March 28, 1985

Mark Nicas, M.S., M.P.H.
Associate Industrial Hygienist
Research and Standards Development Unit
Department of Industrial Relations
Post Office Box 603
525 Golden Gate Avenue
San Francisco, California 94101

Dear Mr. Nicas:

This is in response to your letter of March 28, concerning respiratory protection for abrasive blasting with silica sand.

Provision 29 CFR 1910.94(a)(5)(ii)(b) of the ventilation standard requires that "abrasive-blasting respirators" shall be worn by all abrasive-blasting operators when using silica sand in manual blasting operations where the nozzle and blast are not physically separated from the operator in an exhaust ventilated enclosure. This is a work practice provision. That is, whenever the described operation takes place, "abrasive-blasting respirators" must be worn.

The Occupational Safety and Health Administration (OSHA) interprets provision 29 CFR 1910.94(a)(5)(iii)(b) as providing for a limited deviation from this requirement by permitting the use of "dust-filter respirators" as interim protection for an operator while "abrasive-blasting respirators" are temporarily unavailable for use for a justifiable reason. A "dust-filter respirator" may not be used as interim protection, however, if the airborne concentration of crystalline silica exceeds the protection factor allotted the respirator. If the "dust-filter respirator" is not designed to protect the operator's eyes and face from the impact of abrasives, supplementary equipment providing such protection will have to be provided in accordance with 29 CFR 1910.94(a)(5)(v)(b).

Thank you for contacting OSHA. If we can assist you again in the future, we shall be glad to do so.

Sincerely,



John B. Miles, Jr., Director
Directorate of Field Operations




March 28, 1985

Directorate of Field Operations
Occupational Safety and Health Administration
U.S. Department of Labor
Room N3603
200 Constitution Avenue, N.W.
Washington, D.C. 20210

Dear Sir or Madam:

I am requesting an interpretation of 29 CFR 1910.94(a) pertaining to the use of negative-pressure, air purifying dust filter respirators for silica sand blasting.

Where silica sand is the abrasive and the nozzle and blast are not physically separated from the operator by an exhaust- ventilated enclosure, 1910.94(a)(5)(b) requires that an approved continuous flow, supplied-air respirator (presumably Type CE) be worn by the operator; no minimum blasting time (e.g., 15 minutes or less) is specified before such respirator use is required. With regard to negative-pressure dust filter respirators, 1910.94(a)(5)(iii) states they may be used for "short, intermittent, or occasional dust exposure such as cleanup, dumping of dust collectors, or unloading shipments of sand at a receiving point;" I note that short-term (e.g., 15 minutes or less) blasting with silica sand is not one of the "short, intermittent, or occasional dust exposures" given as an example for permissible use of negative-pressure dust filter respirators. Therefore, 1910.94 (a)(5)(ii)(b) and 1910.94(a)(5)(iii) appear consistent in not permitting the use of negative-pressure dust filter respirators when manually blasting with silica sand, even for short time periods (e.g., 15 minutes or less), where the nozzle and blast are not physically separated from the operator by an exhaust-ventilated enclosure. Further, the language is that of a design standard; that is, a continuous flow, supplied-air respirator (presumably Type CE) is required without regard to the silica dust concentration (and without the need to demonstrate an exposure above the permissible exposure limit for silica-containing dust) where silica sand is the abrasive and the nozzle and blast are not physically separated from the operator by an exhaust-ventilated enclosure.

However, 1910.94(a)(5)(iii)(b) subsequently states that negative pressure dust filter respirators shall not be used for "continuous protection" where silica sand is the abrasive. The term "continuous protection" is ambiguous in that it appears to allow negative-pressure dust filter respirator use for "non- continuous" protection, for example, for a 15 minute manual blasting operation with silica sand where the nozzle and blast are not physically separated from the operator by an exhaust- ventilated enclosure. Yet such "non-continuous" use of a negative-pressure dust filter respirator would be inconsistent with the respirator use practice intended by 1910.94(a)(5)(iii).

Please note that I have already discussed this apparent inconsistency with Mr. Donald Webster, CIH, who was chairman of the subcommittee which developed the ANSI Z9.4-1979 standard; the 1979 standard is identical to the 1968 standard (on which 1910.94(a) is based) with respect to the specific respirator provisions in question. If I may paraphrase part of our conversation, Mr. Webster recollected that ANSI Section 6.2 (corresponding to 1910.94(a)(5)(ii)) specifically required a positive-pressure respirator for manual blasting with silica sand because the ANSI committee considered silica to be "such a bad actor", and that silica was specifically singled out from other toxic dusts due to the ANSI committee's concern with silica's pulmonary toxicity. He also stated that the "continuous protection" language in ANSI Section 6.3.2 (corresponding to 1910.94(a)(5)(iii)(b)) did not appear to be consistent with the respirator use practice specified in ANSI Section 6.2; however he could not locate any ANSI committee meeting comments on this respirator question.

Given this preface, my questions are the following:

1) Does 1910.94(a) permit the use of a negative-pressure dust filter respirator for manual blasting with silica sand for short time periods where the nozzle and blast are not physically separated from the operator by an exhaust- ventilated enclosure?

2) If the answer to question (1) is affirmative, for what time period (e.g., 15 minutes, 120 minutes, etc) during an 8- hour shift is negative-pressure dust filter respirator use permitted for manual blasting with silica sand where the nozzle and blast are not physically separated from the operator by an exhaust-ventilated enclosure?

3) If the answer to question (1) is affirmative, up to what silica dust exposure level (e.g., 10 times the PEL) is negative-pressure dust filter respirator use permitted during manual blasting with silica sand where the nozzle and blast are not physically separated from the operator by an exhaust-ventilated enclosure? (I note that 1910.95(a) (5)(ii)(c) requires the use of a continuous flow, supplied-air respirator (presumably Type CE) where concentrations of toxic dust (presumably including crystalline free silica) exceed one-time the PEL and the nozzle and blast are not physically separated from the operator by an exhaust-ventilated enclosure).

4) If the answer to question (1) is affirmative, and given that you specify some multiple of the PEL in your answer to question (3), is the employer required to demonstrate by records of personal exposure monitoring during similar previous manual blasting operations, or by use of a "real time" aerosol mass monitor (with a conservative assumption regarding the percent crystalline free silica content of the aerosol), that the silica dust exposure is below this multiple of the PEL?

I appreciate your timely response to these questions.

Respectfully,



Mark Nicas, M.S., M.P.H.
Associate Industrial Hygienist
Research and Standards Development Unit