From the annals of Rikers Island comes a document titled, “Three Adolescents With Mental Illness in Punitive Segregation in Rikers Island.” Punitive segregation means solitary confinement for 23 hours a day. Schoolwork, if it comes, is passed through a slot in the cell door. Toothpaste is available once a day.

Rikers Island exists to make the rest of New York City seem blissful; there, troublemakers, troubled people and the unlucky are hidden behind a cloak of invisibility. It is a campus of jails for people arrested and awaiting trial, or others serving sentences of less than a year. Also, it is the basket into which society drops the disruptive mentally ill.

More than most jail systems, New York City has made extensive use of “punitive segregation” in recent years. Of the people put into solitary confinement, a high percentage have mental illness. On July 23, for instance, 102 of the 140 teenagers in solitary were either seriously or moderately mentally ill, according to a consultant’s report prepared for the city’s Board of Correction.

Few government agencies are subjected to candid oversight, but the city’s Correction Department is an exception. It faces a board that includes mayoral appointees but is controlled by no single politician. The board’s reports are blunt and lack the sanitized spin of most government documents.

The board’s staff prepared the report on the three mentally ill teenagers, each of whom was doing more than 200 days in solitary. Such segregation is a measure most often used to protect other detainees and prison staff when a person is dangerous or violent. But it comes at a steep cost: Even for people with no history of mental health problems, the prolonged isolation can lead to hallucinations, the consultants stated. Among those who are already ill, solitary accelerates existing psychiatric problems and makes treatment very difficult.

One of the three teenagers in the report was a detainee identified only as “Matthew,” 17, with bipolar disorder. He had been sentenced to 250 days in punitive segregation for punching a correction captain in the face. Four other members of his immediate family were also found to have bipolar disorder. The father’s only known job was drug dealer. His mother was a home health aide who worked two jobs. After two months in solitary at Rikers, Matthew had gone to two individual therapy sessions. He did not make other appointments for the following reasons, the report found: a search was underway for contraband in his area; no mental health escort to bring him to the therapy room; multiple floods; an episode involving “use of force” in the area; his refusal on one occasion to leave the cell.

An 18-year-old detainee, “Carlos,” who had depression, was also profiled in the report. He arrived in jail owing 150 days in solitary from a prior incarceration — time that remained on his account when he was released. On his return, Carlos was paying it backward.

“He and his family have had multiple encounters with the criminal justice system,” the report stated. “His earliest memory of the police goes back to his fourth birthday party, when police raided the family’s home to look for his father and drugs. Carlos said that his father did not attend his fourth birthday party because a police officer had tipped him off to the scheduled raid.”

When he was 8, his father was sent to prison, and his mother told him that he was gone “because he was a construction worker doing post-9/11 construction at ground zero.”

Between 2007 and the end of June 2013, the consultants reported, the number of punitive segregation beds increased by about 62 percent, to 998 from 614. For the last year, the board has been critical of the increase, and the city correction commissioner, Dora B. Schriro, said she was working to shift detainees into better spaces. By September, though, the board decided that rules limiting the use of solitary were essential: It voted 9 to 0 in favor of the rules. This week, the city reported that it had reduced the number of solitary cells by 17 percent in October, and was continuing the reduction into November.

As it happens, the city has drastically reduced the Police Department’s use of the stop-and-frisk tactic under pressure of a lawsuit. “What went on in Corrections looks very similar,” Dr. Bobby Cohen, a member of the board, said.

The department spokesman did not respond to messages Tuesday seeking comment. But the reductions in solitary, Commissioner Schriro said, was the fruit of long-planned strategies.

“Whatever the reason is,” Dr. Cohen said, “I’m glad they’re reducing the numbers.”

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