Perhaps the most frustrating aspect of altitude sickness is the randomness with which it can occur. The same hiker, who has no problem at 10,000 feet one year, might suffer at 8,000 feet the next. Some people go decades without symptoms and then, suddenly, begin to have problems.
The consensus is that most hikers will be fine until they get above 8,000 feet, but that’s no comfort on the JMT. Although the lowest point on the trail is 4,000 feet, after the first day or two you’ll be spending much of the first half of the trail above 9,000 feet (assuming you start at Happy Isles, the northern terminus). During the second half you’ll spend most of your time above 10,000 feet. There’s just no way to avoid the potential of altitude sickness.
There really isn’t a foolproof way to avoid it, either, although there are some common sense things you can do. These are roughly in the order of effectiveness (most effective, first):
Transition as slowly as possible. If you are coming from lower elevations, and if your schedule allows it, spend a couple of days in Yosemite Valley (4000 feet), then a couple days at Tuolumne (8600 feet), prior to starting your hike. While you are there resist the temptation to do a bunch of day hiking. The idea is to ease your body into handling the low pressure and thin air, not to shock the body into compliance.
Hike southbound. If you start in Yosemite Valley, you will postpone the highest part of the trail for a week or more, depending on your hiking speed. If you can hike ten-to-twelve miles a day, you should be able to end each day of the first week below 10,000 feet. On a few nights you should be able to get below 9,000 feet. That will be a tremendous help, because you are going to want to…
Sleep as low as possible. Himalayan adventurers have known for years that the best technique is to “climb-high, sleep-low.” Learn from them and plan your campsites accordingly.
Stay well hydrated. Start several days before you leave for the hike. Even on the days you are acclimating you are going to lose water much faster, through respiration, because of the high elevation. Once you get on the trail, DRINK! One of the worst things you can do is to delay your water intake because you don’t want to stop and purify water. Your goal should be for your urine to be almost clear. If it’s a nasty, dark yellow, you’re asking for trouble. Last, stay away from alcohol—few things will dehydrate you faster. The week before the hike and the first on the trail would be good times to abstain.
(Hydration is essential, but there is no reason to overdo it; this is the wrong time to over-hydrate and cause electrolyte problems.)
Eat and rest well. Once you’re on the trail you’ll quickly learn how interconnected the human body is. One ailment can exacerbate another very easily. For example, you might have a mild (and almost unnoticed) case of altitude sickness that can dramatically worsen if you don’t properly replace the calories you’re burning, and you don’t get a good night’s sleep. Think of yourself, while on the trail, as a professional athlete, and take care of your body accordingly.
Consider medication. The drug most often prescribed is acetazolamide, also known as Diamox. Most doctors recommend that you begin taking the medication one or more days prior to going above 8,000 feet. There is substantial difference of opinion regarding the dosage, and there are even some alternatives. Do some web searching and talk to a doctor who is familiar with elevation sickness. (One side-effect I can attest to from the Diamox is tingling hands. I know that sounds pretty benign, but I found it quite unpleasant.)
Next week, what to do if you start to exhibit symptoms on the trail.
Good hiking, Ray