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2150EX Area Velocity Flow Module
Appendix D General Safety Procedures
D-5
Whitney, oxygen is equal to 12.2%t. Many hikers go there,
and to higher peaks as well. After adequate acclimation,
they may climb to the top of Mt. Everest, where oxygen is
equal to only 6.7%.
“The lowest oxygen concentrations that I have observed in
a sewer atmosphere was 13 percent. It was in a sealed
chamber, near sea level, upstream from an inverted siphon
on a metropolitan trunk. A man would be foolish to enter
the chamber. Without ventilation, he might die, but not
from lack of oxygen.
“It seems unlikely that anyone has ever died in a sewer
from suffocation, that is, a lack of oxygen. Deaths have
often been attributed to ‘asphyxiation.’ This is a word
which, according to the dictionary, is used to mean death
from an atmosphere that does not support life. The word
has sometimes been misinterpreted as meaning suffoca-
tion, which is only one kind of asphyxiation.
“In nearly all cases of death in sewers, the real killer is
hydrogen sulfide. It is important that this fact be recog-
nized. Many cities diligently test for explosive gases, which
is very important, and they may measure the oxygen con-
centration which usually is unimportant, but they rarely
measure H
2
S. Death has occurred where it is unlikely that
there was any measurable reduction in the oxygen concen-
tration. Waste water containing 2 mg per liter of dissolved
sulfide, and at a pH of 7.0, can produce, in a chamber with
high turbulence, a concentration of 300 PPM H
2
S, in the
air. This is considered to be a lethal concentration. Many
people have died from H
2
S, not only in sewers and indus-
tries, but also from swamps and from hot springs. In one
resort area, at least five persons died from H
2
S poisoning
before the people were ready to admit that H
2
S is not a
therapeutic agent. Hardly a year passes in the US. without
a sewer fatality from H
2
S as well as deaths elsewhere in
the world.
“The presence of H
2
S in a sewer atmosphere is easily
determined. A bellows-and-ampoule type of tester is very
satisfactory for the purpose, even though it is only crudely
quantitative. When using a tester of this type, do not bring
the air to the ampoule by way of a tube, as this may change
the H
2
S concentration. Hang the ampoule in the air to be
tested, with a suction tube to the bulb or bellows.
“Lead acetate paper is very useful as a qualitative indica-
tor. It cannot be used to estimate the amount of sulfide, but
it will quickly turn black in an atmosphere containing only
a tenth of a lethal concentration.
“Electrodes or other similar electrical indicating devices for
H
2
S in air have been marketed. Some of them are known to
be unreliable, and we know of none that have proved
dependable. Do not use one unless you check it at frequent
intervals against air containing known H
2
S concentra-