Council teams were called in to flush out sewers in the Holton Road area of the town following the discovery of a cleaning agent believed to have leaked into drains during weekend maintenance work.
South Wales police said the evacuation was ordered as a precaution on the advice of environmental health officials. Three people were taken to Cardiff Royal Infirmary and another person treated at nearby Llandough Hospital.
Around 200 staff and customers at the Dan Evans department store were among those evacuated to a nearby leisure centre.
A police spokeswoman said: "Anyone who was in the area who experienced headaches, dizziness or nausea is being advised to seek medical attention."Reuse content