Altitude sickness is a series of natural physiological reactions that occur when a person reaches a certain altitude and the body adapts to the changes in air pressure, low oxygen content, and dry air brought about by the altitude.

Symptoms of altitude sickness are common after 2700m above sea level. Rapidly (e.g. by air) reaching 3000m or more, altitude reactions are more likely to occur. 

High altitude reactions generally occur 6-12 hours after arriving at high altitude, with a few occurring 1-3 days later, with recovery generally occurring within 3-7 days, and in severe cases up to 2 weeks or more.

Symptoms of altitude sickness: everyone is different (of course, we can't rule out people who don't react at all), and the most common ones are headaches (the reason for the increase in brain pressure compared to the mainland). Shortness of breath (heart rate increases as the body needs more blood oxygen).

A small number of people have nausea and vomiting (the stomach reacts to changes in air pressure and thin oxygen). A few people have diarrhea (a result of the bowel not working properly).

Others have trouble sleeping, are very excited, and have fever-like symptoms, with aching hands and feet, fever and headaches. In severe cases, there will be dullness of sensation, emotional restlessness, mental hyperactivity, loss of thinking, memory, abnormal hearing, vision, smell and taste, hallucinations, etc. Swelling, shock, or cramps may also occur.

The degree of altitude sickness varies depending on the surroundings and the way you enter the plateau.

1 . Altitude: The higher the altitude you enter, the more intense the plateau reaction and the greater the probability of suffering from various altitude illnesses. Some reports say that the incidence rate is 37%-51% for those below 3500m and 50% for those between 3600m and 5000m.

2. Entry mode: The speed and magnitude of the ascent into high altitude directly affects the strength of altitude sickness. A rapid rise in altitude is stronger than a gradual "step rise" and results in a higher incidence of altitude sickness. In this respect: a train ride is less likely to cause altitude sickness than a flight.

3. Region and season： Similarly, in the same area, areas with good air exchange are stronger than areas with slower circulation (e.g. valleys), and daytime is better than nighttime. Seasonally, there is a higher incidence of illness in winter than in other seasons when one enters high-altitude areas.

This is because the temperature drops in the winter and with it the atmospheric pressure, which further reduces the oxygen content; moreover, the cold stimulates the metabolism, increases the body's oxygen consumption, and makes it easier to complicate respiratory infections.

The fact that people who live regularly at high altitudes are healthier and live longer than others, and have a relatively low rate of obesity, is perhaps an added advantage.