Incidents, and unfortunately sometimes even accidents, concerning poor quality (contaminated) breathing gas (air, nitrox, trimix or another mixture) are reasonably rare; however, they do happen.
Contamination and its sources vary widely depending on the quality of equipment used for filling gas cylinders and its maintenance, filling station’s locations, operating personnel, and many other factors. The most common contaminations include, amongst others, hydrocarbons from compressor lubricants, carbon monoxide (CO) from engine exhaust and impurities from the surrounding environment such as methane and carbon dioxide (CO2).
Never just assume that the breathing gas in your cylinder is safe. You could be breathing a cocktail which, instead of supporting your life under water, could turn into a deadly poison. It could be a mistake that could prove fatal.
Some of the contaminants can be identified through a rough sniff test. Basically, crack open the cylinder valve just a little bit and give the contents a quick sniff. I always perform a quick sniff test before I attach my regulator to the cylinder. Over the years I had to return some cylinders because the gas smelled of cigarette smoke, paint fumes, engine exhaust, burnt grass, and even sewage. Sometimes the odour of the gas made you instantaneously nauseous. I have also seen cylinders contaminated with oil, water, diesel, and excessive rust.
While the sniff test is OK for contaminants that you can smell, it will not identify presence of contaminants such as carbon monoxide. Oxygen and carbon monoxide monitors are readily available these days. If you travel to remote places where the quality of compressors could be suspect, a gas monitor could add peace of mind and make your diving safer.
When diving on nitrox or trimix, analyse every tank before every dive without exception.
The impact of breathing bad gas on divers varies depending on the contaminant breathed. Among the most severe symptoms are impaired judgment and loss of consciousness, both of which could be deadly underwater.