From the SCCA in the USA - trackday regulars beware

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.