Oxygen Toxicity
WHAT IS OXYGEN TOXICITY, AND CAN IT DEVELOP
WHILE
DIVING?
Oxygen toxicity is any injury or discomfort to the body from
inhaling too much oxygen. High concentrations of oxygen
delivered at atmospheric pressure can harm the lungs. When
diving, any given concentration of oxygen comes under higher
pressure than atmospheric, thus increasing the amount inhaled
and the potential for toxicity.
Above atmospheric pressures, oxygen can also affect the
central
nervous system, and cause seizures and convulsions. Thus
oxygen toxicity is a major potential hazard in some diving but
not, as it turns out, recreational diving.
Oxygen is a vital gas, the absence of which leads to death in a
few minutes. People with healthy lungs only need the amount
of oxygen in the atmosphere, no more or less. Anything more
than 21% oxygen is considered "supplemental oxygen."
Supplemental oxygen, like any drug, can be toxic at high
doses;
since oxygen is a gas the "dose" is based on both the
percentage
of oxygen inhaled and the ambient pressure. Patients who are
ill from low blood oxygen receive a higher than normal
percentage of oxygen as treatment (i.e., greater than 21%).
Scuba divers, because of the increase in gas pressures with
depth, inhale a higher than normal oxygen pressure; the
percentage is the same, since compressed air is still 21% oxygen
at any depth.
However, since pressure increases with depth, the deeper one
dives the higher the total pressure of oxygen that is inhaled.
Too high an inhaled oxygen pressure can be toxic to the lungs
and central nervous system.
Oxygen toxicity is the reason why very deep diving (e.g.,
greater than about 170 fsw) is safely accomplished not with
compressed air, which contains 21% oxygen, but with a gas
mixture that has a much lower percentage, e.g., 10% O2. Such
a
low oxygen percentage would be dangerous at sea level, but at
great depth, due to the high ambient pressure, it is more than
adequate to sustain life.
Recreational scuba divers adhering to the dive tables have no
significant risk of oxygen toxicity. At 35 feet depth, where
RSD
tables allow the diver to spend well over two hours on a
non-repetitive dive, the PAO2 (oxygen pressure in the lungs)
is the same as from breathing 43% oxygen at sea level, i.e.,
non-toxic. At the maximum RSD depth of 130 feet, the PAO2
from breathing compressed air is about the same as from
breathing 100% oxygen at sea level.
This level of oxygen would only begin to cause trouble if
inhaled for at least an hour. The few minutes of bottom time
that the tables allow at 130 fsw is simply not long enough to
pose a significant risk from oxygen toxicity.
WHAT EXACTLY DETERMINES RISK OF OXYGEN TOXICITY?
The occurrence and type of oxygen toxicity correlate with the
O2 concentration, the ambient pressure, the length of time
supplemental O2 is inhaled, and the diver's level of activity.
Range of oxygen concentrations. The concentration of inspired
oxygen can vary from zero to 100% (the maximum). The
concentration in ordinary air is 21% (whether compressed or
not, and regardless of the depth at which it is inhaled). The
higher the concentration of O2 the greater the risk of oxygen
toxicity.
Range of ambient pressures. Ambient pressure can range from
zero (outer space), to one atmosphere (sea level), to several
atmospheres (in a hyperbaric chamber or under water). On
land, outside of a chamber, oxygen can be administered only at
the surrounding atmospheric pressure, which can vary from 1
atmosphere (sea level) to about .33 atmosphere (summit of
Mt.
Everest). The higher the ambient pressure, the greater the risk
of oxygen toxicity.
Length of time oxygen is inhaled. Supplemental oxygen can be
given anywhere from a few seconds to lifelong. How long O2
is
given depends on the condition being treated, the
concentration used, and the ambient pressure. The longer
supplemental oxygen is inhaled, the greater the risk of oxygen
toxicity.
Level of activity. This is the least quantifiable aspect of oxygen
toxicity. Once the threshold of oxygen toxicity is reached
(based on atmospheres of O2), the more active the diver the
greater the risk of developing actual toxicity.
Since air contains 21% oxygen, the amount of oxygen inhaled
at
sea level is .21 atm. O2; this amount is safe to breathe forever.
From clinical experience it appears that patients can breathe
.40 atm O2 indefinitely, and possibly up to .60 atm O2 for
weeks at a time (equivalent to 40% O2 and 60% O2 at sea
level,
respectively), without apparent oxygen toxicity.
In healthy subjects, 100% oxygen at atmospheric pressure (1
atm. O2) causes chest discomfort, pain and cough after only a
few hours. If inhaled continuously over 24 hours, 1 atm. O2
can lead to lung congestion (pulmonary edema) and, if
continued, death. Obviously, doctors try not to use high
concentrations of oxygen unless absolutely necessary. Patients
who require 100% oxygen because of heart or lung disease are
critically ill and will almost always be cared for in a hospital
intensive care unit.
The most serious potential harm from inhaling supplemental
oxygen at sea level pressure is lung injury, which develops
slowly, over many hours. At depth the most serious harm
from
too much oxygen is a seizure, which can occur in just a few
minutes of oxygen breathing.
HOW DO ATMOSPHERES OF O2 RELATE TO OXYGEN
TOXICITY?
Although potentially toxic, 1 atm. O2 does not cause seizures.
However, when 100% oxygen is delivered at pressures two or
more times sea level pressure, the first toxic manifestation can
be a seizure. A seizure is a sudden electrical discharge from the
brain that causes uncontrolled muscle movement. If seizures
occur under water the diver will likely be unable to breathe
through the regulator and will drown (if rescue is not
immediate).
Atmospheres of O2 is the major determinant of oxygen
toxicity; the risk increases directly with the atmospheres of
oxygen inhaled. A diver breathing compressed air (21%
oxygen)
at 4.76 atmospheres (124 fsw) has the same risk of developing
oxygen toxicity as when breathing 100% O2 at sea level
(assuming the same level of activity). In either situation the
diver is breathing one atmosphere of oxygen (1 atm. O2).
Exposure to high oxygen pressures at RSD depths is not long
enough to cause oxygen toxicity. Oxygen toxicity is mainly a
concern for the deep diver, for divers breathing mixtures that
contain more than 21% O2 (e.g., Nitrox), and for patients
undergoing hyperbaric oxygen therapy. The thresh-old beyond
which oxygen toxicity is a major concern is about 1.3-1.5 atm
O2. The box shows some permutations for reaching this
threshold.
HOW CAN OXYGEN TOXICITY BE MINIMIZED?
The risk of seizures from oxygen toxicity begins at 1.3 to 1.5
atm O2. To reach this level on compressed air the diver has to
exceed the RSD depth limits. Divers who go deep (technical or
other) can reduce the risk of oxygen toxicity by decreasing the
concentration of inhaled oxygen.
For example, a diver at 7 atm. (198 fsw) might switch to a
mixture containing just 4% oxygen (mixed with helium or
helium and nitrogen). At sea level, 4% oxygen would not
support human life; at 7 atm., 4% oxygen is about the same as
breathing 28% oxygen at sea level. On the other hand, a diver
breathing 21% oxygen at 7 atmospheres (198 fsw) would be at
risk for oxygen toxicity as he would be inhaling 1.54 atm. O2.
Pure oxygen was used in re-breathing scuba equipment during
World War II. Because of the risk of oxygen toxicity, military
divers were limited to about 25 fsw, or 1.76 atm. O2. (The
military now uses mixed gases with its re-breathing scuba
apparatus for deeper diving). It is also because of oxygen
toxicity that hyperbaric treatment schedules limit the
breathing of 100% oxygen to only about 20 minutes at a time.
In summary, the risk of oxygen toxicity is directly related to
the total atm. of O2 activity.
WHAT IS OXYGEN TOXICITY, AND CAN IT DEVELOP
WHILE
DIVING?
Oxygen toxicity is any injury or discomfort to the body from
inhaling too much oxygen. High concentrations of oxygen
delivered at atmospheric pressure can harm the lungs. When
diving, any given concentration of oxygen comes under higher
pressure than atmospheric, thus increasing the amount inhaled
and the potential for toxicity.
Above atmospheric pressures, oxygen can also affect the
central
nervous system, and cause seizures and convulsions. Thus
oxygen toxicity is a major potential hazard in some diving but
not, as it turns out, recreational diving.
Oxygen is a vital gas, the absence of which leads to death in a
few minutes. People with healthy lungs only need the amount
of oxygen in the atmosphere, no more or less. Anything more
than 21% oxygen is considered "supplemental oxygen."
Supplemental oxygen, like any drug, can be toxic at high
doses;
since oxygen is a gas the "dose" is based on both the
percentage
of oxygen inhaled and the ambient pressure. Patients who are
ill from low blood oxygen receive a higher than normal
percentage of oxygen as treatment (i.e., greater than 21%).
Scuba divers, because of the increase in gas pressures with
depth, inhale a higher than normal oxygen pressure; the
percentage is the same, since compressed air is still 21% oxygen
at any depth.
However, since pressure increases with depth, the deeper one
dives the higher the total pressure of oxygen that is inhaled.
Too high an inhaled oxygen pressure can be toxic to the lungs
and central nervous system.
Oxygen toxicity is the reason why very deep diving (e.g.,
greater than about 170 fsw) is safely accomplished not with
compressed air, which contains 21% oxygen, but with a gas
mixture that has a much lower percentage, e.g., 10% O2. Such
a
low oxygen percentage would be dangerous at sea level, but at
great depth, due to the high ambient pressure, it is more than
adequate to sustain life.
Recreational scuba divers adhering to the dive tables have no
significant risk of oxygen toxicity. At 35 feet depth, where
RSD
tables allow the diver to spend well over two hours on a
non-repetitive dive, the PAO2 (oxygen pressure in the lungs)
is the same as from breathing 43% oxygen at sea level, i.e.,
non-toxic. At the maximum RSD depth of 130 feet, the PAO2
from breathing compressed air is about the same as from
breathing 100% oxygen at sea level.
This level of oxygen would only begin to cause trouble if
inhaled for at least an hour. The few minutes of bottom time
that the tables allow at 130 fsw is simply not long enough to
pose a significant risk from oxygen toxicity.
WHAT EXACTLY DETERMINES RISK OF OXYGEN TOXICITY?
The occurrence and type of oxygen toxicity correlate with the
O2 concentration, the ambient pressure, the length of time
supplemental O2 is inhaled, and the diver's level of activity.
Range of oxygen concentrations. The concentration of inspired
oxygen can vary from zero to 100% (the maximum). The
concentration in ordinary air is 21% (whether compressed or
not, and regardless of the depth at which it is inhaled). The
higher the concentration of O2 the greater the risk of oxygen
toxicity.
Range of ambient pressures. Ambient pressure can range from
zero (outer space), to one atmosphere (sea level), to several
atmospheres (in a hyperbaric chamber or under water). On
land, outside of a chamber, oxygen can be administered only at
the surrounding atmospheric pressure, which can vary from 1
atmosphere (sea level) to about .33 atmosphere (summit of
Mt.
Everest). The higher the ambient pressure, the greater the risk
of oxygen toxicity.
Length of time oxygen is inhaled. Supplemental oxygen can be
given anywhere from a few seconds to lifelong. How long O2
is
given depends on the condition being treated, the
concentration used, and the ambient pressure. The longer
supplemental oxygen is inhaled, the greater the risk of oxygen
toxicity.
Level of activity. This is the least quantifiable aspect of oxygen
toxicity. Once the threshold of oxygen toxicity is reached
(based on atmospheres of O2), the more active the diver the
greater the risk of developing actual toxicity.
Since air contains 21% oxygen, the amount of oxygen inhaled
at
sea level is .21 atm. O2; this amount is safe to breathe forever.
From clinical experience it appears that patients can breathe
.40 atm O2 indefinitely, and possibly up to .60 atm O2 for
weeks at a time (equivalent to 40% O2 and 60% O2 at sea
level,
respectively), without apparent oxygen toxicity.
In healthy subjects, 100% oxygen at atmospheric pressure (1
atm. O2) causes chest discomfort, pain and cough after only a
few hours. If inhaled continuously over 24 hours, 1 atm. O2
can lead to lung congestion (pulmonary edema) and, if
continued, death. Obviously, doctors try not to use high
concentrations of oxygen unless absolutely necessary. Patients
who require 100% oxygen because of heart or lung disease are
critically ill and will almost always be cared for in a hospital
intensive care unit.
The most serious potential harm from inhaling supplemental
oxygen at sea level pressure is lung injury, which develops
slowly, over many hours. At depth the most serious harm
from
too much oxygen is a seizure, which can occur in just a few
minutes of oxygen breathing.
HOW DO ATMOSPHERES OF O2 RELATE TO OXYGEN
TOXICITY?
Although potentially toxic, 1 atm. O2 does not cause seizures.
However, when 100% oxygen is delivered at pressures two or
more times sea level pressure, the first toxic manifestation can
be a seizure. A seizure is a sudden electrical discharge from the
brain that causes uncontrolled muscle movement. If seizures
occur under water the diver will likely be unable to breathe
through the regulator and will drown (if rescue is not
immediate).
Atmospheres of O2 is the major determinant of oxygen
toxicity; the risk increases directly with the atmospheres of
oxygen inhaled. A diver breathing compressed air (21%
oxygen)
at 4.76 atmospheres (124 fsw) has the same risk of developing
oxygen toxicity as when breathing 100% O2 at sea level
(assuming the same level of activity). In either situation the
diver is breathing one atmosphere of oxygen (1 atm. O2).
Exposure to high oxygen pressures at RSD depths is not long
enough to cause oxygen toxicity. Oxygen toxicity is mainly a
concern for the deep diver, for divers breathing mixtures that
contain more than 21% O2 (e.g., Nitrox), and for patients
undergoing hyperbaric oxygen therapy. The thresh-old beyond
which oxygen toxicity is a major concern is about 1.3-1.5 atm
O2. The box shows some permutations for reaching this
threshold.
HOW CAN OXYGEN TOXICITY BE MINIMIZED?
The risk of seizures from oxygen toxicity begins at 1.3 to 1.5
atm O2. To reach this level on compressed air the diver has to
exceed the RSD depth limits. Divers who go deep (technical or
other) can reduce the risk of oxygen toxicity by decreasing the
concentration of inhaled oxygen.
For example, a diver at 7 atm. (198 fsw) might switch to a
mixture containing just 4% oxygen (mixed with helium or
helium and nitrogen). At sea level, 4% oxygen would not
support human life; at 7 atm., 4% oxygen is about the same as
breathing 28% oxygen at sea level. On the other hand, a diver
breathing 21% oxygen at 7 atmospheres (198 fsw) would be at
risk for oxygen toxicity as he would be inhaling 1.54 atm. O2.
Pure oxygen was used in re-breathing scuba equipment during
World War II. Because of the risk of oxygen toxicity, military
divers were limited to about 25 fsw, or 1.76 atm. O2. (The
military now uses mixed gases with its re-breathing scuba
apparatus for deeper diving). It is also because of oxygen
toxicity that hyperbaric treatment schedules limit the
breathing of 100% oxygen to only about 20 minutes at a time.
In summary, the risk of oxygen toxicity is directly related to
the total atm. of O2 activity.
Adventure Dominica
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