The incidence of lung cancer was higher than in the general population. However the fibre types involved and levels of exposure were not known. Several studies have shown no excess lung cancer in persons with plaques beyond that accounted for by exposure level. Extensive calcification may occur in cases of pleural plaque without appreciable thickening and without any increased incidence of lung cancer. It was hypothesized that calcification here might be associated with fibrous tremolite exposure.
Studies reporting high tumour rates also report high levels of asbestosis.
Two inhalation studies have shown both fibrosis and the risk of pulmonary tumours to be dose related but the number of animals used have been inadequate to demonstrate the shape of the exposure response curve. While all 8 species of experimental animal studied with chrysotile develop lung fibrosis, only rats develop pulmonary tumours. Diffuse mesotheliomas of the pleural and peritoneal cavities can occur in animals without exposure to mineral fibres. Inoculation of fibres into the pleural cavities may result in mesotheliomas but the fibre burdens are extremely high. Tumour incidence depends on the dose, source and preparation of the fibres employed.