The unique feature common to all diisocyanates is that they consist of two N=C=O (isocyanate) functional groups attached to an aromatic or aliphatic parent compound. Because of the highly unsaturated nature of the isocyanate functional group, the diisocyanates readily react with compounds containing active hydrogen atoms (electrophiles). Thus, the diiso-cyanates readily react with water (humidity), alcohols, amines, etc.; diisocyanates also react with themselves to form either dimers or trimers. When a diisocyanate species reacts with a primary, secondary, or tertiary alcohol, a carbamate (NHCOO-) group is formed, which is commonly referred to as a urethane. Reactions involving a diisocyanate species and a polyol result in the formation of cross-linked polymers; i.e., polyurethanes. Hence, they are used in surface coatings, polyurethane foams, adhesives, resins, elastomers, binders, and sealants. Many material safety data sheets (MSOSs) use isocyanate-related terms interchangeably. For the purpose of this report, terms are defined as follows. Diisocyanates (Monomers): The difunctional isocyanate species from which poly- isocyanates and polyurethanes are derived. Common examples of monomeric iso-cyanates include 1,6-hexamethylene diiso- cyanate (HOI), 2,4- and/or 2,6-toluene diiso-cyanate (TOI), 4,4' diphenylmethane diisocyanate (MOl), methylene bis (4-cyclohexylisocyanate) (HMOI), isophorone diiso-cyanate (IPOI), and 1,5-naphthalene diiso- cyanate (NOI). Commercial-grade TOI is an 80:20 or 65:35 mixture of the 2,4- and 2,6-isomers of TOI, respectively. Polyisocyanates: Species possessing free isocyanate groups and derived from monomeric isocyanates either by directly linking these monomeric units (a homopolymer) or by reacting these monomers with di- or poly-functional alcohols or amines (a copolymer). Prepolymers: Species possessing free iso-cyanate groups, prepared from the reaction of a polyol with an excess of di- or polyiso-cyanate. Commercially available isocyanate products frequently contain pre polymers in lieu of more volatile isocyanate monomers. Oligomeric Isocyanates (Oligomers): Relatively low molecular weight polyiso-cyanates. Intermediates: Species possessing free isocyanate groups, formed during use of an isocyanate product by partial reaction of the isocyanate species with a polyol. In general, the types of exposures encountered during the use of iso-cyanates (i.e., monomers, prepolymers, polyisocyanates, and oligomers) in the workplace are related to the vapor pressures of the individual compounds. The lower molecular weight isocyanates tend to volatilize at room temperature, creating a vapor inhalation hazard. Conversely, the higher molecular weight isocyanates do not readily volatilize at ambient temperatures but are still an inhalation hazard if aerosolized or heated in the work environment. The latter is important since many reactions involving isocyanates are exothermic in nature, thus providing the heat for volatilization. As exposure limits decrease, the volatility of solid materials becomes an issue. To reduce the vapor hazards associated with the lower molecular weight diiso-cyanates, prepolymer and polyisocyanate forms of these diisocyanates were developed and have replaced the monomers in many product formulations. An example is the biuret of t1DI, which consists of three molecules of t1DI monomer joined together to form a higher molecular weight oligomer having similar characteristics to those found in the monomer. Also, many MDI product formulations consist of a combination of MDI monomer and a MDI-based polyiso-cyanate (such as polymethylene polyphenyl isocyanate). Many prepolymer and polyiso-cyanate formulations contain a small fraction (usually less than 1 %) of unreacted monomer.