A carbon monoxide scare shut down a day of swimming at the Inglewood Pool Sunday.

Around 40 people were evacuated from the building Sunday afternoon after high levels of the gas were detected in the swimming area.

Paramedics assessed and treated nine people at the scene - one as young as five years old.

They used a device to measure the carbon monoxide levels in the patients' blood.

A 24-year-old woman had a reading of 21 per cent and was taken to hospital as a precaution.

Fire crews tell CTV News there were up to 2-hundred parts per million of carbon monoxide in the swimming area. 50 parts per million is considered normal.

They're now trying to determine the cause.