The supermarket in Birmingham has been evacuated alongside several other premises within the cordon.
Seven people have been treated by paramedics after reporting breathing difficulties at a Sainsbury’s.
A cordon has been placed around the store on Longbridge Lane in Longbridge, Birmingham, after emergency services were called shortly before 10:00 BST.
West Midlands Fire Service said the supermarket was evacuated prior to its arrival, with several other premises within the 50m cordon also evacuated.
Sainsbury’s has been contacted by the BBC for comment.
West Midlands Ambulance Service (WMAS) said it was alerted to reports of six people suffering with breathing problems at the shop, with seven treated at the scene.
Two people were taken to hospital as a precaution, the fire service added.
Specialist officers are using air monitoring equipment inside the store, it said, to try to establish the cause of the incident.
In total, the fire service added, around 150 people were evacuated from the store.
A nearby college building was not evacuated, it said, but advised to keep windows closed as a precaution.
People have been advised to stay away from the scene.
“Anyone who has been in the area and is feeling unwell should seek medical advice as a precaution,” WMAS added.
West Midlands Police said specialist officers were working with colleagues across the emergency services to establish exactly what had happened.