Altitude illness is usually filed under expeditions, but the hikers most often caught out are day walkers. A cable car in the Alps delivers you from a valley at 1,000 m to a station above 3,800 m in twenty minutes. A road in the Rocky Mountains takes a car to 4,300 m (14,100 ft). The volcano on Tenerife has a lift to 3,555 m, and the summit is 3,715 m. None of these places require ropes, and all of them put an unacclimatised body in air that it does not handle well. The good news is that the physiology is well understood and the planning is straightforward.
What thinner air actually means
The proportion of oxygen in the air is the same everywhere, about 21 percent. What changes with altitude is pressure, and with it the amount of oxygen forced into the blood with each breath. At 2,500 m (8,200 ft) the barometric pressure is about 75 percent of sea level; at 3,000 m about 70 percent; at 4,000 m roughly 60 percent; at 5,000 m about half. Blood oxygen saturation, which sits at 97 to 99 percent at the coast, typically falls into the low 90s at 2,500 m and the mid 80s at 4,000 m in a person who has just arrived.
The body responds within minutes by breathing faster and deeper, and within hours by raising the heart rate and increasing urination, which concentrates the blood. Over days it makes more red blood cells. The first responses are enough to keep most people functional above 2,500 m; the problem is that “functional” and “comfortable” diverge, and the illness that follows is a matter of how fast you arrived, not how fit you are.
Acute mountain sickness: who gets it and when
Acute mountain sickness, AMS, is the mild form of altitude illness and by far the most common. Studies of visitors arriving rapidly at 2,500 to 3,000 m find that roughly one in four develops it; at 3,500 to 4,000 m without acclimatisation the proportion rises to around half. Symptoms begin 6 to 12 hours after arrival, rarely sooner, which is why a day hiker who goes up and down in eight hours often escapes and the one who sleeps at the high hut does not. The defining symptom is headache, accompanied by at least one of nausea or loss of appetite, fatigue, dizziness and poor sleep. It resembles a hangover, and is often mistaken for one.
Fitness offers no protection; if anything, fit people ascend faster and suffer more. Previous history is the best predictor: someone who was ill at 3,000 m last year will likely be ill at 3,000 m this year. Age, sex and experience matter little. The two severe forms, swelling of the brain and fluid in the lungs, are rare below 3,000 m and usually preceded by untreated AMS, but they are life-threatening and they are the reason the rules that follow exist.
The rules that mountain medicine agrees on
Every guideline reduces to the same handful of numbers, and they concern sleeping altitude, not the height reached during the day:
- Above 3,000 m, raise the sleeping altitude by no more than 300 to 500 m (1,000 to 1,600 ft) per night.
- Take a rest day, sleeping at the same altitude, for every 1,000 m gained.
- Climb high, sleep low: a day trip to 3,500 m from a bed at 2,500 m aids acclimatisation; a bed at 3,500 m does not.
- Never ascend to sleep with symptoms; descend if symptoms worsen or fail to improve after a day of rest.
There is a prescription drug widely used to prevent AMS, which works by speeding up the body’s own adjustment of blood chemistry. It is worth discussing with a doctor before a trip with an unavoidable fast ascent, but it is an aid to the rules above, not a substitute for them. Painkillers treat the headache; they do not treat the altitude.
Planning a day hike that starts high
For the walker who drives or rides to 2,500 m and above, the practical questions are where to sleep and how to pace. If the trip allows it, spend the first night at 1,500 to 2,000 m rather than at the high trailhead; a single night of partial acclimatisation reduces AMS incidence measurably. Do the high day early in the trip only if the sleeping altitude stays modest, and save the high hut, if there is one, for the third or fourth night.
On the hill, expect to be slower. A rule of thumb used by guides is to reduce the expected pace by about 10 percent for every 1,000 m above 2,500 m on the first days, so the ascent rate of 350 m per hour that felt easy at home becomes 300 m per hour at 3,500 m, and a walk that takes six hours in a valley range takes seven. Breathe deliberately on the climb, take the breaks you would normally skip, and treat a headache that starts before noon as information rather than an inconvenience.
The other things that change with height
Altitude does not arrive alone. Air temperature falls by roughly 6.5 C per 1,000 m in average conditions, so the 20 C (68 F) valley morning is 7 C at 3,000 m before any wind is considered, and wind above the tree line is the norm. Ultraviolet radiation increases by about 10 to 12 percent for every 1,000 m, and snow reflects up to 80 percent of it back at the face, so burns on the underside of the nose and the roof of the mouth are a real feature of high snow days. The air is drier, breathing is faster, and thirst is blunted, which means the hydration schedule replaces the thirst signal above 2,500 m. Appetite falls as well, at the moment the body needs carbohydrate most, so small sweet snacks on a timer serve better than a lunch nobody wants.
Weather at altitude also moves faster. Afternoon convection builds over high ground on warm days, and the ridge at 3,000 m is where the storm arrives first; a turnaround time set for early afternoon is a good habit in any mountains and an essential one on a high day trip.
Knowing when to go down
The single most important decision above 2,500 m is the easiest to state and the hardest to make. If a companion has a headache that a painkiller does not shift, cannot eat, and is unusually clumsy, confused or breathless at rest, the treatment is descent, now, by 500 to 1,000 m, accompanied. Do not wait for morning; the severe forms worsen at night. A day hiker has a large advantage here: the car, the lift or the valley path is usually a few hours away, and going down is always available. Using it early is not an admission of anything; it is the whole skill.