A Boston-area hospital is investigating after five nurses who have worked on the same floor have developed brain tumors.

Mass General Brigham Newton-Wellesley Hospital said that in total, 11 employees from the fifth-floor maternity unit identified health concerns. Five had brain tumors, all of which are benign. Two of those have the most common, benign type — meningioma, according to the hospital in Newton, Massachusetts, which is about 10 miles west of Boston.

“The investigation found no environmental risks which could be linked to the development of a brain tumor,” the hospital’s Jonathan Sonis, associate chief medical officer, and Sandy Muse, chief nursing officer, said in a statement.

The hospital said its investigation was completed in collaboration with government health and safety officials and it considered multiple possible sources. It ruled out disposable masks, the water supply, nearby x-rays, and chemotherapy treatment on the floor below, the hospital said.

“Based on these results, we can confidently reassure our dedicated team … and all our patients that there is no environmental risk at our facility,” the administrators said.

The Massachusetts Nurses Association, the union that bargains for nurses’ compensation at Newton-Wellesley Hospital, said it will continue to investigate.

“Right now, the best way we can help is to complete an independent, scientific investigation,” MNA spokesperson Joe Markman said in a statement Friday. “That effort is underway and may take additional weeks.”

The union indicated that nurses came forward with workplace health concerns, which led to the discovery of those with tumors.

“The hospital only spoke to a small number of nurses, and their environmental testing was not comprehensive,” he said in his statement. “The hospital cannot make this issue go away by attempting to provide a predetermined conclusion.” 

A spokesperson for a state agency was unable to provide conclusive information on the matter by deadline. Federal occupational health and safety officials did not immediately respond to a request for comment.

The American Cancer Society says that in order to meet the definition of a cancer cluster, occurrences must be the same type, in the same area, with the same cause, and affecting a number of people that’s “greater than expected” when a baseline for occurrences is established.

“Nearly 4 out of 10 people in the United States will develop cancer during their lifetimes,” the society said on its cancer clusters webpage. “So, it’s not uncommon for several people in a relatively small area to develop cancer around the same time.”