There is a bit of a balancing act involved as if you lower the O2 content to lower the PO2, you increase the deco obligation. But as Mer indicates, oxtox is a serious problem that is difficult to manage without a full face mask and is potentially fatal. Therefore despite the comparative statistical safety of a 1.6 PO2, it is most commonly reserved for the non working deco portions of the dive while a 1.4 PO2 is a much more common PO2 for the working portion of the dive.
In practice the 1.4 PO2 limits a diver on air to a maximum depth of 187' and even a higher 1.6 PO2 sets the limit at 218 ft. In both cases however narcosis is a serious factor and should limit your depth to something far shallower - how much shallower is often debated and opinions very from 60 ft among the very ardent trimix fans to 150 -170 ft for some of the more die hard deep air divers.
In the extreme, you can still get deep air certification to 240 ft and it still gets done routinely with a degree of safety. But even then the people who are allowed to go that deep do it in stages and not all of those who aspire to it will be taken that deep in a course if they show problems as they progressively go deeper. There is research to support the relative safety of short exposures to high PO2's and the US Navy developed extreme exposure guidlines. This is where some deep air record setters (Hal Watts, etc.) have managed to survive. But on the other hand the US navy also explored elevated PO2s and enriched O2 mixes in the 50's and decided there were to many unknows and too many unpredictable problems to want to make either deep air or enriched O2 diving part of it's standard operating procedures. With regard to the eary nitrox problems, the US Navy did studies with divers with extensive hard hat experience who also had very high tolerance to elevated CO2 levels, which it is currently felt can increase the potential for an oxtox incident in addition to the potential narcotic issues with CO2.
Personally I think 240 ft on air is insane and rather pointless as most divers are too severely impacted by narcosis to work effectively or to resolve problems effectively at that depth.
I also agree with those above who indicate redundancy is as much an issue as gas volume. Once you are in a deco situation surfacing is not an option to deal with an emergency - a key difference from recreational diving situations. So you need to have the gas and the redundancy to deal with the problem at depth so that you can meet any deco obligation that has been acquired.
In that regard, I think any type of deco dive on a single tank is a very bad idea. Even if you bring a large pony bottle (30-40 cu ft) your gas reserve is limited if you and/or your buddy have a problem. My preference is to do deeper dives and deco dives with doubles allowing for enough reserve gas to decompress off the back gas and then also taking along at least one additional deco gas for accellerated decompression purposes at a higher PO2.
Even a non deco deep dive (>100 ft) on a single tank is a bad idea. Gas consumption reaches a point where things happen fast and any unexpected delay or problem can cause you to end up in a situation where surfacing at a normal rate with an adequate safety stop may not be possible. Regulators are pretty reliable, but even as a reg tech with very well maintained regulators, I am not comfortable putting my faith in just one reg below 100' so a large pony or doubles are, for me, mandatory.
