The majority of vehicle pollution is produced by a small number of older or "badly-tuned" cars and trucks, according to a study by researchers at the University of Toronto in Canada.

The study measured the emissions of 100,000 vehicles as they drove past air-sampling probes of the university's main laboratory on College Street, a major roadway in Toronto.

According to the findings, 25 percent vehicles on the road produced 95 percent of black carbon (or "soot"), 93 percent of carbon monoxide, and 76 percent of volatile organic compounds such as benzene, ethylbenzene, toluene, and xylenes, some of which are known carcinogens.

"As we looked at the exhaust coming out of individual vehicles, we saw so many variations. How you drive, hard acceleration, age of the vehicle, how the car is maintained – these are things we can influence that can all have an effect on pollution," said study author Greg Evans.

The findings were published in the March edition of the journal Atmospheric Measurement Techniques.