The former OSHA limit for toluene-2,4-diisocyanate (TDI) was a ceiling of 0.02 ppm. OSHA's proposed and final rule limits for TDI are 0.005 ppm as an 8-hour TWA and 0.02 ppm as a 15-minute STEL. The ACGIH (1986/Ex. 1-3, p. 584) and NIOSH (Ex. 8-47, Table N1) both recommend a TWA of 0.005 ppm and a STEL of 0.02 ppm for TDI.
TDI is one of the most frequently encountered occupational sensitizers, and it is also a known cross-sensitizer. The revised limit is based on human data showing that workers can develop sensitization reactions at exposure levels below the 0.02 ppm level. Elkins and colleagues (1962/Ex. 1-138) reviewed the incidence of TDI intoxication in 14 plants in Massachusetts between 1957 and 1962. In eleven instances of TDI intoxication, the average concentration of TDI was 0.015 ppm, and in nine cases the average concentration was below 0.01 ppm. In all plants where the average levels were above 0.01 ppm, TDI had caused respiratory problems. TDI-related respiratory problems were not observed when the average concentration of TDI was maintained below 0.007 ppm (Elkins, McCarl, Brugsch, and Fahy 1962/Ex. 1-138).
Williamson conducted two TDI studies (1964 and 1965, as cited in ACGIH 1986/Ex. 1-3, p. 584) that revealed a 5-percent sensitization rate in 99 workers exposed for 18 months to average levels of TDI below 0.02 ppm. The author believed that accidental spills accounted for the high sensitization rate. Williamson also found that six sensitized workers out of 18 exposed to concentrations of TDI below 0.02 ppm for 14 months showed marked decreases in lung function (Williamson 1964 and 1965, as cited in ACGIH 1986/Ex. 1-3, p. 584).
A NOEL (no-observed-effect level) for TDI has been documented. In 1975, Roper and Cromer (Ex. 1-147) failed to observe any symptoms of respiratory illness or changes in pulmonary function in nine employees working in a plant where breathing zone samples showed TDI concentrations of 0.001 to 0.002 ppm.
Wegman and colleagues (1974/Ex. 1-112; 1977/Ex. 1-171; 1982/Ex. 1-133) observed a dose-response relationship between exposure and long-term decline in lung function as documented by test results among TDI-exposed employees. Only for those workers exposed to less than 0.002 ppm TDI were the results of lung function tests normal (Wegman, Pagnotto, Fine, and Peters 1974/Ex. 1-112; Wegman, Peters, Pagnotto, and Fine 1977/Ex. 1-171; Wegman, Musk, Main, and Pagnotto 1982/Ex. 1-133).
Several commenters submitted comments on TDI. NIOSH (Tr. 3-96, 97) and the United Auto Workers (Tr. 7-38 to 7-44) urged OSHA to designate TDI as a carcinogen, while the Dow Chemical Company (Ex. 106A) argued that TDI should not be so designated. As discussed in the preamble section entitled "Boundaries to Regulation," OSHA is not specifically designating substances as carcinogens; many other organizations, such as the International Agency for Research on Cancer, the ACGIH, NIOSH, etc. do so. The Workers Institute for Safety and Health (Ex. 106) urged OSHA to regulate all six of the isocyanates, rather than the three included in this rulemaking, on the grounds that employees would switch to the unregulated isocyanates, which might present as great a hazard as the regulated ones. In response to WISH, OSHA notes that the scale and scope of the present rulemaking required that OSHA make decisions on substances to be included to facilitate the process; the selection process is described in the preamble section entitled "Boundaries to Regulation."
The Agency concludes that the evidence clearly demonstrates that workers are at significant risk of pulmonary sensitization reactions at the former PEL, as evidenced by declines in pulmonary function observed among workers exposed below this level. OSHA has determined that establishing a 0.005 ppm TWA with a 0.02 ppm STEL will substantially reduce this significant risk. The Agency notes that effects have been observed at levels somewhat below the final rule's PEL; OSHA will therefore continue to monitor the toxicological literature on this substance carefully in the future.