It changes over time and is actually decreasing.
The earth's atmosphere at sea level.
22% oxygen at sea level
Because due to the over pollution and cutting down the tree's level of Oxygen is decreasing .The level of Nitrogen always remains constant in the atmosphere . The level of Nitrogen doesn't' depend on the level of the Oxygen level.
Planning on going to a trip? Take your own oxygen with you, since you can't breath on Mars. 95.32 percent of Mars' atmosphere contains carbon dioxide. Oxygen level is 0.13% on Mars.
The oxygen level in the chaparral biome is similar to the oxygen levels found in other terrestrial biomes, which is around 21% of the Earth's atmosphere. Plants in the chaparral biome, such as shrubs and trees, contribute to producing oxygen through photosynthesis.
The atmosphere becomes highly flammable when the concentration of oxygen reaches about 23.5%. Above this level, the risk of fires and explosions increases significantly.
At 10,000 feet altitude, the percentage of oxygen in the atmosphere remains the same as at sea level, which is about 21%. However, the partial pressure of oxygen decreases at higher altitudes, leading to lower oxygen levels available for breathing.
An oxygen deficient atmosphere has an oxygen concentration less than the normal level of around 21%.
There is oxygen in the atmosphere of Saturn, but it is only a tiny fraction of the deep atmosphere, which is mostly hydrogen. It is not in a usable form as is oxygen on Earth.
Those percentages closely match the composition of Earth's atmosphere at sea level. The atmosphere is made up of mostly nitrogen (around 78%) and oxygen (around 21%), with trace amounts of other gases like carbon dioxide (around 0.04%).
oxygen is a constant 20 percent throughout the atmosphere. Even in space but the air is so thin in space that the oxygen would have to be compressed to be able to breathe it. Here on earth at sea level the atmospheric pressure is 14.7 pounds per square inch (PSI) making it possible for your body to inhale enough usable oxygen for you to breathe. As you go higher the pressure is less so even though the oxygen is still 20% of the atmosphere there is less atmosphere for you to breathe.
Mars' atmosphere is composed of only about 0.13% oxygen, with the majority being carbon dioxide (95.3%), nitrogen (2.7%), and argon (1.6%). This low oxygen level makes it uninhabitable for humans without life support systems.