Epidemiological studies have indicated airborne particulate matter (PM) have significantly negative effects on human health,furthermore,toxicological studies were reported that DNA damage such as DNA strand break and oxidation of DNA base were induced by PM.It is widely accepted the method of Plasmid Scission Assay (PSA) to detect potential damage for cell-free DNA caused by PM suspension.The PSA is the in vitro method to evaluate strength of DNA damage caused by PM as index of TD20 or TD50 (Toxic dosage of PM causing 20 or 50 % plasmid DNA damage) estimated by Dosage (concentration of PM suspension) – Response (DNA damage ratio) relationship.However,the index is not a convenient index because of its limitation of comparisons between different PM samples which were tested with different PM suspension concentrations and reaction time,furthermore,TD20 or TD50 can estimated as far as Dose-Response relationship.In this study,a new index based on the PSA method and the Dose- Response model in the field of radiobiology wad tested for the evaluation to the PM collected at southwestern Japan in March 2014.In this model,the potential of PM to damage DNA was decided by a parameter r,which was the damage ratio of DNA per unit Dose per time.Results showed the effectiveness of the index in evaluating the potential of PM to damage DNA and the convenience for comparison..