UV-B-laser-induced DNA damage in lymphocytes observed by single-cell gel electrophoresis

Human lymphocytes from a cell culture were irradiated with 12 mJ pulses (524 J m-2) of an XeCl excimer laser (308 nm). DNA strand breaks were analysed with the "comet assay", a single-cell method. The DNA damage of individual cells could be investigated depending on the energy and the irradiated volume. DNA damage could be detected already after a single laser pulse of 12 mJ. Since the irradiation time is short compared with the DNA repair time, DNA damage is solely dependent on the total number of photons. A quantitative evaluation of the number of damaged cells vs. number of photons can be best explained by a detection threshold model. The calculated threshold is 1.25 x 10(8) photons cell-1. From this and the principal sensitivity of the comet assay, one can estimate that the upper limit for DNA damaging efficiency is 400,000 photons.