A laboratory assistant and two firefighters were treated after inhaling fumes during a chemical spillage at South Tyneside District Hospital.
An investigation has been launched after the spill forced parts of the site to be evacuated.
It happened in the histopathology department when formalin and nitric acid were dropped and reacted to produce potentially harmful gases including nitrous oxide gas.
Nitrous oxide is commonly known as laughing gas which can cause severe disorientation. Nitrogen oxide compounds can cause serious lung damage.
The lab assistant and the firefighters, who complained of sore throats, were discharged and returned to duty.
The chemicals were isolated safely in a 30m exclusion zone and allowed to evaporate. The lab is working again normally.
The quanities involved were the contents of two 500ml containers.
Fire crews dealt with the incident wearing gas-tight suits and specialist breathing apparatus to protect them from the fumes.
Tyne and Wear Fire and Rescue Service were called to the hospital in South Shields at 12:13 on Tuesday.
Northumbria Police assisted in setting up a cordon to keep the public away from the spillage.
Helen Ray, South Tyneside NHS Foundation Trust’s Chief Operating Officer, Clinical Services, said: “We are pleased to say the laboratory has now resumed full daily operational functions.
"There was a small chemical incident in the histopathology department of the Pathology laboratory at South Tyneside District Hospital.
"Staff immediately took appropriate action, following agreed procedure, and the fire service was contacted.
"The fire brigade have isolated the chemicals involved and securely removed them from the department.
"We have strict safety guidelines in place and we have already launched a full investigation into what happened in this instance.”
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