More than 75,000 Kaiser Permanente employees walked off the job on Wednesday in what their union representatives say is the largest strike of health care workers in U.S. history.

A coalition of labor unions representing the workers notified the company last month that it planned to carry out a three-day strike starting Wednesday at 6:00 a.m. PT if a new contract was not reached in time. The previous contract expired Saturday.

The strike will impact hundreds of hospitals spanning several states, with picket lines in California, Colorado, Washington, Oregon, Maryland, Virginia and Washington, D.C.

The health care workers' unions are calling for increased staffing levels at Kaiser Permanente hospitals, saying the facilities are in the midst of a "short-staffing crisis" that is unsafe and could lead to patients facing dangerously long wait times, mistaken diagnoses and neglect. The unions also accuse the hospital conglomerate of committing unfair labor practices.