Experts are still trying to pinpoint the chemical responsible for leaving two men suffering from blistered skin and vomiting.
Initial reports suggested that three people had been effected but only two required treatment.
Specialist staff from South Central Ambulance Trust's chemical biological radioactive and nuclear (CBRN) Decontamination unit were called to Banbury to treat the victims before they could be admitted to The Horton Hospital.
It was thought a 45 gallon barrel of powder left in a car park close to Bridge Street may have caused their injuries, although a fire brigade spokesman said this was now thought to be "inert" and not harmful.
She said: "We don't even know if this barrel is the cause, and that their reactions are a delayed reaction from something else."
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