Prison and union officials say 18 employees of a central Illinois prison have been treated at hospitals after they became sickened while responding to inmates suffering severe discomfort.

An Illinois Department of Corrections spokesperson says staff were exposed to an unknown substance at Graham Correctional Center in Hillsboro Wednesday but are “stable” after treatment.

Prison officers and other staff were responding to inmates who appeared under the influence of the substance.

Anders Lindall of the American Federation of State, County and Municipal Employees Council 31 says staff required treatment for dizziness and vomiting. A State Police hazardous materials unit is investigating.