ISOPHORONE DIISOCYANATE

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: 4098-71-9; Chemical Formula: C₁₂H₁₈N₂O₂

OSHA previously had no limit for isophorone diisocyanate (IPDI). The Agency proposed an 8-hour TWA of 0.005 ppm for this substance, with a 10-minute short-term limit of 0.02 ppm and a skin notation; these limits are consistent with NIOSH’s recommended limits for all isocyanates, and on Table N1 of Exhibit 8-47, NIOSH indicated its concurrence with the selection of this PEL. The ACGIH has established an 8-hour TWA of 0.01 ppm and a skin notation for IPDI. In the final rule, OSHA is establishing an 8-hour TWA of 0.005 ppm for isophorone diisocyanate, with a 15-minute STEL of 0.02 ppm and a skin notation.

To date, there is little direct information on the health effects associated with exposure to this particular isocyanate. However, diisocyanates, in general, cause irritation of the respiratory tract, decreases in pulmonary function, and sensitization. The ACGIH (1986/Ex. 1-3, p. 334) cited two reports in which workers exposed to isophorone diisocyanate suffered asthma or dyspnea; neither of these reports contained quantitative exposure data (Clarke and Aldons 1981/Ex. 1-475; Tyrer 1979/Ex. 1-396). The ACGIH (1986/Ex. 1-3, p. 334) recommended that the 0.01 ppm TLV-TWA established for 2,4-toluene diisocyanate (TDI) be applied to isophorone diisocyanate until information specific to IPDI becomes available; however, the ACGIH (1986/Ex. 1-3, p. 334) did not agree that the 0.02 ppm TLV-STEL established by the ACGIH for toluene diisocyanate should also apply to IPDI. In its criteria document on isocyanates, NIOSH (1978c/Ex. 1-259) used similar reasoning to reach the conclusion that, on a molar basis, all of the diisocyanates would react in a manner similar to that of TDI. NIOSH thus recommended that the limits established for TDI (0.005 ppm TWA and 0.02 ppm as a 10-minute short-term limit) be applied to all diisocyanates. In support of the recommended short-term exposure limit for all diisocyanates, NIOSH (1978c/Ex. 1-259) cited a study reporting that 12 workers in an automobile plant had developed severe respiratory symptoms after exposure to 0.03 to 0.07 ppm TDI for one week.

NIOSH (Ex. 150, Comments on Isophorone Diisocyanate) reported that IARC has recently (1986) published results of a positive carcinogenesis bioassay involving TDI that found TDI-induced tumors in both rats and mice. In response to IARC’s determination that the evidence in animals is sufficient to classify TDI as a carcinogen in animals, NIOSH is developing a Current Intelligence Bulletin on TDI. OSHA received no comments suggesting that feasibility is a problem at the revised limits, although the proposal specifically requested additional feasibility information from the public. OSHA received several comments on IPDI (Exs. 116, 144, 194). The Workers Institute for Safety and Health (WISH) argued that OSHA should regulate all six of the isocyanates, rather than the three being regulated in this rulemaking, because employers would otherwise tend to substitute the unregulated members of the isocyanate family for those that are regulated, and the unregulated substances might in fact prove as hazardous as the regulated isocyanates (Ex. 116, p. 34). In response to WISH, OSHA notes that the scale and scope of the present rulemaking demanded that OSHA adopt certain methods of selecting substances to include in this rulemaking (see the discussion in the preamble section on “Boundaries to Regulation”); the Agency believes that the isocyanates included in the present rulemaking are those for which the health evidence is adequate to serve as a basis for limit-setting. The AFL-CIO (Ex. 194) commented along the same lines as WISH, but in addition was of the opinion that the proposed 8-hour TWA PEL was not necessary. OSHA does not agree, believing instead that the 8-hour TWA limit will provide additional protection and is appropriate in workplace exposure in workplace exposure situations characterized by steady-state exposures. The New Jersey Department of Public Health (Ex. 144) recommended the use of EPA’s IRIS data base to set a limit for IPDI; the appropriateness of the IRIS data for limit-setting is discussed in Section VI.A. of the preamble.

OSHA is establishing a 0.005 ppm 8-hour TWA, a 0.02 ppm 15-minute short-term limit, and a skin notation for IPDI. The short-term limit of 0.02 ppm is designed to prevent the severe irritation effects associated with exposure to the diisocyanates even in nonsensitized workers, and the skin notation will prevent dermal absorption of this substance. The Agency has established a 15-minute, rather than a 10-minute, short-term limit for isophorone diisocyanate because OSHA has decided, as a matter of policy, to conform all of its revised short-term limits (5, 10, 15, or 20 minutes) to a duration of 15 minutes. The Agency finds that the TWA and STEL limits will both protect nonsensitized workers against IPDI’s sensitizing effects and minimize asthmatic reactions among sensitized workers. OSHA concludes that these revised limits will reduce the significant risk of material health impairment (i.e., immune-system-mediated pulmonary sensitization, which is associated with isocyanate exposure. In addition, the Agency also finds that these limits are feasible. OSHA will continue to monitor the toxicological literature on all of the isocyanates in the future.