The exposure to air pollution from living in a city can be equated to number of cigarettes in terms of cancer risks. Living in San Francisco is about 1/3 cigarette a day, with an average PM2.5 of around 8ug/m^3.

https://www.citylab.com/environment/2018/04/how-much-are-you...

http://www.baaqmd.gov/~/media/files/communications-and-outre...