The differences between fibrous and isometric dust particles in causing lung disease are outlined; the former cause diffuse lung scarring and 2 types of cancer, while the latter do not normally cause cancer; asbestosis and lung cancer can be caused by the retention of <1 g of asbestos, and mesothelioma with <1 mg. The factors causing asbestos dust to be so dangerous are outlined, including retention, clearance, physico-chemical properties, and smoking. Mesotheliomas are more likely where crocidolite exposure occurs. Potency of mineral fibres seems to be dependent on size and independent of the chemistry. Palygorskite is also an active agent. The epidemiology of mesothelioma in Turkey is described; in 2 areas the mesotheliomas appear to be due to fibrous erionite (a zeolite) in the dust. Urgent research and control measures are needed concerning fibres other than asbestos as a cause of cancer, involving collaboration between geologists and physicians interested in environmental health.