OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at http://www.osha.gov.

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).



April 28, 1993

MEMORANDUM FOR:  ROGER CLARK
                Director Directorate of Compliance Programs

ATTENTION:       DALE CAVANAUGH

THROUGH:         LEO CAREY Director Office of Field Programs

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

The attached letter of April 23, 1993 from the Dallas Area Rapid Transit (DART) requests an interpretation of 29 CFR 1926.800(h)(1)(i) concerning the definition of a potentially gassy atmosphere.

The Dallas Area Rapid Transit is currently tunneling in the Dallas area. The tunnel will be utilized for a mass transit system. However, methane gas has been encountered throughout the tunneling operation. At this time, all work has been suspended until a response can be obtained on the definition of a potentially gassy atmosphere.

The question of what is a potentially gassy atmosphere was discussed in recent conversations between Bill Tschappat of your office, Frank Memmott, OTI, and Jose Chapa, Region VI Technical Support. The discussions yielded the following definition of a potentially gassy atmosphere.

A potentially gassy atmosphere is defined as "when monitoring indicates 10% or more of the lower explosive limit of methane, or other flammable gases, would have to be continuous at that level for a 24-hour period."

Is this definition consistent with National Office policy? Your immediate attention and concurrence with this correspondence is requested.



GILBERT J. SAULTER
Regional Administrator

Attachment



April 23, 1993

U.S. Department of Labor/OSHA
525 Griffin Street
Dallas, TX 75202

ATTENTION: MR. GLEN WILLIAMSON

Dear Mr. Williamson:

Request you provide us an interpretation of Occupational Safety and Health Administration (OSHA) Standard 29 CFR 1926.800(h)(1)(i) that defines potentially gassy operations. It seems there can be two (2) different interpretations, one being that underground operations are considered potentially gassy when 10% or more of the lower explosive limit of methane or other flammable gases are reached more than once in a 24 hour period, and the second interpretation indicates 10% or more of the lower explosive limit of methane, or other flammable gases, would have to be continuous at that level for a 24 hour period.

Your prompt assistance in this matter would be greatly appreciated.

Sincerely,



Anthony P. Venturato
Assistant Executive Director

c: Al Brunson Ralph Moody