Today Scientist has discovered that if we go above the sea-level it would be harder for us to breathe as the amount of Oxygem in upper atmosphere is less which makes it difficult to breathe.
We need Oxygen to survive. If we go above sea-level our lungs will become constricted in order to obtain more and more Oxygen, thus producing more Red Blood Cells (RBCs). When you are higher up the air gets thinner, this means that you are not taking in as much oxygen as normal resulting in difficulty of breathing. 
As we go above the sea-level the Oxygen in the air decreases and there are different air-pressures at different levels above sea-level

“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)

Quran tells it about 1400 years ago mentioning that “Allah tell those who have no faith (through his miracles)”. However, Science discovered Oxygen in 1774 by Swedish pharmacist Carl Wilhelm and Joseph Priestley.

After this more research was done on Oxygen and it was found out that the level of Oxygen above sea-level decreases as person get high. It should be noted that to tell that there is less Oxygen above sea level, Oxygen Gas was first to be discovered. Same early and late research includes

1- In the 1500 Leonardo da Vinci observed that a fraction of air is consumed in respiration and combustion.
(Mary Elvira Weeks, The discovery of the elements. IV. Three important gases., J. Chem. Educ., 1932, 9 (2), p 215.)

2- In 1665 Robert Hooke noted that air contains a substance which is present in potassium nitrate [potassium nitrate releases oxygen when heated,] and a larger quantity of an unreactive substance [which we call nitrogen].
(Mary Elvira Weeks, The discovery of the elements. IV. Three important gases., J. Chem. Educ., 1932, 9 (2), p 215.)

3- In 1668 John Mayow wrote that air contains the gas oxygen [he called it nitroarial spirit], which is consumed in respiration and burning.
( John Mayow, Tractatus Quinque Medico-Physici, 1674, Online Book.)