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Respirable Dust Sampling and Measurement 199

minimum number of measurements must be taken to obtain a reliable estimate on dust
exposure. The next article will highlight the importance of a minimum of five shift-
long samples for obtaining a reliable average.

12.5 Respirable Dust Sampling Strategy

3
The enforcement of “respirable dust standard” at 1.5 mg/m in the United States is
based on the average of five samples collected on five consecutive shifts. This is based
on very sound scientific basis because respirable dust concentration is a random var-
iable and averaging is necessary for accuracy. There are some proposals to decide
compliance with the legal standard based on a single sample. To illustrate its inaccu-
racy, a scientific study of variations in silica concentrations was conducted, and the
minimum number of samples needed to arrive at a mean that is within 20% of the
true mean with 95% level of confidence was calculated. The number of samples
needed varied from 5 to 86 depending on the randomness of silica concentrations in
air and reemphasized the need for at least five samples for a better estimate on true
dust concentration [11].
When the respirable dust in the atmosphere of any working place in a mine contains
more than 5% quartz, the operator of the mine is required to reduce the concentration
3
of respirable dust to a value lower than the standard 2.0 mg/m . The reduced standard,
as published in CFR 30 Part 71.101 of the Federal Register, is determined by dividing
the number 10 by the percent of quartz. Thus, the maximum allowable level of quartz
3 3
in the mine atmosphere is 0.1 mg/m (5% of 2 mg/m ).
The respirable dust standard has been determined by this method since 1971. Some
recent changes in the Mine Safety and Health Administration’s (MSHA’s) analytical
procedure for measuring quartz and in the Federal regulations regarding sampling lo-
cations have resulted in reduced standards being applied to a greater number of mine
sections. In 1980, only 130 mine sections were subject to a reduced standard; however,
in 1981, the number of sections on a reduced standard increased by an order of magni-
tude to 1300 [12]. A part of the increase could be due to larger number of samples be-
ing analyzed, but the majority of this increase remains unexplained.
Before February 1981, the analytical procedure used by MSHA’s Pittsburgh labo-
ratory to determine the percentage of quartz in respirable dust required a sample, or a
composite of samples, weighing at least 5.0 mg. In February 1981, the procedure was
revised, and the quartz analysis can now be done on as little as 0.5 mg of dust [13].
Consequently, samples previously too small for quartz analysis, are being analyzed.
In other words, before February 1981, respirable dust samples collected in a mine sec-
tion would have to be added together over many shifts to obtain enough dust for quartz
analysis. This effectively resulted in the quartz exposure being determined on a time-
averaged basis.
With the revised analytical methods, MSHA can now analyze for quartz on a single
shift dust sample, which is not indicative of the workers’ long-term exposure to quartz.
The other factor that contributes to a greater number of reduced standards being
applied lies in the definition of a working place. The original definition of a working