Contaminants. Various constituents of compressed gasses can have
detrimental effects on the health of the diver. Some of these contaminants
and their effects are discussed in the following paragraphs.
(1) Carbon dioxide can be a major contaminant in a breathing-gas
system being supplied by a compressor because of its existence in the air
being compressed, or because of improper function of the compressor. A
significant amount of carbon dioxide is also produced by chamber inhabitants.
Respiration accounts for the largest part of CO2 production. The amount
varies between 0.15 and 0.40 pounds/man/hour, depending upon the level of
physical activity. The normal concentration of CO2 in expired air is 3.75
percent. The physiological reaction to carbon dioxide at atmospheric
pressure as a function of its partial pressure is shown in Figure 6-2. In
order to remain below a given partial pressure, as the total pressure is
increased, requires the attainment of lower and lower concentrations of
carbon dioxide. Acceptable limits of carbon dioxide concentrations as a
function of exposure period and depth are shown in Figure 6-3. Because the
physiological effects of concentration as a function of pressure have not
been investigated in as much detail for carbon dioxide as for oxygen, the
effect of depth has been estimated as shown in Figure 6-3. (See Reference
(4), Roth, Space-Cabin Atmospheres; Part IV -- Engineering Trade-Offs of
One-Versus Two-Gas Systems.) This estimation is based on reported values
(see Reference (5), Elkins, The Chemistry of Industrial Toxicology and
Reference (6), Williams, Some Effects of Water Vapor and System Pressure on
Dynamic Reaction to CO2 with Solid Absorbents) and on the effect of oxygen
concentration as shown in Figure 6-1. For exposure periods shorter than one
hour, carbon dioxide partial pressure of 0.5 percent produces only mild toxic
effects, and serious discomfort may result at 2.0 percent.
(2) Carbon monoxide is sometimes available as a contaminant of a
breathing-gas system being supplied by a compressor. It exists in the air
being compressed and it is a byproduct of an improper functioning compressor.
A small amount of carbon monoxide is also produced by the diver (10 cc per
day at 1 atm) (see Reference (7), Gannoe, Final Report on Gas Monitoring
Equipment Investigation). The toxic effect of carbon monoxide is due
primarily to its affinity for hemoglobin, the oxygen carrier of the blood.
If 500 ppm of carbon monoxide is inhaled at atmospheric pressure until
equilibrium is approached, 50 percent of the hemoglobin in the body will be
combined with carbon monoxide. This will result in slight confusion and
possible fainting or collapse upon exertion. Asphyxiation by carbon monoxide
to the extent of unconsciousness, if not prolonged, usually has no
significant after effects. Prolonged anoxemia, however, may result in
permanent damage to the brain. Exposure to concentrations greater than 1200
ppm can be fatal. Figure 6-4 shows limits for carbon monoxide extrapolated
to a depth of 1000 feet. These curves do not consider the partial pressure
of oxygen in compressed-air exposures which would allow the carbon monoxide
content to be greater than shown here. Work by the Navy Toxicology Unit has
shown that with a constant partial pressure of oxygen the acute toxicity of
carbon monoxide is a linear function of its partial pressure (see Reference
(8), Wands, Some Recommended Standards for Compressed Air for Breathing).