President Barack Obama on Monday announced new restrictions on the use of solitary confinement in federal prisons — a move that will affect as many as 10,000 inmates.

Solitary confinement involves putting someone in a cell for hours, days, weeks, months, and even years with little to no contact with others. It is often used to discipline wrongdoers in prisons, but it's also deployed to protect inmates, including transgender prisoners, from others and segregate groups of people who are causing trouble. As many as 100,000 prisoners are placed in solitary across the US each year.

But many critics call solitary confinement dangerous, citing the deleterious effects it has on a person's physical and mental well-being. These issues led a United Nations report to conclude that anything above 15 days in solitary confinement is torture.

In his announcement, Obama cited solitary confinement's negative effects to justify banning its use against juveniles and as a punishment for low-level infractions in prison, among other new restrictions.

"The United States is a nation of second chances, but the experience of solitary confinement too often undercuts that second chance," Obama wrote in the Washington Post. "Those who do make it out often have trouble holding down jobs, reuniting with family and becoming productive members of society. Imagine having served your time and then being unable to hand change over to a customer or look your wife in the eye or hug your children."

The actual reforms are surprisingly strong. The changes are limited to federal prisons, which house a significant segment of the overall prison population in the US. But the plan could stop thousands of uses of solitary each year. That, advocates say, is a big deal.

What Obama's solitary confinement reforms do

Last summer, Obama asked the US Department of Justice to review the use of solitary confinement in federal prisons. Here are some of the changes announced on Monday:

Solitary confinement is strictly banned for juvenile prisoners.

Inmates with serious mental illnesses will be diverted to alternative forms of housing, such as "secure mental health units" with additional staff for mental health care.

The Federal Bureau of Prisons (BOP) will limit disciplinary use of solitary confinement, particularly to limit how many infractions in prison can be punished with restrictive housing — and outright ban the use of solitary confinement for low-level infractions.

Solitary confinement for a first offense is limited to a maximum of 60 days, down from 365 days.

Inmates placed in restrictive housing for protective custody will be diverted to less restrictive conditions than solitary confinement, particularly "reintegration housing units" that BOP plans to build.

Inmates should be placed in the least restrictive setting possible and told why they're being placed in a restrictive setting. Staff should also regularly review decisions to put inmates in restrictive settings like solitary confinement and develop a clear plan to return the inmate to less restrictive conditions as quickly as possible.

Wardens will be directed to develop plans to increase the number of hours inmates in solitary confinement spend out of their cell, especially to allow for rehabilitation and reentry services. BOP will discourage the use of solitary confinement during the final 180 days of inmates' prison terms.

In addition to the changes, BOP will also publish monthly solitary confinement data on its public website.

The idea behind the reforms is to limit the use of solitary confinement to a last resort, including for discipline but especially juveniles, inmates with mental health issues, and prisoners in protective custody. If solitary is used, it needs to be clearly communicated to an inmate why it's necessary, and staffers need to work to reduce how long it lasts.

Still, the reforms are limited to executive action, since they weren't approved by Congress. So they could be undone by a future administration — just one of a few limitations in the announced changes.

The reforms range from very small to very significant

While the reforms stop well short of banning the use of solitary confinement, as some advocates would like, Obama's actions amount to a very comprehensive list of all sorts of moderate changes that reformers have long been calling for.

But there are limitations. For one, these reforms only apply to federal prisons. That's a minority but significant portion of the US prison population: Federal prisoners made up 13 percent of all US inmates in 2014, according to the Bureau of Justice Statistics.

Furthermore, the ban on juvenile solitary confinement, which got a lot of attention from media outlets like the Washington Post, will affect a tiny number of prisoners overall. Just 26 federal prisoners are juveniles, and 13 juvenile inmates were put in solitary between September 2014 and September 2015.

Still, the rest of the reforms will impact a large group of people — roughly 10,000 federal prisoners are in solitary confinement, the Washington Post reported. In the fiscal year that ended on September 30, 2014, adult inmates were sent to solitary for nonviolent offenses 3,800 times — suggesting that the ban on solitary for low-level infractions could stop thousands of uses of solitary each year.

"Solitary confinement became the default management practice for every problem in corrections — and then some that didn't exist before. It was reflexively used without any thought to what it was doing to human beings or accomplishing for safety," said Amy Fettig, director of the American Civil Liberty Union's Stop Solitary Campaign. "The fact that the Justice Department has homed into one of the most dehumanizing uses … is a strong signal to the rest of the country."

"[Solitary confinement] was reflexively used without any thought to what it was doing to human beings"

Limiting the use of solitary confinement for protective custody inmates could also go a long way to reducing some of its worse uses, Fettig said. In many cases, this type of solitary confinement is the most absurd: Federal officials are in effect placing prisoners in solitary — a practice that can permanently damage a person's mental and physical well-being — to protect them.

Since the research suggests that solitary confinement's negative effects on mental health are most pronounced among juveniles with developing brains and those with existing mental illnesses, limiting solitary's use among such groups could also prevent some of its worst uses.

"It's unquestionable that there are some folks who are at higher risk of being really, really harmed by the practice of solitary confinement," Fettig said. "But it is also true that this is a practice that hurts everyone, and nobody comes out unscathed. That's why I think it's very important that not only did the Obama administration take a look at additional protections for vulnerable populations, they went much deeper."

Although Obama's actions have no effect at the state level, at least a dozen states have taken steps to limit the use of solitary confinement over the past several years, according to the Washington Post.

Overall, Fettig argued that the full breadth of the changes is remarkable — and could send a signal to more states to change their systems.

"This is a game changer," she said. "There's no question that the Bureau of Prisons is the largest, most influential prison system in the country. What they do has influence on what the states do and what's considered best practice in the field. So to have them dramatically change their system, there's no question that is going to be leadership that some states and local jurisdictions are going to follow."

Solitary confinement is terrible but is sometimes used to protect inmates

A large body of research shows that solitary confinement can worsen mental illness and cause it in some circumstances, particularly among younger people, whose brains are still developing, and prisoners with existing mental illnesses.

Symptoms of prolonged solitary confinement include hypersensitivity to stimuli, perceptual distortions and hallucinations, anxiety, revenge fantasies, rage, appetite and weight loss, heart palpitations, headaches, problems sleeping, self-mutilation, suicidal thoughts, and, in rare situations, lower levels of brain activity.

Some of the research goes back to the 19th century. The early research about the health effects was so convincing that the US Supreme Court stated in 1890 that solitary confinement is not "a mere unimportant regulation as to the safe-keeping of the prisoner."

The Court concluded that solitary confinement caused prisoners to go "into a semi-fatuous condition, from which it was next to impossible to arouse them, and others became violently insane; others still, committed suicide; while those who stood the ordeal better were not generally reformed, and in most cases did not recover sufficient mental activity to be of any subsequent service to the community."

A large body of research shows that solitary confinement can worsen mental illness and cause it in some circumstances

The detrimental effects extend to people who are in solitary confinement at no fault of their own. Juvenile and LGBTQ inmates are often put in isolation for their own protection against older or bigoted prisoners. Sometimes certain tattoos can get a person thrown in solitary confinement, because the body art is often associated with gang affiliation. It's a very subjective practice, leading to a lot of misuse and overuse.

Supporters of solitary confinement argue it's sometimes necessary to protect inmates by separating them from others.

Obama acknowledged this protective use, stating that the Justice Department "found that there are circumstances when solitary is a necessary tool, such as when certain prisoners must be isolated for their own protection or in order to protect staff and other inmates. In those cases, the practice should be limited, applied with constraints and used only as a measure of last resort."

Many advocates and experts would like solitary confinement banned even in these instances. But they acknowledge that prison and jail officials don't necessarily have bad intentions with their use of solitary, even though the results can be devastating.

"This is a lesson in the banality of evil — that a practice could arise that so hurts people and so damages people, and the folks who are practicing it don't even notice," Fettig said. "It shouldn't be normal in an American prison or jail that individuals smear themselves in feces, habitually slice themselves, and commit suicide in housing units. But that's what happened in solitary confinement."

Watch: How mandatory minimums contributed to mass incarceration