In a study of workers exposed to cryolite (sodium aluminum fluoride) dust for many years, Roholm (1) found that some had severe chronic skeletal fluorosis but that in a greater number there was radiologic evidence of osteosclerosis which was not associated with symptoms. This observation led Rich and Ensinck (2) to administer sodium fluoride to patients with rarefying disorders of the skeleton in the belief that the production of a mild degree of asymptomatic osteosclerosis would strengthen the
Discussion
The present epidemiological study on the prevalence of respiratory disease was carried out according t o t h e most widely and comprehensively validated MRC questionnaire. The questionnaire is characterized by its making group comparison of the prevalence of individual symptoms singly or in combination, instead of searching for names of specific diseases such as bronchitis. The results obtained from this study clearly indicate the effect of working environment on the respiratory syst