Surely, oxygen is a good friend, excellent because we could not live without it. However, it is a touchy and intolerant friend, above all for divers; in our enjoyment of diving, we continuously change, the pressure to which it is used to work, also behind its preferred limits, by risking bad or very bad consequences, as we know since decades; we changed idea different time about its limits or about ppO2 correspondent. In current times, whoever teaches us to go underwater is all oriented towards maximum safety of diving, towards abolition of any risk also even if only hypothetically foreseeable, consequently decreeing oxygen limits that are much more rigid and restrictive than time ago. The below graphic demonstrates it; it is extracted form an article in the Mondo Sommerso magazine of November 1964.
As you can see, the area in which recreational divers of those times were believed to be able to operate safely had a depth limit of 12 metres for a maximum permanence time of around 7 minutes; time that extended to 30' stopping at 10 m, or even to three hours not exceeding 4 metres. The absolute maximum depth to which pure oxygen could be breathed, as long as you were at rest, is indicated in the graphic in 18 m, the latter quota is clearly reported on many depth gauges prior to 1970. Quotas and limits considered crazy for modern divers, remained valid only for therapies in hyperbaric chambers and during deco stops of the legendary coral hunters.
We are mentioning times in which divers were "sporty" and not "recreational", when tables we used gave an embolism risk probability up to 5% and the adjectives to market diving were "adventure", "discovery", "exploration". Concepts once fascinating and today exclusively attributable to the few who are seriously committed to the so-called technical diving, in open circuit or better with the use of closed circuit rebreathers.
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