Page 222 - Advanced Mine Ventilation

202 Advanced Mine Ventilation

Thus, the US silica and respirable dust standards are already much lower than what
other major mining countries have deemed necessary to protect the health of mine
workers and maintain high productivity. In 1974, NIOSH published a report [17] rec-
ommending that workers should not be exposed to levels of quartz greater than
3
0.05 mg/m , i.e., one-half of the present limit. In April 1978, MSHA circulated a draft
proposal of revisions to 30 CFR Part 71 in which it was recommended that the 2.0 mg/
3
m standard be reduced when the quartz present in respirable coal dust exceeded
3
0.05 mg/m .
There has been no evidence of silicosis in coal mining with the present standard in
effect and therefore there is no apparent need to further reduce the standard. NIOSH’s
reasoning for reducing this standard was based primarily on epidemiologic studies in
granite sheds. In one study [17], no indication of harmful effects to the workers was
3
found for free silica concentrations in the range of 0.03e0.09 mg/m , with an average
3
of 0.06 mg/m . In a separate study [17], no new cases of silicosis were found in men
starting to work in Vermont granite sheds after 1937, where few exposures exceeded
3
0.05 mg/m . Thus, NIOSH apparently found it appropriate to recommend 0.05 mg/m 3
as the new quartz standard.
Also in reference [17], the authors offer evidence that silica in the forms of cristo-
balite and tridymite are much more dangerous than alpha quartz and recommend that
the standard for these forms of free silica be one-half of that recommended for quartz.
3
Then on the same page they offer evidence showing that the exposure of 0.05 mg/m is
safe for cristobalite and tridymite. These forms of silica have not even been found in
3
coal mines, and reducing the current standard to 0.05 mg/m would thus constitute an
overkill.

12.5.2 Measurement of Quartz in Respirable Dust

Three principal methods are used for quantitative determination of silica in dust. These
are the colorimetric chemical procedure, IR spectrophotometry, and X-ray diffraction.
In comparative tests performed by NIOSH for determining quartz percentage in coal
dust, all of the methods were found to have a maximum deviation of 5% from the over-
all mean value [17], thus showing that all three methods can be used to determine
quartz percentage accurately. Owing to their sensitivity, speed, minimum sample prep-
aration, and capabilities for automation, the IR and X-ray methods have been used far
more often than the colorimetric method for routine quartz analysis.
The X-ray diffraction method has the capability of detecting the various forms of
free silica, e.g., quartz, cristobalite, and tridymite. The X-ray method is highly recom-
mended and routinely used by NIOSH. However, NIOSH recommends using IR spec-
trophotometry if interfering materials in the sample decrease the sensitivity of the
X-ray diffraction or when the quantity of the total sample is very small. The IR method
is used by MSHA for analysis of quartz in respirable coal dust, but it has restricted use