“His heartbeat was restored on the field and he was transferred to the UC Medical Center for further testing and treatment. He is currently sedated and listed in critical condition,” the Bills tweeted around 2 a.m. this morning. 

Each year, researchers estimate, there are between 100 and 150 sudden cardiac deaths during competitive sports, according to research published by the American College of Cardiology. 

In some cases, excessive stress can trigger a rare underlying heart issue that has not been previously detected, or has not been picked up by scans, and can “put the heart into a rhythm that is not sustainable for long periods of time, and hence, the patient usually collapses,” Dr. Doris Chan, an interventional cardiologist at NYU Langone Hospital—Brooklyn, tells Fortune. 

Intense sports could be that trigger.

When someone goes into cardiac arrest, also referred to as sudden cardiac arrest, the heart completely stops beating—sometimes all of a sudden, as in the case of Hamlin. Cardiac arrest happens when theaccording to the American Heart Association, albeit different from a heart attack, which happens when a blockage in the heart restricts blood flow. 

A heart attack can cause cardiac arrest, but the two are not synonymous. Some people may experience dizziness, fatigue, nausea, chest pain, heart palpitations, loss of consciousness, or shortness of breath before going into cardiac arrest, but others may not have any symptoms at all. 

Coronary artery disease is most commonly the cause for sudden cardiac death in athletes 35 and over. For those younger than 35, underlying congenital cardiac arrhythmias, or an irregular heartbeat, and structural heart disease are common causes of cardiac arrest. 

Still, people can go into cardiac arrest without having any known heart disease diagnosis. Certain drugs or medications, under what’s called acquired heart abnormalities, can also trigger cardiac arrest, Chan says. Still, extensive and continuous testing, therefore, is critical to uncovering any underlying heart conditions that may be at play, Chan says. 

For younger, more physically active individuals who experience a cardiac arrest episode, a structural or genetic heart problem may be present, or a non-heart-related event like a trauma may have occurred that caused excessive bleeding, for example. It’s unclear what happened in the case of Hamlin, who is 24 years old. 

“If you’re young, then you may have a congenital abnormality of some sort that is so well hidden and has not manifested itself,” Chan says. “Once you reach a certain threshold of stress or trauma, and/or a combination of both, then you have an effect.”

An array of heart problems put people at risk for cardiac arrest during exercise, Dr. Alexander Postalian, an interventional and general cardiologist at the Texas Heart Institute, tells Fortune. For example, when diagnosed with a condition called hypertrophic cardiomyopathy, people’s heart muscle is abnormally thick, and exercise can impact them more harmfully. 

“The more you exercise, the more strain is put on the heart muscle,” he says. 

It’s become commonplace to see a football player escorted off the field after getting injured—whether from concussions, dislocations, or sprains. Organizations and advocates have since been sounding the alarm on the physical dangers of the sport.  

“As players, injuries are always our deepest fear…But often, you know, we tend to try to remove any of those thoughts the moment we hit the field,” Rodney McLeod Jr., an Indianapolis Colts safety, told CNN this morning. “We’ve become very numb to it, unfortunately.”

There’s been a lot of discussion surrounding football players prone to head injuries, but what happened with Hamlin raises the question of how exercise and competitive sports can potentially trigger the heart. 

It’s rare to go into cardiac arrest from exercise alone, especially noncompetitive exercise. But for people who are concerned about their heart health when exercising, it’s important to talk to a doctor before completely avoiding activity, Chan says.  

“If you have any concerns, definitely approach your primary care physician, who can send you to a specialized cardiologist in order to perform any type of screening,” she says. “But the more important thing is, if you’re fearful, or you’re concerned, or you actually have symptoms, you need to have that checked out before you start an exercise program on your own,” including chest pain, shortness of breath, dizziness, lightheadedness, and passing out. 

For those who exercise or play sports, it’s also beneficial to understand your genetic history, Postalian says, as the debate around screening and how extensive screening should be for sports athletes is still ongoing.