DNA damage in frog erythrocytes after in vitro exposure to a high peak-power pulsed electromagnetic field

Till the present time, the genotoxic effects of high peak-power pulsed electromagnetic fields (HPPP EMF) on cultured cells have not been studied. We investigated possible genotoxic effects of HPPP EMF (8.8 GHz, 180 ns pulse width, peak power 65 kW, repetition rate 50 Hz) on erythrocytes of the frog Xenopus laevis. We used the alkaline comet assay, which is a highly sensitive method to assess DNA single-strand breaks and alkali-labile lesions. Blood samples were exposed to HPPP EMF for 40 min in rectangular wave guide. The specific absorption rate (SAR) calculated from temperature kinetics was about 1.6 kW/kg (peak SAR was about 300 MW/kg). The temperature rise in the blood samples at steady state was 3.5 +/- 0.1 degrees C. The data show that the increase in DNA damage after exposure of erythrocytes to HPPP EMF was induced by the rise in temperature in the exposed cell suspension. This was confirmed in experiments in which cells were incubated for 40 min under the corresponding temperature conditions. The results allow us to conclude that HPPP EMF-exposure at the given modality did not cause any a-thermal genotoxic effect on frog erythrocytes in vitro.     

DNA damage in human leukocytes after acute in vitro exposure to a 1.9 GHz pulse-modulated radiofrequency field

Blood cultures from human volunteers were exposed to an acute 1.9 GHz pulse-modulated radiofrequency (RF) field for 2 h using a series of six circularly polarized, cylindrical waveguides. Mean specific absorption rates (SARs) ranged from 0 to 10 W/kg, and the temperature within the cultures during the exposure was maintained at 37.0 +/- 0.5 degrees C. DNA damage was quantified in leukocytes by the alkaline comet assay and the cytokinesis-block micronucleus assay. When compared to the sham-treated controls, no evidence of increased primary DNA damage was detected by any parameter for any of the RF-field-exposed cultures when evaluated using the alkaline comet assay. Furthermore, no significant differences in the frequency of binucleated cells, incidence of micronucleated binucleated cells, or total incidence of micronuclei were detected between any of the RF-field-exposed cultures and the sham-treated control at any SAR tested. These results do not support the hypothesis that acute, nonthermalizing 1.9 GHz pulse-modulated RF-field exposure causes DNA damage in cultured human leukocytes.     

Evaluation of genotoxic effects in human peripheral blood leukocytes following an acute in vitro exposure to 900 MHz radiofrequency fields

Human peripheral blood leukocytes from healthy volunteers have been employed to investigate the induction of genotoxic effects following 2 h exposure to 900 MHz radiofrequency radiation. The GSM signal has been studied at specific absorption rates (SAR) of 0.3 and 1 W/kg. The exposures were carried out in a waveguide system under strictly controlled conditions of both dosimetry and temperature. The same temperature conditions (37.0 +/- 0.1 degrees C) were realized in a second waveguide, employed to perform sham exposures. The induction of DNA damage was evaluated in leukocytes by applying the alkaline single cell gel electrophoresis (SCGE)/comet assay, while structural chromosome aberrations and sister chromatid exchanges were evaluated in lymphocytes stimulated with phytohemagglutinin. Alterations in kinetics of cell proliferation were determined by calculating the mitotic index. Positive controls were also provided by using methyl methanesulfonate (MMS) for comet assay and mitomycin-C (MMC), for chromosome aberration, or sister chromatid exchange tests. No statistically significant differences were detected in exposed samples in comparison with sham exposed ones for all the parameters investigated. On the contrary, the positive controls gave a statistically significant increase in DNA damage in all cases, as expected. Thus the results obtained in our experimental conditions do not support the hypothesis that 900 MHz radiofrequency field exposure induces DNA damage in human peripheral blood leukocytes in this range of SAR.     

Measurement of DNA damage in mammalian cells exposed in vitro to radiofrequency fields at SARs of 3-5 W/kg

In the present study, we determined whether exposure of mammalian cells to 3.2-5.1 W/kg specific absorption rate (SAR) radiofrequency fields could induce DNA damage in murine C3H 10T(1/2) fibroblasts. Cell cultures were exposed to 847.74 MHz code-division multiple access (CDMA) and 835.62 frequency-division multiple access (FDMA) modulated radiations in radial transmission line (RTL) irradiators in which the temperature was regulated to 37.0 +/- 0.3 degrees C. Using the alkaline comet assay to measure DNA damage, we found no statistically significant differences in either comet moment or comet length between sham-exposed cells and those exposed for 2, 4 or 24 h to CDMA or FDMA radiations in either exponentially growing or plateau-phase cells. Further, a 4-h incubation after the 2-h exposure resulted in no significant changes in comet moment or comet length. Our results show that exposure of cultured C3H 10T(1/2) cells at 37 degrees C CDMA or FDMA at SAR values of up to 5.1 W/kg did not induce measurable DNA damage.     

Genotoxicity of radiofrequency signals. I. Investigation of DNA damage and micronuclei induction in cultured human blood cells

As part of a comprehensive investigation of the potential genotoxicity of radiofrequency (RF) signals emitted by cellular telephones, in vitro studies evaluated the induction of DNA and chromosomal damage in human blood leukocytes and lymphocytes, respectively. The signals were voice modulated 837 MHz produced by an analog signal generator or by a time division multiple access (TDMA) cellular telephone, 837 MHz generated by a code division multiple access (CDMA) cellular telephone (not voice modulated), and voice modulated 1909.8 MHz generated by a global system of mobile communication (GSM)-type personal communication systems (PCS) cellular telephone. DNA damage (strand breaks/alkali labile sites) was assessed in leukocytes using the alkaline (pH>13) single cell gel electrophoresis (SCG) assay. Chromosomal damage was evaluated in lymphocytes mitogenically stimulated to divide postexposure using the cytochalasin B-binucleate cell micronucleus assay. Cells were exposed at 37+/-1 degrees C, for 3 or 24 h at average specific absorption rates (SARs) of 1.0-10.0 W/kg. Exposure for either 3 or 24 h did not induce a significant increase in DNA damage in leukocytes, nor did exposure for 3 h induce a significant increase in micronucleated cells among lymphocytes. However, exposure to each of the four RF signal technologies for 24 h at an average SAR of 5.0 or 10.0 W/kg resulted in a significant and reproducible increase in the frequency of micronucleated lymphocytes. The magnitude of the response (approximately four fold) was independent of the technology, the presence or absence of voice modulation, and the frequency (837 vs. 1909.8 MHz). This research demonstrates that, under extended exposure conditions, RF signals at an average SAR of at least 5.0 W/kg are capable of inducing chromosomal damage in human lymphocytes.     

Genotoxicity of three pesticides used in Costa Rican banana plantations

The in vitro genotoxicity of imazalil and thiabendazole fungicides and the insecticide chlorpyrifos, compounds used in Costa Rican banana plantations, was evaluated with the single-cell gel electrophoresis technique (comet assay). The comet assay is a simple, rapid and low cost technique for quantification of DNA damage. This assay detects DNA single-strand breaks and alkali-labile sites in individual cells. The effects were analyzed by using human lymphocytes exposed to doses of 0, 25, 50, 75 and 100 microg/ml of each pesticide for 30 min at 37 degrees C. The cells were embedded in agarose, lysed, subjected to alkaline electrophoresis (pH >13) for 20 min at 25V, neutralized and dehydrated to be stained with a fluorescent dye and later comets visualization with the epifluorescence microscope. Chlorpyrifos and imazalil induced significant DNA damage in a dose-dependent manner. Chlorpyrifos was the major inductor of DNA breaks. These results indicate that both are genotoxic compounds in vitro. Thiabendazole fungicide did not induced DNA damage using the comet assay for all concentrations tested.     

DNA damage and micronucleus induction in human leukocytes after acute in vitro exposure to a 1.9 GHz continuous-wave radiofrequency field

Human blood cultures were exposed to a 1.9 GHz continuous-wave (CW) radiofrequency (RF) field for 2 h using a series of six circularly polarized, cylindrical waveguides. Mean specific absorption rates (SARs) of 0.0, 0.1, 0.26, 0.92, 2.4 and 10 W/kg were achieved, and the temperature within the cultures during a 2-h exposure was maintained at 37.0 +/- 0.5 degrees C. Concurrent negative (incubator) and positive (1.5 Gy (137)Cs gamma radiation) control cultures were run for each experiment. DNA damage was quantified immediately after RF-field exposure using the alkaline comet assay, and four parameters (tail ratio, tail moment, comet length and tail length) were used to assess DNA damage for each comet. No evidence of increased primary DNA damage was detected by any parameter for RF-field-exposed cultures at any SAR tested. The formation of micronuclei in the RF-field-exposed blood cell cultures was assessed using the cytokinesis-block micronucleus assay. There was no significant difference in the binucleated cell frequency, incidence of micronucleated binucleated cells, or total incidence of micronuclei between any of the RF-field-exposed cultures and the sham-exposed controls at any SAR tested. These results do not support the hypothesis that acute, nonthermalizing 1.9 GHz CW RF-field exposure causes DNA damage in cultured human leukocytes.     

Non-thermal DNA breakage by mobile-phone radiation (1800 MHz) in human fibroblasts and in transformed GFSH-R17 rat granulosa cells in vitro

Cultured human diploid fibroblasts and cultured rat granulosa cells were exposed to intermittent and continuous radiofrequency electromagnetic fields (RF-EMF) used in mobile phones, with different specific absorption rates (SAR) and different mobile-phone modulations. DNA strand breaks were determined by means of the alkaline and neutral comet assay. RF-EMF exposure (1800 MHz; SAR 1.2 or 2 W/kg; different modulations; during 4, 16 and 24h; intermittent 5 min on/10 min off or continuous wave) induced DNA single- and double-strand breaks. Effects occurred after 16 h exposure in both cell types and after different mobile-phone modulations. The intermittent exposure showed a stronger effect in the comet assay than continuous exposure. Therefore we conclude that the induced DNA damage cannot be based on thermal effects.     

Primary DNA damage in human blood lymphocytes exposed in vitro to 2450 MHz radiofrequency radiation

Human peripheral blood samples collected from three healthy human volunteers were exposed in vitro to pulsed-wave 2450 MHz radiofrequency (RF) radiation for 2 h. The RF radiation was generated with a net forward power of 21 W and transmitted from a standard gain rectangular antenna horn in a vertically downward direction. The average power density at the position of the cells in the flask was 5 mW/cm(2). The mean specific absorption rate, calculated by finite difference time domain analysis, was 2.135 (+/-0.005 SE) W/kg. Aliquots of whole blood that were sham-exposed or exposed in vitro to 50 cGy of ionizing radiation from a (137)Cs gamma-ray source were used as controls. The lymphocytes were examined to determine the extent of primary DNA damage (single-strand breaks and alkali-labile lesions) using the alkaline comet assay with three different slide-processing schedules. The assay was performed on the cells immediately after the exposures and at 4 h after incubation of the exposed blood at 37 +/- 1 degrees C to allow time for rejoining of any strand breaks present immediately after exposure, i.e. to assess the capacity of the lymphocytes to repair this type of DNA damage. At either time, the data indicated no significant differences between RF-radiation- and sham-exposed lymphocytes with respect to the comet tail length, fluorescence intensity of the migrated DNA in the tail, and tail moment. The conclusions were similar for each of the three different comet assay slide-processing schedules examined. In contrast, the response of lymphocytes exposed to ionizing radiation was significantly different from RF-radiation- and sham-exposed cells. Thus, under the experimental conditions tested, there is no evidence for induction of DNA single-strand breaks and alkali-labile lesions in human blood lymphocytes exposed in vitro to pulsed-wave 2450 MHz radiofrequency radiation, either immediately or at 4 h after exposure.     

Vanadate induces DNA strand breaks in cultured human fibroblasts at doses relevant to occupational exposure

To study possible genotoxic effects of occupational exposure to vanadium pentoxide, we determined DNA strand breaks (with alkaline comet assay), 8-hydroxy-2'deoxyguanosine (8-OHdG) and the frequency of sister chromatid exchange (SCE) in whole blood leukocytes or lymphocytes of 49 male workers employed in a vanadium factory in comparison to 12 non-exposed controls. In addition, vanadate has been tested in vitro to induce DNA strand breaks in whole blood cells, isolated lymphocytes and cultured human fibroblasts of healthy donors at concentrations comparable to the observed levels of vanadium in vivo. To investigate the impact of vanadate on the repair of damaged DNA, co-exposure to UV or bleomycin was used in fibroblasts, and DNA migration in the alkaline and neutral comet assay was determined. Although, exposed workers showed a significant vanadium uptake (serum: median 5.38microg/l, range 2.18-46.35microg/l) no increase in cytogenetic effects or oxidative DNA damage in leukocytes could be demonstrated. This was consistent with the observation that in vitro exposure of whole blood leukocytes and lymphocytes to vanadate caused no significant changes in DNA strand breaks below concentrations of 1microM (50microg/l). In contrast, vanadate clearly induced DNA fragmentation in cultured fibroblasts at relevant concentrations. Combined exposure of fibroblasts to vanadate/UV or vanadate/bleomycin resulted in non-repairable DNA double strand breaks (DSBs) as seen in the neutral comet assay. We conclude that exposure of human fibroblasts to vanadate effectively causes DNA strand breaks, and co-exposure of cells to other genotoxic agents may result in persistent DNA damage.     

DNA damage in mouse lymphocytes exposed to curcumin and copper

Dietary polyphenolics, such as curcumin, have shown antioxidant and anti-inflammatory effects. Some antioxidants cause DNA strand breaks in excess of transition metal ions, such as copper. The aim of this study was to evaluate thein vitro effect of curcumin in the presence of increasing concentrations of copper to induce DNA damage in murine leukocytes by the comet assay. Balb-C mouse lymphocytes were exposed to 50 μM curcumin and various concentrations of copper (10 μM, 100 μM and 200 μM). Cellular DNA damage was detected by means of the alkaline comet assay. Our results show that 50 μM curcumin in the presence of 100–200 μM copper induced DNA damage in murine lymphocytes. Curcumin did not inhibit the oxidative DNA damage caused by 50 μM H₂O₂ in mouse lymphocytes. Moreover, 50 μM curcumin alone was capable of inducing DNA strand breaks under the tested conditions. The increased DNA damage by 50 μM curcumin was observed in the presence of various concentrations of copper, as detected by the alkaline comet assay.     

Genotoxicity of inhalation anesthetics halothane and isoflurane in human lymphocytes studied in vitro using the comet assay

The alkaline single cell gel electrophoresis (comet) assay was applied to study genotoxic properties of two inhalation anesthetics-halothane and isoflurane-in human peripheral blood lymphocytes (PBL). The cells were exposed in vitro to either halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) or isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) at concentrations 0.1-10 mM in DMSO. The anesthetics-induced DNA strand breaks as well as alkali-labile sites were measured as total comet length (i.e., increase of a DNA migration). Both analysed drugs were capable of increasing DNA migration in a dose-dependent manner. In experiments conducted at two different electrophoretic conditions (0. 56 and 0.78 V/cm), halothane was able to increase DNA migration to a higher extent than isoflurane. The comet assay detects DNA strand breaks induced directly by genotoxic agents as well as DNA degradation due to cell death. For this reason a contribution of toxicity in the observed effects was examined. We tested whether the exposed PBL were able to repair halothane- and isoflurane-induced DNA damage. The treated cells were incubated in a drug-free medium at 37 degrees C for 120 min to allow processing of the induced DNA damage. PBL exposed to isoflurane at 1 mM were able to complete repair within 60 min whereas for halothane a similar result was obtained at a concentration lower by one order of magnitude: the cells exposed to halothane at 1 mM removed the damage within 120 min only partly. We conclude that the increase of DNA migration induced in PBL by isoflurane at 1 mM and by halothane at 0.1 mM was not a result of cell death-associated DNA degradation but was caused by genotoxic action of the drugs. The DNA damage detected after the exposure to halothane at 1 mM was in part a result of DNA fragmentation due to cell death.     

DNA damage in rat lymphocytes treated in vitro with iron cations and exposed to 7 mT magnetic fields (static or 50 Hz)

The present study was undertaken to verify a hypothesis that exposure of the cells to static or 50 Hz magnetic fields (MF) and simultaneous treatment with a known oxidant, ferrous chloride, may affect the oxidative deterioration of DNA molecules.The comet assay was chosen for the assessment of DNA damage. The experiments were performed on isolated rat lymphocytes incubated for 3h in Helmholtz coils at 7 mT static or 50 Hz MF. During MF exposure, part of the cell samples were incubated with 0.01 microM H(2)O(2) and another one with 10 microg/ml FeCl(2,) the rest serving as controls.Lymphocyte exposure to MF at 7 mT did not increase the number of cells with DNA damage in the comet assay. Incubation of lymphocytes with 10 microg/ml FeCl(2) did not produce a detectable damage of DNA either. However, when the FeCl(2)-incubated lymphocytes were simultaneously exposed to 7 mT MF, the number of damaged cells was significantly increased and reached about 20% for static MF and 15% for power frequency MF. In the control samples about 97% of the cells did not have any DNA damage.It is not possible at present to offer a reasonable explanation for the findings of this investigation - the high increase in the number of lymphocytes showing symptoms of DNA damage in the comet assay, following simultaneous exposure to the combination of two non-cytotoxic factors -10 microg/ml FeCl(2) and 7 mT MF. In view of the obtained results we can only hypothesise that under the influence of simultaneous exposure to FeCl(2) and static or 50 Hz MF, the number of reactive oxygen species generated by iron cations may increase substantially. Further studies will be necessary to confirm this hypothesis and define the biological significance of the observed effect.     

Comparison of dose dependences for bioeffects of continuous-wave and high-peak power microwave emissions using gel-suspended cell cultures

The study compared bioeffects of continuous wave (CW) microwaves and short, extremely high power pulses (EHPP) at the same carrier frequency (9.3 GHz) and average power (1.25 W). The peak transmitted power for EHPP was 250 kW (0.5-micro s pulse width, 10 p.p.s.), producing the E field of 1.57 MV/m in the waveguide. A biological endpoint was the density of yeast cells, achieved after a 6 h growth period in a solid nutrient medium (agarose gel) during EHPP or CW exposure. Owing to power losses in the medium, the specific absorption rate (SAR) ranged from 3.2 kW/kg at the exposed surface of the sample to 0.6 mW/kg at 24 mm depth. Absorption and penetration of EHPP was identical to CW, producing peak SAR values 200 000 times higher than the average SAR, as high as 650 MW/kg at the surface. CW and EHPP exposures produced highly nonuniform but identical heating patterns in exposed samples. Following the exposure, the samples were sliced in a plane perpendicular to the wave propagation, in order to separate cell masses exposed at different SAR levels. Cell density in the slices was determined by nephelometry and compared to unexposed parallel control samples. Cell density was strongly affected by irradiation, and the changes correlated well with the local temperature rise. However, the data revealed no statistically significant difference between CW and EHPP samples across the entire studied range of SAR levels (over six orders of magnitude). A trend (P<0.1) for such a difference was observed in slices that were exposed at a time average SAR of 100 W/kg and higher, which corresponded to peak SAR above 20 MW/kg for the EHPP condition. These numbers could be indicative of a threshold for a specific (not merely thermal) exposure effect if the trend is confirmed by future studies.     

Assessment of DNA sensitivity in peripheral blood leukocytes after occupational exposure to microwave radiation: the alkaline comet assay and chromatid breakage assay

DNA sensitivity in peripheral blood leukocytes of radar-facility workers daily exposed to microwave radiation and an unexposed control subjects was investigated. The study was carried out on clinically healthy male workers employed on radar equipment and antenna system service within a microwave field of 10 μW/cm²–20 mW/cm² with frequency range of 1,250–1,350 MHz. The control group consisted of subjects of similar age. The evaluation of DNA damage and sensitivity was performed using alkaline comet assay and chromatid breakage assay (bleomycin-sensitivity assay). The levels of DNA damage in exposed subjects determined by alkaline comet assay were increased compared to control group and showed inter-individual variations. After short exposure of cultured lymphocytes to bleomycin cells of subjects occupationally exposed to microwave (MW) radiation responded with high numbers of chromatid breaks. Almost three times higher number of bleomycin-induced chromatid breaks in cultured peripheral blood lymphocytes were determined in comparison with control group. The difference in break per cell (b/c) values recorded between smokers and non-smokers was statistically significant in the exposed group. Regression analyses showed significant positive correlation between the results obtained with two different methods. Considering the correlation coefficients, the number of metaphase with breaks was a better predictor of the comet assay parameters compared to b/c ratio. The best correlation was found between tail moment and number of chromatid with breaks. Our results indicate that MW radiation represents a potential DNA-damaging hazard using the alkaline comet assay and chromatid breakage assay as sensitive biomarkers of individual cancer susceptibility.     

Evaluation of DNA damage in workers occupationally exposed to pesticides using single-cell gel electrophoresis (SCGE) assay. Pesticide genotoxicity revealed by comet assay

The comet assay, also called the single-cell gel electrophoresis (SCGE) assay, is a rapid and sensitive method for the detection of DNA damage (strand breaks and alkali-labile sites) in individual cells. The assay is based on the embedding of cells in agarose, their lysis in alkaline buffer and finally subjection to an electric current. In the present study, alkaline SCGE was used to evaluate the extent of primary DNA damage and DNA repair in peripheral blood lymphocytes of workers employed in pesticide production. After the period of high pesticide exposure, lymphocytes of the occupationally exposed workers manifested increased tail length and tail moment compared to the control group. After the workers spent 6 months out of the pesticide exposure zone, both endpoints were still above that of the control but significantly decreased as compared to the results of the first analysis.     

Oxidative DNA damage in peripheral blood cells in type 2 diabetes mellitus: higher vulnerability of polymorphonuclear leukocytes

Since oxidative stress is thought to play an important role in the pathogenesis and complications of diabetes, we used the comet assay (single cell alkaline gel electrophoresis) to evaluate DNA strand breaks and DNA base oxidation, measured as FPG (formamidopyrimidine DNA glycosylase)-sensitive sites, in peripheral blood cells (PBC) from type 2 diabetes patients and healthy controls. Oxidative DNA damage in leukocytes was increased in diabetic compared to normal subjects. However, no differences in the levels of DNA damage in isolated lymphocytes were found between the two groups. These data indicate a higher vulnerability to oxidative damage of polymorphonuclear as compared to mononuclear leukocytes in type 2 diabetes. Thus, the measurement of oxidative DNA damage in leukocytes by means of the comet assay is a suitable marker for the evaluation of systemic oxidative stress in diabetic patients.     

DNA repair of hydrogen peroxide-induced damage in human lymphocytes in the presence of four antimutagens. A study with alkaline single cell gel electrophoresis (comet assay).

We assessed four antimutagenic compounds' influences on DNA repair in human lymphocytes exposed in vitro to hydrogen peroxide (20 microM, 5 min, at 4 degrees C). DNA damage and repair were estimated by means of alkaline single cell gel electrophoresis (comet assay). It was noticed that the enhancement of DNA repair was relatively strongest when fluphenazine was present in the cell culture medium. In the cases of anthocyanins and alkylresorcinols, the effects were almost 6-9 times weaker than that of FPh. The effect of todralazine on DNA repair was relatively weakest. Further study should be done on fluphenazine as a potential DNA repair-enhancing compound.     

DNA strand breaks are not induced in human cells exposed to 2.1425 GHz band CW and W-CDMA modulated radiofrequency fields allocated to mobile radio base stations

We conducted a large-scale in vitro study focused on the effects of low level radiofrequency (RF) fields from mobile radio base stations employing the International Mobile Telecommunication 2000 (IMT-2000) cellular system in order to test the hypothesis that modulated RF fields may act as a DNA damaging agent. First, we evaluated the responses of human cells to microwave exposure at a specific absorption rate (SAR) of 80 mW/kg, which corresponds to the limit of the average whole body SAR for general public exposure defined as a basic restriction in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. Second, we investigated whether continuous wave (CW) and Wideband Code Division Multiple Access (W-CDMA) modulated signal RF fields at 2.1425 GHz induced different levels of DNA damage. Human glioblastoma A172 cells and normal human IMR-90 fibroblasts from fetal lungs were exposed to mobile communication frequency radiation to investigate whether such exposure produced DNA strand breaks in cell culture. A172 cells were exposed to W-CDMA radiation at SARs of 80, 250, and 800 mW/kg and CW radiation at 80 mW/kg for 2 and 24 h, while IMR-90 cells were exposed to both W-CDMA and CW radiations at a SAR of 80 mW/kg for the same time periods. Under the same RF field exposure conditions, no significant differences in the DNA strand breaks were observed between the test groups exposed to W-CDMA or CW radiation and the sham exposed negative controls, as evaluated immediately after the exposure periods by alkaline comet assays. Our results confirm that low level exposures do not act as a genotoxicant up to a SAR of 800 mW/kg.