Self-harming affects nearly 1 in 5 teens. But we hardly talk about it.

Every day of his senior year in high school, Connor Quealy felt lonely.

It wasn’t just that he wanted more friends. It was a profound sense that he was alone in the world.

One day, Quealy was sitting in his family’s home in a Pittsburgh suburb, feeling depression envelop him, when he noticed a pocket knife on his desk.

“I don’t know what possessed me to do it,” said Quealy, now a 22-year-old senior at Saint Joseph’s University in Philadelphia. “I just cut myself.”

For one instant, the pain inside him seemed to transform into a tangible, physical pain.

It’s difficult to explain, Quealy said. “It got to the point where I didn’t know how else to express this pain, how to get rid of it.”

Turning to self-injury can be a surprisingly common outlet among adolescents, with 18 percent intentionally harming themselves, research shows. A study of more than 65,000 public high school students found one in four girls and one in 10 boys engage in self-injury — prompting researchers to call it a public health problem.

“It’s far more common than people recognize,” said Edward Selby, an associate professor of psychology at Rutgers University who has treated patients who self-injure and researches the topic. “Especially in high school, this is going on at a frequent rate.”

Yet discussion on the topic remains fairly muted, advocates and public health experts say, even as similar subjects such as suicide are entering the public domain.

“It’s just a naturally visceral antagonistic reaction people have,” said Janis Whitlock, director of Cornell University’s Research Program on Self-Injury and Recovery. “It’s like, close your eyes, look away, and not think about it again.”

That stigma can make it difficult for individuals like Quealy to get help.

But it’s vital that they do. Although people who hurt themselves are not necessarily suicidal in that moment, self-injury is one of the most powerful predictors of future suicidal thoughts. Suicide is the second leading cause of death for Americans ages 15 to 34.

For Quealy, it’s still difficult to talk about his history of self-injury for fear of judgment, but he wants others in pain to know that self-harm is not the answer.

“When you wake up in the morning and your arms hurt, you realize what you did. It’s not going to get better,” he said. “It’s going to make things worse.”

Tracking the numbers on self-injury is more difficult than other mental-health topics, experts say.

The U.S. Centers for Disease Control and Prevention and other large surveyors started asking adolescents about self-injury only in the early 2000s, Whitlock said.

Then there’s the question of what is considered self-injury. Cutting is the most prevalent form, but there’s also severe scratching to the point of drawing blood, biting or hitting oneself, and burning. Some researchers will include repetitive hair pulling, while others classify that as its own disease called trichotillomania.

Though drug and alcohol misuse, starving oneself, binging and purging are all harmful, substance use and eating disorders typically are not counted among self-injury conditions.

Some studies have reported significantly higher rates of self-injury among girls, while others say the gender difference is negligible.

Studies on emergency room visits suggest that rates of self-harm are climbing — a 19 percent annual increase from 2009 to 2015 — but it’s difficult to know whether more people are intentionally hurting themselves or a greater proportion are coming to the hospital than before.

After that first time in October 2014, Quealy began cutting himself regularly. Nearly every other day for more than two months.

He did it on his upper arm so it would be hidden, even by short-sleeved shirts.

“I put on a mask every day,” he said. “A smiling face mask and walked around pretending I was happy.”

Most people who self-injure don’t tell anyone, Whitlock said. It’s a myth that people do this to get attention or manipulate others.

Research shows the most common reason for self-injury is to relieve emotional distress.

People who have trouble regulating emotions will often ruminate about an upsetting event and get caught in a cycle of negativity, Selby said. Some people binge eat or use drugs and alcohol as a release. Some turn to self-injury.

For those who self-injure, the physical pain often has two purposes: It provides a distraction from emotional pain, and it causes the body to release endorphins, which creates a sense of relief. Endorphins interact with opioid receptors in the brain, similarly to such drugs as morphine.

In a new study published in March, Selby found that people who said they felt less physical pain during self-injury were likely to repeat the actions during a given episode or self-injure in more extreme ways — cutting deeper, for example.

“They’re purposely doing this to feel pain,” Selby said, “because the pain has a function.”

Some research indicates there may be an addictive quality to self-injury, too.

A few days after Christmas 2014, Quealy was playing a game with his younger brother when he absentmindedly lifted his sleeve to scratch his arm.

His brother spotted the scars, and immediately ran to their mother.

“The dog scratched me,” Quealy told her.

But she wouldn’t accept that. She told him to pull up both of his sleeves.

“It was that persistence that helped me,” Quealy said. He wouldn’t have shared the truth otherwise.

Soon after, Quealy started therapy for the first time — something he continues today.

“Talking about it, I felt relieved,” he said. “I realized the best medicine for me is to talk about my problems with someone.”

There’s no medication specifically for self-injury, Selby said. But research has shown that therapy helps.

Dialectical behavior therapy is the most common approach, focusing on emotional acceptance and learning healthy ways to regulate emotions. Therapists will also address underlying conditions such as depression or anxiety that might be fueling the behavior.

It can be a slow process, Whitlock said. “A lot of people don’t feel like they have a choice” but to hurt themselves, she said.

Today, when Quealy feels overwhelming emotions, he watches a TV show, cooks, or plays a video game -- anything to take his mind off them.

Sometimes he'll text a friend to grab food. They don’t necessarily talk about what’s upsetting him, but just feeling that human connection helps, Quealy said.

It’s still difficult and recovery is an ongoing process, Quealy said. But he hasn’t self-injured in four years.

“I look at the way my life is now compared to where it was four years ago and it’s full of hope,” he said. “I have hope for today, hope for tomorrow.”

If you are in crisis, call the National Suicide Prevention Lifeline at 1-800-273-8255 or text TALK to the Crisis Text Line at 741741. For additional resources, visit http://www.selfinjury.bctr.cornell.edu/.