Technical Report of the Study Group on Radiation and Visual Display Unit

Technical Report of The Study Group on Radiation and Visual Display Unit

• Foreword
• Introduction
• Radiation from VDUs
• Standards Relating to VDU Radiations
• Measurements of Radiation From VDUs
• Quality Control of VDUs
• Installation and Maintenance of VDUs
• VDUs and Pregnancy Outcomes
• In-service Testing of VDUs
• A. TERMS OF REFERENCE
• B. MEMBERSHIP OF STUDY GROUP
• C. EXPERT FROM TELECOM AUSTRALIA SPECIFICATION
• D. SOME QUESTIONS AND ANSWERS CONCERNING VDUS AND RADIATION
• E. UNITS AND SYMBOLS

Source: National Occupational Health and Safety Commission. Worksafe Australia

Date Published: December 1989

© Commonwealth of Australia 1989 ISBN 0 644 09155 X

This work is copyright. Apart from any use as permitted under the Copyright Act 1968 (Cwlth), no part may be reproduced by any process without prior written permission from the Director, Publishing and Marketing, Australian Government Publishing Service. Inquiries should be directed to the Manager, AGPS Press, Australian Government Publishing Service, GPO Box 84, Canberra ACT 2601.

Foreword

The National Occupational Health and Safety Commission is a tripartite body established by the Commonwealth Government to develop, facilitate and implement a national occupational health and safety strategy.

This includes standards development, the development of hazard-specific preventive strategies, research, training, information collection and dissemination and the development of common approaches to occupational health and safety legislation.

The National Commission comprises representatives of the peak employee and the employer bodies - the Australian Council of Trade Unions (ACTU) and the Confederation of Australian Industry (CAI) - as well as the Commonwealth, State and Territory governments.

Consistent with the National Commission's philosophy of consultation, tripartite standing committees have been established to deal with issues relating to standards development and research. Expert groups may be established to provide advice to the standing committees on those issues with which the National Commission is concerned.

Introduction

In late 1988 the Commonwealth Department of Industrial Relations and the ACTU requested the National Commission to establish an expert study group to advise on radiation and visual display units (VDUs), with terms of reference as reproduced in Appendix 1. The study group, whose members are listed with their qualifications and affiliations in Appendix 2, met in December 1988 and presented its report after a second meeting in June 1989. Its response to each term of reference forms the body of this report, which includes a further three appendixes as detailed below.

Appendix 3 provides an example of how a purchasing document can be used to ensure that VDU emissions conform with all appropriate standards, while Appendix 4 deals with the study group's eighth term of reference: it provides questions and answers in non-technical language to address possible worker concerns. In the final appendix, some of the units and symbols used in the report are explained.

It should be noted that the study group has prepared this document as a technical report. The report has not been subjected to any review by, nor received any endorsement from, the National Commission or any of its members.

1. Radiation from VDUs

Term of Reference 1: The types of radiation emitted from VDUs.

1.1 The key element in any VDU is a display device. This is normally a television-type picture tube, known as a `cathode ray tube', which incorporates a large glass vacuum tube with a phosphor coating on the inside surface of its thick glass front screen. Screens using liquid crystal or similar displays emit extremely low radiation levels and are not considered further.

1.2 To produce text or diagrams on the screen, a scanning electron beam is turned on and off to light different spots on the screen phosphor to generate the display. This electron beam is controlled by magnetic fields produced by deflecting coils mounted near the back of the tube. These coils and associated circuitry give rise to emissions in both the radio frequency (RF) and the low frequency (LF) regions of the electromagnetic spectrum.

1.3 Mechanical effects in various components and circuits can produce acoustic emissions or noise, sometimes in the ultrasonic region.

1.4 Electrons are released from a heated electrode and accelerated towards a second electrode called the `anode' by a voltage (normally around 15-20 kV). They pass through a hole in the anode and proceed through direction-controlling fields to the screen. Heat generated by the high voltage power supply and the various electrical circuits all contribute to emission of infrared (IR) radiation.

1.5 All electromagnetic radiation emissions from VDUs are in the non-ionising region of the electromagnetic spectrum.

X-Ray Emission

1.6 X-rays are a form of ionising electromagnetic radiation with extremely high frequency and very short wavelength photons. Very weak X-rays are produced by the electron beams being decelerated as they strike the phosphor. However, these `soft X-rays' are totally absorbed within the thick glass front screen of the VDU because of the relatively low accelerating voltage on the electron gun. Normally at least a 30 kV accelerating voltage would be necessary to produce X-rays which could pass through the screen.

Ultraviolet Radiation

1.7 Ultraviolet (UV) radiation is emitted at low levels from some VDU screen phosphors. It is greatly attenuated by the thick glass screen of the display tube because glass is an excellent absorber of UV.

Visible Light

1.8 Visible light is the useful and essential component of radiation emission from a VDU. The type of phosphor used in the VDU determines the colour of the display, for example, green or orange. The brightness level of the display is adjustable for operator comfort.

Infrared Radiation

1.9 IR radiation is commonly referred to as heat, and is what you feel when standing near any warm or hot object. Since VDUs contain electronic circuitry, some heat is generated whenever the system is on and current flows. Much of this heat is carried away by the warming and movement of surrounding air, but very low levels of IR are radiated.

Microwave Radiation

1.10 Microwaves are another category of non-ionising radiation, with wavelengths longer than those of IR, between 1mm and 1m. Microwaves are not deliberately produced, but may be emitted at very low levels from VDUs as part of an electronic noise emitted from warm objects. Microwaves form part of the RF region of the electromagnetic spectrum.

Radiofrequency Radiation

1.11 RF radiation is also a non-ionising radiation but with wavelengths in the range of millimetres to kilometres. RF radiation is used commonly for broadcast AM and FM radio and television signals. VDUs and their associated computers, like other electrical and electronic equipment, operate on, and generate, RF signals. The common source of RF radiation in a VDU is the flyback transformer used in the circuit to scan the electron beam backward and forward across the face of the display tube.

Low Frequency Fields

1.12 These emissions can result from keying, paging, line scrolling and other control operations. There are also detectable fields in the low frequency region that result from display circuits and the mains frequency (50 Hz) power connection, as can be expected from any mains-operated equipment.

Static Fields

1.13 Static electric fields occur when the surfaces of objects collect an electric charge that is not immediately carried to ground or discharged. A static electric charge may build up on the screen of a VDU tube because of the electrons striking the phosphor coating on its inside surface. Touching the outside of the screen can result in small discharges similar to, but generally smaller than, those sometimes experienced by a person leaving a car on a very dry day or walking in rubber-soled shoes on a synthetic carpet.

Ultrasound

1.14 Ultrasonic emissions may be produced by mechanical vibrations associated with the electron beam scanning control circuits. Some VDUs produce these emissions which are annoying in the same way that the hiss from air conditioning systems or the hum from some lighting fixtures may be irritating, but at intensities much too low to produce any harm. For those people who perceive the ultrasound, the annoyance can be minimised by conventional sound deadening techniques.

1.15 In summary, VDUs emit visible light, together with very low levels of RF, IR, UV, microwave and ultrasonic radiation, but no X-rays.

2. Standards Relating to VDU Radiations

Term of Reference 2: What standards have been established by recognised international and national standard setting bodies in relation to harmful effects to health of exposure to such radiation.

X-Ray Emission

2.1 Both national and international limits have been set for X-ray emission from television receivers. It has been generally accepted that X-ray emission from such devices should not exceed 0.5 millirontgen (1.29 x 10-7 C/kg) per hour at 5 cm from the surface of the tube. As VDUs use cathode ray tubes similar to television receivers, this standard can be applied to VDUs.

Ultraviolet Radiation

2.2 Limits of exposure to UV radiation of wavelengths between 180 and 400 nm have been set by the International Radiation Protection Association (IRPA) (Health Physics, vol.49, pp.331-40, 1985). These limits are identical to the threshold limit values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH) and very similar to the American National Standards Institute (ANSI) exposure limits. Australia has formally accepted the IRPA limit through the National Health and Medical Research Council (NHMRC - Occupational Standard for Exposure to Ultraviolet Radiation, 1985).

Visible Light

2.3 As the useful component of radiation emission from VDUs, visible light levels can be adjusted to suit the comfort of the operator. Although no formal national or international standards exist for this region of the electromagnetic spectrum, the ACGIH has given notice of intent to establish a TLV for visible light and near IR.

Infrared Radiation

2.4 The ACGIH and ANSI standards limit IR radiation exposure to less than 10 mW/cm2. No equivalent Australian standard exists.

Microwave Radiation

2.5 IRPA recently issued guidelines on limits of exposure to RF electromagnetic fields in the frequency range from 100 kHz to 300 GHz (Health Physics, vol.54, pp.115-23, 1988). In the microwave region (300 MHz to 300 GHz), this standard is very similar to Australian Standard AS 2772 (Maximum exposure levels - Radio frequency radiation - 300 kHz to 300 GHz, Sydney, 1985), except in the GHz region where the latter's power density limit is five times lower for occupational and general exposure than the IRPA standard.

Radiofrequency Radiation

2.6 Although the IRPA guidelines (1988) and Australian Standard AS 2772 (1985) cover part of this range, the limits do not apply to all of the low frequency region. However, a standard adopted by the ACGIH (1988) has TLVs for the frequency range down to 10 kHz. At this frequency the ACGIH allows a power density of 100 mW/cm2 or an electric field strength of 614 V/m or a magnetic field strength of 1.63 A/m.

Low Frequency Fields

2.7 IRPA (1989) approved an interim guideline on limits of exposure to 50/60 Hz electric and magnetic fields (Health Physics in press). For 50 Hz the continuous exposure limit for the general public to electric fields is 5 kV/m and to magnetic fields is 0.1 mT (80A/m).

Static Fields

2.8 No Australian or international standard exists for static electric fields.

Ultrasound

2.9 In 1984 the IRPA produced interim guidelines on limits of human exposure to airborne ultrasound (Health Physics, vol.46, pp.969-74, 1984). No equivalent Australian standard exists.

3. Measurements of Radiation from VDUs

Term of Reference 3: What studies have been conducted on radiation emission levels from VDUs used in Australia and how the levels of radiation found in VDUs used in Australia compare with established standards.

3.1 Since 1982, the Australian Radiation Laboratory (ARL) has measured the electromagnetic radiation emissions from 108 colour and monochrome VDUs. RF, microwave, IR, visible, UV and X-ray emissions were measured initially. However, measurements of the IR and visible radiation emissions were soon discontinued. The levels of the IR emissions were comparable to that from a human hand. Also, the visible radiation emissions were directly adjustable by the operator using the brightness control. No microwave or X-ray emissions have been detected from any of the 108 VDUs.

3.2 The electric and magnetic field components of the RF emissions were compared with the exposure limits that are recommended for members of the public by IRPA. These limits allow 24 hour daily exposure. Since VDU operators generally have, at most, 8 hour daily exposure, there is a built-in safety factor of three. Emissions from the VDU screen were, for a small number of VDUs, up to 40 percent of these limits. But for most VDUs the screen emission levels were less than 10 percent of these limits. Emissions from the top, rear and sides of each VDU were, in all cases, less than the exposure limits.

3.3 Telecom Australia staff have measured the RF radiation emissions from 13 VDUs. Again, no emission levels in excess of the relevant recommended limit were found.

3.4 The UV emissions measured by ARL were compared with the occupational exposure limit recommended by IRPA. The measurements were made with the VDU screen adjusted to maximum brightness and contrast. Even under these worst-case conditions, the highest emission level measured was more than two orders of magnitude below (that is, less than one-hundredth) the recommended limit. Most emission levels were more than four orders of magnitude below (that is, less than one ten-thousandth) this limit.

3.5 A contractor has measured the electromagnetic radiation emissions from the 8500 VDUs that are in the offices of the Department of Social Security across Australia. No emission levels in excess of the relevant recommended limit were found.

3.6 The United States Centre for Devices and Radiological Health has measured the emissions of ultrasound radiation from 25 VDUs and found them to be much less than the recommended limit.

3.7 In conclusion, the radiation emission levels from VDUs used in Australia have been compared with recommended international exposure limits and found to be less (and, in most cases, very much less) than the relevant limit.

4. Quality Control of VDUs

Term of Reference 4: What measures can be taken to determine that manufacturing standards and testing of VDUs used in Australian Government Employment provide an adequate quality control, thus ensuring that there are no harmful emissions.

4.1 Quality control in the manufacture of VDUs ensures that units function correctly, that is, provide a visual interface with a computer. As stated previously, many VDUs have been tested, both in a normal and abnormal mode of operation. No units have been found to exceed generally accepted exposure limits at the nominal operator position of 30 cm from the screen.

4.2 Therefore, no quality control testing, additional to that routinely performed by manufacturers to ensure their units function correctly, is necessary. However, if required, VDU manufacturers should be able to supply typical emission levels for electromagnetic radiations outside the visible range.

5. Installation and Maintenance of VDUs

Term of Reference 5: Whether circumstances in the workplace contribute to increased exposure of workers to radiation emission. For example, installation, the siting of individual VDUs, the grouping of large numbers of VDUs, maintenance regimes, breakdowns of VDUs and subsequent repairs.

5.1 Proximity of a number of VDUs is considered a possible way of enhancing exposure of workers to radiation emissions, in particular, emissions in the RF range. The magnitude of RF emissions, however, has been found to decrease very rapidly with distance. For example, if the distance from the screen of the VDU were doubled, the emissions would drop by a factor of between four and eight. It would therefore require at least four to eight extra VDUs 60 cm away from an operator to double the emission levels to which that operator would be exposed if positioned 30 cm from their own screen. Even then the levels would remain below relevant standards. Being surrounded by four to eight VDUs all within 60 cm would not appear to be a realistic situation. However, appropriate workplace layout should ensure that VDU operators are no closer to any other VDU than they are to their own VDU.

5.2 Like other electronic equipment, VDUs sometimes break down and need to be repaired. While awaiting repair, VDUs should be switched off, not because of radiation emissions, but to avoid possible further damage. There are no components in a VDU which could malfunction and cause an increase in emission levels while continuing to produce an acceptable display.

5.3 Due to the nature of the electronic circuits used in VDUs, they will only display clear and usable images if their voltages and currents are within design tolerances, so that any radiation from repaired VDUs will be similar to that produced by new equipment.

6. VDUs and Pregnancy Outcomes

Term of Reference 6: Whether there has been a link established between exposure to radiation emissions from VDUs and adverse pregnancy outcomes.

6.1 Research into whether or not such a link exists has followed two courses: animal studies, where pregnant mice or chicken eggs have been exposed to RF radiation similar to, but not the same as, that emitted by VDUs; and epidemiological studies, in which attempts have been made to correlate VDU use with adverse pregnancy outcomes. Reliable studies that seek to correlate the radiation emissions of a particular VDU with the pregnancy outcome of that VDU's operator have not been reported.

6.2 Some research groups have reported various adverse effects in the offspring of mice and in eggs due, apparently, to their exposure to the radiation. Other groups have not been able to replicate these results, so that the evidence from animal studies reported so far is inconclusive. At best, the animal studies may indicate possible biological mechanisms involved in adverse effects. These studies cannot predict human experience. It is very difficult, for example, to relate the exposure conditions of a pregnant mouse to those of a VDU operator.

6.3 Epidemiological studies that compare the rates of adverse pregnancy outcome (particularly spontaneous abortion or miscarriage and birth defect), between VDU operators and office workers who do not operate VDUs, have been reported. In general, these studies do not indicate an increased risk for VDU operators. Some results, in part, have suggested an increased risk which, after further analysis, could reasonably be ascribed to confounders, random variations and/or methodological deficiencies. Others suggested a decreased risk which could be similarly accounted for. The results of the more complete studies are summarised in the table on the next page.