The union that represents about 2,200 nurses is accusing the University of Chicago Medical Center of jeopardizing patient safety and creating long wait times by failing to adequately staff their departments.

It filed complaints Tuesday with the Illinois Department of Public Health and the U.S. Occupational Safety and Health Administration. The university has denied the claims. UCMC is one of Chicago’s largest hospital systems.

“[Hospital administrators] have failed to comply with their own policy of what they have defined as safe staffing,” said Marti Smith, Midwest director at the National Nurses United union.

Hospitals in Illinois are required by law to publish and implement written staffing plans. The union claims UCMC fails to meet its own staffing standards 46% of the time, according to their analysis.

The union said that’s left nurses without backup in high-pressure situations, and has forced them to work overtime. For instance, the union claims, one nurse was so concerned her unit had two nurses instead of four that she cut her vacation short to return to work.

“If your heart stops, there’s no do-over,” UCMC cardiology nurse Cincerlyn Lewis said at a news conference outside a UCMC hospital Tuesday. “ … so 50% of the time is not good enough.”

In response, a U. of C. spokesperson said in a statement that “distributing sensational allegations through press releases is consistent with a national [NNU] playbook that prioritizes media attention over productive dialogue.”

The statement points to UCMC’s high rankings in national hospital safety guides such as Leapfrog, which gives the hospital an A grade, and said the hospital “takes issues of nurse staffing and workplace safety very seriously.”

Nurses have been working under the terms of an expired contract since mid-April. They’re pushing for mandatory minimum staffing levels and increased oversight of workplace violence.

Mariah Woelfel is a producer at WBEZ. You can follow her on Twitter at @MariahWoelfel.