Sour gas is a natural gas that contains significant amounts hydrogen sulphide (H2S). It is extremely poisonous to humans and animals even in small quantities, corrosive, flammable and smells like rotten eggs.

Turner Valley, a community southwest of Calgary, was placed under an emergency alert Thursday after a sour gas pipeline ruptured. The alert urged people to move indoors and to prepare for a possible evacuation.

Sour gas accounts for approximately 30 per cent of all natural gas in Alberta, according to the Alberta Energy Regulator. It is usually found in high-pressure natural gas deposits.

Sour gas poses a public health risk, Watershed Sentinel reports:

Humans are likely to smell the rotten egg associated with hydrogen sulfide well below one part permillion (1ppm). Ten times that amount will irk the eyes and the lungs, while a hundredfold increase in dosage can be dangerous to say the least. Around 500 ppm the central nervous system is strongly affected, leading to loss of balance, loss of reasoning, unconsciousness and asphyxiation. Exposure to sour gas in excess of 1000 ppm is lethal over any duration.

More on sour gas from Alberta Energy:

Sour gas makes up about one-third of the gas produced in Alberta; the province also accounts for nearly 85 per cent of Canada's sour gas production. British Columbia's natural gas industry contributes most of the remainder of Canada's sour gas. There are over 6,000 sour gas wells and 18,000 kilometres of operating sour gas pipelines in Alberta. Sour gas is processed at approximately 250 plants in Alberta, including more than 50 larger facilities that produce elemental sulphur.

The Turner Valley alert says the rupture is potentially life-threatening. The Turner Valley area is currently experiencing significant flooding due to heavy rains.

Two neighbourhoods are already under a mandatory evacuation order.

With files from The Canadian Press