As we have seen, a single cigarette can cause very high pollutant concentrations indoors due to the large quantity of particulate mass and other pollutants it emits. Furthermore, the pollutants emitted by a cigarette tend to linger indoors for many hours due to the relatively low air change rates found in most homes. An EPA measurement study of 178 homes in Riverside, CA, found that homes with smokers had indoor concentrations that averaged 30 μg/m$^3$ higher than homes without smokers $^{5-7}$, an important number that has been confirmed in other measurement studies in U.S.