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Clearing the air on cars and greenhouse gases

Exhaust fumes coming out of automobile tailpipes

Tibor Kolley/Tibor Kolley/The Globe and Mail

I read an article elsewhere that said getting old cars off the road would help solve the greenhouse gas emission issue. It said that aging vehicles emit "37 times the greenhouse gas" of one new vehicle. I thought the factor affecting greenhouse gas emissions was the quantity of fuel burned, not the emission equipment on the vehicle. Can you set me/them straight? – Robin in Bedford, N.S.

You're right and that the article is wrong. "Greenhouse" gas emissions are pretty much made up entirely of carbon dioxide (CO2). Commonly felt to be the chief contributor to global climate change, these emissions come from the combustion of gasoline – about 20 pounds of them per gallon, or according to the U.S. Department of Energy, about six to nine tons of CO2 per year from a typical family car.

I suspect the article you referred to was referring to other harmful emissions. Gasoline is composed of hydrogen and carbon atoms. Under ideal combustion conditions, the oxygen in the incoming air stream would convert that hydrogen into water and the carbon into carbon dioxide. But internal combustion engines are not ideal.

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When gasoline is burned in an engine, the vast majority of the exhaust stream is composed of nitrogen, water vapour and CO2, none of which are toxic. But a tiny portion of that exhaust is made up of nasty stuff like carbon monoxide, hydrocarbons, nitrogen oxides and particulate mater – all of which are harmful to humans.

Carbon monoxide reduces the flow of oxygen in our bloodstream. Hydrocarbons form ground-level ozone, the major component of smog. Ozone irritates the eyes and is harmful to the lungs and respiratory system. Some hydrocarbons are also toxic and linked to cancer. Nitrogen oxides are another contribution to ozone and acid rain. Carbon dioxide, while not toxic or harmful to humans, traps the earth's heat and is blamed for global warming.

Cleaner fuels and a variety of pollution control devices and methods have all but eliminated the harmful emissions in a modern vehicle. But they have no effect on CO2 emissions; the only way to reduce that is to use less fuel.

The claim that older vehicles produce more CO2 can only apply to lower fuel mileage. It should be pointed out that the average mileage of passenger vehicles today has been vastly improved by a number of measures and regulations – but it is nowhere near 37 times better.

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