HBO A new HBO documentary looks at the psychological effects of solitary confinement.

About 100,000 prisoners are housed in solitary confinement across the U.S., spending 23 to 24 hours per day alone in an 8-foot-by-10-foot cell.

At Virginia’s Red Onion State Prison, inmates are housed in segregated units with virtually no social contact, sometimes for years and even decades. Red Onion is one of more than 40 “supermax” prisons in the U.S., which takes inmates from other prisons who have been deemed difficult or dangerous.

Solitary confinement is a common practice in U.S. prisons, but one that raises significant ethical concerns. Isolation can be psychologically harmful, and research has shown typical effects of solitary confinement include anxiety, depression, an inability to think clearly, obsessive thoughts, paranoia and psychosis. Self-harm, including suicide, is also common.

Over 90 percent of prisoners who are housed in solitary will be released. While Red Onion has initiated a new “Step Down” program to help prisoners return to the general population, after months or years in isolation, they often face psychiatric issues that leave them poorly equipped to reintegrate themselves into society.

These effects are particularly pronounced for inmates with severe mental illness. One federal judge said that housing mentally ill prisoners in isolation “is the mental equivalent of putting an asthmatic in a place with little air.”

In 2011, United Nations experts called for the practice to be banned, insisting that prolonged solitary confinement is a cruel and inhumane punishment that can amount to torture, particularly for vulnerable individuals such as juveniles and the mentally ill.

“Solitary confinement is a harsh measure which is contrary to rehabilitation, the aim of the penitentiary system,” U.N. Special Rapporteur on torture Juan E. Méndez said at the time.

In a new HBO documentary, “Solitary: Inside Red Onion State Prison,” Kristi Jacobson gets to know the inmates and prison guards at Red Onion, offering an intimate glimpse into life inside a supermax prison, where cameras and journalists have rarely been allowed.

We spoke to Jacobson to hear more about what she learned during a year spent probing inside a supermax prison:

Why has solitary confinement become such a common practice in the U.S.? What’s the argument in favor of it?

You’re probably familiar with the story of the growth of the prison population. In the 1970s there were tougher sentencing laws, and then in the ‘80s and ‘90s came the “tough on crime” ethos, which was so strong in this country. That ethos led to a prison boom and increased sentencing ― there were more people inside our prisons with less to do, which led to more violence inside the prisons.

In 1983, two guards were killed on the same day at a high-security prison in Marion, Illinois. They put the entire prison on lockdown and kept it on lockdown as punishment for everyone. Experts around the country began studying this Marion model, which was locking people down for 23 to 24 hours a day. The idea was that they would build these “supermax” prisons, take the bad apples and put them in these facilities, and make the rest of the prison system safer. That was the rationale. Supermax prisons were designed to punish, and to limit contact between prisoners.

HBO Red Onion State Prison is a "supermax" correctional facility in Virginia that houses roughly 800 prisoners in solitary confinement.

How do prisoners end up in “segregation”? Is it often those who are mentally ill?

What happened is that we built these supermax prisons, so then we needed to fill them. It became a go-to solution for people running prisons to deal with a range of inmates, including those who may pose a danger to other inmates or officers, but also people who have disciplinary problems ― people who are a problem for the management. It kind of became the de facto way of dealing with problems.

There are a lot of people inside our prisons who have some mental illness. Those are people who have trouble fitting into the prison environment and following the rules, and they are disproportionately sent to isolation ― where then they are susceptible to becoming more ill as a result of being in that situation. It seems to not make any sense, but yet many billions of dollars were spent on these prisons with the idea of isolating prisoners indefinitely.

In your year at Red Onion, what were some of the most striking things you observed of how these conditions seemed to affect the prisoners?

One of the things that struck me the most was the screaming, the crawling, the yelling that was coming out from behind the cell doors. That reflected a helplessness and a pain that’s hard to describe. There was the man you see in the film who appears to be catatonic. There was talk of depression and “It’s either I kill myself or I make it.” There’s a really high level of depression.

I found myself connecting with a handful of prisoners who had an extraordinary ability to articulate what they were feeling and going through in such a profound and almost poetic way. I think part of that had to do with a certain inner strength that was pre-existing.

HBO This young man was sentenced to prison for committing two armed robberies and sent to Red Onion after getting into a fight with some other inmates.

A 2011 U.N. report denounced prolonged solitary confinement as amounting to torture. After witnessing it firsthand, is that something you’d agree with?

Yes. Look, I don’t run a prison, and I know that there are a lot of competing decisions that are made when you’re managing a prison. But I think that holding people in isolation for long periods of time ― and especially for indefinite periods of time ― absolutely amounts to what is effectively state-sponsored torture.

Under these conditions, it’s common for the inmates’ mental health to deteriorate before they are sent back into society. Is this a public health concern?

It can be dangerous to release people from this system back into society. The statistic that I hear most often is that 90 to 95 percent of people inside our prisons will go home. So to not be thinking about in what state psychologically, emotionally and physically they will return to their homes, I think, is dubious if you care about your community. But in my opinion, we should also care about the human beings that are doing time ― because they are that, they’re human beings.

What needs to begin shifting for us to build a more humane and effective correctional system?

A number of state prison systems have begun looking at this issue of solitary confinement and how they’ve relied on it for many years. The results have been bad, and they need to rethink it. We can’t ignore that.

And in dealing with that, we’re forced to look at what our prisons are doing more broadly. Are they designed to create an environment in which individuals can better themselves and become more productive when they leave, or is it about punishment, punishment, punishment? I think we’ve hit a point where we’ve realized that this punitive system isn’t serving anyone ― we have a more than 66 percent recidivism rate.

The more people talk about it as citizens and constituents, the more that politicians have to address it. We need a big paradigm shift around understanding what our prisons should and could do.

“Solitary” premieres at 10 p.m. ET/PT Monday on HBO.

This interview has been edited for length and clarity.