To idle or not to idle? On a cold winter day, downtown Whitehorse shows how divided Yukoners are on this issue. Many people leave their vehicles idling when they stop to run errands, while others religiously turn off their engines -- willingly returning to a cold car in the belief that they are creating less pollution.

Just about all of the available information on air pollution says that idling one's car is unnecessary; that it wastes gas and produces carbon dioxide, which is the most significant of the greenhouse gasses contributing to global warming.

Brochure after brochure admonishes drivers that vehicles should not be "warmed up" for longer than 30 seconds, and that engines should be turned off for stops longer than 30 seconds. It is pointed out that restarting uses less gasoline than idling.

But -- unfortunately for all of us -- cars do not work efficiently in frigid conditions. Both idling and restarting a cold car creates pollution of one type or another, so deciding what to do can be a matter of picking your poison.

The problem is that cars -- and their emission systems -- are designed to work most effectively at around 25 degrees Celsius or warmer. When you start your cold car on a frigid northern day, the vehicles systems are not working at their best.

Environment Canada addressed the idling question back in 1994 when its Mobile Sources Emission Division tested three different models of cars under controlled conditions. The experiment was designed to measure the emissions produced at different temperatures, to determine whether idling or restarting in cold conditions was the better environmental choice.

The three cars -- ranging in age from a 1990 model to a 1993 model -- were tested at -5°C, -10°C, -20°C and -35°C. The vehicles were left for at least 18 hours at these temperatures, and block heaters were used.

After starting the vehicles were driven for about eight minutes, idled for 30 minutes, driven again for about eight minutes and then turned off. After sitting for 30 minutes they were turned on again, and driven for another eight minutes.

In general it was found that idling produced significantly less carbon monoxide (CO) and unburned hydrocarbons, but more carbon dioxide (CO₂). Neither CO nor CO₂ is something that we need more of in our atmosphere, but these pollutants have quite different impacts.

Carbon monoxide is a colourless, odourless gas that -- among other things -- reduces the blood's ability to carry oxygen. Although carbon monoxide is not a main component in smog, unburned hydrocarbons (fuel) and nitrogen oxides are and if the vehicle is producing high levels of carbon monoxide it generally will also be producing proportionally high levels of hydrocarbons, which can collect in Whitehorse during inversions.

Carbon dioxide is the main culprit among the greenhouse gasses contributing to climate change, which has been called the most significant environmental issue the world has ever faced. Forecasts show that a continued warming of the earth's temperature could trigger a wide range of changes in our climate. While some northerners might welcome the thought of warmer winters, scientists say rising temperatures will come at significant costs.

So there lies the dilemma. If you leave your car idling when running errands, you produce less carbon monoxide -- an immediate concern for human health -- but more carbon dioxide -- which can threaten the long-term health of the planet.

So what is an environmentally responsible driver to do? How does one decide which is the lesser of two evils -- carbon monoxide or carbon dioxide? Even a publication published jointly by the Yukon government and the City of Whitehorse advises drivers not to idle their cars when stopping to do errands.

Peter Barton, the head of engineering and vehicle testing with the Mobile Sources Emission Research and Measurement Division, says that understanding why your car pollutes can help drivers make a more informed decision.

When it is cold, vehicles are designed to "richen up" the fuel mixture, meaning that there is more fuel and less air in it. This boost of hydrocarbons helps the car to start, but this also occurs before the catalytic converter has time to warm up to operating temperature and thus spews more carbon monoxide and other pollutants out of the tailpipe.

A warm car emits considerably less CO exhaust emissions because the engine and emission control hardware operate most efficiently at their design temperatures. warms the gasoline, helping to transform the liquid fuel into a vapour. The fuel ignites more readily in this form, burning more completely and reducing the amount of CO emitted.

This burst of pollution on cold starts is compounded by the fact that catalytic converters are designed to work at very high temperatures. They need to reach a temperature of about 250°C before they start helping to oxidize unburned fuel.

Barton says that as it gets colder, you can probably idle your car longer. "At minus 20, I would not idle any longer than 10 minutes in the morning. With most cars five minutes is probably enough starting off in the morning. If you go into a store for ten minutes, it is probably better to let the car sit there and idle if you are concerned about exhaust emissions," he says.

This is all assuming that you are driving a passenger car or light truck built after 1987 and that the catalytic converter is working efficiently. If you are driving an older vehicle or one without a catalytic converter, then the only option is to not idle.

The exhaust emission control equipment on a vehicle was guaranteed by the the manufacturer for 80,000 kilometres or five years, until 1997. At that point the exhaust emission standards changed and the manufacturers had to ensure that the emissions control equipment would operate for 160,000 kilometres or 10 years. So if your vehicle is pre-1997 and older than five years your catalytic converter is probably not working very efficiently and idling would produce higher levels of carbon monoxide, unburned hydrocarbons and extra CO₂.

Barton, who lives in Ottawa, says that he does not idle for very long. "The best thing to do is plug your car in. On a short trip of 18 kilometres at a temperature of -18°, if you plug your car in for 2-3 hours you can save 25 percent of your fuel consumption. This is just because the engine comes up to normal operating temperature much more quickly and is able to run most efficiently on that short trip," he says.