yup, up to that point the earth's atmopshere was mostly ammonia and methane and such. When cyanobacteria started producing oxygen the life that had evolved to that point couldn't survive the toxicity of the oxygen and died off. For the next billion years cyanobacteria pretty much ruled the earth until about 1.7 billion years ago when more complex life that could handle oxygen toxicity started to evolve to take advantage of an oxygenated atmosphere.
For humans there are two types of oxygen toxicity. Long term oxygen toxicity and, you guessed it, short term oxygen toxicity. If you breath 100% oxygen for 12hrs or longer you run the risk of your lungs filling with fluid similar to pneumonia and this can be fatal. Short term oxygen toxicity generally occurs at 200% or greater. This can only occur under pressure. For example, a diver at 33ft is experiancing twice the pressure of someone at the surface. Breathing normal air at 33ft underwater, the oxygen content of 21% would be double at 42%. If you were breathing 100% oxygen, it would be at the 200% threshhold, and you run the risk of passing out, going into convulsions, and dying within 30 seconds.
In the world of chemistry, there is no such thing as a toxic chemical, only toxic ammounts. I forget where I heard that. hehe.
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