25% of vehicles cause 90% of emissions — study 

Source: By Justine Hofherr, Boston Globe • Posted: Tuesday, May 19, 2015

Just a small percentage of cars produce most of the traffic-related air pollution, according to a new study.

The study by researchers at the University of Toronto found 25 percent of vehicles were responsible for about 90 percent of vehicle-related air pollution.

To conduct the research, the scientists took real-time measurements of the exhaust of around 100,000 cars driving past air monitors on a highway in Toronto.

The culprits were a subset of especially old vehicles, said Jonathan Wang, an author of the study and a chemical engineering doctoral student at the University of Toronto