More than 900 nurses at Cook County Health went on strike Thursday morning, forcing the health system to postpone some appointments and surgeries and send ambulances to other hospitals.

The nurses walked off the job for the one-day strike after their last contract expired in November. The nurses work at Stroger and Provident hospitals, the system’s clinics and Cermak Health Services, which provides health care to the detainees at the Cook County Department of Corrections.

Cook County Health said in a statement Wednesday that it planned to use temporary agency nurses to fill in gaps Thursday in certain areas, including trauma and emergency departments, operating rooms and units for patients hospitalized overnight.

The system also postponed some elective and nonurgent procedures and appointments and moved other appointments to telehealth.

Stroger Hospital went on ambulance bypass for emergency department and advanced life support cases Thursday morning, meaning it asked ambulances to take patients to other hospitals instead of Stroger. The safety-net hospital continued to accept trauma patients.

The nurses union, National Nurses Organizing Committee/National Nurses United, says a major sticking point in negotiations has been staffing. The union says the hospital system is short hundreds of nurses, which has led to delays in patient care and burnout among nurses. The health system, however, has said it is continually recruiting nurses and hired nearly 800 in the past year.

The nurses picketed outside Stroger, Provident and Cermak. At Stroger, hundreds of nurses wearing red scrubs waved noisemakers and held signs reading, “Safe staffing saves lives” and “Our patients deserve better.” Cars and trucks honked in support as they passed on Damen Avenue.

Nurse Cathleen Armstrong said in her 23 years at Cook County Health, she’s never seen staffing and morale as bad as they are now.

“The staffing is so bad that patients can be here for several days without someone to wash their face,” Armstrong said, noting that washing a patient’s face is usually the job of certified nursing assistants, and nurses are not the only workers in short supply. “We’re always short staffed.

“I go home and wonder did I get done everything I needed to get done,” Armstrong said. “That brings me a lot of sleepless nights because you want to advocate for your patients.”

The nurses say that Cook County Health isn’t hiring enough nurses or doing a good job retaining them.

“When someone leaves or retires, they’re not replacing them,” said Adrienne Cleveland, a trauma nurse who’s been with the system for 28 years. “They’re just putting more workload on us, and it’s not fair to the patients at all.”

Emergency department nurse Consuelo Vargas, who is the union’s chief nurse representative for Cook County Health, said the union also wants to see pay improve so Cook County Health can be competitive with other hospitals in the West Side medical district where Stroger sits.

Cook County Health said in a statement Thursday that it “remains prepared to provide safe care for our patients during today’s nursing strike.”

When asked about short-staffing and nurse burnout at the health system, Cook County Board President Toni Preckwinkle said in an interview with the Tribune Wednesday, “I’m grateful for the work that our employees do across the board, in particular for the good work that our nurses do in the health and hospitals and that’s all I want to say about that.”

Preckwinkle’s administration on Wednesday projected a $121 million budget deficit for next year, compared with a deficit of $410 million this fiscal year.

Originally, about 1,250 nurses had been poised to strike, but the county received a court-ordered injunction Wednesday night to prevent nearly 330 nurses from striking. The order followed the Illinois Labor Relations Board’s finding earlier this week that those nurses should not be allowed to strike because their absence would pose a “clear and present danger to the health and safety of the public.”

As of Thursday morning, a separate strike by members of Service Employees International Union Local 73 was planned to begin Friday.

Those employees work in offices under the Cook County president, in the county clerk’s office, in civilian positions in the sheriff’s office, and for Cook County Health. About 1,473 of those workers are part of Cook County Health, working at Stroger and Provident hospitals, clinics, and in mental health services at Cermak. They include technicians, physician assistants and service and maintenance workers, among others.

Points of disagreement include pay equity, pandemic pay and retiree health benefits, among other issues, said Eric Bailey, a union spokesman. SEIU Local 73′s last contract expired in November.

About 2,500 members of that union had originally planned to strike, though the Illinois Labor Relations Board found Wednesday that about 370 of those workers, many of whom work in health care, should not be allowed to strike because of the danger it could pose to the public. Cook County said it would seek a court-ordered injunction to formalize that ruling Thursday.

Typically, employers can take findings from the board to circuit court, where they can pursue an injunction to limit or prohibit a strike.

A number of Illinois hospitals have faced nurse strikes in recent years, with nurses often citing staffing as a main concern. Hospitals where nurses went on strike in recent years include University of Chicago Medical Center, University of Illinois Hospital and Amita Health Saint Joseph Medical Center Joliet.

Cook County Health patients with questions about appointments, tests or procedures scheduled for Thursday should call 312-864-0200.

Alice Yin contributed.