Various hazards associated with grouting of mine roofs were discussed. A polyurethane (9009545) grouting system has been used in mines since the early 1970s in Europe to strengthen and consolidate coal and rock. In the United States, such operations have been gaining in number since 1978. The chemicals involved in the operation include a polymeric material containing methylene-diphenyl- diisocyanate (101688) (MDI) and higher molecular weight isocyanates which react with one or more polyether polyols to produce an expanded rigid binder. At concentrations of diisocyanates as low as Respiratory sensitization is possible and symptoms include wheezing, shortness of breath, and bronchial constriction. Skin sensitization is also possible along with direct irritation to the eyes and mucous membranes. Studies have been conducted for some time to determine possible ill effects to miners involved in injecting the materials or in working in the immediate vicinity of an injection site. Isocyanate concentrations have been measured during trial injections in several coal mines in the United States. The Mine Safety and Health Administration standard of 0.02ppm as a ceiling for isocyanate concentrations has not been surpassed in any of these tests. The NIOSH recommended time weighted average of 0.005ppm has been equaled in two of the 234 samples, and exceeded in one. In further studies made under general working environmental conditions, no detectable airborne concentrations of MDI have been noted. Self contained breathing devices should be used during injection, along with protective clothing, gloves and safety glasses. There is no safe level of exposure for individuals who have been previously sensitized to isocyanates.