(CNN) -- Last year, cops in Manhattan arrested people for smoking or possessing small amounts of marijuana a little more than 5,500 times. A disproportionate number of those arrested were minorities.

In a few months, most similar cases won't be prosecuted. On Tuesday, Manhattan District Attorney Cyrus Vance Jr. announced his office would decline to take up those cases, effective August 1.

"The dual mission of the Manhattan DA's office is a safer New York and a more equal justice system," Vance said. "The ongoing arrest and criminal prosecution of predominantly black and brown New Yorkers for smoking marijuana serves neither of these goals."

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Vance, a Democrat who is in his third term, said his office was discussing with New York City police and Mayor Bill de Blasio what exceptions there should be to the policy.

New York Police Commissioner James O'Neill on Tuesday announced a working group will take the next 30 days to look at the enforcement measures by the department.

He said that, while the department doesn't target minorities, "there are differences in arrest rates, and they have persisted going back many years, long before this current administration. We need an honest assessment about why they exist ... ."

O'Neill said NYPD officers should not make arrests that don't impact public safety.

Under the DA's office new policy, people who violate the law would be issued summonses. The NYPD does this in cases where possession is the most serious charge a person would face, O'Neill said.

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Under the current policy in Manhattan, people are arrested, fingerprinted and have to appear in court.

The DA's office said this creates enormous costs for the legal system and alienates too many people.

"Such arrests can significantly impact job searches, schooling, family members, immigration status, and community involvement," the DA said. There are often no punitive, rehabilitative or deterrent purposes in these cases, the DA said.

CNN reached out to the offices of district attorneys in other boroughs to see whether they are considering similar measures. Brooklyn already has a similar policy, which began four years ago and is still evolving.

"Since 2014, the Brooklyn DA's office has led the way in declining to prosecute marijuana possession cases, resulting in a 75% decline in arrests citywide and confirming that such a policy does not adversely affect public safety," District Attorney Eric Gonzalez said.

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Gonzalez said Brooklyn three months ago began to decline to prosecute some cases involving smoking in public when a nuisance did not occur.

The DA added his office also will work with the police and the mayor to determine the "very small number of exceptions" to the policy.

Queens County wants to wait to comment until after the 30-day NYPD working group analysis.

"It is our understanding that Mayor Bill de Blasio has directed the New York City Police Department to review its policy and practices. We will await the results of that review," a spokesperson for Queens District Attorney Richard A. Brown said.

Marijuana is a Schedule I drug under federal law and is illegal. Some states, like New York, have decriminalized marijuana, making it a violation and not a crime to possess small amounts of cannabis.

Medical marijuana is legal under New York law, but cannot be smoked.

Other counties that have policies like the one being adopted in New York include Philadelphia. In February DA Larry Krasner told his assistants to stop prosecuting simple possession cases, CNN affiliate KYW reported.

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