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The Shell Scotford plant flares off extra gas after a fire Monday near Fort Saskatchewan. ((CBC))

A third sour gas leak in two years at a Shell refinery northeast ofEdmonton has some residents wondering how safe it is to live in the areaknown as Upgrader Alley.

A giant fireball erupted above Shell's Scotford upgrader near Fort Saskatchewan on Monday afternoon, sparked by aleak of the highly toxic and flammable gas. About 3,000 workers were sent home, but no one was injured in the incident.

Worker Rod Whitford said it wasn't a big scare.

"Just a routine evacuation that lasted maybe 2.5 hours," he said.

But his wife, Vicky, was more concerned. "This really kind of wakes you up, makes you wonder what's going on," she told CBC News.

The Scotford upgrader, whichrefines bitumen from Alberta's oilsands into lighter crude oil, is the first facility in Upgrader Alley.There are two moreupgraders currently being built and another 10 planned on paper.

Shell officials saidMonday's leak of hydrogen and sour gas happened in the hydro conversion unit, the same place where a cloud of sour gas caught fire in September 2006. That forced a plant evacuation and a warning to residents nearby to stay indoors.

One week later, hydrogen sulphide and sulphur dioxide were released from another unit of the plant, prompting another warning.

"I want to move. My husband doesn't want to move, but I want to move, yes. I don't like it being so close," said Vicky Whitford.

'My husband doesn't want to move, but I want to move yes. I don't like it being so close.' —Vicky Whitford, resident

"We take these types of incidents very seriously. And given that we had two incidents about 15 months ago and here we are with another one, it certainly isn't what we want to see," said Shell spokesman Randy Provencal.

"We're going to do a full investigation and we're going to find out what the root cause was."

Government agencies continue to monitor air quality levels and have started their own investigations at the Scotford plant, whichhas beguna $5-billion expansion.

Also on Monday, a Shell pipeline in southwestern Alberta leaked sour gas, prompting the evacuation of 10 homes near Pincher Creek.