More than 200 people were evacuated from the William Hedley pub, on North Road at 5.45pm on Saturday (8 July), and fire and rescue services were called to the scene.
“A small number of staff and customers reported symptoms of irritation to the nose and eyes and difficulty breathing, however, there have been no serious injuries or lasting effects,” Durham Constabulary confirmed in a statement.
The statement continued: “The premises were quickly declared safe, but remained closed as a precaution. The substance has not yet been identified and inquiries are ongoing.”
The pub was busy because an annual event was being hosted in the city, which is believed to have attracted more than 200,000 people.
“This was understandably alarming for those present, but despite the premises being busy, there have fortunately been no reports of any lasting effects,” DS Richards of Durham City CID said.
DS Richards added: “A police investigation is now under way to determine the nature and source of the substance.”
The Stonegate-managed pub reopened the following morning.
A spokeswoman said: “There was an incident early Saturday evening that led to the quick and safe evacuation of customers and staff before reopening as normal on Sunday morning. We are assisting police with ongoing enquiries.”