Ten people were take to hospital suffering suspected carbon monoxide poisoning from a petrol generator being run in a shop.
Emergency services were called to computer sales and repair shop Sana Tech in Ashton under Lyne after concerns were raised for public safety.
Several businesses on Market Street were evacuated.
The North West Ambulance Service sent two ambulances and a hazardous area response team to the scene.
The people affected were taken to the Manchester Royal Infirmary and Tameside Hospital as a precaution but nobody was seriously harmed.