How high can you go?
I am not a fitness buff. I am not even the outdoor-type. But I have summited Mount Fuji (3776m) and gone up to altitude as high as 5248m without falling sick.
Once you are at an altitude of about 2000m, you will inevitably feel breathless when you strain yourself. But this is not Acute Mountain Sickness (AMC) or Chronic Mountain Sickness (CMS), maybe not yet. Hypoxia is a condition in which the body as a whole or a region of the body is deprived of oxygen resulting in what we generally called ACS/CMS. When ACS/CMS becomes serious resulting in fluid build-up in the brain (HACE) or in the lungs (HAPE), it is fatal if not attended to immediately.
As a matter of fact, Acute Mountain Sickness (ACS) or Chronic Mountain Sickness (CMS) has little to do with a person's level of fitness, gender or age. Herein lies the danger. It is therefore important to be prepared, recognise the signs and symptoms and know what to do when faced with it.
Now, if you are going to Tibet (which is an interesting place for holiday), most of the time you will be above 2000m. The altitude of Lhasa is already 3650m. The first thing to do is acclimatisation. When you arrive at Lhasa, for example, you do nothing on the first day. Just rest. Don't be a hero and insist on venturing out. Some people think that they have paid a lot of money for their holiday and they are not going to idle around and waste a day or two. You are only asking for trouble if you do so.
Just be mindful that you are at high altitude and do things slowly; don't run, don't jump, don't bent down to pick up something abruptly. I even tell people to brush their teeth slowly.
Some people think that when going to Lhasa, Tibet, taking the train will enable them to acclimatise better as the ascent is more gradual compared to taking a plane. Sure the ascent is more gradual but the trains are pressurised and therefore do not help in acclimatising.
Many people believe in consuming the Chinese herbal medicine HongJinTian as an antidote for ACS. I am skeptical. But if you believe in it, please go ahead; I cannot underwrite your life. I would suggest that before your trip go and see a GP and get a prescription of Diamox.
If you experience giddyness, nausea, headache, perspiration, light headedness, prolonged breathlessness, consume the medicine based on the doctor's prescription. If the symptons still persist, go to see a doctor as soon as possible. The doctor will most likely put you on a drip. Now, if the doctor recommends that you evacuate to a lower level, comply as soon as possible. Even Everest climbers have died for refusing to follow advice to evacuate.
You may also come across tourist shops selling oxygen cannisters that look like insecticide spray. I am doubtful of its content. Again, I don't want to underwrite your life; go ahead and buy if you want it as a precaution. If you are travelling with an organised tour, make sure you that the tour coach carries a real oxygen bottle, the type used in hospitals.
One final point, avoid consuming alcohol at high altitude.