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Regulations (Preambles to Final Rules) - Table of Contents
• Record Type: Occupational Exposure to Asbestos, Tremolite, Anthophyllite and Actinolite [1988]
• Section: 5
• Title: Section 5 - V. Feasibility of Measuring Excursion Limit


OSHA also has determined, based on the rulemaking record of the revised standard, that the lowest feasible short term limit which can be reliably measured for purposes of the OSHA compliance programs, is 1 f/cc measured over 30 minutes. OSHA reaffirms that the OSHA Reference Method (ORM) provides the optimal technology for assessing worker exposure to airborne asbestos.

A brief review of the ORM is necessary to an understanding of this determination. The ORM is based largely on NIOSH Method 7400, a method widely acknowledged in the record as superior to the earlier NIOSH P&CAM 239 method previously prescribed by OSHA [Exs. 117-A; 123-A; 328; 330; TR. 6/20, p. 10; Tr. 6/21, p. 186; Tr. 7/81, p. 69].

In the preamble to the revised standards OSHA explained the relationship to the ORM to NIOSH P&CAM 239 and to the revised NIOSH 7400 method (51 FR 22688).

Because the NIOSH 7400 method takes advantage of technological improvements that have been adopted worldwide for asbestos sample analysis, but retains the same counting rules as the NIOSH P&CAM 239, OSHA has used the major features of the NIOSH 7400 method as the basis for developing a required standardized sampling and analytical method measuring airborne asbestos concentrations. The method required by the revised asbestos standards for both general industry and construction, referred to as the OSHA Reference Method (ORM), is detailed in the mandatory Appendix A of each standard. (1910.1001 and 1926.58).

These appendices require that the employer collect airborne asbestos samples using 25 mm diameter mixed cellulose filters and a 50 mm electrically conductive extension cowl. Samples must be analyzed using a phase contrast microscope calibrated using a phase shift test slide and equipped with a Walton-Beckett graticule. The ORM also requires that filter samples be prepared using acetone-triacetin clearing solution and be counted in accordance with the rules specified.

The ORM differs from the NIOSH 7400 method in two important respects. The ORM mandates a flow rate for asbestos sampling of between 0.5 and 2.5 lpm, which is similar to the flow rate range permitted by the NIOSH P&CAM 239 method (1.0 to 2.5 lpm). In contrast, the NIOSH 7400 method permits the use of any flow rate between 0.5 lpm and 16 lpm. Secondly, the ORM permits the use of the large 37 mm diameter filter when the employer has written justification explaining the need to use a larger filter to obtain readable samples. Both of these departures from the NIOSH 7400 method were made in response to commenters who pointed out that the use of high flow rates (e.g., 4 lpm) combined with the use of the smaller 25 mm filter may result in samples that are too overloaded with dust to permit the counting of asbestos fibers. This is particularly true in construction where nonasbestos dust particles released to the air as a result of demolition or renovation activities may interfere with analyzing samples that were collected using high flow rates and the smaller filter. OSHA believes that, by limiting the flow rate and permitting the use of the 37 mm filter in certain circumstances, employers will be more likely to obtain readable samples in dusty environments. As explained below however, the 37 mm filter will be allowed to measure short term exposures only when they are above the El. Since short term exposures in impacted construction activities are likely to exceed the EL, OSHA believes that many employers will continue to have the flexibility to pick the filter and flow rate to best assure reliable measurement results. In addition, record evidence suggests that the use of high flow rates may increase electrostatic charges in the filter apparatus, preventing some fibers from reaching the filter and resulting in lower fiber counts [Ex. 84-478: Tr 7/6., p. 99]. OSHA adopted these specific provisions to establish uniformity to the asbestos exposure determination.

To determine whether the ORM could be used to analyze short-term samples, and what the lowest feasible excursion limit is, the limit of reliable detection for 15-and 30-minute samples was evaluated. OSHA calculated the lowest reliable limit of quantitation using the following formulas:

                  E = ---------------


E  is the fiber density in fibers per square millimeter;
f  is the total fiber count;
n  is the number of microscope fields examined;
Af is the field area (0.00785 mm(2) for a properly calibrated
   Walton-Beckett graticule); and

                  C = ---------------


E  is as above;
Ac is the effective area of the filter (nominally 385 mm(2) for a
   25-mm-diameter filter and 855 mm(2) for a 37-mm-diameter filter; and
V  is the sample volume.

Prior to the ORM, analysts could use different procedures which resulted in different asbestos counts from one laboratory to the next. In addition the ORM method contains procedures that reduce variability in asbestos counts within a laboratory. In the final rule OSHA acknowledged that the use of the phase contrast light microscope method was approaching its limits of use with the new PEL, but OSHA determined the method, which the procedures required by the ORM, could reliably measure 8 hour TWA exposures at 0.1 f/cc for purposes of the OSHA compliance program.

Using the minimum filter loading that is suggested for the ORM (i.e., 80 fibers/100 fields, or 100 fibers/mm(2)), OSHA examined the relationships among these two sampling periods (15 and 30 minutes), the two filter sizes (25- and 37-mm in diameter), and various possible flow rates ranging between 2.5 lpm and 0.5 lpm.

The results set out in the Table show that 1 f/cc measured over 30 minutes is the lowest level which can be reliably measured for most operations likely to be affected by an excursion limit.

The ORM has been designed to provide needed flexibility to reliably measure exposures in the wide variety of operations where asbestos, tremolite, anthophyllite and actinolite are used. As explained in the preamble to OSHA's revised standards, filter overload or interference by other particles in dusty environments is accommodated by the ORM by permitting the use of the 37 mm filter when justified, and by reducing the flow rate. OSHA believed that in most cases reducing the flow rate will minimize filter overload for TWA exposure measurements, but allowed the 37 mm filter for stubborn situations, with written justification (51 FR 22690-1).

The major industries and operations affected by the imposition of an excursion limit; construction, and maintenance and brake repair in general industry, expose employees to the kinds of dusty environments which may result in filter overload. In addition, short term bursts of dust containing asbestos may contribute to overloading the filter.

The flexibility needed to reliably measure excursions in these operations, requires the ability to sample at low flow rates. Table X shows that only at the relatively high flow rates of 1.6 lpm and above are levels less than 1 f/cc over 30 minutes quantifiable. We note, based on the results in the Table that the use of the 37 mm filter is precluded for measuring short term limits down to 1 f/cc over 30 minutes. OSHA therefore finds 1 f/cc measured over 30 minutes is the lowest level feasibly measured for the operations impacted by this amendment.

OSHA notes that these considerations apply to measurements at or below the excursion limit, the level which must be capable of being measured for most enforcement and compliance purposes. The employer is not precluded from using the 37 mm filter to reliably measure short term exposures above the excursion limit so long as the level measured falls within the limits or reliability set out in the table. OSHA therefore will allow the use of the 37 mm filter for measuring short term exposures for the same reasons and requiring the same justification as time-weighted average measurements. If an employer uses measurement results to show exposures below the excursion limit, he must use the 25 mm filter.

Also, OSHA has determined that employers can comply with the 1 f/cc excursion limit within the accuracy requirements of the revised asbestos standards. As discussed at length in the preamble to the final rules (see 51 FR 22686-22691), the key factor in sampling precision is fiber loading. Using the minimum loading suggested by the ORM (80 fibers/100 fields, or 100 fibers/mm(2)), employers can be confident that they are measuring the actual airborne concentrations of asbestos in their workplaces within a standard sampling and analytical error (SAE) or +/-25%.(1)

  Footnote(1) OSHA evaluates the precision of the ORM (implemented as
NIOSH 7400) as follows: NIOSH has estimated that the overall
precision, expressed as the coefficient of variation (CV(t)), of the
7400 method ranges from 0.13 to 0.115 for samples in which 80 to 100
fibers per 100 fields have been counted [Ex. 84-444]. For filters at
the minimum loading suggested by the ORM, (80 fibers/100 fields) the
CV(t) is 0.13. This yields a 95% One Sided Upper Confidence Interval
of 21.4%. This is lower than the SAE of 25% currently listed for this
method in OSHA's Industrial Hygiene Technical Manual.

OSHA points out, as stated earlier, that a superficial contradiction exists between OSHA's finding the 1 f/cc measured over 30 minutes is the lowest reliable level of detection, and data cited regarding lower levels in brake repair (51 FR 22662). Those measurements, mainly derived from studies, were made by NIOSH with expert analytical capabilities under controlled conditions. In addition these measurements do not reflect the differences in results that occur due to common statistical sampling factors. As stated above, OSHA does not believe, based on a full rulemaking record, that such low levels can reliably be measured by employers for regulatory requirements, OSHA considers the recorded levels indicative of a range of exposures for the brake repair industry, and has not used these results for any other regulatory purposes.

Thus, OSHA's finding that the excursion limit of 1 f/cc for 30 minutes is the lowest that can be reliably measured is based upon the enforceability of the limit, recognizing that in some situations, lower exposures could theoretically be measured and are reported in the rulemaking record. In reaching this decision, OSHA has relied upon the asbestos rulemaking record, the equations described above being part of the record.

[53 FR 35609, Sept. 14, 1988]

Regulations (Preambles to Final Rules) - Table of Contents

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