Contents
Search publications
Previous chapter
Next chapter
Screen technology
Throughout this book we have emphasised the fact that many of the health hazards which have come to be associated with VDUs are to do with the way in which work is organised ie. the volume, pace, degree of operator control, hours of work and breaks, or the environment in which the work is carried out. In this chapter we focus on VDUs themselves and the hazards which they may pose for the people who use them.
The majority of VDUs currently in use (as many as 10 million in the UK alone) are based on cathode ray tube (CRT) technology, and much of this chapter is therefore about CRTs. However we also look ahead to the development of alternative methods for the display of computerised information onto screens and the benefits and drawbacks that these may bring.
How CRTs work
The screen is backed with fluorescent radioactive powder or phosphor which is bombarded at very high speed by electron particles fired from an electron gun at the back of the CRT. The electron beam is directed by powerful magnets to the back of the screen which it scans in a rapid, continuous motion, visiting the spot several times a second to 'refresh' the fluorescent powder, making it glow to produce the bright characters which you can read on the screen.
Radiation emissions from CRTs
Electromagnetic radiation is the term used to describe electrical and magnetic energy travelling in the form of waves. The waves differ enormously in frequency (how many go past a given point in every second) and cover a wide spectrum from the very high frequency gamma rays of nuclear fall-out to the extremely low frequencies given off by electrical appliances operating a high voltages. Table 1 summarises the main types of radiation, with examples of what might produce them, whether they are present in VDUs and some of their known hazards.
[insert 'types of radiation table']
Ionising radiation
In the course of producing the image on the screen X-rays are generated inside the CRT. The tube face of modern machines is made of a type of glass designed to filter out the X-rays so none are emitted from the screen. Manufacturers have hastened to assure the public that measurements of X-rays from modern VDUs are below official safety limits. Some critics have argued that this does not necessarily give VDUs a clean bill of health as far as X-rays are concerned because:
It is unlikely that exposure to X-rays alone during VDU work poses a serious threat to health, but the only way to be sure that this work does not add to the risk of radiation-related illness later in life is to develop equipment which emits no ionising radiation at all.
Non-ionising radiation
The types of non-ionising radiation emitted by CRTs which have been the subject of greatest controversy during the last 10 years or so are very low frequency (VLF) and extremely low frequency (ELF) electromagnetic fields (EMFs). An EMF is made up of an electric field (measured in Volts per metre or V/m) and a magnetic field (measured in milli-, micro or nano- Tesla (mT, m T, nT = 1000th, millionth, on thousand millionth or a Tesla, respectively). Research suggests that the type and shape of the waveform may be as important as the strength of the fields in determining their biological effects.
Biological effects of EMFs
The main biological effects of EMFs of the type emitted by CRT-based VDUs which have concerned researchers and campaigners are the effects on reproduction and the possible cancer-causing properties. The evidence for these effects has already been explored in Chapters 4 and 6. A number of mechanisms for these effects have been suggested, some which implicate the electric field component of EMFs and some which indicate that the magnetic field component is responsible for adverse effects.
One debate has focused on the effects of living near high-voltage power lines (which emit EMFs with similar characteristics to VDUs) - which appears to increase the incidence of depression, suicide and general susceptibility to illness as well as childhood and adult cancers - caused, it is thought by suppression of the immune system. In a Swedish study of residential exposure, researchers found that childhood leukaemia rates rose with increasing magnetic field exposure, showing a clear dose-response relationship. Workers exposed to these types of EMFs have an increased risk of leukaemia and brain cancer.
Recent research into an increased risk of breast cancer for VDU workers has shown that power frequency magnetic fields can disrupt the body's defences against breast cancer. It appears that exposure to these fields inhibits the ability of the hormone melatonin to control the growth of human breast cancer cells (VDT News March/April 1993). This finding is particularly worrying in the light of recent studies showing that male electrical workers may be at increased risk of developing breast cancer, and the report of a recent cluster of breast cancer cases among VDU users at a telephone exchange in Queensland, Australia. These reports contrast with the UK's National Radiological Protection Board's view that ELF electromagnetic fields are not cancer-causing (although the NRPB acknowledges that EMFs may promote the growth of previously initiated tumours).
Other research has shown that low frequency magnetic fields exert their effects by interfering with the genetic material involved in cell division. Some researchers believe that effects can be attributed to the emission of localised electric fields from the flyback transformer in unshielded CRTs - these fields can reach extremely high values as a result of the associated high voltage and are capable of inducing greater currents in the exposed user than the magnetic field emissions. The currents may in turn interfere with the body's own electrical impulses such as those involved in the transmission of nervous and heart muscle signals.
A major research project, involving six laboratories in several different countries, known as the Henhouse Project, undertook to establish whether or not pulsed magnetic fields, of the type emitted from VDUs, had effects on embryo development. The experimental protocol attempted to replicate the conditions in each laboratory exactly so as to rule out influence from external environmental factors. The pooled results, reported in Spring 1988 confirmed that pulsed magnetic fields did have a detrimental effect on chick embryo development. The importance of this result was that it demonstrated that magnetic fields can be biologically active at very low levels (VDT News March/April 1988). Subsequent research projects have shown that the embryos of chicks, mice and several other species are most susceptible to the effects of exposure to pulsed magnetic fields very early on in their development.
As well as EMF emissions from CRTs, VDU workers are inevitably exposed to EMFs from other electrical sources, such as fluorescent lights and building wiring. (The effects of these sources of EMFs can sometimes be seen in the form of a distortion of the characters on the CRT screen as the background magnetic fields interfere with those of the CRT). Very little work has been done which will allow an accurate prediction of the cumulative effects of exposure to all sources of EMFs. Whilst the UK's National Radiological Protection Board is satisfied that we have nothing to worry about, the USA has authorised $65 million for a five year EMF research and public information program.
Electrostatic fields
In a typical CRT-based VDU the voltage difference between the screen and the user can amount to 10-15kV for monochrome (black-and-white) units and 18kV for colour VDUs. The electrostatic field around the head of a user depends not only on the fields surrounding the VDU, but also on the voltage differences between the user and their surroundings.
Effects of electrostatic fields
As discussed in Chapter 3, one of the effects of these electrostatic fields is that negatively charged particles are attracted to the screen and positively charged particles are attracted in the opposite direction - towards the user, sometimes causing skin and eye irritation. Over the years there has been speculation that the low levels of negative ions and high levels of positive ions often found in offices where VDUs are used, are responsible for impairing health in a number of ways, one of which is to stimulate increased production of the neurohormone serotonin. High levels of serotonin are associated with depression and tiredness.
As the intensity of the electrostatic field depends on the operating voltage inside the CRT, the fields emitted from non CRT screens in normally very low. Where CRTs are in use, electrostatic fields can be eliminated by:
Standards for CRTs
Despite the almost universal denial among computer manufacturers of the possibility of any health hazards associated with electromagnetic radiation emissions from their machines, many have now produced VDUs which are shielded to varying degrees against these emissions.
Partly, development of low-emission VDUs is a 'litigation avoidance' exercise - the companies want to avoid huge law suits if the adverse effects can ever be conclusively proven - and partly to comply with standards which have been introduced in a number of European countries and regions in the USA.
Sweden has pioneered the introduction of ergonomic and emission standards for VDUs in the form of guidelines issued by the Swedish National Board for Measurement and Testing known as MPRII (SWEDAC 1990). The guidelines include the following limits for VLF and ELF electric and magnetic fields:
- ELF (5Hz-2kHz)
**measured 50cm from the front of the VDU
***measured 50cm from all sides of the VDU.
The Swedish Confederation of Professional Employees (TCO) has gone further in calling for a 200nT limit for ELF magnetic fields at 30cm from the front of the VDU and at 50cm from the sides and back, as well as a scheme for 'environmental labelling' of VDUs to enable purchasers to evaluate the hazard they present to both users and the environment. This has become known as the TCO standard.
The Swedish National Institute of Occupational Health has suggested ten ways to reduce exposure to EMFs in the office:
To this list we would add the relatively simple remedy adopted by the US foundation The Fund for the City of New York which has redesigned its offices so that all operators now sit at least 70cm from their VDUs and approximately 1 metre from other terminals. Magnetic and electric field strength is reduced with increasing distance from the source.
The effect of purchasers' demands on manufacturers should not be underestimated - for example, The New York City Public [state] School system has had no difficulty in finding several computer manufacturers only too happy to supply monitors which comply to even stricter standards than the Swedish standards - especially as they purchase between six and seven thousand monitors each year.
In the UK many of the major manufacturers now have available CRT monitors which comply both with the Swedish MPRII and TCO standards. In most cases they cost only 20-30 more than ordinary CRT VDUs. The Schedule to the Display Screen Equipment Regulations requires all radiation to be 'reduced to negligible levels'. This provides a useful back-up to the argument for provision of monitors which at least meet the Swedish standards. Better still, are VDUs which emit virtually no radiation at all. In the next section we look at some alternatives to CRT-based VDUs.
Alternatives to CRTs
The two main alternatives to CRT-based monitors are Liquid Crystal Display (LCD) and Active Matrix Display (which is an advanced form of LCD. LCDs have much lower current requirements and emit much weaker electromagnetic fields than CRTs. Up until recently, LCD have had the disadvantage that they produce lower quality visual images than CRTs. However, the modern generation of LCDs, particular active matrix displays, are being produced with better and better visual characteristics. Most lap-top computers rely on LCD technology and ferroelectric liquid crystals displays are being developed which promise lighter, brighter displays with less of the problems of poor contrast experienced by users of some currently available LCDs.
As the technology is improved, and as employers find themselves closer to the 1996 deadline when all VDUs will have to comply with the 'negligible radiation' requirements of the DSE Regulations, it makes sense wherever possible to replace CRT-based monitors with alternative monitors. Some unions have already negotiated this as a right for pregnant women concerned about radiation emissions from conventional VDUs.
VDU Work and the Hazards to Health - Chapter 7
© 1993 London Hazards Centre, Interchange Studios, Hampstead Town Hall Centre, 213 Haverstock Hill, London NW3 4QP, UK