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Permissible Exposure Limit (PEL)
PEL’s are promulgated & enforced by OSHA. However, MSHA has
used the term PEL in recent rule making, such as Part 62 (noise exposure)
and at times refers to PEL’s in the DPM regulations. In most part, the term
PEL has the same meaning as TLV, however refers to an enforcing agency
(OSHA or MSHA).
Recommended Exposure Limit (REL)
This limit is developed by the National Institute of Occupational Safety
& Health. Often the REL is a time-weighted average for a 10-hour work day
during a 40-hour work week.
Action Level (AL)
This is a level at which action is required. OSHA & MSHA requires an
action level for some specific substances as well as for noise exposure. Many
industrial hygiene professionals use the action level to evaluate workplace
exposure: It is usually identified as half the PEL or TLV.
Categories of Exposure Limits
There are three important categories of exposure limits that apply to
TLVs, PELs, and RELs: time-weighted average, short-term exposure limit,
and ceiling.
Time-Weighted Average
This is the average concentration for an 8-hour workday or 40-hour
workweek, to which nearly all workers may be repeatedly exposed, day after
day, without adverse effect.
Short-Term Exposure Limit (STEL)
This is a short-term TWA exposure to which workers can be continu-
ously exposed for up to 15 minutes without suffering from irritation, chronic
or irreversible tissue damage, or narcosis of sufficient degree to increase the
likelihood of accident or injury.
Ceiling (C)
This is the concentration that should not be exceeded during any part of
the work day.
Skin Designation
In looking up exposure limits, you may see a skin designation. This
alerts you that there is a potential for significant exposure due to skin absorp-
tion. This designation is an alert that air sampling alone is insufficient to
quantify exposure.
Chemical agents – crystalline silica, coal dust, diesel particulate matter
(DPM), welding fumes, solvent vapors, oxides of nitrogen, etc.
Physical agents – noise, heat, and cold.
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