The A&E department and part of the car park was closed.
Bosses at Doncaster and Bassetlaw Teaching Hospitals NHS Foundation Trust have issued at update on the incident.
In a statement they said they anticipated that the A&E department would reopen at 9.30am.
The statement added: “The service was temporarily closed earlier this morning as a patient presented after exposure to an unknown chemical substance.
"The appropriate decontamination procedures were followed and colleagues worked with the relevant agencies.
"Following final checks, the emergency department will safely reopen - no additional harm occurred during this incident and we thank colleagues for their response and our communities for their patience in this matter.”
Nottinghamshire Fire and Rescue Service said crews from across Notts, Leicestershire and Derbyshire attended the scene.
“Enquiries so far have not led to the identification of a noxious substance.
“Investigations are ongoing at the hospital and at an address in Albert Road, Retford, where the man was injured.”