Demystifying Hazmat Chemistry
Demystifying Hazmat Chemistry - Firebelle Productions
Demystifying Hazmat Chemistry - Firebelle Productions
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STUDENT HANDOUT<br />
DEMYSTIFYING HAZMAT CHEMISTRY<br />
Workplace environmental exposure limits (WEEL) apply more to normal occupational exposures<br />
than to emergency response. However, these values may become significant in a protracted incident.<br />
Workplace environmental exposure limits are established for workers without any special<br />
protective equipment. The first three below represent the maximum concentration to which an<br />
average person in average health may be exposed repeatedly on a day-to-day basis (40 hours per<br />
week, 8 to 10 hours per day) with no adverse health effects. Each is established by a different<br />
agency. Often the limits established by each agency are the same, but that’s not always the case.<br />
Some of these published limits will have a “[skin]” notation, meaning that there’s a potential for<br />
dermal absorption and that workers should protect against skin exposure.<br />
• Permissible Exposure Limits (PEL) are established by the Occupational Safety and Health<br />
Administration (OSHA). These are time-weighted average (TWA) concentrations that, unless<br />
otherwise noted, must not be exceeded during any 8-hour shift of a 40-hour workweek. (Timeweighted<br />
average means that one can exceed the identified concentration, within limits, as<br />
long as the average exposure over an 8-hour shift does not exceed the PEL.) Because PELs<br />
are OSHA limits, compliance with PELs is mandated by law.<br />
• Recommended Exposure Limits (REL) are similar to PELs in that they are also time-weighted<br />
averages for a 40-hour workweek. However, RELs differ from PELs in that they allow for a<br />
10-hour shift versus an 8-hour shift and RELs are recommended limits established by the<br />
National Institute for Occupational Safety and Health (NIOSH), not regulatory limits from<br />
OSHA.<br />
• Threshold Limit Value—Time-Weighted Average (TLV-TWA) is also a recommended limit, one<br />
from the American Conference of Governmental Industrial Hygienists (ACGIH). It’s based on<br />
an 8-hour shift and 40-hour workweek.<br />
Workers can exceed the permitted or recommended exposure limits if they stay within the shortterm<br />
and ceiling exposure limits explained below. Short-term and ceiling exposure limits may<br />
be either regulatory limits or recommended limits, depending on whether they’re established by<br />
OSHA or another agency. (If reference books don’t cite the source, err on the side of safety and<br />
assume it’s a regulatory limit.)<br />
• A Short-Term Exposure Limit (STEL) is the maximum concentration to which a worker may be<br />
exposed for short durations (15 minutes, unless otherwise noted). Exposures above the STEL<br />
should be at least 60 minutes apart and not repeated more than 4 times per day. (Often this<br />
is written as TLV-STEL.)<br />
• A Ceiling Exposure Limit is the concentration that may not be exceeded at any time. (Often<br />
this is written as TLV-C.)<br />
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