According to this article http://www.huffingtonpost.com/2014/03/25/air-pollution-deaths_n_5027320.html?ref=topbar

Air pollution kills about 7 million people a year, according to World Health Organization. In another words, this is one in eight people. We all have to breathe, and unavoidable polluted air forces us to breathe in something poisonous, causing health risk.

On a personal experience, I am a bicyclist and it is a major pet peeve of mine when I smell and breathe in vehicle emissions.