Altitude sickness or acute mountain sickness (AMS) or mountain sickness is the inability of our body to adjust with the low levels of oxygen at high altitudes, here’s what considered as high-
High altitude: 1,500–3,500 metres (4,900–11,500 ft)
Very high altitude: 3,500–5,500 metres (11,500–18,000 ft)
Extreme altitude: above 5,500 metres (18,000 ft)
The most extreme accent I have done so far is around 7000 ft in a day, of which approximately 3000 ft were done on a bike and rest of the 4000 ft by trekking, I was walking uphill from 7:00 AM in the morning till 6:00 PM in the evening.
That day I was pushing myself so hard, to the point that I messed up my left leg very well which took nearly a month to get better.
Being totally exhausted, when I tried sleeping that night in the campsite, I started feeling like I was going to catch a fever because my body temperature was quite high, but it came back to normal after a few minutes of just laying down.
At first, I didn’t want to believe that I got hit by Altitude sickness but after doing a ton of research about it, I now know that I was brushed by Altitude sickness that day. I spent that night at 12500 ft of altitude after literally getting lost for around 3 hours at approximately 13500 ft (which also made me mentally tired).
The next day I woke up with an excruciating headache and pain in my left leg, the headache felt more like a muscular type, connected with my neck muscles, so I thought that one possible reason for it could be my heavy backpack.
My headache went away after doing some stretching exercises, even after having all types of medicine like Diamox, Paracetamol… I chose not to take them.
On my second trek, the highest altitude I reached was 12500 ft and I gained around 6000 feet of altitude in a day. After reaching the valley I pitched my tent and tried to sleep but was unable to do so. This time it was mainly because of the fear of camping alone, but I did experience a mild headache for a few minutes which was due to the altitude.
We camped at 3600 meters on my recent Deo Tibba base camp trek where I again experienced a mild headache, 3 of our group members also got hit by Altitude sickness but their case was more severe as they were showing clear and obvious symptoms like nausea, hallucinations, headache, body weakness etc, reason being them living at sea levels their whole life and then climbing up to a high altitude in a good pace.
I never cared much about Altitude sickness initially but after doing some serious research on the subject I learned how important it is to have complete knowledge about it.
AMS is caused by going up high too fast and can be fatal if all the warning signals are ignored
If you have not done a high altitude trek ever before and you have lived near sea level your whole life, then you should not choose to go, let’s say something higher than 4000 meters on your first trek, choose a lower-high altitude trek instead (somewhere near 12500 feet) to see how your body responds to lower levels of oxygen in the atmosphere, forget anything above 5000 meters.
Anything above 5000 meters is a dangerous territory for a new trekker because at that altitude AMS can quickly turn into HACE (High-altitude cerebral edema) or HAPE (High-altitude pulmonary edema), both being fatal conditions if not taken care of immediately.
Even if you have done many high altitude treks in the past you still need to acclimatize before going further up.
India Hikes has a very informative archive for Altitude sickness and you should read it before going on your next high altitude trek.
I have bought a pulse oximeter to test my oxygen saturation levels at high altitude but I am yet to test that on me and some locals (shepherds), to compare my readings with theirs.