About 18 percent of Los Angeles County jail inmates could be diverted into community-based mental health treatment programs, according to a study released this week by a Santa Monica-based think tank.

Roughly 30 percent of inmates in county jail suffer from mental illness and are either housed in special units and/or prescribed psychotropic drugs, according to the RAND Corp. study that was presented to the Los Angeles County Board of Supervisors. More than 60 percent of those inmates are appropriate candidates for diversion, based on a set of clinical and legal factors reviewed by RAND.

The situation is not unique to Los Angeles County, as county jails nationwide have become America’s largest mental health facilities.

“There are quite a high number of people in our jails every day who are not there because they actually pose a great risk to public safety but because their offense was committed through some aspect of their mental illness,’’ Supervisor Sheila Kuehl said. “Most of them do not actually have to be in jail.’’

Calculating the size of that group is important, a RAND researcher said.

“Knowing how many people are appropriate for diversion is a first step toward understanding the types of programs, staff and funding that would be needed to treat those individuals in the community,’’ said Stephanie Brooks Holliday, a behavioral scientist and the study’s lead author.

The study, “Estimating the Size of the Divertible Jail Mental Health Population in Los Angeles” looked at the jail population on June 9, 2019, and its findings were similar to those of an earlier study by the county’s Office of Diversion and Reentry.

RAND researchers focused on a single day’s population, and determined that roughly 61 percent of mentally ill inmates—more than 3,300 people—would fit the criteria for diversion, and another 7 percent might be eligible.

That calculation would be important to the Board of Supervisors’ planning for alternatives to the deteriorating and outdated downtown Men’s Central Jail. Justice reform advocates have pushed for a decentralized system of community facilities, while the county has struggled with how to provide quality care for mentally ill inmates who need to remain behind bars.

Los Angeles County officials have long acknowledged that jail is not the best place to treat mental illness and can make things worse. Authorities say it is cheaper to pay for community programs than to keep people in custody. the county’s Office of Diversion and Reentry estimates the daily cost of incarceration at $600 per inmate versus $70 for community care.

The board previously approved $100 million in 2019-20 budget dollars for efforts related to diversion, part of a shift toward a “care first, jail last’’ policy.