this study demonstrates the ability of tar isolated from environmental tobacco smoke ( ets ) to nick dna in mammalian cells . 
 solutions of ets tar behave similarly to aqueous solutions of cigarette tar from mainstream smoke . 
 both solutions contain the tar semiquinone radical , and this radical associates with the dna in viable rat alveolar macrophages . 
 solutions of tar from ets cause single - strand dna breaks in rat thymocytes in proportion to the amount of tar present , until a plateau is reached . 
 ets tar solutions , like mainstream tar solutions , produce hydrogen peroxide . 
 hydrogen peroxide appears to be an essential component of the mechanism by which both ets tar and mainstream tar cause dna damage in rat thymocytes , as catalase substantially protects against dna damage . 
 glutathione also protects against dna nicking by both ets and mainstream tar solutions by scavenging radicals and/or oxidants . 
 the chelator diethylenetriamine pentaacetic acid also provides partial ( 40% ) protection . 
 the studies demonstrate that the water - soluble components of ets tar can enter cells , associate with , and then nick dna.imagesfigure 1.figure 2.figure 3.figure 4.figure 5 .