664 HAZWOPER for General Site Workers V

Threshold Limit Value (TLV)®

These values have been derived for many substances and can be found in Threshold Limit Values for Chemical Substances and Physical Agents, which is published annually by the American Conference of Governmental Industrial Hygienists (ACGIH). The ACGIH defines three categories of TLVs:

1. time-weighted average (TWA);
2. short-term exposure limit (STEL); and
3. ceiling (C).

All three categories may be useful in selecting levels of protection at a hazardous waste site.

Permissible Exposure Limit (PEL)

Permissible exposure limits are enforceable standards promulgated by OSHA. In many cases, they are derived from TLV values. The PEL for a substance is the 8-hour time-weighted average or ceiling concentration above which workers may not be exposed.

Although personal protective equipment may not be required for exposures below the PEL, its use may be advisable where there is a potential for overexposure. See the tables and substance-specific standards in 29 CFR Part 1910, Subpart Z, for additional details.

Recommended Exposure Limit (REL)

A NIOSH recommended exposure limit (REL) is the workplace exposure concentration recommended by NIOSH for promulgation by OSHA as a PEL, but is not enforceable as is the OSHA PEL. In some cases, NIOSH has described time-weighted average concentrations in terms of 10-hour, rather than 8-hour, averages.

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