Guidelines for care & Use of Dry Solvent Stills [Example]

K. Provisions for medical exams, consultation & exposure assessments
written by Julianne Braun, grad student, and M. Thompson, Lab Manager
1. A Guide to OSHA Air Concentration Acronyms
OSHA regulations require workers to be guarded against excessive exposure to chemical vapors
when working in a lab. If you are routinely exposed over the Permissible Exposure Level, you must request
that the chemical vapor be monitored by the University appointed Industrial Hygienist. Before you decide
whether or not a particular exposure situation in your laboratory requires air-monitoring, you must have a
clear conception of what OSHA means by a permissible exposure level. They were derived from threshold
limit values.
Threshold limit values (TLVs) are definite numerical chemical air-concentration safety limits
determined by the American Council of Governmental and Industrial Hygienists (ACGIH, an advisory data
review agency) and are expressed in 3 different ways. They are quantitative limits to which lab workers
may be exposed without adverse effects. The TLVs may take the form of Time Weighted Averages
(TWAs), Short-term Exposure Limits (STELs), or Ceiling Limits (CLs). You may sometimes see these
limits described in an incidental 4th way in MSDS sheets as NOEALs (No Observable Adverse Effect
Levels). TWAs are usually based on 8-hour time periods. TWAs are not necessarily concentrations to
which a person's exposure should not be exceeded at any point in time, but are really time weighted
averages. For example, if a TWA is given as 100 parts per million, it may be OK to be exposed to 200 ppm
for 4 hours and to 0 ppm for the next four hours (time weighted exposure would then be 200 ppm x 4 hours
+ 0 ppm x 4 hours = 800 ppm divided by a total of 8 hours = 100 ppm). STELs are similar to TWAs, but
are based on exposure for only 15 minutes. Ceiling Limits are levels which cannot be exceeded for any
length of time, period. They are assigned for chemicals which are reported to be acutely toxic.
"TWAs permit excursions above the TLV provided they are
compensated by equivalent excursions below the TLV during the workday".
"For the vast majority of substances with a TWA, there is not
enough toxicological data available to warrant a STEL. Nevertheless,
excursions above the TWA should be controlled even where the 8- hour TWA is
within recommended limits".
"Excursions in worker exposure levels may exceed 3 times the TLV
for no more than a total of 30 minutes during a workday, and under no
circumstances should they exceed 5 times the TLV, provided that the TWA is not
exceeded." (Threshold Limit Values for Chemical Substances and Physical
Agents in the workroom environments, ACGIH, 1995).
When OSHA adopted the ACGIH threshold limit values (TLVs), confirming their validity by
incorporating them into employee safety laws, they were subsequently referred to as permissible exposure
levels (PELs). These are the legal concentrations of hazardous chemicals the government permits
employees to be exposed to for the associated length of time. PELs include TWAs, STELs, and CLs. In
addition, OSHA defined "action levels" as TWAs. There are many references in the relevant law in the
code of Federal Regulations (29 CFR 1910. 1450 or "Laboratory Standard") to certain provisions for airmonitoring
which go into effect when these values are exceeded in the lab. One encounters descriptions of
employee exposure over the "action level (or in the absence of an action level, the PEL)". This essentially
means that if a particular chemical hasn't been assigned a TWA, it is covered by one or the other type of
PEL, namely a STEL or a CL.
“Many of the newer legal limits established by OSHA include an action level concept,
typically 50% of the OSHA PEL, at which an employer is to take action to reduce the
level or to ensure that the 8-hour time-weighted PEL is not exceeded.” Furr, Keith A.
CRC Handbook of Laboratory Safety, 4th Ed. Boca Raton, Fl: CRC Press, 1995, p. 414
117