Tim Curry is scary, right? Right.

What is the most frightening thing in the world? A giant spider? A clown? A clown riding a giant spider? Actually, it’s carbon dioxide.

I can see you require further explanation.

Scientists at the University of Iowa researched people with damage to their amygdala, the part of the brain that controls fear. According to the study in Nature Neuroscience, these people reported that they had never felt fear, even one woman who had once had a knife pulled on her. To test the veracity of her statement, the researchers took her on a haunted house tour and made her watch horror movies. Even The Shining didn’t send shivers up her spine.

After gathering two other people with the condition, two sisters that both suffered the same rare brain disease, the researchers tried another tack. They gave the participants carbon dioxide masks, carbon dioxide being known to evoke feelings of fear and panic in people without amygdala damage.

After only a few seconds of carbon dioxide in their bodies, the participants started waving frantically to the researchers to take the masks off. They reported feeling panic and one subject even described her emotion as “fear.”

Researcher Justin Feinstein says this study not only proves that people with amygala damage can experience fear, it shows that the way they process fear is probably different. Feinstein suggests that the brain damage prevents fear in response to learned scary situations, but when something unexpected and unexplained is happening to the body, the afflicted person will feel fear just like anyone else. All from a little CO₂.