Imperial Oil sells the dirtiest gas in Canada at its Esso gas stations, according to records obtained by environmental group Friends of the Earth.

Esso's gas has the highest sulphur content of any in the country. Sulphur is naturally present in gasoline, and is costly to remove. It's a leading contributor to smog and acid rain and causes problems for people with respiratory problems.

The worst of the gas is made and sold in Ontario, Canada's most populous province, say the Environment Canada documents. Imperial Oil's Sarnia refinery reported the highest sulphur level in regular grade gasoline during July-Sept of 1998: - 810 parts per million (ppm). Shell Canada's refinery in Sarnia produced gasoline during the same period with a sulphur content of 700 ppm, while Sunoco's Sarnia refinery's levels were around 200 ppm.

By contrast, the world leader in reducing sulphur content levels in its gas, California, has set a limit of just 30 ppm. Canada plans to meet this level in 2005.

Friends of the Earth is calling on Ontario drivers to boycott Esso for the entire summer season, starting this Saturday, the 30th anniversary of Earth Day.

"Motorists who care about their family's health should send a message to Imperial Oil by avoiding Esso gasoline during the summer smog season," said Beatrice Olivastri, Chief Executive Officer of FOE.

Six of Canada's major oil companies (including Imperial Oil) tried to block the release of information on sulphur levels in their gasoline in Federal Court. It took a legal action by FOE, represented by the Sierra Legal Defence Fund to finally force the data to be made public.

In an affidavit during the court case, an Imperial Oil official expressed fear over "the high potential for boycotts" if the numbers were made public.

"A significant percentage of current customers will shift their gasoline purchases away from companies with relatively high sulphur content levels should this information be disclosed," Imperial vice-president Roger Purdie said in the affidavit.

Imperial Oil defends its gas, saying its sulphur content levels remain below current government limits. The company says it is working to reduce those levels even further and plans to have sulphur levels down to 30 ppm by 2004, a year ahead of schedule.

The Canadian Petroleum Products Institute, which represents Canadian refiners and gas retailers, says suggesting that reducing sulphur content in gasoline will alleviate the smog problem Canada's cities is inconsistent with the facts. It says lower sulphur levels do not significantly reduce smog unless they are coupled with new car technologies.