How much oxygen is there?

[Daehawk]

Ok before people were around breathing it all up and before we started cutting down huge amounts of trees I imagine there was a crap ton of O2. Of course its not just us breathing it but we are a large part that is growing. Now with nearly 9 billion people and lots of trees and plants gone shouldn't the Earth's O2 amount be less. How much is there now? How much less is there now? At what point would we notice less O2? And at what amount of people on Earth does the O2 count drop below life lvls?

[LawBeefaroni]

Geological History of O2.

We're at approximately 21% today. Peak was maybe 35%, all time low was 0%.

[hepcat]

Don't worry, there's enough oxygen left that you'll be able to eat thousands more meatloaf and sausage patties for breakfast.

...or 10. 10 is my guess before fate jumps in.

[Isgrimnur]

Geological history of oxygen

Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2).

Earth's Atmospheric Oxygen Levels Continue Long Slide

Atmospheric oxygen levels have declined over the past 1 million years, although not nearly enough to trigger any major problems for life on Earth, a new study finds.
...
Although a drop in atmospheric oxygen levels might sound alarming, the decrease the researchers found "is trivial in regard to ecosystems," Stolper told Live Science. "To put it in perspective, the pressure in the atmosphere declines with elevation. A 0.7 percent decline in the atmospheric pressure of oxygen occurs at about 100 meters (330 feet) above sea level — that is, about the 30th floor of a tall building."

Minimum Oxygen Concentration for Human Breathing

The normal air in our environment consists of a few different gases. Approximately 78 percent of the air is nitrogen gas while only about 20.9 percent is oxygen.
...
When oxygen concentrations drop from 19.5 to 16 percent, and you engage in physical activity, your cells fail to receive the oxygen needed to function correctly. Mental functions become impaired and respiration intermittent at oxygen concentrations that drop from 10 to 14 percent; at these levels with any amount of physical activity, the body becomes exhausted. Humans won't survive with levels at 6 percent or lower.