"High g-forces are common and well tolerated during many daily activities," says Smith. "Fighter pilots have high g-forces of long duration, causing pooling of blood. G-forces alone are a very poor measure of a risk of brain trauma. Pilots can endure fairly high g-forces, as much as 8 or 9 g's, for an average of 43 seconds, as opposed to the brief g-forces found on coasters," Smith says. "Pilots may pass out from these extended high g-forces, but that's not found on roller coasters, and pilots are not the victims of brain injuries as a result of those g-forces. Unconsciousness is not from brain injury, but from lack of blood flow."
"The key to producing brain trauma is head accelerations that can be caused by g-forces," Smith says. "In an auto accident, sudden stopping creates extremely high g-forces, causing brain deformation. If the brain is deformed rapidly, various types of injuries to the brain will occur, including ruptured blood vessels. We see auto crashes well documented, but that's been extrapolated to roller coasters without any scientific link. There have been no studies done that indicate an increased risk. We aren't seeing any numbers that concern experts, especially considering that bleeds in the brain from pre-existing aneurisms is not an uncommon thing. The woman's death in the Southern California incident is typical. Aneurisms commonly rupture, even during sleep. Her death could have been a coincidence, it could have been from the excitement of the ride increasing blood pressure, or it could have been from g-forces, but that's not proven. There simply is no evidence."