This is consistent with the results of previous studies whichindicate that cigarette smoke condensate is mutagenic in Amesbacterial strains TA98 and TA100 in the presence of metabolicactivation (S9) (Doolittle et al. 1990; Shin et al. 2009). S9, thefraction of liver homogenates containing the metabolic en-zymes, is routinely employed in the Ames assay to allow formetabolic activation of test materials, as most mutagens re-quire metabolic activation (Johnson et al. 2009). The resultsindicate that the mutagenicity of sidestream cigarette smokePM larger than 1 μm is significantly lower than that from PMsized from 10 nm to 1 μm.It is believed that the smaller size particles have greatersurface area per unit mass compared to larger particles, whichresults in the hypothesis that the smaller particles would carrymore toxic constituents than the larger particles. Moreover, thesmaller particles can easily cross cell membranes and react