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However, [previous posts may not] address...
the question that Herb Gottlieb asked when he
said, "Please tell me what the writer is trying to say?"
The writer has made some errors... here is what he should have been saying.
"Ordinary air is about 21 percent oxygen and 78 percent nitrogen. Each gas
exerts its own pressure, independently of the other. At one atmosphere this
means the partial pressure of oxygen is 0.21 atmospheres. That means the
compressed air that a diver breathes at 10 meters (two atmospheres total
pressure) actually has 0.42 atmospheres partial pressure of oxygen if it
were compressed from normal air. That's approaching the danger level. Much
higher than 0.42 atmospheres partial pressure of oxygen, and the diver may
go into convulsions. Prolonged exposure to high partial pressure of oxygen
is dangerous. The solution is to use less oxygen in the mixture of
compressed gases that divers breathe. Around 300 meters, the gas mixture
may contain as little as 2 percent oxygen in order to keep the partial
pressure of oxygen near 0.2 atmospheres."