People can pass out, gray out, or experience loss of peripheral vision on coasters for a number of reasons. But according to the Rice Undergraduate Science Research Journal, the main reason is positive G-forces. G-force is the measure of acceleration divided by the gravitational constant.

Yes, it is possible for people to pass out on roller coasters, a phenomenon often referred to as G-LOC (G-force induced Loss of Consciousness). This typically occurs on high-intensity rides that pull significant positive G-forces, which can cause blood to drain away from the brain and pool in the lower extremities. While most modern coasters are engineered to keep these forces within safe limits for the general public, individuals may experience "greyouts" (loss of color vision), "blackouts" (loss of vision while conscious), or a brief fainting spell if the forces are sustained. Another common cause is vasovagal syncope, where the intense emotional stress, anxiety, or excitement of the ride triggers a sudden drop in heart rate and blood pressure. Most "fainting" videos seen online involve short-lived episodes from which the rider recovers almost immediately once the G-forces neutralize, often feeling slightly disoriented for several seconds afterward.