Toxicology of Industrial Compounds

94 CARCINOGENIC POTENTIAL OF MAN-MADE VITREOUS FIBERS
major studies, comprise sub-groups within them, or represent smaller
worker populations outside of them.
A Canadian study was reported by Shannon at the WHO Occupational
Health Conference on Biological Effects of Man-Made Mineral Fibers at
Copenhagen in 1982 and 1986 (Shannon et al., 1987). It followed 2557
male workers at a Canadian glass wool plant through 1977 and was later
updated to extend the follow-up to the end of 1984. In the updated study,
the authors reported a statistically significant excess of lung cancer. In
discussing this excess, the authors concluded that the interpretation of the
information was difficult since there was no relationship between the
excess of lung cancer and the length of time since first exposure to the
fibrous glass manufacturing environment.
Two recent case-control studies have addressed the lung cancer mortality
of FG and slag wool production workers. Chiazze et al. (1992) have
investigated the potential impact of confounding factors such as smoking
and other occupational exposures for workers at the oldest and largest US
fiber glass manufacturing facility. In particular, Chiazze helped clarify the
heavy smoking patterns in those workers and verified the large impact that
smoking has on their lung cancer experience. Wong et al., (1991)
investigated the potential impact of smoking on the lung cancer deaths at
nine US slag wool manufacturing plants. Wong also found heavy smoking
among the slag wool workers and advanced the understanding of the
modest increase in lung cancer seen in the historical cohort studies cited
above.
Users of MMVFs generally have experienced mixed exposures, making
the study of any potential health effects of MMVF difficult, if possible at
all. For example, in a study of Swedish construction workers, Engholm et al.
(1987) discussed the difficulty caused by overlapping of reported exposures
to asbestos and MMVFs. In addition, essential employment and exposure
histories for users of MMVFs are lacking.
The mortality studies of FG workers, while showing a small but
statistically significant increase in lung cancer, have failed to show any
consistent relationship with exposure to FG (i.e. no dose-response
relationships have been found). It is recognized that uncontrolled
occupational and/or non-occupational confounding factors may be
associated with the slight increase. The IARC review (IARC, 1988)
concluded that there is ‘inadequate evidence’ for carcinogenicity in
humans. Other reviews have reached similar conclusions. In addition,
reports subsequent to the IARC review have further clarified potential
confounding factors and, if anything, shown weaker evidence of a
relationship between exposure and lung cancer.
The cohort mortality studies of rock wool and slag wool workers have
shown a somewhat larger statistically significant excess of lung cancer
deaths, but have also provided no clear dose-response relationship with