One thing I was not aware before is ozone. It is not visible and levels become much higher on sunny days, because of photochemical smog. And if you have asthma, for example, you will feel it because accepted quantities are toxic to the lungs.
Also, because the chemicals that catalyze the creation of ozone in the sun, also help to reduce it during the night, ozone levels in the suburbs and in the countryside near cities are often much higher in the morning, than in the city. Not healthy any more to do sports in such mornings.
If there is missing any argument for people in the suburbs why we should reduce cars in cities, this is it: Ozone from air pollution is really bad for their health and for their children.