Standard Interpretations - Table of Contents|
| Standard Number:||1910.94(a)(2); 1926.57(f)(2); 1926.62(d); 1910.1018(e); 1910.1027(d)|
August 13, 1999
Ms. Irene S. Fanelli, CIH
Dear Ms. Fanelli:
We are in receipt of your letter of January 8, 1999, requesting assistance regarding exposure monitoring requirements for the Lead-in-Construction (29 CFR 1926.62), Arsenic (29 CFR 1910.1018), and Cadmium (29 CFR 1910.1027) Standards. Please excuse this delay in response. Your questions, repeated below, related specifically to monitoring for abrasive blasting operations.
The correct placement for air sampling cassettes is near the breathing zone of the employee. It should be as close as possible to the employee's nose and mouth, i.e., in a hemisphere forward of the shoulders within a radius of 6 to 9 inches. Since OSHA is concerned with an employee's potential exposure, the cassette must also be located outside of any protective equipment.
This measurement will, in large part, determine the actions the employer must take to provide the employee with protection, such as appropriate personal protective equipment, medical monitoring, and hygiene facilities. Each of the standards you mentioned (Cadmium, Lead-in-Construction, and Arsenic) recognize ingestion as a route of exposure. Particles which are too large to contribute to the employee's airborne exposure nevertheless contribute to the overall exposure through the potential for ingestion. The standards were written to include protection from ingestion of these toxic metals.
Therefore, while it is true that sampling inside the hood would eliminate particles driven into the cassette by the force of the blasting operation, it would also result in a measurement which is not representative of the employee's potential exposure. Measuring inside the shroud would create an artificially low exposure concentration as it would be composed of a mixture of the air passing out of the respirator and the contaminated air outside the hood.
The current, acceptable sampling method for determining employee exposure is to place the sampling cassette outside of all personal protective equipment.
The HHE looked at the effect of blasting materials, specifically steel grit, on employee sampling results. It concluded that non-inhaleable particles of steel grit trapped in the cassette inflate the employee's airborne lead-exposure level. The study did not determine an effective method of sampling which would overcome this problem.
The monitoring methods OSHA uses for lead, cadmium, and arsenic meet the accuracy requirements as stated in the standards. The overall accuracy for monitoring, which is conducted in the field and analyzed in a laboratory, is customarily broken into two constituents: sampling and analytical error. The analytical error is derived from both the recovery efficiency of the compound from the collection substrate and the precision of the analytical technique. Conventionally, the sampling error is plus-or-minus five percent to account for variations in pump flow. Both of these errors are summed to derive the accuracy of the method of monitoring. The collection efficiency of the sampler, in this case a 37mm closed face cassette containing a mixed cellulose ester filter, is assumed to represent the exposure of interest. There is specific information on analytical and sampling procedures listed by subject on the OSHA web site in the Chemical Information File, the OSHA Technical Manual, and the Analytical Methods.
The sampling procedure followed by OSHA Compliance Officers is to place the cassette outside of all personal protective equipment. Further information is described in OSHA's Technical Manual, TED 1.15, Section 1, Chapter 1, a copy of which is enclosed for your convenience.
OSHA adopted its current particulate methodology based upon a general consensus at the time. The Agency is aware of the continuing discussion of the particle-size-dependent efficiency of the different sampling devices and of the human body at inhalation or ingestion. The Agency's Salt Lake Technical Center is closely following the efforts to reach a consensus of which sampler configuration best represents the exposure of occupational concern.
The effects of simply putting a deflector of unknown specifications at the opening of a sampling train are not well studied. NIOSH's HHE Report 94-0122-2578 used a deflector and found the device to be ineffective at shielding the sampler from abrasive blast grit. One sampler manufacturer has recently introduced a sampler with an inlet port shield by a perforated screen. The effect of this screen on sampler efficiency is currently being studied by NIOSH. The use of any deflectors or screens would be evaluated by the Agency based upon the available information supporting the sampling characteristics of such devices.
The employee's potential exposure is the contaminant air concentration in the employee's personal breathing zone without regard to personal protective equipment, including the respirator and its abrasive blasting shroud. If the sampling inlet is not located outside of all personal protective equipment, OSHA could cite the employer for failing to adequately assess employee exposure.
We hope that this adequately answers your questions. If further information is needed, please do not hesitate to contact the Office of Health Compliance Assistance on 202-693-2190.
Richard E. Fairfax, Director
Standard Interpretations - Table of Contents|