A medical team in full decontamination suits have treated patients at Epsom Hospital as part of an exercise to check how it would cope with a nuclear, chemical, biological or radiological disaster.
At the same time a real chemical incident was taking place at St Helier Hospital in Carshalton at 8am when a fridge started leaking ammonia.
The exercise at Epsom Hospital
The major exercise started at 7am on Thursday when urgent pager alerts called on doctors, nurses and other staff from Epsom and St Helier NHS Trust to respond to a chemical leak on a farm.
A&E staff put on hazardous material suits and set up decontamination tents to treat 10 actors posing as patients to wash off the chemicals before they were allowed into the hospital buildings for treatment.
Behind the scenes staff set up a command centre to co-ordinate the emergency response, communicate with the public and keep the rest of the hospital running smoothly.
On Thursday chief operating officer Jackie Sullivan, responsible for the day to day running of the trust's hospitals, said: "This morning’s exercise was a vital opportunity for us to test how well we can respond to an emergency situation whilst keeping our hospitals running smoothly.
"I’m absolutely delighted with the way all of our staff handled the situation and having identified just a few small amendments to our plans, believe that we can be ready for anything."
An hour after the exercise started there was a real chemical incident at St Helier Hospital.
A fridge near the CT scanner suite malfunctioned and leaked ammonia, which can cause burning of the nose, throat and respiratory tract. The fire brigade removed the fridge and ventilated the area.
Ms Sullivan said: "The minor incident at St Helier just goes to show that we have incidents that can happen at anytime, and as a large organisation caring for thousands of patients every day, we cannot afford to be underprepared.
"I am pleased to say that the team at St Helier, led by general manager Lesley Nolan, dealt with the chemical leak very well, and with a timely response, we were able to avoid any impact on our patients.
"I would like to thank all staff and volunteers involved in the exercise at Epsom, and those involved in the real thing at St Helier."
A trust spokeswoman said they put up posters, gave out leaflets and posted information online to inform patients of the exercise and ensure they were not alarmed by it.