If you’re a roller coaster enthusiast, chances are you’ve been called a “thrill-seeker” or an “adrenaline-junkie.” But what is it about roller coasters that gives the rider a high-impact, thrilling experience? Many cite the high speeds and inversions as the reason they ride the massive metal machines, but in reality, it’s probably the acceleration into these components of the ride that truly creates the experience. But coaster designers can’t simply throw human bodies into high speeds with abandon — they must also carefully calculate the G-forces at work in order to make sure the coaster is safe.

G-forces are a measurement of the type of acceleration that causes weight, like the kind you feel when you’re pressed into your seat during a roller coaster loop. Though they’re capable of providing fun, G-forces are also a formidable foe to the human body, capable of taking us out within a few seconds if we underestimate them. But how exactly can acceleration harm us, and how fast can we go before our need-for-speed mentalities get us killed?

Things Just Got Heavy

G-forces, short for gravitational forces, act on us a lot more than you’d probably think. Any time you speed up or slow down, they’re there (think being thrown forward against your seatbelt in a rapidly halting car, or when you experience intermittent moments of weightlessness on a theme park ride or on a flight). These forces are capable of getting much more intense; fighter pilots and astronauts, for example, often subject themselves to rapid accelerations to high speeds. Depending on how you’re sitting, lying, or standing when you experience G-forces, they can occur front-to-back, side-to-side, or top-to-bottom — or vice-versa. We as humans are much better at tolerating horizontal G-forces, or those that are perpendicular to the spine, than we are the head-to-toe, vertical kind.

The danger with G-forces lies in two areas, the first of which is the fact that our body is flexible and soft. Though we contain bones, we’re more than 60 percent water, with only a thin layer of skin protecting many soft organs like the stomach, kidneys, and liver. Dealing with aggressive horizontal G-forces can result in broken bones, shifting of organs, and burst blood vessels. It makes sense; the higher the G-forces, the more weight we’re essentially laying upon our bodies.

U.S. Air Force pilot John Stapp, experiencing intense g-forces during a 421-mph ride as a part of an experiment to see what humans could take. U.S. Air Force

When it comes to vertical forces, the body’s second weakness comes into play: our blood pressure. Deadly vertical G-forces go for the head; since they’re aligned with the spine, these accelerations can drive blood down toward the feet, and away from the blood-needy brain. With the development of stronger head-to-toe G’s, our bodies need to work harder and harder to try and pump blood back up to the brain, and after a while, the heart simply can’t summon enough pressure to do so.

Lack of blood in the brain means a lack of oxygen, which is a dire situation. As vertical G-forces are increased, we would experience, in order: loss of hues in vision, tunnel vision, blacking out (while still conscious), loss of consciousness, and death.

Many factors can affect an individual’s ability to withstand G-forces, some of which were outlined by the Federal Aviation Administration.

“G tolerance is degraded as a result of alcohol, fatigue, and dehydration,” they wrote. “Lack of physical conditioning and a sedentary lifestyle can also degrade G tolerance and increase the aviator’s susceptibility.”

They noted, however, that G-protective clothing, known as a “G suit” can help a pilot deal with intense G’s. Being well-rested, hydrated, and fit will also be advantageous.

What’s The Magic Number?

As we’re just standing at sea level, a standard 1 G of G-force is acting on us. The record for highest G-force on a roller coaster is 6.3, and it’s only manageable because it lasts just a few seconds. Fighter pilots may have to endure up to 8 or 9 Gs while wearing special compressed suits, designed to keep blood in the upper body and prevent fainting.

It’s difficult to calculate the exact level of G-force that would kill a human, because the duration of exposure is such an important factor. There are isolated incidents of humans surviving abnormally high G-forces, most notably the Air Force officer John Stapp, who demonstrated a human can withstand 46.2 G’s. The experiment only went on a few seconds, but for an instant, his body had weighed over 7,700 pounds, according to NOVA.

Check out the video below for an interesting example of lethal, high-intensity G-forces from a design project called the Euthanasia Coaster. It would, hypothetically of course, kill anyone who rode it by cutting off oxygen to their brain. This particular design places the lethal exposure level at one minute of 10 Gs.