In the event of doubling the oxygen levels on Earth, the most significant changes would be the speeding up of processes like respiration and combustion. With the presence of more fuel, i.e. oxygen, forest fires would become more massive and devastating. ... Anything and everything would burn more easily.
Oxygen toxicity is lung damage that happens from breathing in too much extra (supplemental) oxygen. It's also called oxygen poisoning. It can cause coughing and trouble breathing. In severe cases it can even cause death.
With the increased levels of oxygen humans would have an increased chance of oxygen toxicity. This happens when humans are exposed to increased oxygen levels than their bodies are normally used to. Over time, this could lead to lung damage, poor eyesight and cells not being able to reproduce.
