Emergency services were called after two customers collapsed and others, including staff, started to feel unwell at Firm, Figure and Beauty Salon in Newtown.
The fire service confirmed carbon monoxide was present at the salon but the cause of the leak was unknown.
The matter will now be investigated by the Health and Safety Executive.
Salon owner Claire Spalding told a newspaper traces of carbon monoxide had been found in her blood after tests.
She told the Shropshire Star that two customers collapsed at the salon on Wednesday afternoon and others started to feel
I had 100% 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”
Claire SpaldingSalon owner
Police, fire and ambulance crews were sent to the salon and nine people were taken to the Royal Shrewsbury Hospital for checks.
“I was released on Wednesday night after they found carbon monoxide in my blood which I am told is the same as the others,” she told the newspaper.
“I had 100% 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 into it.”
The Welsh Ambulance Service said it was called to the salon at 15:20 BST on Wednesday.
A doctor and a number of vehicles were sent to the scene including three emergency ambulances.
“Nine patients were taken to the Royal Shrewsbury Hospital for a precautionary check-up,” a spokesperson added.
Mid and West Wales Fire and Rescue Service crews were also called to the salon just before 15:45 BST and used breathing apparatus to enter the property.