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 https://www.osha.gov.

July 23, 2010

Letter #20100303-9658

Re: Standards for decompression chambers for use with pressurized face-tunnel boring machines.

This letter responds to your question of February 9, 2010, to Richard S. Terrill, Administrator of OSHA Region X.

Question:  Are pressure vessels for human occupancy involved in tunnel boring required to be fabricated and documented in accord with the ASME PVHO-1 standard, "Pressure Vessels for Human Occupancy"?

Answer:  Yes; employers must use decompression chambers for workers involved in pressurized tunnel boring that meet the requirements of American Society of Mechanical Engineers (ASME) standard PVHO-1 ("Safety Standard for Pressure Vessels for Human Occupancy"), published by the American National Standards Institute (ANSI).

OSHA issued the current Compressed Air standard for underground construction, §1926.803, in 1971.  This standard addresses bulkheads used to separate a working chamber from free air or from another chamber having less pressure than the working pressure.  Accordingly, §1926.803(m)(3) requires:

Bulkheads used to contain compressed air shall be tested, where practicable, to prove their ability to resist the highest air pressure which may be expected to be used.

As described in your letter of February 9, 2010, modern tunnel-boring machines (TBMs) can pressurize the face area up to 261 psig.  To access this area for repairs or inspection, workers must enter the area through a decompression chamber.  When they finish their work, they must reenter the decompression chamber.  Chamber operators then reduce the atmospheric pressure on the workers in a controlled manner, following an prescribed decompression schedule.  Current §1926.803 does not address these working conditions.1 However, the general-duty clause specified by Section 5(a)(1) of the Occupational Safety and Health Act of 1970 (29 U.S.C. 654) requires:

Each employer shall furnish to each of his employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.

ANSI developed national consensus standard A10.16-2009 ("Safety Requirements for Tunnels, Shafts, and Caissons") to address the recognized hazards associated with using improperly designed and fabricated chambers to decompress workers.  Section 12.3.2 of this standard states:

Except where a lock in a tunnel is constructed by bulk heading a length of the tunnel, all air locks shall be designed and fabricated in accordance with the provisions of Sections VIII and IX of the ASME Boiler and Pressure Vessel Code.2

A footnote to the  2010 ASME Boiler & Pressure Vessel Code, Section VIII, Division 1, Paragraph U-1(c)(2)(j) provides the specific requirements for pressure vessels for human occupancy, as follows:

Requirements for pressure vessels for human occupancy are covered by ASME PVHO-1.

OSHA recognizes the ASME PVHO-1 standard as specifying the construction industry's usual and customary practices for preventing death and serious injury associated with worker exposure to the recognized hazards of decompression from pressurized workplaces; therefore, OSHA would enforce the standard under the general-duty clause.  Accordingly, decompression chambers associated with pressurized TBMs and used for decompressing workers must meet the requirements of ASME PVHO-1, or provide an equivalent means of worker protection.


Ben Bare, Acting Director
Directorate of Construction

1  for example, §1926.803(e)(5) states, "No employee shall be subjected to pressure exceeding 50 pounds per square inch except in emergency." [back to text]

2  The term "air locks" includes decompression chambers. [back to text]