Pascal was the first to explain that air pressure decreases along with altitude in the atmosphere. The density of the lower layers of the atmosphere is high, because the air present above exerts pressure over here. As a result, both the density and pressure of air in the lower layers of the atmosphere is greater. On the contrary, the air of upper layers is less pressurized, so its density and pressure are both low.
That is why the air pressure always decreases along with elevation, but its rate of decline is not always the same. It depends on the density of the air, the amount of water vapor and the gravitational force of the Earth. Because of all these factors being variable, there is no direct proportional relationship between altitude and air pressure.
Even then, the average rate of decreasing air pressure in the troposphere is approximately 34 millibars at an altitude of 300 meters. At higher altitudes, the gases become more sparse and lighter in a rapid form. As a result, air pressure is greatly reduced. That is why a person uses oxygen cylinder and special suit while climbing high mountain peaks.