OSHA had no previous permissible exposure limit for caprolactam as vapor. The Aency proposed a TWA of 5 ppm (20 mg/m3) for the vapor, supplemented by a STEL of 10 ppm (40 mg/m3), based on the limits adopted by the ACGIH. These limits are established in the final rule. Caprolactam is a white crystalline solid at room temperature; thus, high vapor levels occur only at elevated temperatures.
The health effects of exposure to caprolactam vapor are identical to those described for caprolactam dust, except that contact with the vapor is reported to be even more irritating (Hohensee 1951, as cited in ACGIH 1986/Ex. 1-3, p. 95). Workers exposed to the vapor at approximately 12 ppm complained of a bitter taste in the mouth, nervousness, epistaxis, upper respiratory tract congestion, and dry and splitting skin; other workers reported experiencing heartburn, flatulence, and a heavy feeling in the stomach (Hohensee 1951, as cited in ACGIH 1986/Ex. 1-3, p. 95).
In another report of industrial exposure to the vapor, Ferguson and Wheeler (1973/Ex. 1-1108) reported that workers routinely exposed to unspecified levels and occasionally to concentrations as high as 100 ppm for 18 years reported severe discomfort from burning nose, throat, and eyes. This irritation response was dose-related, with no workers reporting effects at 7 ppm or below, some experiencing transient upper respiratory tract irritation at levels above that, and others reporting eye irritation at concentrations of 25 ppm and above (Ferguson and Wheeler 1973/Ex. 1-1108). Ferguson (1972, as cited in ACGIH 1986/Ex. 1-3, p. 96.1) noted that a group of 143 workers, some of whom were exposed for as long as 17 years to vapor concentrations of 5 to 10 ppm, showed no evidence of adverse effects. At higher vapor exposures (13 to 130 ppm), all subjects experienced eye irritation (Ferguson 1972, as cited in ACGIH 1986/Ex. 1-3, p. 96.1). Human volunteers exposed at low relative humidities to concentrations of the vapor in the range of 10 to 100 ppm showed a dose-related response, but at higher relative humidities, no irritation was observed below a concentration of 14 ppm (Ferguson and Wheeler 1973/Ex. 1-1108).
NIOSH (Ex. 8-47, Table N2; Tr. p. 3-86) did not concur with the Agency's proposal to establish an 8-hour TWA exposure limit of 5 ppm (20 mg/m3) and a 10-ppm (40-mg/m3) STEL for caprolactam vapor. NIOSH (Ex. 8-47) noted that the 1986 ACGIH Documentation (Ex. 1-3) lists a TLV-TWA of 1 mg/m3 for the combined vapor and aerosol of caprolactam and 0.22 ppm if the substance is present only as a vapor. The proposed change in the 1986 TLV was recommended to prevent early signs of irritation in some workers. NIOSH observed that "the proposed PEL does not appear to provide a sufficient margin of safety to caprolactam vapor," based on available human exposure responses. No other comments on this substance were submitted to the rulemaking record.
As explained in Section IV.D. of this preamble, which discusses the boundaries of today's rulemaking, the Agency confined its attention to the substances and exposure limits listed in the 1987-1988 edition of ACGIH's Threshold Limit Values and Biological Exposure Indices (ACGIH 1987/Ex. 1-16). Caprolactam vapor is listed on ACGIH's Notice of Intended Changes but new limits have neither been reviewed nor adopted by that organization to date. Under these circumstances, OSHA believes it prudent to promulgate the limits as proposed for caprolactam vapor. The Agency is establishing an 8-hour TWA limit of 5 ppm (20 mg/m3) and a 15-minute STEL of 10 ppm (40 mg/m3) for this previously unregulated substance. OSHA concludes that these PELs will substantially reduce the significant risk of eye, upper respiratory tract, and skin irritation that are permitted in the absence of any exposure limit for caprolactam vapor. OSHA considers the irritant effects resulting from exposure to caprolactam vapor to be material impairments of health. The Agency will continue to monitor the health evidence for this substance to determine whether further action is warranted.