Documents of the NRPB: Volume 5 , No. 2
Sir Richard Doll (Chairman)
Health Effects Related to the Use of Visual Display Units: Report of an Advisory Group on Non-Ionising Radiation
Summary and conclusions
- This report considers first the types of radiation to which ppeople working with visual display units (VDUs) may be exposed. The VDU or video display terminal (VDT) is a device that converts a digital electrical signal into an image displayed on a screen. Although most VDUs resemble television sets and work on similar principles, people generally sit closer to the VDU than to the television screen. New display technologies, such as liquid crystal panels, are becoming more common but, for the present, the majority of VDUs depend on cathode ray tube (CRT) technology.
- In VDUs based on CRT technology, the processes necessary to generate and steer the electron beam in the conversion of electrical energy to light produces electromagnetic fields and radiation across the spectrum. These emissions include x-rays, optical radiation (ultraviolet, visible and infrared radiation), radiofrequency fields, very low and extremely low frequency fields, and static electric fields.
- X-ray emissions are thousands of times lower than normal background levels of ionising radiation; short wavelength ultraviolet is absorbed in the glass screen and exposure to long wavelength ultraviolet emission is negligible compared with that from winter sunlight; visible light is at levels necessary for satisfactory operation of the VDU and long wavelength infrared emissions are comparable with those from a heated object at around 30–35 degrees Celsius.
- Radiofrequency fields at very low frequencies (VLF) are generated by the horizontal deflection coils and by the flyback transformer which produces a high voltage used to accelerate the electron beam. VLF electric field strengths are generally around 1 volt per metre at 30 centimetres from the screen, although electric field strengths up to 15 volts per metre have been reported. Magnetic flux densities at the same distance range from 5 to 600 nanotesla but are generally less than 200 nanotesla. Close to the casings of VDUs it is possible to measure magnetic flux densities of several microtesla and electric field strengths of a few hundred volts per metre.
- Extremely low frequency (ELF) electric fields and magnetic fields are generated by the power supply transformer and vertical deflection coils. ELF electric fields range typically from less than 1 volt per metre to 10 volts per metre at operator positions. Magnetic flux densities are typically in the range 100–700 nanotesla but close to the casing may be several microtesla.
- Static electric fields may exceed 100 kilovolts per metre within a few centimetres of non-conductive screens, whilst at operator positions they are likely to be in the range 5–20 kilovolts per metre. Materials used in offices can give rise to electrostatic fields, which can exceed those experienced at the VDU operator position.
- A large number of animal studies have been carried out to investigate the potential for ELF and VLF electric and magnetic fields to affect embryo and fetal development. In general, the studies have utilised exposure conditions that will have induced current densities equal to or, more often, very much greater than those induced in VDU operators. No well-conducted study of the effects of exposure to power frequency electric fields on mammalian development have obtained consistent and reproducible teratogenic effects.
- A number of studies have reported abnormal development, particularly at the head end, of chick embryos exposed to pulsed or sinusoidal magnetic fields greater than about 1 microtesla at frequencies between 10 and 1000 hertz. In contrast, other studies, including one that examined the effect of exposure to VLF sawtooth magnetic fields characteristic of VDU emissions, failed to find such effects. A large-scale study in which replicate experiments were carried out in six different laboratories failed to establish that such effects occur reproducibly.
- Studies of the possible effects of magnetic field exposure on mammalian development are considered to be more relevant to humans. In general, most studies report a lack of effect on any developmental endpoint; the few positive effects reported are not consistently demonstrated in different studies and may perhaps reflect the spurious significance seen when large numbers of variables are analysed separately. In particular, most studies of mice and rats exposed throughout gestation to power frequency fields up to 20 milletesla or VLF sawtooth magnetic fields up to 200 microtesla have not found decreased levels of post-implantation survival during either early or late post-implantation development. Thus, in general, the studies offer no support to reports of increased spontaneous abortion rates (following implantation) in women using VDUs during pregnancy.
- Similarly, most studies of rodents exposed during gestation to power frequency magnetic fields up to 20 milletesla, or VLF fields up to 200 milletesla, have not found significant effects either on the proportion of gross external abnormalities or on the proportion of visceral or skeletal abnormalities; some studies have observed an increased number of skeletal variants in exposed fetuses, but others did not. In addition, most studies report a lack of consistent effect on postnatal developmental and juvenile and adult behaviours. Thus, in general, these studies lend no support to the view that the exposure of pregnant women to the weak ELF and VLF fields generated by VDUs will result in an increased proportion of congenital malformation.
- There is some very limited evidence from experimental studies in animals that prenatal exposure to power frequency electric and/or magnetic fields might transiently affect the development of reflexive behaviour and motor coordination in neonates and juveniles and may also reduce scent marking behaviour and task learning in adults. However, these various behavioural endpoints involve different regions of the brain and are therefore unlikely to share a common aetiology. Nevertheless, in view of the evidence indicating that nerve cell growth may be affected by large applied direct current (DC) fields, and possibly by localised electric fields, the behavioural consequences of perinatal exposure to electromagnetic fields much larger than those generated by VDUs should be further investigated.
- Of the nine epidemiological studies of spontaneous abortion and VDU use reviewed here, six found no increase in risk, even in heavy users of VDUs, and three reported some increase in certain subgroups. The studies which showed no evidence of an increased risk of spontaneous abortion in VDU users had far fewer problems in their design and conduct than the ones reporting some increase in risk. Overall the results indicate that VDU use does not increase the risk of spontaneous abortion.
- The risk of congenital malformations does not appear to be increased in women who have used VDUs in early pregnancy. Little information is available on the association of other adverse outcomes such as low birthweight with maternal VDU exposure.
- Some investigators have studied related exposures, such as domestic exposure to electromagnetic fields, and outcome of pregnancy. There is some suggestive evidence that work with, or the manufacture of, electrical appliances might be associated with an increased risk of prematurity, but this could be due to chemical exposures rather than exposure to electromagnetic radiation.
- Skin diseases do not appear to be caused by the electromagnetic fields from VDUs, although there is anecdotal evidence unsupported by epidemiology that in conditions of low humidity the associated electrostatic fields may aggravate existing skin problems and the strain of the actual work may give rise to skin problems among those with a predisposition to them. Where such problems are considered to be aggravated by electrostatic fields, either from VDUs or from the other characteristics of the workplace furnishings, they can possibly be alleviated by grounding the operator and by electrostatic shielding of the screen of the VDU.
- There is no evidence that work with VDUs results in a predisposition to the formation of cataracts, although there is an absence of long-term follow-up studies of users of VDUs. Minor opacities of the eye lens that do not affect visual acuity are to be expected among a large fraction of the population whether or not they use VDUs.
- It is concluded that the totality of the epidemiological and experimental evidence provides no good reason to suppose that low frequency electromagnetic fields encountered through the use of VDUs cause any harm to the fetus in utero. Skin diseases do not appear to be caused by electromagnetic fields from VDUs, although existing conditions may be aggravated. Work with VDUs does not appear to cause a predisposition to the formation of cataracts.
- In the absence of stronger reasons for suggesting that the use of VDUs has any harmful effects on operators there does not appear to be any urgent need for further epidemiological studies.
Electromagnetic Fields and the Risk of Cancer: Supplementary Report by the Advisory Group on Non-Ionising Radiation
- The first report of the Advisory Group on Non-Ionising Radiation was published in March 1992 1. In it, the Group reviewed the experimental and epidemiological data concerning the possibility that electromagnetic fields might be a cause of cancer. The Group concluded that the epidemiological findings that had been reviewed provided no good evidence of a cancer risk, to either children or adults, from normal levels of power frequency electromagnetic fields, or from radiofrequency microwave radiation. The experimental evidence strongly suggested that these radiations did not harm genetic material and so would not initiate cancer. The only possibility was that they might act as promoters: that is, they might increase the growth of potentially malignant cells. The epidemiological evidence for such an effect was, however, weak, with the least weak evidence pointing to the possibility of causing an increased risk of tumours of the brain. In the absence of any unambiguous experimental evidence to suggest that exposure to these electromagnetic fields was likely to be carcinogenic, the findings could be regarded only as sufficient to justify formulating a hypothesis for testing by further investigation.
- Since the publication of that report, several more epidemiological studies relating to domestic and occupational exposure to electromagnetic fields have been reported and reviewed 2. The Group concluded that three occupational studies, from Denmark, Norway and Sweden 3–5, strengthened the evidence for believing that some groups of workers in industries where exposure to electromagnetic fields may have been particularly elevated have had an increased risk of leukaemia, but not of brain cancer. More recently still there has been a report of an increased risk of leukaemia in electrical utility workers in France and Canada, whose jobs involved exposure to electromagnetic fields above the median values for all workers in the groups studied 6. The results of the new studies are, however, neither consistent in the type of leukaemia found to be increased nor consistent in finding a progressive increase with increasing exposure, and further research is clearly required.
- Two residential studies of childhood cancer previously reviewed 2 were from Sweden and Denmark 7,8. A further study from Finland has now been examined 9. The Group has concluded that all these studies were well controlled and substantially better than those that previously reported associations with childhood cancer. The studies do not establish that exposure to electromagnetic fields is a cause of cancer but, taken together, they do provide some evidence to suggest that the possibility exists in the case of childhood leukaemia 10. The number of affected children in the studies is, however, very small.
- Experimental studies to date have failed to establish any biologically plausible mechanism whereby carcinogenic processes can be influenced by exposure to the low levels of electromagnetic fields to which the majority of people are exposed 11,12. Most experimental evidence continues to suggest that extremely low frequency (ELF) electromagnetic fields cannot damage DNA directly. The possibility that ELF fields do affect carcinogenesis at the level of promotion has been investigated, principally by exploring the possibility that cell signalling pathways may be affected leading to increased cellular proliferation. Many of these studies have centred on the cell membrane, an important organelle in cell signalling and a possible site of interaction with ELF fields. Other studies have examined the expression of cancer-related genes known to be involved in regulating cellular behaviour. In addition, a number of animal studies have been performed. Amongst these studies, some positive findings have been reported but, taken overall, the evidence obtained so far is inconclusive with no consistently reproducible effects of ELF fields being established.
- There has been recent evidence suggesting that ELF fields may act as co-promoters, essentially enhancing the effects of chemical tumour promoters, possibly via an effect on cell to cell communication, but again these studies have proved difficult to replicate 12. Suggestions that tumour progression could be affected, either by suppression of immune surveillance or by inhibition of melatonin synthesis, remain largely a matter for conjecture given the tentative nature on which they are based 12.
- Thus, at present, there is no persuasive biological evidence that ELF electromagnetic fields can influence any of the accepted stages in carcinogenesis. There is no clear basis from which to derive a meaningful assessment of risk, nor is there any indication of how any putative risk might vary with exposure.
- In the absence of any convincing experimental support, the Advisory Group stresses the urgent need for large and statistically robust epidemiological studies based on objective measurements of exposure to electromagnetic fields and the need to investigate further the basis for any interactions of environmental levels of electromagnetic fields with the body. In the UK the National Childhood Cancer Study, which should have the required characteristics, is examining the possible influence of a number of agents, including magnetic fields, on the incidence of childhood cancer.
Last reviewed: 12 October 2010