The problem is the proximity of humans to cars. The results are form a study done at UofT Eng Dept. The crux of their findings were that it takes much longer than anticipated for the levels of 'pollution', in various form, to drop in concentration with distance from the source. I think the following excerpt from the UofT press release is the most intriguing[1];
“The ultrafine particles are particularly troubling,” says Evans. “Because they are over 1000 times smaller than the width of a human hair, they have a greater ability to penetrate deeper within the lung and travel in the body.”
On a typical summer day in Toronto, Evans’ instruments measure approximately 20,000 ultrafine particles in each cubic centimetre of air. This means that for every average breath, Torontonians are inhaling 10 million of these nano-sized particles. These numbers increases to 30,000 and 15 million in the winter, when there is more stagnant air and less evaporation of the compounds."
“The ultrafine particles are particularly troubling,” says Evans. “Because they are over 1000 times smaller than the width of a human hair, they have a greater ability to penetrate deeper within the lung and travel in the body.”
On a typical summer day in Toronto, Evans’ instruments measure approximately 20,000 ultrafine particles in each cubic centimetre of air. This means that for every average breath, Torontonians are inhaling 10 million of these nano-sized particles. These numbers increases to 30,000 and 15 million in the winter, when there is more stagnant air and less evaporation of the compounds."
[1]http://news.engineering.utoronto.ca/traffic-emissions-may-po...