Hi 'buff':
Can't add very much to Doc Vikingo's explanation above. I suspect the problem in your mind is getting things squared away between O2 at sea level and O2 under pressure.
Maybe some basics about oxygen toxicity might be helpful. This can be found at our web site at
http://www.scuba-doc.com/oxtox.html .
Oxygen toxicity is cumulative and can be measured in units called 'oxygen toxicity units' (OTUs). Not only does it cause pulmonary damage but it can cause seizures under water - uniformly fatal unless a knowledgeable buddy is close by.
The equation for calculating toxicity units is as follows:
OTU=t(PO2 - 0.5 divided by 0.5) to the 0.83 power
t=duration of exposure in minutes, PO2 is the oxygen partial pressure in bars, and 0.5 is the exposure threshold in bars.
For a diver starting fresh, daily exposures in OTUs are totalled and compared to a curve for allowable exposure. Divers whose cumulative exposure falls below the curve normally have few symptoms. Recovery takes place at about 300 OTUs/day. This curve is published in Bove's textbook on page 47. This also has a Repex table of operational oxygen exposure limits:
Repex Operational Exposure Limits are as follows:
1 day exposure=850 OTUs limit
2 day exposure=700 OTUs Limit (Total= 1400)
3 day exposure=620 OTUs Limit (Total=1860)
4 day exposure= 525 OTUs Limit (Total=2100), etc.
NOAA O2 PP and exposure time limits are also in a Table in Bove's text, 'Diving medicine' on page 46.
There is also more information on our web page at
http://www.scuba-doc.com/gasesprbs.html#O2toxicity