On Tuesday, tens of thousands of Kaiser Permanente health care workers in California and Hawaii began a five-day strike, citing ongoing disputes over wages, staffing shortages, and contract negotiations.

According to the United Nurses Associations of California/Union of Health Care Professionals (UNAC/UHCP), around 31,000 union members across more than 500 hospitals and clinics will be on strike through Sunday morning. Union leaders are calling it the largest labor action in the organization’s 50-year history.

UNAC/UHCP is part of the Alliance of Health Care Unions. If other member unions choose to participate, the total number of striking workers could rise to 46,000.

“After months of stalled negotiations, Kaiser leadership has failed to agree on a fair contract that ensures safe staffing, fair compensation, and a stronger voice for frontline workers,” the union said in a statement released Monday.

The strike involves a wide range of medical professionals, including registered nurses, pharmacists, nurse practitioners, anesthetists, midwives, physician assistants, rehabilitation therapists, speech pathologists, dietitians, and more.

Kaiser officials have criticized the strike as both unnecessary and disruptive. They note that employees represented by the Alliance currently earn about 16% more than peers in similar roles at other organizations.

Lionel Sims, senior vice president of human resources for Kaiser Permanente Northern California, said in a statement that the union originally sought a 38% wage increase over four years and is now demanding 25%. Sims argued that the current economic climate and rising health care costs make such a request unrealistic. He added that Kaiser’s latest offer — a 21.5% increase — would raise payroll by nearly $2 billion through 2029.

To maintain operations during the strike, Kaiser is deploying roughly 7,600 temporary nurses, clinicians, and health professionals. The health system will also shift some in-person visits to telehealth services and may need to postpone certain non-urgent procedures or appointments.

The strike officially began at 7 a.m. Tuesday, with the Bay Area’s only picket line set up outside Kaiser Permanente’s Oakland Medical Center at its Broadway locations.