SPRINGFIELD, Ill. (AP) — Several employees of a central Illinois prison were treated at hospitals Wednesday after they became sickened while responding to one or more inmates suffering severe discomfort, according to a prison spokesperson and a prison employees’ union representative.

Officials said 18 staff members at John A. Graham Correctional Center in Hillsboro required treatment at area hospitals.

An undisclosed number of inmates received treatment in the health care unit of the prison about 65 miles (105 kilometers) northeast of St. Louis.

An Illinois State Police hazardous materials team is investigating, state Department of Corrections spokesperson Naomi Puzzello said in a statement.

The staff members became ill after a prison employee responded to a “medical incident involving individuals in custody who appeared to be under the influence of an unknown substance,” Puzzello said.

“All staff members who may have potentially encountered the unknown substance were also sent to a local hospital, as a precaution. All staff are stable currently and many have already been discharged,” Puzzello said.

Prison officers and other staff who responded to the emergency call became ill, some violently, when they came near the affected inmates, said Anders Lindall of the American Federation of State, County and Municipal Employees Council 31, an employee union.

“Those who were in proximity to the inmate were immediately overcome with a variety of symptoms: Lightheadedness, dizziness, vomiting or feeling nauseous,” Lindall said.

Many staff members received medical care at Hillsboro Area Hospital. When the facility reached capacity, others sought treatment at HSHS St. Francis Hospital in Litchfield, 10 miles (16 kilometers) west. Most were released but several were kept for observation and testing to determine the cause, Lindall said.

AFSCME’s state council has expressed growing concern about prison employees coming in contact with dangerous substances.

“The issue of exposure to harmful substances in prisons is increasing,” Lindall said, adding that prisons need better searches. “AFSCME has been sounding the alarm for months to tighten up the protocol for both incoming mail and visitor screening.”

Lindall said a union meeting was underway at the prison when the call went out and members broke up the gathering to transport colleagues to the hospital, alert family members and provide other assistance.

Graham Correctional Center is a medium-security lockup for adult males, which opened in 1980 with room for 1,596 inmates. The prison currently houses 1,328 inmates.

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