed is that Diamox should not substitute for proper acclimatisation — even with the drug, the same gradual ascent schedule applies.

Practical Prevention Tips

Hydration is one of the most underappreciated altitude sickness prevention measures. Dehydration worsens AMS symptoms significantly — drink three to four litres of water daily on the trail, regardless of thirst. Alcohol and sleeping pills both suppress breathing and worsen altitude illness; avoid them above 3,000 metres. Avoid overexertion on the first day at a new altitude. Eat regular meals even when appetite is reduced — your body needs fuel for acclimatisation. Communicate honestly with your guide every morning about how you feel — early detection of worsening symptoms is far easier to manage than a crisis.

Pulse Oximeters

A pulse oximeter is a small clip-on device that measures blood oxygen saturation (SpO2) through the fingertip. It provides objective data about acclimatisation that complements symptom assessment. A healthy SpO2 at sea level is 95-100%. At 5,000 metres, readings of 75-85% are common in well-acclimatised trekkers. Readings below 70% at altitude, or a rapid decline from previous readings alongside symptoms, indicate a problem requiring attention. Our guides carry pulse oximeters and check trekker readings each morning on acclimatisation-critical sections of the route.

When to Descend: Non-Negotiable Signs

Descend immediately, without delay or negotiation, if any of the following occur: AMS symptoms that worsen overnight despite rest; inability to walk in a straight line (ataxia); confusion, disorientation, or unusual behaviour; breathlessness at rest or severe breathlessness with minimal activity; productive cough with any blood-tinged sputum. These are not signs to monitor for another day — they are emergencies. Descent is the treatment. Every additional hour at altitude worsens the condition. Helicopter evacuation from the Khumbu to Kathmandu hospitals takes twenty to forty-five minutes and is covered by most travel insurance policies that include trekking altitude coverage.