When taking about air quality one must understand that there are certain components in the air. There are different levels on earth known as lower atmosphere, atmosphere, and stratosphere.
Water vapor is also present.
The atmosphere is mixed with argon and carbon dioxide as high as the stratosphere. Ozone is a gas in the stratosphere that absorbs large amounts of ultraviolet light from the sun, heating the stratosphere and protecting life on earth from solar radiation. There are other gases which are human-made highly poisonous, like dioxin. The level of oxygen must be high enough as for every living thing to survive breathing, but it also cannot be so high or it will cause damage to the whole planet, like catching on fire. Every single one those gases, no matter what the smallest amount may be, is important, playing a vital part in the protection of life on earth.