A man has been arrested after seven police officers came into contact with a suspected noxious substance at an address.
During a routine arrest at a property in Elizabeth Jennings Way, Oxford, they were made aware of a potential chemical hazard.
Around 60 people were evacuated from a block of flats after the incident at 5.16am, and at around 9pm Thames Valley Police said they had been allowed back into their homes.
Two of the affected police officers are being kept in hospital overnight.
The only people affected were officers and the arrested man, Thames Valley Police said.
The 34-year-old, from Oxford, is in police custody on suspicion of harassment without injury.
Police said the incident is not being treated as terror-related and the property is now safe.
Group Manager Kerry Blair of Oxfordshire Fire and Rescue Service said: "Specialist officers from Oxfordshire Fire and Rescue Service, working with Thames Valley Police and South Central Ambulance Service, have carried out extensive tests throughout the building and can now confirm that there is no risk to any residents returning to their homes."
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