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Dennis & George, There have now been several posts pointing out that *safe* not an appropriate scientific term to use in the above context, as it depends on what degree of risk you are prepared to assume. I agree with the authors on that and that "local standards" seem to prevail for pPO2s as they do for depth limits for sport divers. Nontheless, I do believe that better estimates of risk can be made when they are based on cases rather than opinion. If indeed there has only been one episode of O2 toxicity at 1.3 ATA, then most people would say "interesting, but probably not relevant to my risk calculations". On the other hand, if someone says "every time we push the limits. someone takes a hit", then one is likely to be more conservative. Risk estimates take at least two forms: (1) Where you get a percent risk over the population; say what's my risk for getting lung cancer if I smoke?. Most of us are comfortable with this kind of calculation, but also hold back by saying "Well, it still may not happen to ME". The better way of estimating risk has to do with calculating them for an individual by looking at co-factors that likely contribute to the 100% hit you take if the event happens to you. Certain factors, notably CO2 retention (due say to exercise loading) and rapid change/increase in FI O2 are believed to contribute to O2 hits - at least at high (? >1.3 ATA) pPO2s. This is from the US Navy Divers Handbook and most dive medicine texts. So this is a logical way of estimating your own risk potential: Several people have posted or called me to say "In cold water, or in high effort situations, I'll plan to limit my pPO2 to 1.2 (even), with 1.6 ATA for deco." But I'm left with an empiric issue: Brett says no-way, no-how, *anyone* has ever experience an O2 hit below 1.6 ATA unless they had already exceeded their CNS clock. If that's true, then *no one* need worry until either they have exceeded their clock or get to a pPO2 of 1.6. But it's clear that others in the dive community think that there is enough risk for them to limit their pPO2 as noted in other posts. Other than opinion - do we have facts? George IV, you've been very silent. Can you direct me to someone(s) in the commercial diving industry who have facts or are willing to share their opinion on this? If anyone has extensive experience under high work load and high pPO2's related to depth (not just breathing O2 at 50 fsw), it must be commercial divers. - ph
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