Someone at 5,000-7,500 meters (16,500-24,500 feet) above sea-level may faint and go into a coma because of breathing difficulties. That explains the presence of oxygen equipment in airplanes. There are also special systems that regulate air pressure when planes fly at 9,000-10,000 meters (29,500-33,000 feet) above sea-level.
Anoxia occurs when oxygen fails to reach the tissues. This oxygen deficiency occurs at heights of 3,000-4,500 meters (10,000-15,000 feet). Some people even lose consciousness at such elevations, but can be saved by immediate oxygen treatment.
In the comparison made in the verse below, this physical truth, the changes that take place in the chest with increasing height, is indicated in these terms:
"فَمَن يُرِدِ اللَّهُ أَن يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلْإِسْلَامِ ۖ وَمَن يُرِدْ أَن يُضِلَّهُ يَجْعَلْ صَدْرَهُ ضَيِّقًا حَرَجًا كَأَنَّمَا يَصَّعَّدُ فِي السَّمَاءِ ۚ كَذَٰلِكَ يَجْعَلُ اللَّهُ الرِّجْسَ عَلَى الَّذِينَ لَا يُؤْمِنُونَ" سورة الأنعام:125
"When Allah desires to guide someone, He expands his breast to Islam. When He desires to misguide someone, He makes his breast narrow and constricted as if he were climbing up into the sky. That is how Allah defiles those who have no faith." (Qur'an, 6:125)
