Effects of Exposure to Continuous Low Doses of Ionising Radiation

Ionising radiation has the ability to induce, inter alia, DNA damage and is well established as a causative agent of carcinogenesis and mutagenesis. The effects of high doses of short duration have been well documented, whereas the effects of continuous exposure to low doses of ionising radiation have not, nor are they as clearly understood and current risk estimates are largely extrapolated from high-dose data of atomic bomb survivors. This study evaluated the clastogenic effects of low dose ionising radiation on a population of bats (Chiroptera) residing in an abandoned monazite mine. Bats were sampled from two areas in the mine, with external radiation levels measuring around 20 µSv/h (low dose) and 100 µSv/h (high dose). A control group of bats was collected from a cave with no detectable radiation above normal background levels.The most frequently encountered genetic event in human malignancy is the alteration of the p53 gene. Mutant p53 proteins have a longer half-life than the wild-type variant and accumulate to high levels in the nucleus of tumour cells. The study showed that not only was there a significant increase in p53 positive cells of radiation exposed bats, but also in the degree of positivity, especially in the cells lining the bronchioles of the lungs.Although experimental studies have shown that exposure to radiation may lead to the onset of fibrosis and an inflammatory response in the lung and other tissues, the magnitude of the dose exposure was not comparable to this study and histological examination of bat lung and liver tissues showed no morphological changes in radiation exposed bats when compared to the control group.It has been documented that chronic radiation exposures may give rise to a number of specific haematological defects which are collectively termed "preleukemia" or myelodysplastic syndrome. Full blood counts on bat samples showed a significant decrease in the MCV indicating microcytic erythrocytes from the radiation exposed bats. Differential counts performed on the peripheral blood of the bats showed a marked neutropenia. Neutrophils also showed marked dysplasia including psuedo-Pelger Huet cells in radiation-exposed bats. Cytochemical analysis using DAB myeloperoxidase showed that control bats had hypogranular neutrophils and radiation-exposed bats had largely agranular neutrophils. Bone marrow biopsies were taken from both control and radiation-exposed bats and evaluated for cellularity, granulocyte: lymphocyte: erythrocyte (GLE) ratio and megakaryocyte morphology. A hypocellular bone marrow, a decreased granulocytic haematopoeisis and dysplastic megakaryocyte morphology were observed in radiation-exposed bats. Mineralisation of bone osteoid was determined using image analysis and showed a highly significant decrease in the bone matrix from radiation-exposed bats. All haematological features observed are congruent with current literature describing secondary (radiation-induced) myelodysplastic syndrome. As there is considerable debate as to what dose radiation exposure is harmful, the results of these findings may prove significant in establishing haematopoeitic effects of continuous low dose exposure to ionising radiation.The micronucleus assay was used to evaluate residual radiation damage in binucleated lymphocytes and showed that the micronucleus frequency was higher in the test group than the control group. This study showed that bats exposed to radiation also presented with an increased number of micronuclei in reticulocytes compared to the control group. The Single Cell Gel (Comet) assay further showed increased DNA damage as indicated by the length of the comet tails. The results of the micronucleus and the comet assays showed not only a statistically significant difference between test and control groups, but indicated a dose dependent increase in DNA damage. These findings were supported by the presence of chromosomal aberrations in metaphases of radiation exposed bats. These assays may thus be useful in evaluating the potential clastogenecity of exposure to continuous low doses of ionising radiation.Although one has to be cautious about extrapolating data from bats to humans, the results of this study clearly indicate the potential risk of continuous exposure to low doses of ionising radiation. As a monazite mine at Steenkampskraal, South Africa, is likely to reopen in the near future, it will be essential to make use of a biodosimetry service to monitor radiation exposed individuals on a regular basis for early signs of DNA damage thereby ensuring the right of all employees to a safe working environment The biodosimetry service can be extended to incorporate exposure to any potential mutagen such as in the paint, pesticide and plastics industries.