NORTH BAY -- The National Union of Healthcare Workers  is holding a day-long strike at Kaiser Permanente facilities throughout Northern California, including hospitals in Santa Rosa, San Rafael, Vallejo and Vacaville, as the union negotiates a new contract with the health system.

The union, representing mostly mental health and optical workers, was joined in solidarity by registered nurses from the 17,000-strong California Nurses Association and 650 members of the Stationary Engineers Local 39, according to NUHW, which said it was the largest walkout in Kaiser's history.

Across Northern California, Kaiser said more than 66 percent of its nurses crossed picket lines today. The work stoppage is only for today and does not include a five-day "lockout," as other CNA strikes have included in the past, most recently last September at Sutter Health facilities in Northern California. Such lockouts can bar striking workers from returning to work for a certain number of days.

It's the fourth work stoppage NUHW has engaged in since 2010, when negotiations began, according to 4,000-member union.

The union says Kaiser management is insisting on "major reductions to workers' health care coverage and retirement benefits" at a time when the Oakland-based HMO has made $5.6 billion in profits over the last three years. It also says Kaiser has not addressed members concerns about staffing levels and its impact on patient care.

Kaiser disputes those allegations, saying in a statement that will continue to provide employees with "highly desirable, market-leading salary and benefits.

"We are disappointed in NUHW’s decision to strike," said Gay Westfall, senior vice president of human resources for Kaiser. "We recognize their legal right to conduct a strike, but believe differences are best resolved at the bargaining table.  We are disappointed and concerned that CNA is calling for yet another work stoppage by (Kaiser Permanente) nurses."

Kaiser said its  hospitals, emergency and urgent care departments, pharmacies and laboratories would remain open during the strike.  Some elective procedures may be rescheduled, according to Kaiser.

Kaiser also said it has been bargaining in good faith with NUHW "to reach a fair and equitable agreement as soon as possible."