Firefighters responded to reports of a chemical incident at Guy's Hospital, SE1.
The incident is believed to have been caused by the mixing of chemicals inside a plant room which produced a chlorine gas.
One person was injured as a result of a chemical reaction. Several people, who came to the aid of the person, have been treated for the inhalation of chlorine gas.
Firefighters have carried out a sweep of the area to check for no elevated readings of chlorine gas. The building has also been ventilated. Crews also supported with the precautionary evacuation of the basement and ground floors of the building.
Two fire engines, two Fire Rescue Units, a Command Unit and specialist hazardous materials officers attended the scene.
The Brigade was first called about the incident at 0849, with crews from Whitechapel, Dowgate, Euston and surrounding fire stations sent to the scene. The incident was over for firefighters by 1109.