New York City police will stay out of many mental health crisis calls and social workers will respond instead in parts of northern Manhattan starting this spring, an official told lawmakers Monday.

The test program will begin in three Harlem and East Harlem police precincts that together accounted for a highest-in-the-city total of over 7,400 mental health-related 911 calls last year, Susan Herman, who heads a wide-ranging city mental health initiative called ThriveNYC, told the Associated Press.

The experiment “will be a critical step forward in the city’s commitment to treat mental health crises as public health problems, not public safety issues,” Herman told a City Council committee. She said the city hopes to make the program citywide as quickly as possible.

Some important specifics remain to be seen in practice, including how real-time decisions about when to summon police — who will still respond to calls involving a weapon or “imminent risk of harm” — will play out. And the union that represents the city’s EMTs says the city needs to make sure they’re safe before going ahead.