Toxicology of Industrial Compounds

98 CARCINOGENIC POTENTIAL OF MAN-MADE VITREOUS FIBERS
exposure group at 3, 6, 12 and 18 (rats only) months and held without
further treatment until the end of the exposure period, when they were
killed to assess progression or regression of lung lesions and lung retention
and clearance of fibers after cessation of exposure. To assure quality
control, the lofting technique and exposure level were consistently
monitored during the study by both gravimetric measurement and fiber
counting techniques. The terminal sacrifice was carried out when only 20 per
cent of the animals survived. A complete necropsy was performed on each
animal. Gross pathological examination and diagnoses were performed
using a dissecting microscope. Uniform sections of the left lung and right
diaphragmatic lobe were embedded in paraffin, cut at a thickness of 4 mm
and replicate sections were routinely stained with hematoxylin and eosin
(H&E) and Masson-Goldner’s trichrome stain for collagen staining to
assess the presence of lung fibrosis. In addition, sections were made from
all grossly visible lesions from that and other portions of the lung.
Proliferative lesions of the pulmonary parenchyma were designated as
bronchoalveolar hyperplasia, pulmonary adenoma or adenocarcinoma.
Other types of lesions, including those in the pleura were noted where
appropriate. All research and analyses were conducted using good
laboratory practices.
Lung fiber burden
Immediately after necropsy, the infracardiac lobe of each animal’s lung was
removed and frozen for later analysis of lung fiber burden. To recover
fibers from the lung, the tissue was rapidly dehydrated with acetone and
ashed using a low-temperature process. Recovered fibers were dispersed in
distilled water and examined using scanning electron microscopy. Number,
dimensions and other physical characteristics of the inhaled lung fibers
were determined, and reported as fibers per mg of dry lung weight.
Results from recent animal inhalation studies of MMVFs
Refractory ceramic fibers
In the first RCC studies, rodents were exposed to the MTD of the sizeselected
RCF test fiber, 30 mg m −3 and approximately 200–250 fibers cm
−3 . Rats were exposed for 6 h per day, 5 days a week to aerosols containing
one of four different types of RCF: kaolin, RCF 1; zirconia, RCF 2; high
purity kaolin, RCF 3; and ‘after service’ (a kaolin based ceramic fiber
containing 27% crystalline silica that had previously been exposed to high
temperature), RCF 4. Hamsters were exposed to only kaolin RCF fibers.
Positive controls (chrysotile asbestos) and negative controls (filtered air)