OSHA's former PEL for insoluble uranium compounds is 0.25 mg/m3. The proposed limits were 0.2 mg/m3 as an 8-hour TWA and 0.6 mg/m3 as a 15-minute STEL, based on the ACGIH recommendation. These limits are being established in the final rule. Uranium is a silver-white radioactive metal.
OSHA's former limit for the insoluble compounds of uranium was based on several early studies of uranium's toxic effects in animals; these effects included kidney damage and blood changes (Voegtlin and Hodge 1953, as cited in ACGIH 1986/Ex. 1-3, p. 617). In the intervening years, a considerable body of evidence has accumulated based on the actual occupational exposures of uranium plant workers over periods as long as 25 years. This evidence shows that, before 1950, workers were often exposed to uranium levels between 0.2 and 1.5 mg/m3, but that after 1950, only about 6 percent were exposed at 0.05 mg/m3 or above; despite these relatively high early exposures, the incidence of all diseases, whether or not linked to radiation exposure, has been no higher than is the case for workers in the general population (ACGIH 1986/Ex. 1-3, p. 617). However, there is also evidence that several workers were exposed to brief excursions during which exposure levels reached a concentration as much as five times the TLV (Wing, Heatherton, and Quigley 1963, as cited in ACGIH 1986/Ex. 1-3, p. 617).
NIOSH (Ex. 8-47, Table N6A) indicated that uranium compounds may present a carcinogenic hazard, but concurred with the proposed limits. OSHA has reviewed the scientific evidence on insoluble uranium compounds and notes the results of a five-year inhalation toxicity study of natural uranium dioxide (UO2), which involved monkeys, dogs, and rats (Leach, Hodge, Wilson et al. 1970, as cited by H.E. Stokinger in Patty's Industrial Hygiene and Toxicology, 3rd rev. ed., Vol. 2A, p. 2002, Clayton and Clayton 1981). This study found that the two major sites of uranium accumulation, the lungs and tracheobronchial lymph nodes, accounted for over 90 percent of the uranium found in the body. Fibrotic changes suggestive of radiation injury were seen occasionally in the tracheobronchial lymph nodes of both dogs and monkeys, as well as in the lungs of monkeys after exposure periods longer than three years. The lung and lymph node data obtained in this study show that the animal body can accumulate sufficient uranium, from prolonged exposures to insoluble uranium dust at 5 mg/m3, to create potential radiological hazards. The lung and tracheobronchial lymph node radiation values were high enough, "in fact, to anticipate radiation hazards in these tissues from exposures at or lower than the occupational TLV (200 ug U/m3) recommended by the ACGIH..." (Patty's Industrial Hygiene and Toxicology, 3rd rev. ed., Vol. 2A, pp. 2002-2003, Clayton and Clayton 1981).
Laurence Hecker, representing Abbott Laboratories (Ex. 3-678), commented that there was no health basis for the proposed STEL for uranium. OSHA believes that the findings from the study discussed above illustrate the importance of maintaining employee TWA exposures at or below the 0.2-mg/m3 PEL. Therefore, in accordance with the policy described in Section VI.C.17, OSHA is establishing a STEL for insoluble uranium compounds to ensure that adequate process control is achieved to maintain exposures at or below the TWA PEL.
In the final rule, OSHA is establishing an 8-hour TWA PEL of0.2 mg/m3 and a STEL of 0.6 mg/m3 for the insoluble forms of uranium. The Agency concludes that these limits are required to protect workers exposed to uranium from the significant risks of kidney or blood disorders and radiological damage potentially associated with both full-shift and excursion exposures to these compounds. The Agency considers these adverse effects material impairments of health. OSHA finds that these limits will substantially reduce these risks.