Carbon Monoxide Toxicity
HOW DOES CARBON MONOXIDE TOXICITY OCCUR IN
SCUBA DIVING?
Carbon monoxide (CO) is a tasteless, odorless, highly
poisonous gas given off by incomplete combustion of
petroleum fuel. Virtually every gasoline powered motor,
including all cars that use hydrocarbon fuel, emit some
carbon
monoxide. All lighted cigarettes also give off carbon
monoxide. The extreme toxicity of CO arises from the fact
that, compared with oxygen, it combines about 200 times
more readily with hemoglobin. As a result, any excess CO
readily displaces some oxygen from the blood; the more CO
there is, the more oxygen will be displaced.
CO-related problems while diving can occur two ways, one
more infamous than the other. Probably the less appreciated
problem is simply from smoking. All smokers (cigarette, cigar,
pipe) have an elevated blood CO level and, sadly, many divers
smoke (even on the dive boat!). There is no evidence that
diving increases the blood CO level in smokers, but since CO
competes with oxygen, the smoking diver is more hypoxic on
entering and exiting the water than otherwise. Any stressful
situation thus puts the diver at increased risk for a
hypoxic-related event, such as heart attack.
While at depth, the hypoxic effect of excess CO will be
somewhat (but not completely) mitigated by the higher
blood oxygen level that also occurs at depth. In final analysis,
we really don't know to what extent smoking causes problems
in divers, but common sense (and basic physiology) makes it
a
dumb practice to smoke and dive.
The toxicity mechanism we hear more about is when enough
CO is in the tank air to act as a life-threatening impurity.
Fortunately this is a rare occurrence, but it happens, and the
result can be truly disastrous. According to news reports in
April 1994, soon after a German scuba diver's body was
recovered off Key West, Florida,
"investigators suspected something unusual...analysis [of air in
the diver's tank] revealed carbon monoxide nearly three times
the level considered acceptable."
The analysis reportedly showed 2500 parts per million of CO
in the tank's air, an extraordinary level. Non-smoking city
dwellers inhale about 10 p.m. (Ten p.m. is considered the
maximal CO level permissible level in scuba tank air).
Cigarette
smokers inhale between 30 and 60 p.m. of CO; this amount
binds from 5 to 10 per cent of the blood with CO, which
means 5-10% of the smoker's blood is unable to carry oxygen.
An inhaled CO level of 2500 would tie up over 60% of the
blood and make anyone fatally hypoxic.
It was speculated that this diver's tank air was contaminated
from a faulty air compressor. Air can certainly become impure
when tank filling takes place near machine exhaust; the
exhaust fumes can be taken up and compressed along with the
surrounding air. At depth the pressure of any CO inhaled from
a scuba tank is increased just like every other inhaled gas.
However, unlike any other gas likely to be in the tank, even
small amounts of CO can be harmful, because CO has a great
affinity for hemoglobin and easily displaces oxygen from the
blood.
Depending on the concentration of CO in the tank and the
depth at which it is inhaled, the effects of CO toxicity may
range from mild headache to confusion to a state of
unconsciousness and death. Any CO impurity must be
considered potentially dangerous at depth.
The incidence of faulty tank air is very rare, at least at
reputable fill stations, so it is impractical to do on-site
chemical analysis of every tank. Until air analysis becomes
routine (if ever), testing must be up to the diver's senses,
which means taking several breaths from the tank before
entering the water. This practice helps provide a regulator
check as well as a cursory check of the tank air. Certainly any
headache (from CO) or bad taste (from other impurities) is
warning that something may be wrong with the air. (Such a
cursory check will likely not detect low levels of impurities, so
sticking with a reputable fill station is probably your best
protection.)
WHAT IS CARBON DIOXIDE TOXICITY?
Carbon dioxide is a gas byproduct of metabolism. Our body
makes about 200 cc's of CO2 every minute (more when we
exercise) and excretes it in the air we exhale. Plants take up
the
CO2 and give off oxygen (photosynthesis). The concentration
of carbon dioxide in the atmosphere is almost zero, and poses
no risk when fresh air is compressed inside a scuba tank.
The partial pressure of carbon dioxide in a scuba diver's blood
is
a function only of metabolism and the rate and depth of
breathing the same factors that determine blood CO2
concentration on land. Unlike other gases normally inhaled
(nitrogen and oxygen), or gases that could be inhaled under
abnormal conditions (CO and other gas impurities), the CO2
level in the blood is unchanged by the ambient pressure (i.e.,
the depth) per se.
Scuba apparatus used in recreational diving is "open circuit,"
so
exhalation of carbon dioxide is through the mouthpiece and
into the water (it's all in the bubbles). Abnormal carbon
dioxide accumulation in the blood can occur from too high a
level of metabolism (from exercise) and/or inadequate
breathing (usually not breathing deep enough). The medical
term for high carbon dioxide in the blood is hypercapnia;
when
the level is high enough it can cause "CO2 toxicity," which
can
lead to shortness of breath, headache, confusion and
drowning
(depending on severity).
Air density increases with depth; so the deeper you go the
greater the work of breathing. Increased resistance to
breathing can cause the diver to take shallow breaths, and
shallow breaths make carbon dioxide elimination less
efficient.
If the diver also exerts herself heavily, her body will produce
more CO2, resulting in a "vicious cycle" of carbon dioxide
buildup: heavy work (more CO2 production) ---> shallow
breathing (less efficient elimination of CO2) ---> higher
blood CO2 (CO2 toxicity).
Hypercapnia (and resulting CO2 toxicity) is a major concern
among deep divers, and also any diver who has to perform
heavy work. It is much less of a concern for the typical
recreational diver. Regular, deep breathing, and a properly
functioning regulator, should eliminate risk of carbon dioxide
buildup in recreational diving.
Some experienced divers practice "skip" breathing, which is
holding the breath (on inhalation or exhalation) in order to
conserve air. This might save air but it could also lead to CO2
buildup, since by breath holding the diver is, in effect, under
ventilating; if the diver under ventilates he will soon want to
breathe even more, from the stimulus of an increasing CO2
level. As a result, the diver who skip breathes enough to
increase his CO2 could end up depleting air supply faster than
with normal breathing! Even without the obvious risk of
pulmonary barotrauma (particularly if near the surface), skip
breathing is definitely not recommended.
Adventure Dominica