Researchers at the University of Pittsburgh used a Pennsylvania trauma database that included approximately 12,700 patients with spinal injuries -- 8,500 drivers and 4,200 passengers from 1990 to 2002. Of these, 5,500 patients had fractures of the cervical spine.

Dr. William F. Donaldson III and colleagues says the rate of cervical spine fractures was 54 percent in drivers using an airbag only, compared to 42 percent for drivers using both an airbag and seat belt.

The study, published in the journal Spine, finds that with adjustment for other factors, the relative risk of cervical spine fracture was 70 percent higher for drivers using an airbag without a seat belt, compared to drivers using both protective devices. This was even greater than the 32 percent increase in cervical fracture risk for drivers using neither an airbag nor seat belts.

Airbags were specifically designed to be used with seat belts -- serious injuries may result in victims who, because they are not properly restrained by seat belts, are "out of position" when airbags deploy, Donaldson says.