they experience breathing problems because the mountains have high altitudes.When we go up high the oxygen supply becomes less. So they carry oxygen cylinders with them.
When we go above the sea level the atmosphere pressure goes on decreasing and the the amount of oxygen also decreases at higher altitude. so mountaineers carry oxygen cylinders with them.
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Aryambikhaa Babu answered this
The oxygen content keeps on decreasing as we go higher and higher . This is why mountaineers usually stay in a lower altitude so that they get temporarily adaptd to less oxygen and then start climbing.