Inorganic mercury exposure limit as eight-hour average

September 3, 1996

MEMORANDUM FOR: REGIONAL ADMINISTRATORS
STATE DESIGNEES

FROM: JOHN B MILES, JR., DIRECTOR DIRECTORATE
OF COMPLIANCE PROGRAMS

SUBJECT: PEL for Inorganic Mercury

The purpose of this memorandum is to address the permissible exposure limit (PEL) for mercury. As you may know, Table Z-2 of 29 CFR 1910.1000 incorrectly lists the inorganic mercury PEL as a ceiling value of 1 mg/10m³ (0.1 mg/m³). It has come to our attention that compliance officers may be citing workplace mercury exposure as a ceiling PEL rather than as an 8-hour time weighted average (TWA).
While this error has not been corrected in Table Z-2 of 29 CFR 1910.1000, OSHA originally issued a correction notice in the old Field Operations Manual. Enclosed for your use and information is a [June 30, 1976 letter of interpretation] and a copy of the October 30, 1978 ["Inorganic Mercury and its Compounds" [CPL 02-02-006 (formerly CPL 2-2.6)] directive] which provides documentation for the 8-hour TWA mercury PEL as opposed to a ceiling value.
Please assure that all field personnel receive a copy of this memorandum and when a mercury over-exposure is cited, it is cited as an 8-hour TWA PEL. If you have any questions please contact the [Office of Health Enforcement at 202 693-2190].
[Corrected 6/2/2005]