The fire service was called to deal with an incident at the Princess Royal Hospital, in Haywards Heath.
Several fire engines were seen responding at 5.08pm on Tuesday (December 3).
A spokesman for West Sussex Fire said crews were called to reports of a fire at the hospital.
He added: “We found on arrival that there were fumes issuing from a sterilisation unit but there was no fire.
“Firefighters entered and ventilated the building to remove the fumes.”
The incident did not appear to affect the main building and no patients or staff were evacuated, an eyewitness said.