Emergency officials are updating a situation that forced hundreds of people from their homes and left others sitting in traffic.
It happened just after 3pm on Tuesday, the transport trailer crashed and flipped onto the centre median on the Fort Erie bound QEW near Martindale Road.
The truck was carrying a chemical called phosphine.
Phosphine was being transported as a compressed liquid in a compressed cylinder.
It is a highly flammable product that poses an inhalation hazard.
Representatives from OPP, EMS, and St. Catharines Fire spoke with CKTB's Lee Sterry.
OPP Sergeant Kerry Schmidt said there were about 45 cylinders on board.
The truck was properly labeled, and the driver was able to give officers information sheets on the chemical.
A two-tiered evacuation situation took place.
First, an Evacuation Order was put into effect, and people in the immediate area were evacuated shortly after the crash.
Fire Chief Dave Wood explained that they have a manual that lists chemicals, where they can determine how dangerous the chemical is.
The evacuation was a precaution until they knew if anything had been spilled.
A Shelter-in-Place was put into effect later, which advised people within a 2km radius of the crash to stay indoors, with the air conditioning off, and chimneys closed.
Wood says the chemical company that was shipping the product was very diligent in getting to the scene to help.
They were able to determine that there were no leaks.
NRP say about 3,500 local residents were affected by the Evacuation Order.
A shelter was set up at the Seymour-Hannah Sports and Entertainment Centre for displaced residents who needed it.
Both orders were lifted around 1am, and the highway reopened around 3am.
Niagara EMS Chief Kevin Smith commends social media for helping get the word out.
He stressed the importance of making sure your family has a 72-hour plan. Read more by clicking here.
Smith says there were no casualties and no serious injuries as a result of this accident.
The cause of the accident is still unknown.