Ergonomic Guidelines

EHS-F073 - Workstation Assessment Checklist

Ergonomic Guidelines for Video Display Terminals


Source Material provided by: US Department of Labor

Introduction

VDT Components

Health Effects

Visual Problems

Fatigue and Musculoskeletal Problems

Radiation

Ergonomic Interventions

Lighting

Work Station Design

Work Practices Job Organization

 


Introduction

The applications of computer technology and the accompanying use of video display terminals (VDTs) are revolutionizing the workplaces of America, and their use will continue to grow in the future. For example, in 1984 only 25 percent of the US population used computers at work, in 1993, more than 45 percent of the population used computers at work and the number continues to grow. Also, more than 18 million workers are in jobs that often require intensive keying.

Along with this expanding use of VDTs have come reports about adverse health effects for VDT operators. This guide briefly examines the potential hazards and interventions you can use to prevent or reduce the potential harmful effect of working with VDTs. Some of the common stressors, their related health effects and their means of prevention are discussed. A checklist to assess workstations is included.

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VDT Components

VDT -- comprised of a display screen, a keyboard, and a central processing unit have rapidly replaced the use of typewriters and other office machines.

The display screen is the output device that shows what the computer is processing. Display screens can be monochrome (green, white, or orange on a black background), or full color.

The keyboard is the input device that allows the user to send information to the ‘brains’ of the computer. Keyboards are commonly used for data entry and inquiry. The keyboard is similar to a standard typewriter keyboard but with additional special keys and functions. The central processing unit is referred to as the "brains" of the computer. It is the center of operation for all the computer processing and performs calculations and organizes the flow of information into and out of the system.

The VDT operates at high voltages, but the power supplies generating these voltages produce very little current. All data processing equipment, including VDTs, must meet stringent international safety standards in this regard.

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Health Effects

In the wake of the expanding use of VDTs, concerns have been expressed about their potential health effects. Complaints include excessive fatigue, eye strain and irritation, blurred vision, headaches, stress, and neck, back, arm, and muscle pain. Research has shown that these symptoms can result from problems associated with the equipment, work stations, office environment or job design, or from any combination of these. Concerns about potential exposure to electromagnetic fields have also been raised.

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Visual Problems

Visual problems such as eyestrain and irritation are among the most frequently reported complaints by VDT operators. These visual symptoms can result from improper lightingglare from the screenpoor positioning to the screen itself, or copy material that is difficult to read.

These problems usually can be corrected by adjusting the physical and environmental setting where the VDT users work. For example, workstations and lighting can and should be arranged to avoid direct and reflected glare anywhere in the field of from the display screen, or surrounding surfaces. Many VDT jobs require long sessions in front of a display screen, consequently some people may need corrective lenses to avoid eye strain and headaches. Vision examinations are therefore recommended to ensure early detection and correction of poor vision. Eyecare specialists should be informed of computer use by VDT operators.

It is also recommended that VDT operators reduce eyestrain by taking rest breaks, after each hour or so of operating a VDT. Changing focus is another way to give eye muscles a chance to relax. Relaxation of the eye muscles simply involve glancing across the room or out the window from time to time and focus on an object at least 20 feet away.

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Fatigue and Musculoskeletal Problems

Work performed at VDTs may require remaining in one position for considerable time and usually involves small frequent movements of the eyes, head, arms, and fingers. Retaining a fixed posture over long periods of time has been shown to cause muscle fatigue.

VDT operator may also be subject to the potential risk of developing various musculoskeletal disorders such as carpal tunnel syndrome, and tendonitis. Musculoskeletal disorders are injuries to the muscles, joints, tendons, or nerves that are caused or made worse by work related risk factors.

Early symptoms of musculoskeletal disorders include pain and swelling, numbness and tingling (hands falling asleep), loss of strength, and reduced range of motion. Any of these symptoms should be evaluated, as soon as possible, failure to treat these symptoms early can result in loss of strength in affected area, chronic painor disability.

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Radiation

Another issue of concern for the VDT operator is whether the emission of radiation, such as X-ray or electromagnetic fields in the radiofrequency and extreme low frequency ranges, poses a health risk. Many people, including pregnant women, are concerned that their health could be affected by electromagnetic fields emitted from VDTs. The threat from X-ray exposures is largely discounted because of the very low emission levels. The radio frequency and extreme low frequency electromagnetic fields are still at issue despite the low emission levels. To date, however, there is no conclusive evidence that the low levels of radiation emitted from VDTs pose a health risk to VDT operators. Some workstation designs, however, have incorporated changes such as increasing the distance between the operator and the terminal and between work stations in order to reduce potential exposures to electromagnetic fields.

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ERGONOMIC Interventions

There are a variety of interventions that you can implement to reduce or prevent any harmful effects that may be associated with VDT use.

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Lighting

Light should be redirected so that it does not shine directly into the operator's eyes when the operator is looking at the display screen.

Further, lighting should be adequate for the operator to see the text and the screen, but not so bright as to cause glare or discomfort.

There are four basic lighting factors that must be controlled to provide suitable workstation illumination and to avoid eyestrain: quantity, contrast, and direct and reflected glare.

Quantity:In most offices, light fixtures and daylight provide illumination for work surfaces of approximately 50-100 foot-candles. High illumination "washes out" images on the display screen and illumination levels of 28-50 foot-candles are often satisfactory.

Contrast:Contrast is the difference in luminance or brightness between two areas. To prevent the visual load caused by alternate light and dark areas, the difference in luminance between the VDT display screen, horizontal work surface, and surrounding areas should be minimized. Most of the tasks associated with VDTs do not require precise visual acuity, and diffuse (indirect) lighting is appropriate. The advantages of diffuse lighting are twofold: there are usually fewer sources of glare in the visual field, and the contrasts created by the shape of objects tend to be "softer." The result, in terms of luminous intensities, is a more uniform visual field. Where indirect lighting is not used, parabolic louvers on overhead lights are probably the next best way to ensure that light is diffused.

Glare:Glare is usually defined as a harsh, uncomfortably bright, light. Glare is dependent upon the intensity, size, angle of incidence, luminance, and proximity of the source to the line of sight. Glare may be the result of direct light sources in the visual field (e.g., windows), or reflected light from polished surfaces (e.g., keyboards,) or from more diffuse reflections which may reduce contrast (e.g., improper task lighting). Glare may cause annoyance, discomfort, or loss in visual performance and visibility.

To limit reflection from walls and work surfaces around the screen, these areas should have a non-reflective finish. Work stations and lighting should be arranged to avoid reflected glare on the display screen or surrounding surfaces.

In many cases, reorientation of your workstation may help remove sources of glare from the line of sight. Ideally, the face of the display screen should be at right angles to windows and light sources. Care should be taken, particularly when terminals are installed within 20 feet of windows, to ensure that there is some method of blocking the sun’s light such as blinds or curtains. The proper "treatment" for window glare includes baffles, venetian blinds, draperies. shades, or filters.

Screen glare filters that attach directly to the surface of a VDT screen can help reduce glare. Two types of filters are available: natural density filters, which scatter and diffuse some of the light reflected off the glass display screen, and micromesh filters, which not only scatter the light but also absorb most of the light reflected from the surface of the screen by means of an imbedded interwoven grid of dyed nylon fibers. These should be used as a last resort since filters can reduce visibility and legibility of screen. Filters should be cleaned regularly.

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Work Station Design

Proper work station design will reduce visual and musculoskeletal discomfort associated with VDT use when the following work practices are observed:

  • Ensure that the operator has a comfortable sitting position sufficiently flexible to reach, use, and observe the display screen, keyboard, and document.
  • Provide posture support for the back, arms, legs, and feet, as well as an adjustable display screen and keyboard.
  • Ensure that VDT tables or desks are vertically adjustable to allow for operator adjustment of the screen and keyboard.
  • Ensure proper chair height and support to the lower region of the back.
  • Ensure that document holders are used to allow the operator to position and view material without straining the eyes or neck, shoulder, and back muscles.

VDT Arrangement

The type of task performed at the VDT may also influence the development of fatigue. In designing a workstation, the type of tasks involved should be considered when determining the placement of the display screen and keyboard. For example, if the job requires the operator to look mainly at the source document than the display screen, the source document should be in front of the operator and the screen may be to the side. (See Figure 2.) You must arrange to have adequate work space to perform each of the tasks required by the job as your body size influences your access to various accessories.

In general, VDT work stations should provide as many adjustable features as possible. Also, adequate leg room should be provided to allow stretching of the legs and relief of some of the static load that results from sitting with the legs in a fixed position for long periods.

Work Station Design

The work station consists primarily of a work surface, a chair, VDT equipment, and other related accessories.

Chairs. The chair is a crucial factor in preventing back pain as well as in improving performance. A properly designed and adjustable chair, from which all adjustments can be easily made from the seated position is recommended. Specific chair criteria are discussed in the following paragraphs.

Chair Height:For any individual who spends from 6 to 8 hours in a chair, the height of the chair and the work surface are critical. The human body dimension that provides a starting point for determining correct chair height is the "popliteal" height. This is the height from the floor to the point at the crease behind the knee. The chair height is correct when the entire sole of the foot can rest on the floor or footrest and the back of the knee is slightly higher than the seat of the chair. This allows the blood to circulate freely in the legs and feet.

Seatpan Design:Size and shape are two factors to consider in the design of the seatpan of the chair. The seatpan should be slightly concave with a softly padded rounded, or "waterfall," edge. This will help distribute the weight and may also prevent sliding forward in the chair. The angle of the seatpan should also be considered. Some options include a seatpan that slopes slightly down at the back or one that has a forward tilt that produces less stress on the lower region.

Armrests:Armrests should be low and short enough to fit under work surfaces to allow users to get close enough to the work surface. Chairs are available with adjustable armrests.

Backrest:A proper backrest should support the entire back including the lower region. The seat and backrest of the chair should support a comfortable posture that permits frequent variations in the sitting position. The backrest angle and chair height should also be easily adjustable.

VDT Design

Display Screen:Most new VDTs have separate, adjustable keyboards, and display screens that allow both the keyboard and display screen to be positioned to suit the individual. This is important as VDT operators may spend a considerable amount of time looking at the display. Screens should have user controls for character brightness, should swivel horizontally and tilt or elevate vertically to enable the operator to select the optimum viewing angle.

The topmost line of the display should not be higher than the user's eye level. The screen and document holder should be the same distance from the eye (to avoid constant changes in focus) and close enough together so the operator can look from one to the other without excessive movement of the neck or back. Operators who wear bifocals often have to tilt their head back to read through the bottom portion of their lenses. They should avoid tilting their head back by lowering the display or using single lens glasses while using the VDT.

The preferred viewing distance for VDTs ranges between 18 and 24 inches. To this distance must be added the depth of the display itself. Some displays are as much as 20 inches deep. The best way to deal with this, other than increasing table depth, is to install a keyboard extender or tray underneath the desk.

Legibility is also a primary consideration in selecting a display screen. Legibility factors to be considered include symbol size and design, contrast, and sharpness.

Keyboard:The keyboard should be detachable and adjustable to ensure proper position, angle, and comfort for the operator. A lower-than-normal work surface may be required to keep the operator's arms in a comfortable position.

This can be achieved by installing a keyboard extender or tray. The thickness and the slope of the keyboard are critical in determining the preferred height.

The preferred working position for most keyboard operators is with the forearms parallel to the floor and elbows at the sides, which allows the hands to move easily over the keyboard. The wrist should be in line with the forearm. A padded and detachable wrist rest for the keyboard can help keep the operator's wrists and hands in a straight position while key stroking.

Mouse:The mouse should be positioned at the operator's side with his or her arm close to the body for support, while maintaining a straight line between the hand and forearm. The upper arm should not be elevated or extended while using the mouse. The top surface of the wrist should also be flat, not angled. A mouse pad or rest can be used to help maintain straight wrists.

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Work Practices Job Organization

Operating a VDT, like any form of sustained physical or mental work, may lead to visual, muscular or mental fatigue. Rest pauses, as recommended by NIOSH (as per note on page 3), to alleviate or delay the onset of fatigue, are necessary. Jobs should be designed so that the operator can vary VDT tasks with non-VDT tasks. In addition, open and positive working relationships between and among the employees and supervisor can be factors in reducing muscle tension and musculoskeletal disorders.

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