Atmospheric Pressure

Atmospheric pressure decreases with altitude; The higher the altitude the more difficult breathing becomes. Moslems say that this is what Allah says. The Quran says that as we ascend to the sky it becomes more difficult for us to breath:

[Quran 6.125] Those whom Allah wants to guide, He opens their chests to Islam; And those whom He wants to leave astray, He makes their chests tight and constricted, as if they are ascending to the sky: Such is the penalty of Allah on those who refuse to believe.

How could an illiterate man who lived 1400 years ago have known that we cannot breath with increased altitude?