A single transport truck carrying a flammable, hazardous chemical has rolled over on the Queen Elizabeth Way, shutting down a section of the highway and forcing an evacuation in St. Catharines on Tuesday afternoon.

The crash happened around 3 p.m. along the Fort Erie bound QEW at Martindale Road when a tractor trailer flipped onto its side over the centre median guardrail.

A fire broke out as a result of rollover, Ontario Provincial Police Sgt. Kerry Schmidt told CP24. It was not immediately clear if the product held in the tanker's trailer had leaked.

Toxic chemical identified as phosphine

The transport truck was hauling a toxic substance -- phosphine, a flammable fumigant and clear, colourless gas in its natural state, according to Schmidt.

It is classified as "very toxic" by WHMIS, and when inhaled, causes cell damage in lungs and affects the nervous system.

“This product is very flammable if exposed to the atmosphere and we’re using all precautionary measures necessary right now,” he said.

There is concern of inhalation as the wind is blowing eastward, according to Niagara Regional Police.

A hazardous goods cleanup crew from Hamilton later determined "the phosphine is contained and there is no leak," Niagara Police Const. Phil Gavin told CP24 at 8:30 p.m. 

'Shelter in place' order for residents, evacuation of businesses

The portion of the roadway has been closed between Ontario Street and Highway 406 for more than five hours due to an evacuation order by police. It was later downgraded to a "shelter in place" order.

This is in effect for a two-kilometre radius from the crash site. This is an expansion of the original 1.2-kilometre radius set up around the site to reduce the public's exposure to the substance. For now it is a "precautionary measure."

“We’re taking all the precautions necessary to make sure no one gets hurt in getting this situation rectified,” Schmidt said.

Crews are currently evacuating nearby businesses and are advising local residents to take shelter inside their homes. 

Schmidt explained the "shelter in place" order is expected to only affect a small number of residences and businesses.

"If you live in the area an officer might knock on your door to evacuate and you may hear sirens," he warned.

Affected residents are advised to stay inside, shut their windows, turn off their air conditioning and close their chimney, Niagara Regional Police Const. Phil Gavin said. 

St. Catharines Fire and Niagara paramedics are creating a barrier around the site to protect the area, according to city officials. 

Phosphine is contained, hazmat team says

The hazmat team called in to determine if the toxic substance spilled out from the trailer where it was being carried, according to Niagara police, has determined it was contained. 

"At this point we believe the chemical is contained and it's not an airborne issue, but we don't want people moving around," Gavin told reporters in St. Catharines on Tuesday evening.

So far, the transport truck is still overturned and lying on its side, he added. 

The hazmat team is expected to remain on site for at least five more hours, according to investigators. 

"Residents are urged to avoid travelling near the area due to the increased traffic caused by the closure of the QEW," St. Catharines officials say.

Schmidt said it appeared to be a mechanical issue that caused the crash, but crews are still working to determine the cause of the crash. No injuries were reported as a result.

Heavy delays are expected to last in the area for several more hours.