Recent discussion regarding what levels of oxygen constitute a safety hazard
in NMR labs may have left some readers with the impression that brief
exposure to oxygen deficient atmospheres is acceptable. It should be noted
that the OSHA limit (see 29 CFR 1915.12) is 19.5% oxygen, and anything less
is defined as an oxygen deficient atmosphere.
According to the Compressed Gas Association (http://cganet.com), the
following physiological effects of low oxygen availability (percentages are
by volume) are observed:
15-19% Decreased ability to perform tasks. May impair
coordination and induce early symptoms in persons with head, lung or
circulatory problems.
12-15% Breathing increases, especially in exertion. Pulse
rate up. Impaired coordination, perception, and judgement.
10-12% Breathing further increases in rate and depth, poor
coordination and judgement, lips slightly blue.
8-10% Mental failure, fainting, unconsciousness, ashen
face, blueness of lips, nausea and vomiting.
6-8% 8 minute exposure may be fatal in 50-100% of cases; 6
minute exposure may be fatal in 25 to 50% of cases; 4-5 minutes, recovery
with treatment.
4-6% Coma in 40 seconds, followed by convulsions, breathing
failure and death.
The Compressed Gas Association website also contains the following warning:
Exposure to atmospheres containing 8-10% or less oxygen will
bring about unconsciousness without warning and so quickly that the
individuals cannot help or protect themselves....
Since gas exchange in the lungs is a process of simple diffusion that
depends upon the difference in partial pressures on both sides of the
alveolar membrane, insight into the hazard can be gained by examining the
partial pressure of oxygen in the lungs. According to the US Naval Flight
Surgeons Manual, (http:\\www.vnh.org/FSManual/01/02RespPhysiology.html), the
partial pressure of oxygen in the lung alveoli (PO2(alv)) is given in mm Hg
by:
PO2(alv) = (PB-47)FI - 40
where PB is the atmospheric pressure, 47 is a term resulting from the
partial pressure of water (in the trachea), and 40 is a term associated with
the partial pressure of CO2 present. FI is the fraction of oxygen in the
inspired gas (0.21 for a normal atmosphere).
At sea level with PB=760, and FI = 0.21 the PO2(alv) is found to be 109.7 mm
Hg which is in good agreement with the measured value of 103.
If the same calculation is done for a 50% reduction of oxygen by nitrogen
displacement (PB=760, FI=0.105) the result is a PO2(alv) value of 34.9 mm
Hg. This roughly corresponds to the value of 33 measured at a 20,000 foot
altitude. For comparison, pilots in unpressurized aircraft are required to
begin using supplemental oxygen at 14,000 feet. Although one might be able
to acclimate to the environment at 20,000 feet over a period of days or
weeks, it seems reasonable to expect that a sudden encounter with this level
of oxygen deficiency, even for a healthy person, is hazardous.
Since atmospheric pressure drops with altitude, if one is not at sea level
the situation becomes much worse. At only 2500 feet, the PO2(alv) is
calculated (PB=698, FI =0.105) as 28.3 mm Hg, a value corresponding to the
normal atmosphere at over 22,000 feet. At 5000 feet (PB =632, FI = 0.105)
the PO2(alv) is found to have dropped to 21.4 mm of Hg. This probably
corresponds to a much higher altitude but the published chart does not go
beyond 22,000 feet.
All of the physiological effects noted are for a healthy person; smokers or
persons with otherwise impaired ability to utilize oxygen would be expected
to show symptoms earlier and to a greater degree. The oxygen carrying
capacity of blood is a sigmoid curve (oxygen dissociation curve) so as you
move to lower levels of inspired oxygen, you reach a point of precipitous
decline in function. This probably explains the well-known fact that
hypoxia, whether due to displacement of oxygen by nitrogen or any other
cause, leads to sudden unconsciousness without warning.
I am by no means an expert in this area and welcome further information or
alternative viewpoints. Personally however, if I were maintaining a magnet
or magnets in a room characterized as "poorly ventilated", I would want to
determine by testing, particularly during cryogen fills, that the oxygen
content was acceptable.
All of the above represents my personal opinion, is intended for discussion
purposes only and should not be relied upon otherwise.
Michael D. Meadows
Principal Research Chemist
Eastman Chemical Company
Kingsport, Tennessee USA 37662-5150
Phone: 423-229-8826