Rikers Island (Seth Wenig/AP)

New York City officials approved a series of sweeping changes Tuesday to the way its prisons use solitary confinement, headlined by the decision to stop putting nearly all inmates 21 and younger in isolation. The city will also limit how long any inmate can be sentenced to solitary and prohibit putting any inmates with serious mental illnesses or physical disabilities in isolation.

The change impacting younger inmates follows a recent wave of reforms across the country targeting how state prison systems use solitary confinement. Pennsylvania, one of the largest prison systems in the country, said last week that it would stop putting inmates with serious mental illnesses in solitary confinement.

Several states, including New York, have moved in the past year to limit the amount of time juveniles spend in solitary and the overall number of inmates held in solitary. New York Mayor Bill de Blasio and Joe Ponte, commissioner of the city’s Department of Correction, announced last month that the city would no longer put 16- and 17-year-old inmates in solitary.

The New York City Board of Correction voted to approve the change affecting inmates ages 18 to 21 during a public meeting Tuesday morning. These shifts, which go into effect Jan. 1, 2016, are predicated upon the department being able to hire additional staff members, it said after the vote. (The only exceptions to the new ban: Inmates age 18 to 21 who commit certain serious infractions, according to the rules adopted by the board.)

New York’s corrections department has about 14,000 inmates on any given day, more than the state and federal prison populations of 20 states. Most of the city’s inmates are held on Rikers Island, a facility that has been excoriated by the U.S. government and civil liberties groups. Rikers was lambasted for mistreating mentally ill inmates by placing them in solitary confinement and skewing the numbers it reported about inmate fights. It has also come under fire for issues involving guards smuggling contraband and gaps in security.

An investigation carried out by the office of Preet Bharara, the U.S. attorney for the Southern District of New York, found that adolescent inmates suffered “serious physical harm from the rampant use of unnecessary and excessive force” by Rikers staff. (Adolescents are defined in this prison system as inmates ages 16 through 18.) This inquiry focused solely on how the correction department protected adolescent inmates, and it looked at a period before de Blasio was sworn into office.

Ultimately, the report issued by Bharara’s office decried the “deep-seated culture of violence” it found at Rikers. Investigators found that young inmates had suffered serious harm and brutal injuries and said the inmates were at constant risk in Rikers. The use of solitary confinement was singled out, as significant numbers of young inmates were found to be placed in solitary for periods of up to several months. Corrections officers isolated adolescent inmates, many of them mentally ill, “at an alarming rate and for excessive periods of time,” the report said.

There were more than 680 adolescent inmates in Rikers on an average day in 2013, according to the city’s corrections department. Bharara’s report said that on any given day in 2013, up to a quarter of these young inmates could be in some form of solitary, which typically involved spending 23 hours a day in a six-by-eight-foot cell.

The report from Bharara’s office noted that prolonged time in solitary confinement can cause significant harm, something that has been found in multiple studies. Under the new rules approved Tuesday, inmates can no longer be sentenced to more than 30 consecutive days in solitary, a significant drop from the previous limit of 90 days. They also cannot be held in solitary for more than 60 days over a six-month period, with certain exceptions in place for inmates committing “persistent acts of violence.” In addition, inmates cannot be placed in isolation for any time they owe from a previous incarceration.

The board also approved the creation of housing with heightened supervision for the inmates deemed to be the most dangerous, supervised by specially-trained corrections officers and with specific programming. These new units could include restrictions on time inmates can spend out of cells, but these limits are to be built around each inmate’s particular situation. (Inmates ages 21 and younger and those with serious mental or physical ailments are banned from these units as well.)

The changes were praised by the New York Civil Liberties Union on Tuesday. Donna Lieberman, the group’s executive director, said in a statement that “these rules are a major step forward.” De Blasio also issued a supportive statement saying that Rikers would now “be at the forefront of national jail reform efforts.”

[This post has been updated. I originally misspelled “Bharara" as “Bhara," but I’ve fixed that.]