An investigation's underway after sixteen residents at a block of flats in Cardiff Bay were taken ill.
Over 60 residents were evacuated from the Bay Chambers building on West Bute Street on Thursday night.
Five people were taken to hospital after they reported suffering from nausea and headaches - and one person was detained overnight for observation.
Four fire crews and specialist officers attended the scene amid reports of a release of carbon monoxide, but they did not find any evidence of hazardous materials at the site, despite extensive monitoring.
Further air quality tests have since ruled out carbon monoxide poisoning.
The evacuated residents, who were given shelter at the nearby Coal Exchange building, were later allowed to go home, but they have been advised not to drink tap water as a precaution.
South Wales Fire and Rescue Service said they were called out to the area at just before 10.20pm.
A number of residents had reported feeling ill and had shown the symptoms of carbon monoxide poisoning.
A Fire Service spokesperson said: "Our crews at scene used detection, identification and monitoring equipment to try to establish the origin and cause of any suspected hazardous gas escape."
They added: "Approximately 60 residents were evacuated from the building as a precaution and a rest centre was established at the Coal Exchange."
"Multi-agency briefings were held at scene and a Tactical Coordination Group was established to allow partners from South Wales Fire Service, South Wales Police, Cardiff Council, Public Health Wales, Welsh Ambulance Service Trust, Welsh Water and other agencies to discuss tactical priorities to allow the incident to be managed successfully with a view to returning to a normal state as soon as possible."
"All residents were allowed to return to their properties and further investigations are ongoing."
Public Health Wales issued 'Do Not Drink' advice to the affected block of flats as a precaution, after contacting Dŵr Cymru - with bottled water now being provided to affected residents.
A spokesperson for PHW said: "Residents have been allowed to return to their homes."
"Cardiff Council's Incident Support Team and Family Housing Support staff were on site during the incident to support the emergency services."
"Alternative accommodation was arranged, and transport placed on standby for use in the event that that residents needed to be moved further afield, though none of this was required."
South Wales Police said all five people taken to the University Hospital of Wales in Cardiff for further tests have now been discharged,
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