The liquid Bacillus subtilis/microsome rec-assay method was applied to twenty chlorinated chemicals and fourteen aldehydes which may occur in chlorinated and ozonated waters. The results showed that substances could be classified into four categories: strongly DNA damaging, DNA damaging, not DNA damaging, and reverse. A new indicator, ‘rec-gram', was introduced to evaluate quantitatively the DNA damaging potential of substances. Bromoform, chloroform, dibromo-chloromethane, dichlorobromomethane, carbontetrachloride, tetrachloroethylene, and six other chlorinated chemicals showed direct or indirect DNA damaging potential. Heptachlor, trans-chlordane, p,p'-DDT, and three other chemicals showed the reverse effect with or without S9 activation. Trichloroethylene and pentachlorophenol showed neither DNA damaging potential nor the reverse effect. Acetaldehyde, benzaldehyde, formaldehyde, glyoxal, acrolein, acetylacetone, isophorone, and 3-methyl-2-butanone showed direct or indirect DNA damaging potential. Propionaldehyde, furfurol, and two other aldehydes showed the reverse effect with or without S9 activation. Carvone and n-valeraldehyde showed neither DNA damaging potential nor the reverse effect. On the rec-gram scale, hexachlorocyclopentadiene and hexachloropentadiene showed the strongest DNA damaging potential of the test substances. Seven chlorinated chemicals and aldehydes showed DNA damaging potential although they had been shown to be not mutagenic with the Ames system.

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