Suggestion of a Cause-and-Effect Relationship
Among Coal Rank, Airborne Dust, and
Incidence of Workers’ Pneumoconiosis

Steven J. Page and John A. Organiscak

Department of Health and Human Services, Centers for Disease Control and
Prevention, National Institute for Occupational Safety and Health,
Pistsburgh Research Laboratory, 2.0, Box 18070, Pittsburgh, PA 15236;
e-mail sep8@cdc.goo

rolonged exposure to airborne

spirable coal mine dust is responsi-

ble for coal workers’ pneumoconi-

osis (CWP). Furthermore, miners
who show evidence of higher radiographic
categories of simple CWP are at increased
risk of developing progressive massive fibro-
sis (PMF). As of 1990, there were nearly
130,000 coal miners in the United States.”
This excludes mines that produce less than
10,000 tons annually and the anthracite
coal mines. Estimates indicate that at age
58, an average of 7/1000 U.S. workers and
89/1000 U.K. workers will have developed
PMF.2+) Health research studies have iden-
tified that the risk of developing and the
severity of CWP are directly related to (1)
the amount of respirable dust exposure and
(2) the coal rank.'?+) The rank of coal is its
stage of formation in the series peat, lignite,
sub-bituminous, bituminous, and anthra-
cite (in order of increasing rank). The rank
of coal can also be represented by the com-
mercially important characteristics specified
by its proximate analysis: fixed carbon, vol-
atile matter, mineral matter, and moisture
content,

CHARACTERISTICS OF COAL
MINE DUST

Mi: research has been devoted to
identifving cause-and-effect relation-
ships between the properties and charac-
teristics of coal mine dust and the inci-
dence of CWP, Although evidence sug-
gests both physical and chemical
Properties of coal mine dust contribute to
CWP, no definitive conclusions have bee:
reached. A great part of the difficulty i
due to the confounding nature of “coal
mine dust,” which is not limited to coal

dust itself but includes all mineral dusts
produced from cither within the coal seam
or the rock strata surrounding the coal
seam. The question is, what characteristics
of coal mine dust, some of which are as-
sociated with rank, are related to the ob-
served differences in exposure-effect rela-
tionships that have been observed?

To examine these questions, numerous
studies’ have investigated the correla-
tion between mineral coatings that oc-
clude the surface of a crystalline silica par-
ticle core and the rank of coal. The
hypothesis is that the degree of surface
occlusion determines, at least in part, the
biologically available crystalline silica in
an inverse relationship. Other stud-
ies"! have investigated the free radicals
(electric charges) found on fresh-
fractured rock ‘and coal from high rank
coal areas. These studies have suggested
these radicals on the surface of mineral
dusts may be relevant to the pathophysi-
ology that results from their inhalation.
In particular, it has been found that fresh-
ly broken silica dust (responsible for sili-
cosis) is more reactive in vivo than aged
silica, Although not all inclusive, the
above-mentioned studies represent per-
haps the most viable argument for caus-
ative coal rank-related effects to date.

Current knowledge appears to suggest
a significant cause-and-effect relationship
between coal rank-related charging char-
acteristics and the increased incidence of
CWP in high rank coal regions. The two
mechanisms most affected by the charg-
ing characteristics are enhanced respira-
tory deposition and inherent toxicity of
airborne respirable dust (ARD). This sug-
gestion is based on the following known
facts: (1) higher rank coals produce a
higher CWP incidence rate;'*) and (2)

higher rank coals obtain a higher electro-
static charge on breakage,(!7-

COAL RANK AND DUST
GENERATION

rior research has identified several re-

lationships between coal rank and dust
generation. Laboratory coal breakage
studies have shown a significant and con-
sistent positive correlation with the
amount of respirable-size particles found
in the product increasing with coal
rank.2- These studies show conclusively
that either a grinding or crushing process
yields specific total dust and respirable dust
generation rates (milligrams of dust in
product per kilogram of product) that in-
crease with increasing coal rank. It is im-
portant to note these results were mea-
surements of dust in the product and not
measurements of airborne dust. This paper
invites the reader to consider recently pub-
lished research that may establish an im-
portant link. between the electrostatic
charging characteristics of coals and the
high incidence of CWP in higher rank
coals,

The Dust and Toxic Substance Control
Branch at the Pittsburgh Research ‘Labo-
ratory has investigated ARD generation
and electrostatic charging characteristics as
they relate to coal rank. Organiscak and
Page” and Page and Organiscak'" per-
formed laboratory crushing experiments
on a range of low- to high-volatile bitu-
minous coals to investigate the various tac-
tors influencing fresh ARD generation and
liberation from the coal product. It was
shown in these studies that the amount of
ARD is positively correlated with moisture
content and negatively correlated with
electrostatic field. This was confirmed in-
dependently by Page.” The resultant ef-
fect of these factors is that different per-
centages of coal particles of respirable size
are dispersed as ARD, with the largest
amount of ARD produced occurring in a
narrow range of low rank coals (more than
one order of magnitude range in ARD
compared to higher rank coals). It is sig-
nificant that, for the first time, the amount
of ARD produced from different coals can
be predicted, based on well-known com-
mercial coal rank parameters.

These same parameters that establish



the charge/rank relationship of coals may
partially explain the increased prevalence
of CWP observed with high rank coals
through the mechanism of increased lung
deposition. It has long been known that
airborne dust particles can have a signifi-
cant amount of electrostatic charge'#2”
and that this charge can place dusts in
varying degrees of agglomeration.282”
Lifshits et al.” found that respirable-sized
aluminum dust particles had lung deposi-
tion of 34% when uncharged and 66%
when charged. Melandri” found that
lung deposition of airborne particulate in-
creased by up to 30% with charging prop-
erties of the airborne particulate, relative
to a similar but otherwise neutral acrosol.
Furthermore, Melandri suggested the lung
deposition efficiency can be greatly affect-
ed by the charge distribution. This in-
creased deposition may also be attributed
to the higher lung deposition efficiency of
agglomerated ultrafine submicron parti-
cles2238

As referenced earlier, numerous studics
have shown that fresh-broken coal and
quartz surfaces contain highly reactive free
radicals (clectric charges). This establishes
the potential for cytotoxicity through
bond formation with the cell membrane
upon inhalation. Dalal et al.''*' noted that
preliminary tests using the sheep red-
blood cell hemolysis procedure indicated
that fresh-crushed anthracite coal dust ¢x-
hibits significantly higher homolytic activ-
ity than the same dust left in air for several
hours. Dalal et al. also remarked that since
exposure of fresh coal dust to air causes a
decrease in the amount of the free radicals,
as well as a decrease in the cytotoxicity, it
is possible that the free radicals do con-
tribute to the coal dust’s cytotoxicity. Val-
lyathan et al." observed that freshly
ground silica dust exhibits a greater cyto-
toxic effect on cellular membrane integrity
and that, because acute silicosis is fre-
quently associated with occupations in
which freshly fractured crystalline silica of
respirable size ate generated, fracture-gen-
crated silicon-based radicals may play a sig-
nificant role in the pathogenesis of silico-
sis. Although the work by Organiscak and
Page focused on the charging characteris-
tics of freshly crushed coals, brief tests on
various sandstones indicated the ARD
generated exhibited much higher degrees
of electrostatic charging.

CONCLUSION

In view of the work by Lifshits et al.,°
(Melandri et al." Dalal et al.) and
Vallyathan et al.,?! the investigations
of Organiscak and Page” on the charging
characteristics of coal suggest a significant
cause-and-effect relationship between the
coal rank-related charging characteristics,
enhanced respiratory deposition and tox-
icity of ARD, and the increased incidence
of CWP in high rank coal regions.
Recommendations to improve the
health of coal miners would necessarily fo-
cus on providing more effective dust gen-
eration and abatement in the mining of
higher ranked coals through engineering
control technology. The Pittsburgh Re-
search Laboratory is currently pursuing
more in-depth studies of coal charging
characteristics and the use of water addi-
tives (surfactants) for water spray systems,
which are widely used in coal mines to im-
prove dust suppression and reduce miner
dust exposure.

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