From section 4.3.1 of the SCCA Solo Rules:
4.3.1 Helmets
Helmets meeting the following standards must be worn while on course:
Snell Memorial Foundation standards EA 2016, SA2020, SA2015, SA2010,
SAH2010, M2020D, M2020R, M2015, M2010, K2020, K2015, K2010;
SFI standards 31.1/2020, 31.1/2015, 31.1/2010, 41.1/2020, 41.1/2015,
41.1/2010; FIA standards 8859-2020, 8860-2018, 8860-2018-ABP,
8859-2015, 8860-2010.
Full face or modular helmets shall be worn while competing in an open-
wheel car, formula car, or kart. Face shield, goggles, or similar face protec-
tion (conventional eyeglasses are not sufficient) shall be worn while com-
peting in any other vehicle with less than the standard-size windshield.
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.
Helmets sold in the United States typically have a Snell sticker affixed to the inside of the shell, beneath the helmet lining. DOT-approved helmets are not allowed. The sticker will look like one of the following: