The source of a hydrogen sulfide gas leak in Wheatley remains under investigation four days after some residents were evacuated.
Emergency crews are still on the scene at Erie Street North, where the leak of the toxic, flammable gas was first reported in a restaurant building on Wednesday.
"Provincial officers will be on scene today to continue the investigation into the source of the gas, which is a technical and complex challenge," the municipality said in a media release on Monday.
More than two dozen people have been forced from their homes, some businesses were evacuated, and a state of emergency was declared by the mayor of Chatham-Kent on Thursday.
The area cannot reopen until its safe to to do, the municipality said.
"We completely understand the concerns and frustrations of the families and business owners affected by the evacuation in what is already a challenging time and I want to thank them for their patience and understanding," Chatham-Kent Fire Chief Chris Case said in a media release.
According to the Provincial Hazmat Team, the level of hydrogen sulfide in the air has fallen but it's still not known where it is coming from.
Scott Mundle, one of Canada's foremost experts in identifying gases from abandoned wells, said hydrogen sulfide is flammable and highly toxic.
"Really small amounts of this gas can cause adverse health effects, and even death," said Mundle, who is a geochemistry professor at the University of Windsor.
He explained on CBC Radio's Windsor Morning on Monday that the gas is present in naturally occurring reservoirs as well as in sewer gas.
The municipality's fire chief has speculated that the leak could be coming from an old oil or gas well.
Such wells are decommissioned through capping and the pouring of cement, Mundle explained.
"This is something that's routinely done and is highly, highly regulated in western Canada. It's a little bit more challenging in Ontario because the oil and gas sector is actually a lot older, the records aren't quite as good, and the risks are a little bit higher because of the differences in regulation."