PCM by NIOSH 7400

In NIOSH Method 7400, asbestos is collected on a 25 mm cellulose ester filter (cassette-equipped with a 50 mm electrically-conductive cowl). Sampling is done at 0.5 L/min or greater. Adjust sampling flow rate, Q (L/min), and time, t (min), to produce a fiber density, E, of 100 to 1300 fibers/mm 2 for optimum accuracy.

When preparing the sample in the lab, the filter is treated with acetone (to collapse) and triacetin (to immerse) to make it transparent and then is analyzed by microscopy at 400x to 500x magnification, with phase-contrast illumination, using a Walton-Beckett Graticule. A fiber is defined as any particle with a length >5�m and a length-to-diameter ratio of >3:1.

Counting rules

a. Count any fiber longer than 5�m which lies entirely within the graticule area.
(1) Count only fibers longer than 5�m. Measure length of curved fibers along the curve.
(2) Count only fibers with a length-to-width ratio equal to or greater than 3:1.

b. For fibers which cross the boundary of the graticule field:

(1) Count as 1/2 fiber any fiber with only one end lying within the graticule area, provided that the fiber meets the criteria of rule a above.
(2) Do not count any fiber which crosses the graticule boundary more than once.
(3) Reject and do not count all other fibers.

c. Count bundles of fibers as one fiber unless individual fibers can be identified by observing both ends of a fiber.

d. Count enough graticule fields to yield 100 fibers. Count a minimum of 20 fields. Stop at 100 graticule fields regardless of count.

Comparison of B rules and A rules

A number of counting rules are available to quantify airborne concentrations of synthetic vitreous fibers (SVFs), including the National Institute for Occupational Safety and Health (NIOSH) Method 7400 A and B rules and the World Health Organization European Reference Method for Man-Made Mineral Fibers. Differences in counting rules have led to a lack of comparability of sampling results across studies. The purpose of this article is to compare NIOSH Method 7400 A and B fiber counts using samples collected as a part of a large assessment of end-user exposures to SVFs. This comparison provides a link between previous investigations that utilized A rules and more recent investigations that used B rules. A total of 792 airborne fiber samples were collected according to NIOSH Method 7400 and separately evaluated using both NIOSH A and B counting rules. The results of this investigation indicate that A-rule fiber counts give statistically higher fiber density estimates than corresponding B-rule fiber counts, with fiber glass (except fiber glass blowing wool without binder) and mineral wool products A-rule counts producing counts approximately 70% greater than corresponding B-rule counts. A-rule fiber counts for refractory ceramic fiber products averaged approximately 33% greater than corresponding B-rule estimates. In contrast, primarily due to their smaller diameter distribution, fiber glass blowing wool without binder produced B-rule fiber counts only 8% greater than the corresponding A-rule fiber counts.