What he said although id add there is no statistically significant link that it reduces the chance of DCI when dived on an air profile when compared to air.
Interesting point, and one that is hard to argue. The hypothesis is that if you reduce the amount of nitrogen in your breathing gas for any given dive, you will have less bubble formation, and less DCI. From a purely physiological point of view, it is hard to see how that would not be true. However, that type of reasoning is different from demonstrating a statistically significant difference experimentally.
To do that, you would need to have a large dataset of air and nitrox dives with matched profiles, and then look for the incidence of undeserved hits. As undeserved hits are pretty rare, that would mean that your N (number of study dives) would have to be very large to allow any real difference to reach the level of statistical significance. Note that this doesn't mean that a difference doesn't exist, it's just that subtle differences may be harder to pick up in this fashion.
So for this reason, I think that most people who use nitrox use it to extend bottom time and don't just spring for the expense and dive air tables anyway for the theoretical added safety benefit. Unlike the commonly held but unproven assumption that nitrox makes you less tired, there is sound physiological theory behind the claim of reduced DCI when diving nitrox on air tables.
Finally, remember that all tables are really just very educated guesses based on lots of dive data and understanding of physiology, but not something that can be "proven" definitively (like calculating the tensile strength of a bar of steel in a lab). That is why undeserved hits still happen. All you are doing is stacking the odds in your favor.
Mike
