There is something enticing about roller coasters: the drop in your stomach as you plummet down a hill, the weightless feeling as you hover at the top of a loop, or the pressing sensation on your head as you fight to maintain consciousness...no, not that last one.
On certain coasters, it is common for riders to experience a momentary loss of vision and/or consciousness. Commonly called “greying out,” this phenomenon is a result of high amounts of G-force acting on your body during high-intensity points of the ride. G-force is a measure of acceleration divided by the gravitational constant, g (acceleration due to gravity), and can induce serious physiological effects on the human body [1]. In particular, a strong force in the +z direction (from your head downwards, as shown in Figure 1) will push blood towards your feet and away from your brain [2]. This is often caused by powerful upward movement. The sudden decrease in brain oxygen can cause a wide variety of symptoms, ranging from the loss of peripheral vision, to the loss of color vision (greying out), to temporary blindness (blackout), to G-Induced Loss of Consciousness (G-LOC) [1].
G-force directions and resulting responses. –Gz acts upwards (weightlessness), and can cause redouts (blood rushes to the brain and causes vision to redden). +Gz acts downwards (pressure), and can cause greyouts, blackouts and G-LOC. Created with Biorender.com.
Greyouts commonly occur in airplane maneuvers or certain roller coasters with high levels of G-forces. The human body is equipped to handle a max +Gz-force of up to 4.7 G, based on a +Gz-time tolerance curve predicting the point at which G-force exceeds human tolerance [3]. To put this number in perspective, the force of gravity we experience daily has a magnitude of 1 G; however, there are several Six Flags roller coasters that have reported much higher G-forces, such as Taz’s Texas Tornado in Six Flags AstroWorld (6.5 Gs) and Batman and Robin in Six Flags Great Adventure (5.0 Gs) [1].
However, although prolonged exposure to high G-forces can cause serious brain damage, the vast majority of roller coasters are engineered to keep the rider under safe force thresholds [2]. The negative physiological effects of G-forces are time-dependent: a study has shown that although exposure to a force of any G level for more than 4.2 seconds can induce G-LOC, the human body can withstand up to 10 Gs for a very brief period of time [1]. In the cases of rides with extreme G-force measurements, as long as these values are only maintained for a fraction of a second, there is not enough time for blood to pool in the extremities and induce G-LOC [2]. In fact, it is very common to experience high levels of G-force––for safe amounts of time––in everyday activities such as sneezing (2.9 Gs) or sitting down quickly (10.1 Gs) [1]. In the case of amusement parks, greyouts are likely exacerbated by a variety of outside factors: hypoxia (low blood oxygen), heat stress, dehydration, fatigue, and consecutive rides can all increase the risk of greying out [4].
So the next time you buckle yourself into a scary-looking coaster, make sure you’ve had plenty to drink and adequate rest time between rides; this way, you can enjoy every loop or corkscrew, and keep plenty of blood in your brain!
References
Braksiek, R. J.; Roberts, D. J. Amusement Park Injuries and Deaths. Ann. Emerg. Med. 2002, 39 (1), 65–72.
Smith, D. H.; Meaney, D. F. Roller Coasters, g Forces, and Brain Trauma: On the Wrong Track? J. Neurotrauma 2002, 19 (10), 1117–1120.
Whinnery, T.; Forster, E. M. The +Gz-Induced Loss of Consciousness Curve. Extrem. Physiol. Med. 2013, 2 (1), 19.
McMahon, T. W.; Newman, D. G. G-Induced Visual Symptoms in a Military Helicopter Pilot. Mil. Med. 2016, 181 (11), e1696–e1699.
Batman and Robin [Image]. https://www.screammachine.net/rideinfo.php?ridecode=11139&active= (accessed Oct 15, 2020).