The U.K. Health and Safety Commission has recently introduced a new two-part control limit for organic isocyanates. Permissible concentrations in the working environment are now defined in terms of functional NCO-group, so that for the first time all isocyanate species are brought within the scope of a common unifying concept. The decision to widen the basis of the British approach beyond the handful of difunctional monomers currently quoted by most countries is discussed in relation to the incidence of adverse respiratory effects and sensitizations from the inhalation of isocyanate prepolymers in spray-mist and aerosol forms. The relationship between the new control limit and the previous values (in p.p.m.)applying to difunctional monomers such as TDI and MDI is explained and an example of its use in calculating the permissible airborne concentrations of an isocyanate prepolymer of known structure is given.
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