see 1915.152(d)
Reissued equipment. The employer shall ensure that all unsanitary PPE, including that which has been used by employees, be cleaned and disinfected before it is reissued.
and
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9777sounds like they may be inviting trouble, if their insurance carrier knew what they were doing i doubt they would have insurance long!
If you have no choice and must follow their rules, tell your employer you want it in writing that you have to share, and that if you catch dermititus or ANY other desease they are fully responsible to provide medical care and lost time wages to your family. also I would want them to say that all medical bills will be paid until its established that you didn't get sick from anyone else that used the helmet. that may be enough to scare them.
If it was me I would simply refuse to wear it based on safety issues unless they sanitized it and cleaned it each time.
best bet is to continue using yours and if you get terminated, expose them!!!!