A contaminant in compressed gas may have been in the air before compression, added during compression because of some fault in the compressor system or have been present or generated within the storage system.
There are many potential contaminants in the atmospheric air taken in by compressors, particularly if the compressors are located in an industrial area or near any running internal combustion engines (e.g. near a boat jetty). Carbon monoxide and nitrogen oxides are components of polluted city air in levels that may be toxic, particularly when the inspired partial pressure is raised by breathing at depth. They may also enter the compressed air if the compressor is driven by, or operated near, an internal combustion engine that produces these compounds. Volatile hydrocarbons and organic compounds, such as methane, may also be present in environmental air.
Compressor lubricating oil may also contaminate the air if excessive oil vapour or even liquid oil passes around the piston rings to enter the supply and overload filters. This is most likely when the rings are damaged or if the air intake is restricted. In reality, some oil is present in the air delivered by even the best-maintained oil-lubricated compressor, and this may increase at higher temperatures. Hence, the drainage from the interstage and final aftercooler separators will always be oily. A well-maintained, modern compressor will have lower levels present in the compressed air produced, but an older, less well-maintained compressor will have higher levels.
More complex forms of contamination can also arise from within the compressor. High temperatures (‘hot spots’) and high pressures within the compressor produce an ideal environment for contaminants to form. Both these factors promote chemical reactions and hence the production of contaminants. If an unsuitable lubricating oil is used in the compressor, it may produce oil vapour, which may contaminate the air as oil, break down to produce volatile hydrocarbons or burn and form carbon monoxide. The same trouble can also result if the compressor overheats, causing ‘cracking’ (oil breakdown) or ‘flashing’ (oil combustion). The air becomes contaminated with volatile hydrocarbons or combustion products such as carbon monoxide and nitrogen oxides. Many low molecular weight volatile contaminants can cause some level of anaesthesia, and the effect is magnified by increased pressure and inert gas narcosis. This may lead to impaired cognition, a reduced seizure threshold and a greater potential for cardiac arrhythmias while diving. It has been suggested that these contaminants could be contributory to some morbidity and mortality in divers.
Overheating may also be caused by poor design or maintenance of the compressor – a restriction in the compressor intake, a dirty filter, excessive length of intake or a kinked intake hose can all cause problems such as overheating and reduced output.
Therefore, a compressor that may produce satisfactory air when tested running under normal circumstances may deliver contaminated air if it temporarily overheats. This may lead to part of a batch of cylinders being filled with contaminated gas, whereas others are not.
Other problems include leaks between the compressor stages, via piping, loose fittings or head gaskets or around the pistons.
Overall, the most common dangerous contaminant remains carbon monoxide, whether from excessive intake levels or from within the compressor. In some ways this is most dangerous when the air is otherwise clean because carbon monoxide is odourless and tasteless and will not be detected by the diver. Conversely, many potential contaminants have readily noticeable and generally unpleasant smell and taste and are obvious from the first breath.
Contaminants in compressed gas may have been present in the gas before compression, added during compression or resident in the storage system.
Some divers believe that using an electric compressor prevents carbon monoxide contamination. It removes one common cause of carbon monoxide – that of the driving motor exhaust. However, it does not reduce the other external and internal sources of carbon monoxide described earlier.
With more and more divers visiting dive destinations in developing countries, especially in the tropics, gas contamination is becoming an increasing problem. Problems are most likely when small, low-cost, compressors designed for filling a few cylinders are overused to fill cylinders for hours at a time in hot and humid environments, thus leading to overheating and ineffective filtration through water-saturated filters. These risks are compounded where there is lack of knowledge of appropriate purity standards, poor regulatory oversight or just lack of a proper air quality testing program.
Contamination from other sources
Air contamination from residues within the storage cylinder is not common. Residues of cleaning and scouring materials and scale formed by rusting can contribute vapours or dust if the cleaning operation is not conducted properly or if the cylinder or storage vessel is allowed to deteriorate. Water may be introduced into storage cylinders if there has been a failure of the drying system. There have been problems with paint systems that have been used to protect the interior of steel storage cylinders, where pressure cycling can cause the release of solvent vapour.
Lubricants used on regulators and reducers can sometimes cause fears of contamination. A diver may taste the lubricant in the regulator and think that the air is contaminated, and, if the wrong lubricants have been used, these fears may be justified. With enriched oxygen mixtures, only lubricants approved for use with oxygen should be used, to reduce the risk of fire.
The most difficult source of contamination to isolate is intermittent inlet contamination. For example, one report details a company that had an air compressor inlet on the roof (a not uncommon position). On rare occasions, its air was contaminated with organic chemicals. Only after much work and customer dissatisfaction was the source identified. A nearby factory sometimes used spray painting equipment with an exhaust fan. With a particular wind direction, these fumes would blow across to the compressor inlet.
There is a low risk of contamination in mixture diving when the source gases are produced by liquefaction. This reduces the risk of contamination because most of the potential impurities are separated by their higher boiling points. For mixtures prepared by mixing compressed air with other components, there are potential problems with the air and in the mixing process.
There is a possibility of increasing the concentration of trace contaminants when using recycled and reclaimed gases for deep diving operations and gases compressed in submarines. The compounds that are not removed in any purification process can accumulate with recycling until they are at level that causes a problem. For this reason, a thorough risk analysis should be done to determine likely contaminants, including those that could be infrequent (e.g. methane from animals or from sewerage). A suitable screening gas analysis program for the identified contaminants is highly desirable.
In rebreather diving, contamination with expired carbon dioxide as a result of failure of the ‘scrubber’ is a potential problem that periodically leads to fatalities.
With an increase in depth, and hence par-tial pressure of contaminants, toxicity will increase.