Darrell,
 
I doubt you'll find a regulation.  There is a Standard, the ANSI/ISEA Z-358.1-2009 Standard.  The last I have heard, ISEA are meeting to OSHA representatives about the possibllity of incorporating the Standard into the approprite regulations, but this is a slow process.
 
I was on the revision committee and we had a great debate over many months and I did some pretty thorough literature searching on the topic.  In the Standard, you will find the best consensus temperature values our committee could come up with.  Basically, as a medical guy, I want water that is not so cold that the chemical splash victim risks hypothermia or won't stay under the shower long enough for as effective a decontamination that water can give, nor do I want such victims scalded.  If you look in a dictionary, they all pretty much define "tepid water" as water that is neither too hot or too cold.  Now, Ol' Doc Hall's trained fever thermometer (the back of my hand) tells me that but doesn't give me something I can test with a thermometer unless I'm comfortable with the temperature and have an appropriate instrument in the other hand.
 
It is best to obtain your own copy of the Standard from:
 
ISEA (International Safety Equipment Association)
1901 North Moore Street
Arlington, VA 22209-1762
(703) 525-1695
FAX:  (703) 528-2148
www.safetyequipment.org
Good contact person there:  Cristine Fargo
 
NOTE:  I receive no renumeration of any kind from the sale of copies of the standard.
 
Alan
Alan H. Hall, M.D.
President and Chief Medical Toxicologist
Toxicology Consulting and Medical Translating Services, Inc. (TCMTS, Inc.)
210 Wilson Street
Lordsburg, NM 88045
Cell Phone:  (307) 399-1564
email:  ahalltoxic**At_Symbol_Here**msn.com
 
Clinical Assistant Professor
Colorado School of Public Health
Denver, CO