Firefighters were called to Watford General Hospital today after a liquid nitrogen cylinder started leaking.
Crews from Watford and Rickmansworth attended the incident at the dermatology unit at about 1pm.
Watch Commander Lee Haward, from Watford Fire Station, said: "The staff had gone to turn it (the cylinder) off but there was a fault with the valve. I think it was approximately an hour before we were called. It was a small problem in terms of a hazard."
The building was evacuated while two firefighters wearing breathing masks moved the canister to a safe place outside, which was cordoned off. The hospital then called the company, which supplies its nitrogen, to deal with the canister.
Mr Haward said there was no real danger to patients in the incident, but added: "If nitrogen spills or comes into contact with anything organic, i.e. human flesh, it will freeze."