There have been a comparatively large number of studies devoted to visual discomfort in visual display unit (VDU) workers, many of which have yielded contradictory results. From one survey to another, there are discrepancies in reported prevalence of disorders ranging from practically 0 per cent to 80 per cent or more (Dainoff 1982). Such differences should not be considered too surprising because they reflect the large number of variables which can influence complaints of eye discomfort or disability.
Correct epidemiological studies of visual discomfort must take into account several population variables, such as sex, age, eye deficiencies, or use of lenses, as well as socio-economic status. The nature of the job being carried out with the VDU and the characteristics of the workstation layout and of the work organization are also important and many of these variables are interrelated.
Most often, questionnaires have been used to assess the eye discomfort of VDU operators. The prevalence of visual discomfort differs thus with the content of questionnaires and their statistical analysis. Appropriate questions for surveys concern the extent of symptoms of distress asthenopia suffered by VDU operators. Symptoms of this condition are well known and can include itching, redness, burning and tearing of the eyes. These symptoms are related to the fatigue of the accommodative function in the eye. Sometimes this eye symptoms are accompanied by a headache, with the pain located in the front portion of the head. There may also be disturbances in eye function, with symptoms such as double vision and reduced accommodative power. Visual acuity, itself, however, is rarely depressed, provided the conditions of measurement are carried out with a constant pupil size.
If a survey includes general questions, such as “Do you feel well at the end of the working day?” or “Have you ever had visual problems when working with VDUs?” the prevalence of positive responses may be higher than when single symptoms related to asthenopia are evaluated.
Other symptoms may also be strongly associated to asthenopia. Pains in the neck, shoulders and arms are frequently found. There are two main reasons that these symptoms may occur together with eye symptoms. The muscles of the neck participate in keeping a steady distance between eye and screen in VDU work and VDU work has two main components: screen and keyboard, which means that the shoulders and arms and the eyes are all working at the same time and thus may be subject to similar work-related strains.
User Variables Related to Visual Comfort
Sex and Age
In the majority of surveys, women report more eye discomfort than men. In one French study, for example, 35.6% of women complained of eye discomfort, against 21.8% of men (p J 05 significance level) (Dorard 1988). In another study (Sjödren and Elfstrom 1990) it was observed that while the difference in the degree of discomfort between women (41%) and men (24%) was great, it “was more pronounced for those working 5-8 hours a day than for those working 1-4 hours a day”. Such differences are not necessarily sex-related, however, since women and men seldom share similar tasks. For example, in one computer plant studied, when women and men were both occupied in a traditional “woman’s job”, both sexes displayed the same amount of visual discomfort. Furthermore when women worked in traditional “men’s jobs”, they did not report more discomfort than men. In general, regardless of sex, the number of visual complaints among skilled workers who use VDUs on their jobs is much lower than the number of complaints from workers in unskilled, hectic jobs, such as data entry or word processing (Rey and Bousquet 1989). Some of these data are given in table 1.
Table 1. Prevalence of ocular symptoms in 196 VDU operators according to 4 categories
|
Categories |
Percentage of symptoms (%) |
|
Females in "female" jobs |
81 |
|
Males in "female" jobs |
75 |
|
Males in "male" jobs |
68 |
|
Females in "male" jobs |
65 |
Source: From Dorard 1988 and Rey and Bousquet 1989.
The highest number of visual complaints usually arise in the 40–50-year-old group, probably because this is the time when changes in accommodation ability of the eye are occurring rapidly. However, although older operators are perceived as having more visual complaints than younger workers, and, as a consequence, presbyopia (vision impairment due to ageing) is often cited as the main visual defect associated with visual discomfort at VDU workstations, it is important to consider that there is also a strong association between having acquired advanced skills in VDU work and age. There is usually a higher proportion of older women among unskilled female VDU operators, and younger male workers tend to more often be employed in skilled jobs. Thus before broad generalizations about age and visual problems associated with VDU can be made, the figures should be adjusted to take into account the comparative nature and skill level of the work being done at the VDU.
Eye defects and corrective lenses
In general, about half of all VDU operators display some kind of eye deficiency and most of these people use prescriptive lenses of one type or another. Often VDU user populations do not differ from the working population as far as eye defects and eye correction are concerned. For example, one survey (Rubino 1990) conducted among Italian VDU operators revealed that roughly 46% had normal vision and 38% were nearsighted (myopic), which is consistent with figures observed among Swiss and French VDU operators (Meyer and Bousquet 1990). Estimates of the prevalence of eye defects will vary according to the assessment technique used (Çakir 1981).
Most experts believe that presbyopia itself does not appear to have a significant influence on the incidence of asthenopia (persistent tiredness of the eyes). Rather, the use of unsuitable lenses appears to be likely to induce eye fatigue and discomfort. There is some disagreement about the effects in shortsighted young persons. Rubino has observed no effect while, according to Meyer and Bousquet (1990), myopic operators readily complain of undercorrection for the distance between eye and screen (usually 70 cm). Rubino also has proposed that people who suffer from a deficiency in eye coordination may be more likely to suffer from visual complaints in VDU work.
One interesting observation that resulted from a French study involving a thorough eye examination by ophthalmologists of 275 VDU operators and 65 controls was that 32% of those examined could have their vision improved by good correction. In this study 68% had normal vision, 24% were shortsighted and 8% farsighted (Boissin et al., 1991). Thus, although industrialized countries are, in general, well equipped to provide excellent eye care, eye correction is probably either completely neglected or inappropriate for those working at a VDU. An interesting finding in this study was that more cases of conjunctivitis were found in the VDU operators (48%) than in the controls. Since conjunctivitis and poor eyesight are correlated, this implies that better eye correction is needed.
Physical and Organizational Factors Affecting Visual Comfort
It is clear that in order to assess, correct and prevent visual discomfort in VDU work an approach which takes into account the many different factors described here and elsewhere in this chapter is essential. Fatigue and eye discomfort can be the result of individual physiological difficulties in normal accommodation and convergence in the eye, from conjunctivitis, or from wearing glasses that are poorly corrected for distance. Visual discomfort can be related to the workstation itself and can also be linked to work organization factors such as monotony and time spent on the job with and without a break. Inadequate lighting, reflections on screen, flicker and too much luminance of characters can also increase the risk of eye discomfort. Figure 1 illustrates some of these points.
Figure 1. Factors that increase the risk of eye fatigue among VDU workers
Many of the appropriate characteristics of workstation layout are described more fully earlier in the chapter.
The best viewing distance for visual comfort which still leaves enough space for the keyboard appears to be about 65 cm. However, according to many experts, such as Akabri and Konz (1991), ideally, “it would be best to determine an individual’s dark focus so workstations could be adjusted to specific individuals rather than population means”. As far as the characters themselves go, in general, a good rule of thumb is “bigger is better”. Usually, letter size increases with the size of the screen, and a compromise is always struck between the readability of letters and the number of words and sentences that can be displayed on the screen at one time. The VDU itself should be selected according to the task requirements and should try to maximize user comfort.
In addition to the design of the workstation and the VDU itself is the need to allow the eyes to rest. This is particularly important in unskilled jobs, in which the freedom of “moving around” is generally much lower than in skilled jobs. Data entry work or other activities of the same type are usually performed under time pressure, sometimes even accompanied by electronic supervision, which times operator output very precisely. In other interactive VDU jobs which involve using databases, operators are obliged to wait for a response from the computer and thus must remain at their posts.
Flicker and eye discomfort
Flicker is the change in brightness of the characters on the screen over time and is more fully described above. When characters do not refresh themselves frequently enough, some operators are able to perceive flicker. Younger workers may be more affected since their flicker fusion frequency is higher than that of older people (Grandjean 1987). The rate of flicker increases with increase in brightness, which is one reason why many VDU operators do not commonly make use of the whole range of brightness of the screen that are available. In general a VDU with a refresh rate of at least 70 Hz should “fit” the visual needs of a large proportion of VDU operators.
The sensitivity of the eyes to flicker is enhanced by increased brightness and contrast between the fluctuating area and the surrounding area. The size of the fluctuating area also affects sensitivity because the larger the area to be viewed, the larger the area of the retina that is stimulated. The angle at which the light from the fluctuating area strikes the eye and the amplitude of modulation of the fluctuating area are other important variables.
The older the VDU user, the less sensitive the eye because older eyes are less transparent and the retina is less excitable. This is also true in sick people. Laboratory findings such as these help to explain the observations made in the field. For example, it has been found that operators are disturbed by flicker from the screen when reading paper documents (Isensee and Bennett as quoted in Grandjean 1987), and the combination of fluctuation from the screen and fluctuation of fluorescent light has been found to be particularly disturbing.
Lighting
The eye functions best when the contrast between the visual target and its background is maximum, as for example, with a black letter on white paper. Efficiency is further enhanced when the outer edge of the visual field is exposed to slightly lower levels of brightness. Unfortunately, with a VDU the situation is just the reverse of this, which is one reason that so many VDU operators try to protect their eyes against excess light.
Inappropriate contrasts in brightness and unpleasant reflections produced by fluorescent light, for example, can lead to visual complaints among VDU operators. In one study, 40% of 409 VDU workers made such complaints (Läubli et al., 1989).
In order to minimize problems with lighting, just as with viewing distances, flexibility is important. One should be able to adapt light sources to the visual sensitivity of individuals. Workplaces should be provided to offer individuals the opportunity to adjust their lighting.
Job characteristics
Jobs which are carried out under time pressure, especially if they are unskilled and monotonous, are often accompanied by sensations of general fatigue, which, in turn, can give rise to complaints of visual discomfort. In the authors’ laboratory, it was found that visual discomfort increased with the number of accommodative changes the eyes needed to make to carry out the task. This occurred more often in data entry or word processing than in tasks which involved dialogues with the computer. Jobs which are sedentary and provide little opportunity for moving around also provide less opportunity for muscular recovery and hence enhance the likelihood of visual discomfort.
Job organization
Eye discomfort is just one aspect of the physical and mental problems that can be associated with many jobs, as described more fully elsewhere in this chapter. It is not surprising, therefore, to find a high correlation between the level of eye discomfort and job satisfaction. Although night work is still not widely practised in office work, its effects on eye discomfort in VDU work may well be unexpected. This is because, although there are few data as yet available to confirm this, on the one hand, eye capacity during the night shift may be somehow depressed and thus more vulnerable to VDU effects, while on the other hand, the lighting environment is easier to adjust without disturbance from natural lighting, provided that the reflections from fluorescent lamps on dark windows are eliminated.
Individuals who use VDUs to work at home should ensure that they provide themselves with the appropriate equipment and lighting conditions to avoid the adverse environmental factors found in many formal workplaces.
Medical Surveillance
No single, particular hazardous agent has been identified as a visual risk. Asthenopia among VDU operators appears rather to be an acute phenomenon, although there is some belief that sustained strain of accommodation may occur. Unlike many other chronic diseases, misadjustment to VDU work is usually noticed very soon by the “patient”, who may be more likely to seek medical care than will workers in other workplace situations. After such visits, spectacles are often prescribed, but unfortunately they are sometimes ill adapted to needs of the workplace which have been described here. It is essential that practitioners be specially trained to care for patients who work with VDUs. A special course, for example, has been created at the Swiss Federal Institute of Technology in Zurich just for this purpose.
The following factors must be taken into consideration in caring for VDU workers. In comparison to traditional office work, the distance between the eye and the visual target, the screen, is usually of 50 to 70 cm and cannot be changed. Therefore, lenses should be prescribed which take this steady viewing distance into account. Bifocal lenses are inappropriate because they will require a painful extension of the neck in order for the user to read the screen. Multifocal lenses are better, but as they limit rapid eye movements, their use can lead to more head movements, producing additional strain.
Eye correction should be as precise as possible, taking into account the slightest visual defects (e.g., astigmatism) and also the viewing distance of the VDU. Tinted glasses which reduce the illumination level in the centre of the visual field should not be prescribed. Partially tinted spectacles are not useful, since eyes at the workplace are always moving in all directions. Offering special spectacles to employees, however, should not mean that further complaints of visual discomfort from workers may be ignored since the complaints could be justified by poor ergonomic design of the workstation and equipment.
It should be said, finally, that the operators who suffer the most discomfort are those who need raised illumination levels for detail work and who, at the same time, have a higher glare sensitivity. Operators with undercorrected eyes will thus display a tendency to get closer to the screen for more light and will be in this way more exposed to flicker.
Screening and secondary prevention
The usual principles of secondary prevention in public health are applicable to the working environment. Screening therefore should be targeted towards known hazards and is most useful for diseases with long latency periods. Screening should take place prior to any evidence of preventable disease and only tests with high sensitivity, high specificity and high predictive power are useful. The results of screening examinations can be used to assess the extent of exposure both of individuals and of groups.
Since no severe adverse effects on the eye have ever been identified in VDU work, and since no hazardous level of radiations associated with visual problems have been detected, it has been agreed that there is no indication that work with VDUs “will cause disease or damage to the eye” (WHO 1987). The ocular fatigue and eye discomfort that have been reported to occur in VDU operators are not the kinds of health effect which generally form the basis for medical surveillance in a secondary prevention programme.
However, pre-employment visual medical examinations of VDU operators are widespread in most member countries of the International Labour Organization, a requirement supported by trade unions and employers (ILO 1986). In many European countries (including France, the Netherlands and the United Kingdom), medical surveillance for VDU operators, including ocular tests, has also been instituted subsequent to the issuing of Directive 90/270/EEC on work with display screen equipment.
If a programme for the medical surveillance of VDU operators is to be set up, the following issues must be addressed in addition to deciding on the contents of the screening programme and the appropriate testing procedures:
- What is the meaning of the surveillance and how should its results be interpreted?
- Are all VDU operators in need of the surveillance?
- Are any ocular effects which are observed appropriate for a secondary prevention programme?
Most routine visual screening tests available to the occupational physician have poor sensitivity and predictive power for eye discomfort associated with VDU work (Rey and Bousquet 1990). Snellen visual testing charts are particularly inappropriate for the measurement of visual acuity of VDU operators and for predicting their eye discomfort. In Snellen charts the visual targets are dark, precise letters on a clear, well illuminated background, not at all like typical VDU viewing conditions. Indeed, because of the inapplicability of other methods, a testing procedure has been developed by the authors (the C45 device) which simulates the reading and lighting conditions of a VDU workplace. Unfortunately, this remains for the time being a laboratory set-up. It is important to realise, however, that screening examinations are not a substitute for a well-designed workplace and good work organization.
Ergonomic Strategies to Reduce Visual Discomfort
Although systematic ocular screening and systematic visits to the eye specialist have not been shown to be effective in reducing visual symptomatology, they have been widely incorporated into occupational health programmes for VDU workers. A more cost-effective strategy could include an intensive ergonomic analysis of both the job and the workplace. Workers with known ocular diseases should try to avoid intensive VDU work as much as possible. Poorly corrected vision is another potential cause of operator complaints and should be investigated if such complaints occur. The improvement of the ergonomics of the workplace, which could include providing for a low reading angle to avoid a decreased blinking rate and neck extension, and providing the opportunity to rest and to move about on the job, are other effective strategies. New devices, with separate keyboards, allow distances to be adjusted. The VDU may also be made to be moveable, such as by placing it on a mobile arm. Eye strain will thus be reduced by permitting changes in viewing distance which match the corrections to the eye. Often the steps taken to reduce muscular pain in the arms, shoulders and back will at the same time also allow the ergonomist to reduce visual strain. In addition to the design of equipment, the quality of the air can affect the eye. Dry air leads to dry eyes, so that appropriate humidification is needed.
In general the following physical variables should be addressed:
- the distance between the screen and the eye
- the reading angle, which determines the position of the head and the neck
- the distance to walls and windows
- the quality of paper documents (often very poor)
- luminances of screen and surroundings (for artificial and natural lighting)
- flicker effects
- glare sources and reflections
- the humidity level.
Among the organizational variables that should be addressed in improving visual working conditions are:
- content of the task, responsibility level
- time schedules, night work, duration of work
- freedom to “move around”
- full time or part time jobs, etc.