From the link Yom posted in the other thread.
http://scca.com/documents/Solo%20Rules/2008_Solo_Rules_2.pdf"4.3.1 Helmets
Helmets meeting the following standards must be worn while on
course: All helmets meeting the current or two immediately preceding
Snell Foundation standards (SA, K, or M), or SFI standards
31.1A, 31.2A, 41.1A, or 41.2A are acceptable. Helmets meeting
British spec BS6658-85 type A/FR are also acceptable.
For maximum protection, helmets must fit securely and should
provide adequate peripheral vision. The chin strap must be
securely fastened. Loaner helmets should be available to vehicle
occupants not having their own."