Some critics try to diminish the impact of outdoor air pollution from smoking by saying that air pollution from other combustion sources (for example, cars, trucks, and power plants) is much worse. This argument is flawed.
In fact, when no smokers were present, we consistently measured background air pollution levels that were practically zero. We found the air in typical urban locations in California, even near roadways, to be generally quite clean compared to air in the vicinity of smokers.
It is important to remember that one's proximity to a source of air pollution is most important. Unless you are standing near a smokestack or right at a tailpipe, the air pollution from cars and power plants mixes in the atmosphere and becomes very diluted before it reaches your lungs. In contrast, a cigarette, while it has much smaller emissions than a power plant or car, is more likely to be very close to people and, therefore, to expose them to concentrated levels.
To put things in perspective, we typically measured background air pollution (from all distant sources) to be less than 10 or 20 micrograms per cubic meter of air. In contrast, we sometimes measured air pollution near a cigarette to be over 1000 micrograms.