Five members of staff and four customers at Firm, Figure and Beauty Salon in Newtown were taken ill and an investigation is under way to determine the cause.
Claire Spalding, owner of the salon at Merchants House in the town's High Street, said traces of carbon monoxide had been found in her blood following tests at the Royal Shrewsbury Hospital.
But Dyfed-Powys Police said they did not believe this was the cause.
She said two customers collapsed at the salon on Wednesday afternoon and others started to feel unwell shortly after the emergency services were called.
Police, fire and ambulance crews were sent to the salon and nine people were rushed to hospital for checks.
They have all since been released from hospital and the salon is open for business as usual.
Miss Spalding said: "Two customers had just finished having a treatment when they collapsed. We called the emergency services and a member of staff said she also felt unwell and within minutes a few of us were feeling poorly.
"We were all sent to hospital in Shrewsbury.
"I was released on Wednesday night after they found carbon monoxide in my blood which I am told is the same as the others.
"I had 100 per cent oxygen treatment at the hospital and just feel like I have a bad hangover now, but we are open for business now and easing back in to it."
Stephen Bowen, landlord of the building, ruled out a carbon monoxide leak.
He said: "All boilers are serviced on a regular basis by a regulated service engineer. He was on site yesterday along with the fire brigade and found no evidence whatsoever of a carbon monoxide leakage."
Sian George, Dyfed-Powys Police spokeswoman, said: "At 4.04pm on Wednesday we received a call regarding an incident at the salon.
"The fire and ambulance services were already at the scene with nine people feeling unwell.
"They attended hospital and at the moment enquiries into the cause are continuing but it is not believed at this stage that carbon monoxide was involved."
She said further updates would be issued when the cause is found.
A Welsh Ambulance Service spokesman said a basics doctor, two rapid response vehicles, a patient care service vehicle and three emergency ambulances were sent to the scene.
The spokesman confirmed nine patients were taken to the Royal Shrewsbury Hospital for a precautionary check-up."
A spokesman for Mid & West Wales Fire and Rescue Service said three crews attended the scene to deal with a suspected carbon monoxide leak but that the building had been given the all-clear.