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.

December 2, 2005

Ms. Laura Johnson
Assistant Director
Iowa - Illinois Safety Council
8013 Douglas Avenue
Urbandale, Iowa 50322-2453

Dear Ms. Johnson:

Thank you for your August 2, 2005, letter to the Occupational Safety and Health Administration's (OSHA) Peoria Area Office. Your letter has been referred to the Directorate of Enforcement Program's (DEP) Office of General Industry Enforcement for an answer to your questions regarding compliance with OSHA's Permit-required Confined Spaces Standard, 29 CFR 1910.146. Your scenario and questions have been restated below for clarity.

Scenario: Employees must enter a permit-required confined space (PRCS) to perform maintenance work on mechanical apparatus. The enclosure is a circular concrete wall approximately 10 feet in height, tapering outward to the top and 25 feet in diameter. The enclosure sits on top of a concrete cooling tower structure and houses a large fan. The top of the enclosure is completely open to the atmosphere. Below the enclosure (approximately 10 feet) inside of the cooling tower is the "Fill Pack" through which the cooling tower water is distributed. This fill pack is an open cellular media through a hatchway on the side of the enclosure to access catwalks along the drive shaft and to a platform at the main gearbox in the center of the enclosure. Complete control of hazardous energy (lockout/tagout) is performed externally before entry is allowed. The only reason this confined space is considered a permit space is because of the mechanical hazards of the fan blades and drive shaft. It is the intent of the employer to reclassify the space to a non-permit confined space following the regulations in 29 CFR 1910.146(c)(7) once the lockout/tagout is in place and tests to confirm all other hazards are eliminated are completed. Atmospheric testing of the confined space prior to entry occasionally indicates what appear to be erroneous readings for LFL and H2S due to sensitivities of the atmospheric monitoring equipment sensors in the high humidity area; however no sources for flammables or hydrogen sulfide exist within the facility.

Question 1: Please provide an interpretation of potential as it is used in 29 CFR 1910.146(c)(7)(i) as it relates to hazardous atmospheres in the definitions for "Non-permit confined space" and "Permit-required confined space" in 1910.146(b). Specifically, does a background or ambient reading of flammable gas, below the specified 10 percent LFL, constitute potential even when there is no source or when all sources have been isolated?

Reply: In your scenario you state that the reason the confined space is a permit space is due to the mechanical hazards of the fan blades and drive shaft. If these are the only hazards or potential hazards within the permit space, then why is testing of the space conducted? Further you state that the instrumentation may give erroneous readings due to humidity. Any erroneous readings would need to be verified as erroneous prior to entry into the space. We would recommend that you contact the equipment manufacturer for technical assistance in determining how to eliminate this problem. Perhaps a different type of equipment could be used to atmospherically test the space during high humid conditions.

Question 2: Does documentation from a previous PRCS entry into this space that shows that a low level LFL (5 percent) reading was present constitute potential? If so, does this historical record continue to constitute potential even after all processes and procedures are evaluated and it is determined that there is no source of flammables?

Reply: If there is no source of the flammable including residual contamination, then the potential would be removed. However, any flammable which is brought into the space could trigger a potential atmospheric hazard.

Question 3: If, subsequent to the tests that revealed the low level LFL, evaluations determine that there is no source of flammables and repeated tests showing zero LFL are conducted, can it be accepted that there is no potential for a hazardous flammable atmosphere? At what point and by what additional methods can it be accepted there is no potential?

Reply: If the source has been truly removed, i.e. no piping, containers, residual, etc., and the flammable material is not brought into the space by any mechanism, then there would not be a potential atmospheric hazard for that material.

Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA requirements are set by statute, standards, and regulations. Our interpretations letters explain the 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. In addition, 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
www.osha.gov. If you have any further questions, please feel free to contact the Office of General Industry Enforcement at (202) 693-1850.

Sincerely,



Richard E. Fairfax, Director
Directorate of Enforcement Programs