30 CFR § 33.33 Allowable limits of dust concentration.
(a) The concentration of dust determined by the control sample shall
be subtracted from the average concentration of dust determined by the test
samples collected at each drill operator's position, and the difference shall
be designated as the net concentration of airborne dust. Calculations of the
average concentration of dust determined from the test samples shall be based
upon the results of not less than 80 percent of each set of test samples.
(b) Under each prescribed test condition, the net concentration of
airborne dust at each drill operator's position shall not exceed 10 million
particles (5 microns or less in diameter) per cubic foot of air when
determined in accordance with the method given in §33.32(a).
[Sched. 25B, 25 FR 6473, July 9, 1960, as amended at 26 FR 2599, Mar. 28,
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