7-2
180 liter LN2 Supply Tank
Thermo Scientific
Section 7
Safety Bulletin
Oxygen Deficient
Atmospheres
The normal oxygen content of the air is approximately 21%. Depletion of
oxygen content in air, either by combustion or by displacement by inert
gas, is a potential hazard and users should exercise suitable precautions.
One aspect of this possible hazard is the response of humans when exposed
to an atmosphere containing only 8 to 12% oxygen. In this environment,
unconsciousness can be immediate with virtually no warning.
When the oxygen content of air is reduced to about 15 or 16%, the flame
of ordinary combustible materials, including those commonly used as fuel
for heat or light, may be extinguished. Somewhat below this concentration,
an individual breathing the air is mentally incapable of diagnosing the
situation because the onset of symptoms such as sleepiness, fatigue,
lassitude, loss of coordination, errors in judgment and confusion can be
masked by a state of “euphoria”, leaving the victim with a false sense of
security and well-being.
Human exposure to atmospheres containing 12% or less oxygen leads to
rapid unconsciousness. Unconsciousness can occur rapidly, rendering the
user essentially helpless. This can occur if the condition is reached by an
immediate change of environment, or through the gradual depletion of
oxygen. Most individuals working in or around oxygen deficient
atmospheres rely on the “buddy system” for protection – obviously, the
“buddy” is equally susceptible to asphyxiation if he or she enters the area to
assist an unconscious partner unless equipped with a portable air supply.
Best protection is obtained by equipping all individuals with a portable
supply of respirable air. Life lines are acceptable only if the area is
essentially free of obstructions and individuals can assist one another
without constraint.
If an oxygen deficient atmosphere is suspected or known to exist:
1. Use the “buddy system”. Use more than one “buddy” if necessary to
move a fellow worker in an emergency.
2. Both the worker and the “buddy system” should be equipped with self-
contained or air-line breathing equipment.
