29 CFR 1910.1000 (USDOL); U.S. National Archives and Records Administration's Electronic Code of Federal Regulations. Available from, as of December 27, 2017: <http://www.ecfr.gov/>.
Vacated 1989 OSHA PEL STEL 4.7 ppm (5 mg/cu m), skin designation, is still enforced in some states.
NIOSH. NIOSH Pocket Guide to Chemical Hazards. DHHS (NIOSH) Publication No. 97-140. Washington, D.C. U.S. Government Printing Office, 1997. 365.