As some 900 FDNY paramedics and EMTs have left to become firefighters in the last year, the city’s ability to respond to medical emergencies has reached a crisis point, union leaders warn.

Over the last 12 months, official data show, the average time to reach people in cardiac arrest or choking rose by 24 seconds — an increase that can mean life or death.

“Every second counts — every second is lost brain cells,” a veteran paramedic explained. “Brain death starts to occur in four to six minutes.”

The overall EMS response time for other life-threatening emergencies rose by 19 seconds, the city data show.

And for all medical emergencies, the response time jumped an alarming one minute and three seconds.

“It’s serious. People are going to die,” said Oren Barzilay, president of Local 2507, the union that represents EMS workers. The union has lost nearly a quarter of its 3,800 members.

The EMT exodus leaves dozens of ambulances idle, said both Barzilay and Vincent Variale, president of the EMS officers union.

“Every day, 40 to 50 ambulance and supervisor units are put out of service due to no staffing available,” Variale said.

A cap on EMS overtime — barring medics from making more than 40 percent of their annual base pay — was lifted by FDNY brass last month to deal with the crisis.

“They’re working 70 to 80 hours a week,” Variale said of his exhausted members.

What’s more, 60 percent of current EMS personnel have less than three years on the job because so many veterans have left to become firefighters.

“They’re rookies,” Variale said. “You couple that with being short-staffed, it’s worrisome.”

Many newbies “don’t know the area as well,” and the GPS devices “suck,” which also contribute to higher response times, said an EMT.

As a result, some EMTs don’t use GPS at all, or use their personal phone GPS, “which can be dangerous,” he said.

Nuisance calls also drive up response times, such as getting dispatched to cardiac arrest that turns out to be a passed-out drunk. “You sometimes can’t get to real cardiac arrests because they’re on these other calls,” the EMT complained.

He added, “Because the call volumes are so high, they’re pulling ambulances from different areas, so you’re traveling longer distances.”

The response data tell only how long it takes for an ambulance to arrive at a street address — not the added time it takes to find and reach the patient.

Since December 2017, the FDNY has promoted about 900 of its EMTs and paramedics to firefighters. The last three Fire Academy training classes have all been made up of former EMS personnel, who get first priority to join the Bravest even with civil-service scores far lower than applicants in the general public.

Firefighters are paid more — $86,000 after five years compared to $48,000 for a top EMT — and get more career opportunities such as becoming a fire marshal or joining the scuba unit.

EMTs spend all day in ambulances stationed on street corners and go on endless calls with no bathroom or meal breaks. Firefighters enjoy cooking, communal meals and a more relaxed atmosphere between less frequent calls.

The EMS shortage means that people who call 911 for help have to wait longer for an ambulance than they should. Firefighters can administer CPR, but paramedics bring life-saving medications, intubation to secure the airway, and transportation to a hospital — crucial in trauma cases.

“On several occasions, especially during peak periods, like weekends and evenings, they’ve run out of ambulances,” an FDNY dispatcher said.

Manhattan had the worst average response time for all medical emergencies over the last 12 months (12:34), followed by the Bronx (12:30), Brooklyn (11:34), Queens (10:28) and Staten Island (9:46).

“The numbers are shameful. They’re indicative of an underfunded EMS system,” the veteran paramedic said.

Barzilay said he will testify at a City Council hearing this month to demand funds for EMS personnel, vehicles and stations in Long Island City and the Brooklyn waterfront, where new housing units have exploded.

FDNY spokesmen did not respond to a request for comment.