The first possible problem involved with breathing air under pressure (underwater) involves contaminates that aren't supposed to be there. This problem is rare, but possible.

Compressors for filling scuba tanks (breathing air) use special filters and separators to keep contaminates such as carbon monoxide or oil vapor out of your breathing air. This is important because pressure proportionately increases the effects of a gas you breathe, so that traces of contaminants that would be harmless at the surface can be toxic underwater.

Contaminated air generally results from a problem with the compressor or its filtering system, and as a result often tastes and smells bad - but it can also be odorless and tasteless. A scuba diver breathing contaminated air may experience headaches, nausea, dizziness and even unconsciousness. A scuba diver afflicted by contaminated air may have cherry-red lips and fingernail beds, though this may be hard to see underwater.

Give a person suspected of breathing contaminated air fresh air, and administer oxygen if available. In severe cases, rescue breathing may be necessary. The scuba diver should have medical attention in Breathe easy all cases. 

Fortunately, as mentioned, contaminated air is rare as long as you buy your air from reputable air sources, such as professional Bali scuba dive stores. These stores recognize the seriousness of contaminated air and have their air checked frequently to be sure of its quality. Don't fill your tank from a compressor or other air source that isn't intended specifically as a breathing air compressor system; for example, you wouldn't use industrial air systems such as those used for filling tires or powering sandblasters. To avoid contaminated air, be certain you have your tanks filled only with pure, dry, filtered compressed air from a reputable air station.

Even though you have a proper air source fill your tank, if the air tastes or smells bad, don't use it. If you feel ill or get a headache during a Bali scuba dive, end the Bali scuba dive immediately. If you suspect you may have contaminated air in your tank for any reason, save the air for analysis and don't Bali scuba dive with it.

There's another way to suffer contaminated air poisoning, and that's by breathing exhaust fumes aboard a bout. Try to stay out of boat's exhaust and in fresh air.

Because you need oxygen to live, it may seem strange that oxygen can become toxic if you breathe it under pressure. But in fact, you can get "too much of a good thing" - if you were to fill your scuba tank with pure oxygen instead of compressed air, you could suffer oxygen poisoning in water as shallow as 6 metres/20 feet. This is why you should never have your tank filled with pure oxygen.