South Yorkshire Police said in a statement they were called to the Park Hill flats off Duke Street on Monday evening, where officers found a “seriously injured” man in his early 30s.
Crews from Yorkshire Ambulance Service and South Yorkshire Fire and Rescue Service also attended the scene, and the man was given CPR.
However, officers arriving at the location also fell ill from an “unknown chemical” – and four received medical attention.
At around midnight, police said tests had been completed and “there is no longer any risk of contamination as no traces of harmful substances were found”.
“Police officers are now collapsing the scene to just the property involved and will be re-entering the property to continue enquiries as to what happened,” South Yorkshire Police said.
The man has been transported to Northern General Hospital where he remains “in a serious condition”, police added.