Batguano

It has been brought to the attention of SCCA Technical Services that
the use of full-face or closed-face helmets while driving vehicles
with active airbag restraint systems may result in injuries in the
event of a crash that deploys the airbag. Because of the location of
the steering wheel relative to a driver’s position, the airbag axis is
on a level with the driver’s chin. In a crash with airbag deployment,
contact with the chin area of a full-face helmet can be so powerful
“that the risk of fractures to the jaw cannot be ruled out“ (Hubert
Gramling, FIA Institute, FT3/AF, 18.5.1999). This applies to vehicle
that may be used in Solo, RallyCross, High Performance Car Control
Clinics, etc. Therefore, it is highly recommended that full-face
helmets not be used in vehicles with functional airbag systems.
Potentially more restrictive language is currently being considered
for 2005, which could appear in an early 2005 issue of FasTrack. If
you have any questions, please contact the SCCA at (800) 770-2055.

DamonB

Discussion here:

https://www.rx7club.com/race-car-tech-103/scca-advisory-use-closed-face-helmet-airbag-369331/