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Appendices
Appendix A
Toxic gas measurement - Ceilings,
TWAs and STELs
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 which 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 which 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 (H2S) 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 three independent exposure
measurements and alarm types built into the PhD5 design.
1. Ceiling level:
OSHA has assigned some, but not all, toxic substances with a
ceiling level. This is the highest concentration of a toxic substance
to which an unprotected worker should ever be exposed, even for a
very short time.
Never enter an environment even momentarily
when concentrations of toxic substances exceed the ceiling
level
.
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. During this
time, STEL and ceiling concentration limits may not be exceeded.
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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