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crystalline silica

updated sat 29 dec 01


Edouard Bastarache on tue 14 aug 01

Hello Matt,

here is what us canadian specialists in the field think of IARC's standpoint
the carcinogenic property of cristalline silica.

TITLE: Silica, silicosis, and lung cancer: a response to a recent working
group report [In Process Citation] AUTHORS: Hessel PA; Gamble JF; Gee JB;
Gibbs G; Green FH; Morgan WK; Mossman BT
AUTHOR AFFILIATION: University of Alberta, Edmonton, Canada.
SOURCE: J Occup Environ Med 2000 Jul;42(7):704-20
[MEDLINE record in process]
CITATION IDS: PMID: 10914339 UI: 20371548
ABSTRACT: The relationship between crystalline silica and lung cancer has
been the subject of many recent publications, conferences, and regulatory
considerations. An influential, international body has determined that there
was sufficient evidence to conclude that quartz and cristobalite are
carcinogenic in humans. The present authors believe that the results of
these studies are inconsistent and, when positive, only weakly positive.
Other, methodologically strong, negative studies have not been considered,
and several studies viewed as providing evidence supporting the
carcinogenicity of silica have significant methodological weaknesses. Silica
is not directly genotoxic and is a pulmonary carcinogen only in the rat, a
species that seems to be inappropriate for assessing particulate
carcinogenesis in humans. Data on humans demonstrate a lack of association
between lung cancer and exposure to crystalline silica. Exposure-response
relationships have generally not been found. Studies in which silicotic
patients were not identified from compensation registries and in which
enumeration was complete did not support a causal association between
silicosis and lung cancer, which further argues against the carcinogenicity
of crystalline silica.

"On October 22, 1996, an International Agency for Research on Cancer (IARC)
working group voted 10 to 7 to reclassify inhaled crystalline silica as a
Group1 carcinogen from a Group 2 carcinogen. IARC considers substances in
its Group 1 category to be carcinogenic to humans, while Group 2 refers to
substances IARC considers probably carcinogenic to humans. IARC classified
inhaled crystalline silica into its Group 2 category in 1987. In reaching
its 1996 reclassification decision, the working group reviewed additional
experimental animal studies and selected human epidemiological studies."

"The evidence from human studies was, according to the working group, based
on "inhalation resulting from workplace exposures." The IARC group reviewed
studies of workers exposed to crystalline silica at concentrations much
higher than are found in controlled work sites today. Even among those
studies, the working group as a whole noted that "carcinogenicity was not
detected in all industrial circumstances," and thus "may be dependent on
inherent characteristics of crystalline silica or external factors
its biological activity or distribution of its polymorphs.""

Matt MacIntire on wed 15 aug 01


Thank you for that information. I do appreciate your informative posts.

It did seem surprising to me that silica was considered by the IARC as MORE
hazardous than mineral fibers. It appears there is still some disagreement
over the danger of these materials. The literature seems to run over a
fairly wide spectrum.

Perhaps risk assessment is like parenting... One can find books that advise
we never spank our children, while other books suggest that it is OK. We
can usually find an author who supports our initial predisposition. Yet we
must all review the literature, make a rationale decision for ourselves, and
get on with our lives.

I am curious to find out how you regard these sorts of risks, in relation to
our everyday life. You seem very informed about hazardous materials. You
seem interested in pottery. I am curious how you draw the line for yourself
when deciding what you will expose yourself to and how the risk from such
exposure fits into the scope of the risks we all take every day. We drive
cars and fly in airplanes. We inoculate our children with new vaccines. We
get blood transfusions. We eat smoked sausage. We work on our tan in the
summer. We are all going to die. Many of us will die from cancer. Maybe
in some real way, we must pick our poisons.

How do you make any sense of the risks we take each day?

Permit one more question, if you don't mind. I have heard, as I mentioned
in another post, that the shape and size of mineral fiber particles as
important as their chemical composition. My old geology professor said that
some types of asbestos are much more dangerous than others, owing to their
physical characteristics. She suggested to me that it was likely that
certain types of mineral fibers might be just as dangerous as asbestos for
this reason. Have you seem any information that implicates physical
characteristics so clearly?

thank you for your thoughts


Edouard Bastarache on fri 28 dec 01


among the different silica polymorphs only quartz has a C2 designation =
in Quebec,
C2 meaning a suspected carcinogen to human beings.
The VEMP is 0.1 mg/m3 as respirable dust.
VEMP means Valeur Moyenne d'Exposition Pond=E9r=E9e.(Exposure Limit)


Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker