Breathing Gas Contamination: A Case for Education and Maintenance

Reported Story

A group of some 22 divers were on a boat-diving day-trip off Catalina Island over the Labor Day weekend. The divers all appeared to be using their own cylinders and the onboard compressor was used to refill these between divers.

While on the surface prior to the third dive, there was no distinct taste or smell in their breathing air. However, as they descended, they noticed an oily taste in their mouths getting worse with depth. Some of the divers then terminated their dive prematurely due to the unpleasant experience.

When back on board, several complained of headaches, nausea, and in some cases they vomited overboard.

While the dive master indicated that the cause was likely a ‘clogged’ breathing air filter, the diver boat operator denied that this was possible.

Two of the affected divers who reported this incident had their cylinders serviced at their local dive shop immediately afterwards; oil was visibly present inside at least one of the cylinders.


Breathing air filtration units can lose their ability to effectively remove contaminants. Manufacturers provide strict run-hour requirements1 to help ensure that the air will meet the required air quality standards. In the US, the required standard is referred to as CGA Grade E — specifically set for scuba breathing air applications. The limits for oil and odor are clearly stated and compliance with these would ensure that oil would not affect divers or accumulate in the cylinders.

The actual quantities of all contaminants increase with depth; explained as increased partial pressure for gas contaminants, or greater mass per breath for liquids. This would explain the lack of odor on the surface.

Most breathing air compressors run on synthetic oil, which in small doses should not be dangerously toxic. However, they can cause ill-effects when inhaled and ingested. Accumulation in the lungs can also cause respiratory issues. The symptoms of headaches, nausea and vomiting can be ascribed to breathing high levels of oil mist. Carbon monoxide contamination does also lead to some of these symptoms.


The acceptable limit for odor in breathing air is ‘none’. Any air that has an odor, whether oily, acrid, sweet or of an unknown scent, is not acceptable as breathing air.

Divers should trust their noses.

When in doubt, the dive operator should be approached for their latest air quality test certificate, compressor maintenance logbook or filter replacement schedule. Portable carbon monoxide analysers and devices that sample scuba cylinder are available.

When the diver cannot be assured of some degree of quality control at a dive operation, they should consider diving elsewhere. Their health and safety is equally their responsibility.

  1. There are cases where run-hours need to be reduced, such as in very hot and humid environments. Where a high-pressure refrigerant dryer is installed before the filter, the run-hours can be substantially increased. In both cases, the filter will eventually become saturated with moisture and the ability to remove carbon monoxide, residual oil, moisture and odor significantly reduced. The only way to determine the correct filter run-time is through regular air quality testing.