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:: Wednesday, June 15, 2005 ::

Confined Spaces: Is 19.5 Percent Oxygen Really Safe? -Occupational Hazards
So What's Acceptable?

Many reference sources suggest that air contains 20.95 percent oxygen. However, this value is based on the assumption that the air is 'bone dry': in other words, it contains no moisture. However, air in most parts of the country does contain a certain amount of water vapor, which we recognize as humidity. While the exact volume of water that air can hold varies with temperature, a relative humidity of 40 to 60 percent at ambient temperatures can lower the oxygen level by about 0.1 percent. As a practical matter, a value of about 20.8 percent oxygen may be more appropriate than 20.9 percent, because the lower value takes humidity into account.

Now, think about this. If ordinary outside air contains 20.8 percent oxygen, and you're ventilating a space with this air, doesn't it stand to reason that the air in the space ought to also be 20.8 percent? If you make an oxygen measurement and your instrument reads 20.0 percent instead, don't you think you ought to be a little concerned? Shouldnt you ask yourself 'Why?'? If you don't know why, should you really let people enter the space?

Summary

Contrary to popular belief, 19.5 percent oxygen is not some magic number. Rather, it's a value established on the basis of adverse physiological effects that may manifest at an oxygen partial pressure less than 148 mm Hg. Even if the oxygen is well above 19.5 percent, hazardous concentrations of other gases and vapors may be present. Some gases and vapors may be present at concentrations well above the TLV while, at the same time, they are below a combustible gas meter's limit of detection.

Because ambient air contains about 20.8 percent oxygen, if the oxygen concentration in a space is anything other than 20.8 percent, you should ask yourself 'Why?'. If you can't come up with a credible answer, you had better not let people enter a space until you can do so.

What is a Confined Space? - Worksafe BC Health & Safety Centre
The Occupational Health and Safety Regulation defines a 'confined space' as an area, other than an underground working, that

(a) is enclosed or partially enclosed,
(b) is not designed or intended for continuous human occupancy,
(c) has limited or restricted means for entry or exit that may complicate the provision of first aid, evacuation, rescue or other emergency response service, and
(d) is large enough and so configured that a worker could enter to perform assigned work.

Entering and Working in Confined Spaces - OSU Environmental Health & Safety
Oxygen-Deficient Atmospheres
The normal atmosphere is composed of approximately 21% oxygen and 79% nitrogen. An atmosphere containing less than 19.5% oxygen shall be considered oxygen-deficient. The oxygen level inside a confined space may be decreased as the result of either consumption or displacement.

There are a number of processes that consume oxygen in a confined space. Oxygen is consumed during combustion of flammable materials, as in welding, cutting, or brazing. A more subtle consumption of oxygen occurs during bacterial action, as in the fermentation process. Oxygen can also be consumed during chemical reactions such as in the formation of rust on the exposed surfaces of a confined space. The number of people working in a confined space and the amount of physical activity can also influence oxygen consumption. Oxygen levels can also be reduced as the result of oxygen displacement by other gases.

Operator Dies in Oxygen-Deficient Compartment of Ice-Making Barge 1992 Alaska FACE report
A second possibility is that iron oxidation due to rust formation resulted in extreme oxygen-depletion. While the level of rusting observed seemed to be normal for a vessel of this age, areas of 'black rust' (areas of very high oxidation levels which are rarely seen) were observed. 'Black rust' has been reported to reduce oxygen content to less than 5 percent. At this time, this seems to be the most logical possibility.

CAUSE OF DEATH
The autopsy report attributed the victim's death to 'asphyxiation''.

RECOMMENDATIONS/DISCUSSION

Recommendation #1: Employers should ensure that all employees potentially exposed to confined spaces receive specific training in confined space safe work procedures.

posted 10:23 AM :: reference link :: 0 comments ::

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