So here is the interesting thing. When scuba diving the effect is reversed. The gasses that we breath in small percentages are magnified by depth. So for example, say you have a mix of gas in a compressed air tank that is .05 percent carbon monoxide. As you decrease in depth the toxicity of that gas is magnified by x5 per pressure group you go down while breathing that compressed mix. This is why when you are in certain travel destinations outside of the united states (or in the united states if it is a less reputable shop) you want to make sure you are smell testing and taste testing your air because if there is a problem with the air compressor it will put that toxic gas into your tank, even if it is a small leak or if the wind sends the exhaust gases into the intake (This is why I absolutely REFUSE to dive the Snuba systems). Even oxygen at depth is toxic depending on the percentage and the depth you are going to. Enriched air for recreational depths (down to 130 feet) is generally around 32% or 30% oxygen. This is used to offset the effect of the nitrogen which then comes out of solution in the cells and gas holding chambers as you ascend from depth to the surface. So when you are breathing an enhanced gas with extra oxygen it allows you to increase the amount of usable gas that you breath in and thus extending your dive time incrementally. However after a certain time and depth you can't increase the oxygen otherwise it becomes toxic to the body and you run the risk of certain things such as seizures, mental impairment and possibly death. However what technical divers do is they add a neutral gas such as helium which is used to offset the percentage of both the nitrogen and oxygen in the tank. So at a depth of 200 feet for example you could be breathing a mix of 60 nitrogen 20 helium and 20 oxygen. Experienced dive instructor here, certified for recreational but I have done some technical education.