The evaluations for both crystalline and amorphous silica pertain to inhalation
resulting from workplace exposures. Lung cancer was the primary focus. The W orking
Group' s evaluation of the epidemiological evidence for potential causal relations
between silica and cancer risk was focused principally on findings from studies that were
1east like1y to have been distorted by confounding and selection biases. Among these
studies, those that addressed exposure-response associations were especially influential
in the Working Group's deliberations.
Crystallne silica
Possible differences in carcinogenic potential among polymorphs of crystalline silica
were considered. Sorne studies were of populations exposed principally to quartz. ln only
one study (that of United States diatomaceous earth workers) was the exposure predominantly
cristobalite. Studies of mixed environments (i.e. ceramics, pottery, refractory
brick) cou Id not delineate exposures specifically to quartz or cristobalite. Although there
were sorne indications that cancer risks varied by type of industry and process in a
manner suggestive of polymorph-specific hazards, the Working Group cou Id only reach
a single eva1uation for quartz and cristobalite. Nonetheless, the W orking Group did note
a reasonab1e degree of consistency across studies of workers exposed to one or both
polymorphs.