O 2 is required for biological processes. Without it, we die. Oxygen reacts with foods during metabolism to produce CO2 and water. This is an example of a combustion reaction. Humidity is a description of the amount of water vapor in air.
Atmospheric pressure is the force with which the atmosphere presses down on a given surface area.
Atmospheric pressure decreases with increasing altitude, while the percentages of the different components remain the same. Atmospheric pressure is measured with a barometer. The troposphere is our immediately surrounding atmosphere. Above that is the ozone layer, stratosphere and, at the top, the mesosphere. Maximum levels are established and measured based on the toxicity of each individual component.