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, SA2005*, M2020D, M2020R, M2015, M2010, M2005*, K2020,
K2015, K2010, K2005*; SFI standards 31.1/2015, 31.1/2010, 31.1/2005*,
41.1/2015, 41.1/2010, 41.1/2005*; ECE 22.05 or R22.05; FIA standards
8860-2018, 8860-2018-ABP, 8859-2015, 8860-2010, 8860-2004; or
British spec BS6658-85 type A* are acceptable.
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.

Formula Junior drivers must use helmets meeting the above; SFI
24.1/2020, 24.1/2015, 24.1/2010, 24.1/2005* (Youth Helmets); or Snell
CMR2016, CMR2007 (Children’s Motorsports Restricted), CMS2016,
CMS2007 (Children’s Motorsports Standard) specifications. Also, For-
mula Junior helmets must be of closed face design incorporating full face
shields and chin bars.

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

* Not valid after 12-31-2020: Snell SA2005, M2005, K2005; SFI
24.1/2005, 31.1/2005, 41.1/2005; British Standards BS6658-85
type A/FR, BS6658-85 Type A.

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:

2020 Autocross Helmet Certification Decals

Note: As long as our Covid-19 protocols continue to be in effect, loaner helmets will not be available.