You're driving up a hill behind another vehicle when suddenly the smell of rotten eggs fills the car. After glaring at the dog, you realize he's innocent -- this time. The belch of sulphur came from the vehicle ahead of you -- and the true culprit is the high level of sulphur in Canadian fuels.

But sulphur levels in your gasoline are about to change, says Dr. Mark Tushingham of Environment Canada's Oil, Gas and Energy Branch in Ottawa.

Beginning on July 1, 2002, refiners and importers will have to keep sulphur levels to a maximum of 170 parts per million (ppm) in every batch of gasoline or to an average of 150 ppm over the next 30 months. At that point, in January 2005, permissible levels will drop even lower, to 40 ppm or an annual average of 30 ppm.

That's a very significant drop, Dr. Tushingham says. "Prior to the Sulphur in Gasoline Regulations, the only standard for sulphur in gasoline was the voluntary commercial standard of 1000 ppm under the Canadian General Standard Board."

The sulphur levels in gasoline depend on a number of things, including the type of crude oil, the age of the refinery, the kind of technology used in the refining process, and the slate of products made at the refinery, Tushingham explains. Because of various combinations of those factors, sulphur levels in Canadian gasoline are among the highest in the developed world.

The consequences are more than a bad smell. Sulphur plays a major role in the level of pollutants released in vehicle exhaust, and the pollution from vehicle exhaust is a significant health hazard. The consequences range from respiratory conditions like asthma, croup and pneumonia to chronic bronchitis and even premature deaths.

Reducing sulphur levels could make a dramatic change. An independent expert panel of North American Health scientists evaluated the impact on the health of Canadians in the next couple of decades when the sulphur level in vehicle fuel is reduced to an average of 30 ppm.

Over 20 years, the panel estimated, the measure would save more than 2000 people from premature death, reduce emergency room visits by almost 7000, and reduce the incidence of acute respiratory complaints by 11 million.

It's not just sulphur that causes health problems, although sulphur contributes to fine particulate matter in urban air. A more significant part of the problem is the havoc sulphur plays with car emission controls.

"The sulphur in the gasoline 'poisons' the catalyst converters that are put on vehicles to reduce pollution," Tushingham explains. "Because the catalyst is less effective, emissions of all pollutants coming from a vehicle are increased."

Remember the bad smell? The one that wasn't the dog's fault?

"Under some driving circumstances, the sulphur on the catalyst can get blown off and combine with the other elements to create the smell," Tushingham explains.

It's not a danger in itself, but it is a reminder of what each vehicle is adding to the air we breathe.

For more information about the science behind the new Sulphur in Gasoline Regulations, go to http://www.ec.gc.ca/oged-dpge/level2e/projectse.htm and follow the links to the studies on sulphur in gasoline and diesel fuel.