A Manhattan federal judge signed off on a class-action settlement Thursday expected to drastically reduce the use of solitary confinement in New York state prisons and improve conditions in “the hole” for prisoners who undergo it.

U.S. District Judge Shira Scheindlin said 3,700 prisoners in New York and about 80,000 nationally are held in solitary confinement, and those in solitary are seven times more likely to harm themselves than inmates in general population.

“Solitary confinement is a drastic and punitive designation, one that should be used only as a last resort,” she wrote in a 30-page opinion. “It is well known that such confinement causes deterioration of the mental and physical condition of inmates.”

The settlement, first announced in December, puts limits on the situations in which solitary can be used and the length of confinement. For example, the judge noted, one plaintiff given three years for improperly keeping legal papers in his cell would be confined for a maximum of 30 days in his own cell, not a solitary housing unit.

It also bans solitary for pregnant prisoners, provides treatment alternatives, and mandates better food and library privileges, phone calls, access to mental health services and reductions for good behavior for those put in solitary.

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The New York Civil Liberties Union, which brought the suit, said the new rules would eliminate solitary for 1,100 people, and cut in half the 87 rule violations that could be the basis for solitary even for a first-time offender.

The judge also approved awards ranging from $9,900 to $80,000 to the three plaintiffs who brought the suit. She said the settlement had been distributed to prisoners throughout the New York prison system, and responses were generally positive, although critics called for broader reforms and damages to the rest of the class.

Scheindlin, who has announced that she is leaving the bench, said she hoped the agreement would reverse a trend that saw the number of prisoners held in solitary rise by 42 percent nationally from 1995 to 2005.

“While there is undoubtedly more work to be done,” she said, “ . . . it is also my hope that the contours of this settlement agreement . . . will serve as a model for other states that are addressing issues of prison reform.”