The 21 percent oxygen in compressed air can also be toxic, but not until you descend well past the recommended maximum limits for recreational diving. So when diving with air within recreational depth limits, oxygen toxicity isn't an issue.

Recreational divers sometimes use enriched air (also known as "enriched air nitrox" or "nitrox"), which has more. than 21 percent oxygen. Enriched air has some advantages regarding how long you can stay underwater at a given depth, but you can have oxygen problems using it within recreatioral depth limits. For this reason, enriched air diving requires special training and some special equipment requirements (to avoid combustion problems possible with high oxygen levels); reputable dive centers will not, provide enriched air without proof of enriched air certification.

So, to avoid oxygen toxicity problems, don't have (or try to have) your cylinder filled with enriched air, unless you're properly trained and certified. Don't use a cylinder that's marked as being an enriched air cylinder, (again, unless you're properly trained and certified).

Nitrogen Narcosis
Although nitrogen has no direct influence at the surface, that changes as you breathe it under sure. Underwater, at depths approaching 30metres/100 feet, nitrogen has a noticeable intoxicating effect that intensities as you go deeper.
A diver affected by nitrogen narcosis behaves as you might expect someone to behave if intoxicated. Narcosis impairs the diver's judgment and coordination, and may create a false sense of security, cause disregard for safety and other foolish behavior. Nitrogen narcosis can make a diver feel anxious or uncomfortable, which can lead to panic or other poor decisions.