Archive Notice - OSHA Archive

NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

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

September 4, 1996

Mr. Macon Jones
Blasting Cleaning Products LTD.
2180 Speers Road
Oakville, Ontario
Canada L6L2X8

Dear Mr. Jones:

This is response to your request of April 10, requesting clarification of the 29 CFR 1910.146 standard. Please accept our apology for the delay. Responses to your questions follow:

Question 1.    If an enclosed space is a "permit required confined
              space" (PRCS) and all of the proper procedures are 
              implemented, can entry be made and work performed 
              (or continued) if the measured lower flammable limit
              (LFL) is greater than 10%?

Answer         Yes.  The permit-required confined spaces standard (29 CFR
              1910.146) does not prohibit working in a permit-required
              space where the atmosphere is above 10% of the LFL.  Once 
              the atmosphere is above 10% of the LFL, all of the 
              requirements of the standard must be met.

Question 2.    Regarding the above question (question 1) are there
              particular procedures or precautions that are required under
              these conditions?

Answer         Since PRCS is a performance standard, it does not specify
              procedures for conditions where the permit-required space
              has a hazardous flammable atmosphere.  However, what the 
              standard does specify in paragraph (d) is that the employer
              must identify and evaluate each hazard to which the
              entering employees will be exposed.  Based on the hazard 
              analysis, the employer must develop and implement the means,
              procedures, and practices necessary for safe permit space 
              entry operations.

              Although the PRCS standard may not specify or necessarily
              apply to specific precautions an employer must take regarding
              a hazardous flammable atmosphere, other OSHA standards could
              apply.  For example, if the flammable atmosphere also 
              presented a respiratory hazard requiring protection, 29 CFR
              1910.134 specifies precautions relative to the selection
              and use of respirators.  If the flammable atmosphere is the 
              result of a process involving equipment, there may be 
              precautions with regard to the equipment that an employer  
              would be required to follow.

Question 3.    Have OSHA or any other government agencies made
              specific studies regarding the difficulties of accurately
              calibrating (LFL) monitoring devices, when multiple solvent
              coatings are used in a spray coating?

Answer         OSHA is not aware of any specific studies that have been
              conducted in this area.  However, we understand that most 
              manufacturers of this type of testing equipment have 
              addressed this issue.  Manufacturers setup and calibrate 
              their equipment using a single calibration gas (usually
              menthane) and then provide their end users with conversion 
              tables or factors for determining the percentage of the LFL
              for other gases.  Where the finish being applied is a mixture,
              the manufacturer of the coating, through the Material Safety
              Data Sheet or other product information, is able to advise
              the employer of the individual solvent characteristics.

Question 4.    Have OSHA or any other government agencies made
              specific studies as to minimum and maximum distances LFL
              monitoring equipment may be located from the spray process, 
              without adversely affecting worker safety?  (Or adversely 
              affecting monitoring equipment reliability?)  Where can  
              copies of these study results be obtained?

              Per your conversation with Don Kallstrom of my staff, the
              root question is, Where and how often is monitoring required
              under 29 CFR 1910.146(d)(5)(ii) to meet the intent of the 
              standard for a spray painting operation within a railroad tank

Answer         The standard does not specify frequency rates because of the
              performance oriented nature of the standard and the unique 
              hazards of each space. However, there will always be, to some
              degree, testing or monitoring during the entry operations 
              which is reflective of the atmospheric hazard. 

              The employer must determine the degree and the frequency of 
              testing or monitoring.  Some of the factors that affect 
              frequency are results of test allowing entry, the regularity 
              of entry (daily, weekly, or monthly), the uniformity of the 
              permit space (the extent to which the configuration, use,
              and contents vary), the documented history of previous 
              monitoring activities, and knowledge of the hazards which 
              affect the permit space as well as the historical experience
              gained from monitoring results of previous entries.

              Knowledge and recorded data gained from successive entries
              (such as ventilation required to maintain acceptable entry 
              conditions) may also be used to document changes in the 
              frequency of monitoring.

              The placement of the testing or monitoring instrument in
              relation to the employee performing spray coating operations
              is also not specified in the standard.  The intent of this 
              paragraph is to ensure that the predetermined acceptable entry
              conditions established by the employer are being maintained 
              during the entry.  Where the employer can demonstrate that
              the hazard concentration to which the employee is being exposed
              is uniform throughout the tank car being sprayed, then the 
              placement of the instrument is not critical.

Should you have further questions on this correspondence please contact Mr. Don Kallstrom of my Office of Safety Compliance Assistance staff (202)219-8031 x 109.

John B. Miles, Jr., Director
Directorate of Compliance Programs