Classifying potentially gassy underground worksites

July 24, 1993

MEMORANDUM FOR: GILBERT J. SAULTER
REGIONAL ADMINISTRATOR
LEO CAREY, DIRECTOR OFFICE OF FIELD PROGRAMS

FROM: ROGER CLARK, DIRECTOR
DIRECTORATE OF COMPLIANCE PROGRAMS

SUBJECT: Definition of Potentially Gassy Atmosphere
[29 CFR 1926.800]

This is in response to your April 28 memorandum requesting clarification of the potentially gassy classification criteria for underground construction operations. I apologize for the delay in responding to your inquiry.

As you know, there are two criteria to consider when evaluating an underground operation for classification as potentially gassy. First, if all air monitoring conducted in a 24-hour period discloses the presence of 10% or more of the lower explosive limit (LEL) of methane or other explosive gasses, then the operation must be classified potentially gassy as per 1926.800(h)(1)(i). However, if some but not all air monitor readings disclose the presence of 10% or more of the LEL, then the operation is not to be classified potentially gassy under the paragraph 1926.800(h)(1)(i). That paragraph is intended to address situations such as an unexpected pocket of gas. Second, if past experience with a geological area indicates that 10% or more of the LEL is likely to be encountered, an underground operation can be classified as potentially gassy before work begins under 1926.800(h)(1)(ii).

[Earlier regional memorandum and requester correspondence reproduced on OSHA page; omitted here.]