Furthermore, side effects such as depressed immunity and decreased resistance to infection that accompanies cancer therapy make this group even more susceptible to contaminants in the ambient air. Thus, exposure to air pollution could further reduce their respiratory function, cause respiratory problems and other complications and/or make them vulnerable to other risk factors [28]. Consequently, the adverse effect of air pollution could significantly decrease survival time for cancer patients and cause an earlier death.
In this study, we selected two study areas with docu- mented significantly different air pollution levels. In Los Angeles, CA, the average annual PM2.5 was 18.1 μg/m3 during 1999–2008, which is well above the U.S. EPA’s annual PM2.5 standard of 15 μg/m3. Meanwhile, the average annual PM2.5 during 1999–2008 in Honolulu, HI was only 4.3 μg/m3, which is much below the annual
standard (Please refer to Additional file 1: Table S1). Over the study period, air pollution level has decreased in the LA but little temporal variation was observed in Honolulu. Overall, the difference of air quality between two places remained consistently significant during 1992–2008 (Please refer to Additional file 2: Figure S1). We selected LA as a highly-polluted area and Honolulu as a low-polluted area in this study and this ecological exposure assignment might reduce misclassification of air pollution exposure because of a significant difference and low temporal variation of air pollution levels between two areas, i.e. variations of air pollution between counties (LA vs. Honolulu) were much greater than vari- ations within a county. In addition, cancer patients were unlikely to move, which would also minimize the poten- tial errors in exposure assessment when we used this ecological exposure assignment. However, we cannot rule out the possibility that the observed differences in