Ten families from the Foothills region were forced from their homes Monday after a pipeline leaked sour gas in the morning, prompting an evacuation order.

Shell Canada said one of its pipelines released sour gas, a kind of natural gas containing toxic and flammable hydrogen sulphide, at about 9 a.m. near Beaver Mines, just west of Pincher Creek.

Larry Lalonde, a spokesman for Shell, said families stayed at the fire hallin Pincher Creek, 200 kilometres southwest of Calgary, until the air quality was ruled good enough to return Monday evening. He said they also had the optionof staying in a motel for the night.

Mike Judd, one of the residents who had to leave the area, saidthis isnot the first sour-gas release in the area. A provincial spokesman confirmed there have been two leaks before in the 1990s.

"It's sort of a constant threat in the background," Judd told CBC News.

"We just went through an extensive hearing with Shell where they argued adamantly that these things cannot happen or that there is an extremely small possibility of them having an uncontrolled [hydrogen sulfide] release."

Judd said he's worried about the safety of his 30 husky sled dogs that he left behind at home.

Shell said the leak has been isolated and the line is being depressurized so it can be repaired.

The Energy and Utilities Board said its staff are onsite with air monitoring equipment and have started an investigation into the incident, adding that pipeline failures in Alberta are rare.