The test program, to be rolled out in two neighborhoods, will give mental health professionals the lead role when someone calls 911 because a family member is in crisis, officials said.
The initiative is modeled on existing programs in cities including Eugene, Oregon, where teams of paramedics and crisis workers have been responding to mental health 911 calls for more than 30 years.
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A main goal of such programs is to avoid bad outcomes from police interactions with people suffering from mental illness or addiction such as the March 30 death of Daniel Prude in Rochester, New York.
Officials in some other major U.S. cities, including Los Angeles, are exploring similar approaches.
In New York, "this is the first time in our history that health professionals will be the default responders to mental health emergencies," said New York City first lady Chirlane McCray. She has been in charge of a broader mental health initiative called ThriveNYC that critics say has shown few results.
Police officers together with emergency medical technicians employed by the Fire Department now respond to nearly all mental health 911 calls regardless of whether there is a risk of violence, city officials said. Under the test program, unarmed mental health teams composed of health professionals and crisis workers from the Fire Department's Emergency Medical Services will instead respond to mental health emergencies, they said.
A police officer will join the mental health team when there is a possibility that the person in crisis is armed or presents a danger, said McCray, who joined de Blasio and other city officials at a news briefing.
Details including which two neighborhoods will be involved have not been determined.
Mental health advocates in New York said they were glad the city was trying out the idea.
"Somebody who is trained to deal with people who are in distress or in emotional crisis and can assess what's happening and then can make recommendations for more ongoing care — it's just such a right response," said Amy Dorin, CEO of the Coalition for Behavioral Health.
Some advocates, though, hoped for a separate mental health emergency number and response teams with peer workers, who are people who have had mental health problems themselves and are trained to help others.
Cal Hedigan, CEO of a mental health organization called Community Access, said she was grateful the city was making changes but "concerned that how they've set it up ... may mean it's less transformative than it could be."
The EMTs' union, meanwhile, expressed concern about their safety and compensation.