Police called to make a well-being check in Woodridge discovered a small amount of what they believe to be of the toxic chemical potassium cyanide inside the man's apartment.
Authorities evacuated nearby residents for about three hours Thursday while the chemical was removed.
The Woodridge Police Department says no one was hurt, but the resident who possessed the chemical was taken to a hospital for medical evaluation.
Police said Friday the man illegally obtained the chemical from his employer. It was in a crystalized form and will be tested to confirm it is potassium cyanide.
Potassium cyanide can release a highly toxic gas that interferes with breathing and can be deadly. It's used in fumigation, extracting gold and silver from rocks and in electroplating.
Police are investigating the incident.