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Appendices
Appendix A Toxic gas measurement
– Warning, Danger, STEL and TWA
alarms
Many toxic substances are commonly encountered in
industry. The presence of toxic substances may be
due to materials being stored or used, the work being
performed, or may be generated by natural
processes. Exposure to toxic substances can
produce disease, bodily injury, or death in
unprotected workers.
It is important to determine the amounts of any toxic
materials potentially present in the workplace. The
amounts of toxic materials potentially present will
affect the procedures and personal protective
equipment that must be used. The safest course of
action is to eliminate or permanently control hazards
through engineering, workplace controls, ventilation,
or other safety procedures. Unprotected workers may
not be exposed to levels of toxic contaminants that
exceed Permissible Exposure Limit (PEL)
concentrations. Ongoing monitoring is necessary to
insure that exposure levels have not changed in a
way that requires the use of different or more rigorous
procedures or equipment.
Airborne toxic substances are typically classified on
the basis of their ability to produce physiological
effects on exposed workers. Toxic substances tend
to produce symptoms in two time frames.
Higher levels of exposure tend to produce immediate
(acute) effects, while lower levels of long-term
(chronic) exposure may not produce physiological
symptoms for years.
Hydrogen sulfide (H
2
S) is a good example of an
acutely toxic substance which is immediately lethal at
relatively low concentrations. Exposure to a 1,000
ppm (parts per million) concentration of H2S in air
produces rapid paralysis of the respiratory system,
cardiac arrest, and death within minutes.
Carbon monoxide (CO) is a good example of a
chronically toxic gas. Carbon monoxide bonds to the
hemoglobin molecules in red blood cells. Red blood
cells contaminated with CO are unable to transport
oxygen. Although very high concentrations of carbon
monoxide may be acutely toxic, and lead to
immediate respiratory arrest or death, it is the long
term physiological effects due to chronic exposure at
lower levels that take the greatest toll of affected
workers. This is the situation with regards to
smokers, parking garage attendants, or others
chronically exposed to carbon monoxide in the
workplace. Exposure levels are too low to produce
immediate symptoms, but small repeated doses
reduce the oxygen carrying capacity of the blood over
time to dangerously low levels. This partial
impairment of the blood supply may lead over time to
serious physiological consequences.
Because prudent monitoring programs must take both
time frames into account, there are two independent
exposure measurements and alarm types built into
the MultiPro design.
1. Warning and Danger Alarms
OSHA has assigned some, but not all, toxic
substances with a ceiling level which represents the
highest concentration of a toxic substance to which an
unprotected worker should ever be exposed, even for
a very short time. The default Warning and Danger
alarm levels in the MultiPro are less than or equal to
the OSHA-assigned ceiling levels for both CO and
H
2
S.
Never enter an environment even
momentarily when concentrations of toxic
substances exceed the level of either the Warning
or the Danger Alarm
.
Time History Graph
Ceiling
2. Time Weighted Average (TWA):
The maximum average concentration to which an
unprotected worker may be exposed over an eight
hour working day is called the Time Weighted
Average or TWA value. TWA values are calculated
by taking the sum of exposure to a particular toxic gas
in the current operating session in terms of parts-per-
million-hours and dividing by an eight-hour period.
Time History Graph
Ceiling
TWA
(8 hour)
3. Short Term Exposure Limits (STEL):
Toxic substances may have short term exposure
limits which are higher than the eight hour TWA.
The STEL is the maximum average concentration to
which an unprotected worker may be exposed in any
fifteen minute interval during the day. During this
time, neither the eight hour TWA or the ceiling
concentration may be exceeded.
Any fifteen minute periods in which the average
STEL concentration exceeds the permissible eight
hour TWA must be separated from each other by at
least one hour. A maximum of four of these periods
are allowed per eight hour shift.
Time History Graph
Ceiling
STEL
TWA
15 Minutes
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