30 CFR 71.700: Inhalation hazards; threshold limit values for gases, dust, fumes, mists, and vapors.

(a) No operator of an underground coal mine and no operator of a surface coal mine may permit any person working at a surface installation or surface worksite to be exposed to airborne contaminants (other than respirable coal mine dust, respirable dust containing quartz, and asbestos dust) in excess of, on the basis of a time-weighted average, the threshold limit values adopted by the American Conference of Governmental Industrial Hygienists in “Threshold Limit Values of Airborne Contaminants” (1972), which is hereby incorporated by reference and made a part hereof. Excursions above the listed threshold limit values shall not be of greater magnitude than is characterized as permissible by the conference. This paragraph does not apply to airborne contaminants given a “C” designation by the conference in the document. This document is available for examination at MSHA's Office of Standards, Regulations, and Variances, 200 Constitution Avenue NW, Washington, DC 20210; 202-693-9440; and at every MSHA Coal Mine Safety and Health District Office. Copies of the document may be purchased from the American Conference of Governmental Industrial Hygienists, 1330 Kemper Meadow Drive, Attn: Customer Service, Cincinnati, OH 45240; 513-742-2020; http://www.acgih.org.

(b) All persons, including employees, shall be withdrawn from any area in which there is a concentration of an airborne contaminant given a “C” designation by the Conference which exceeds the threshold limit value (ceiling “C” limit) listed for that contaminant.