FAQs

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Do computers emit radiation?
How are ELF/VLF fields generated?
Are these ELF/VLF Fields harmful?
Are there exposure standards for magnetic fields?
What about special equipment?

Do computers emit radiation?
Radiation occurs in many different forms characterized by different frequencies and field strengths. The primary concern in computer use is radiation emitted by the visual display unit, or VDT. Most VDTs do emit radiation, particularly low frequency fields.

High-energy ionizing radiation (X-ray) emission is not a VDT health risk. Although radiation within the X-ray band is produced in VDTs, it is absorbed by the thick glass if the screen before it can be emitted.

Likewise, microwave and ultraviolet radiation are well within established limits of safety. Unlike x-rays, these frequencies of no ionizing radiation do not break chemical bonds in molecules. They could cause damage by raising cell temperature, as a microwave oven heats food or overexposure to a sunlamp causes sunburn, but not in the small amounts generated by a VDT.

In short, the amount of X-ray, microwave, and ultraviolet radiation generated by a VDT are all far below the established safety thresholds.

Recently, however, attention has focused on very low frequency (VLF) and extremely low frequency (ELF) no ionizing radiation D frequencies which were previously believed to have no biological effect. Almost all video display terminals emit both VLF and ELF fields.

This radiation has become cause for concern because some studies indicate that even weak VLF or ELF fields can cause miscarriage, birth defects. Or cancer. However, these studies are extremely controversial. The purpose of this paper is to describe the nature of the ELF/VLF controversy and to suggest some ways to manage the alleged risk while waiting for new findings.
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How are ELF/VLF fields generated?
In order to understand how radiation is generated by VDTs, some background information is helpful.

In standard cathode ray tube VDTs, an electron beam strikes phosphors on the inner surface of the screen, creating a spot of visible light. The electron beam scans quickly across each row of phosphors, moves to the beginning of the next row, and when the entire screen has been scanned, returns to the top corner and starts again, as illustrated in figure.

Figure 1: Simplified pattern of an electron beam scanning a screen.

Horizontal movement of the electron beam is controlled by current passing through a deflection coil. High current in the coil causes the beam to be deflected across the screen. When the beam reaches the far edge, the current in the horizontal deflection coil falls rapidly, causing the beam to move back to the other side of the screen, and so on for each row of phosphors on the screen. Thus, current in the horizontal deflection coil rises and falls rapidly each time the beam crosses the screen.

At the same time, another deflection coil controls vertical movement of the beam. Increasing current in this coil causes the beam to move downward on the screen. When the beam reaches the bottom of the screen, the current drops, and the beam returns to the top of the screen. Thus, current in the vertical deflection coil rises an falls less rapidly than that in the horizontal coil D once for each complete scan of the screen.

In order to ensure that the image on the screen does not flicker, the scanning process must occur very rapidly. In a typical 12” monochrome monitor, the beam moves across the screen almost 16,000 times each second, creating a horizontal scan rate of 15.72 kHz (Marha, et.al.,1986). The beam covers the entire screen from top to bottom about 60 times each second. For a vertical scan rate of 60 Hz. Higher resolution monitors require even faster scanning rates to ensure a constant image.

Electric Fields

Electric fields result from the amount of charge and are generally expressed in volts per meter (V/m).

Fields are distinguished according to the frequency of the pulsing current: very low frequency (VLF) fields are generally considered to be between 2 kHz, and 400 kHz, and extremely low frequency (ELF) fields between 5 Hz and 2 kHz.

In a VDT, very low frequency fields are produced by current in the horizontal scan mechanism. Extremely low frequency fields are created by current in the vertical scanning mechanism. The specific ELF and VLF fields emitted by the any given monitor are determined by the scan rates for that monitor, generally between 15 and 70 kHz.

In addition, VLF fields are produced by the step-up transformer (also known as the fly back transformer) used to accelerate and focus the electron beam. This transformer increases the voltage to around 10 to 25 kilovolts; the voltage requirement increases for color or oversize monitors.

Sixty-hertz fields are emitted by the power transformer, but these fields decay rapidly over distance and are measurable only in the immediate vicinity of the transformer (generally at the side or rear of the VDT).

The ELE and VLF fields generated by the step-up transformer and the deflection coils are pulsed fields. In particular, the rapid rise and very rapid fall of current in the deflection coils gives them a distinctive saw tooth shape.

Figure 2: Saw tooth shape caused by rapid current rise and fall.

Because of the location of the deflection coils, field intensity varies with position and distance. The highest levels are generally at the side and rear surface of the VDT, decreasing with distance from the unit.

Magnetic fields

Magnetic fields result from the motion of a charge D current passing through a wire creates a magnetic field around the wire. The proper unit of magnetic fields intensity is amperes per meter (A/m), but magnetic fields are often described in terms of flux density (the number of field lines that cross a unit of surface area), expressed in gauss (G) or teslas (T). One tesla equals 10,000 gauss, and one gauss equals 1/80/ A/m in air or biological tissue. Gauss and teslas are relatively large units of measure; milligauss (mG) or nanoTesla (nT) are more commonly used.

In a VDT, magnetic fields are produced by current flowing in the horizontal and vertical deflection coils. The frequency of the generating current is often used to describe a magnetic field; thus fields generated by the horizontal scan mechanism are frequency referred to in the literature as VLF magnetic fields, while those generated by the vertical scan mechanism are referred to as ELF magnetic fields. Like the electric fields, these are pulsed fields due to the rapid rise and fall of the deflection currents.

Magnetic fields decrease rapidly with distance, but they are very difficult to shield. They are not blocked by walls or partitions, so in some cases fields from other people’s VDT may reach your work area.

Actual Field measures

There are relatively few printed reports of electric field measurements around a VDT. Marha (1983) found intensities of up to 300 V/m at 20 cm from the side of a unit, dropping off to 150 V/m at 30 cm and 50 V/m at 40 cm. In front of the screen, levels were generally low, under 10 V/m at 30 cm from the unit.

Most recent measures have focused on magnetic rather than electric fields. According to Charron (1988), magnetic fields tend to be evenly distributed around the VDT with field strengths ranging from agreement the University of Maine System allows pregnant VDT operators to request reassignment to other work not involving regular VDT use. An extended leave of absence may be requested if reassignment is not possible. All workers area granted periods of non-VDT time in their daily work schedule in an effort to reduce potential eyestrain, muscle aches, or other problems (Sauda, 1991 a and 1991 b).

In Europe, the issue of VLF and ELF exposure is directly addressed. The Swedish National Board for Metrology and Testing (the Mat Oct Provadet, known as MPR) has established guidelines for VLF and ELF electric and magnetic fields at a distance of one-half meter from the unit:

Table 1: Swedish guidelines for VLF and ELF fields (Source: Coughlin, 1991).

These guidelines are actually voluntary in Sweden, but they have been adopted as requirements in other European countries (Chandler, 1991a).

Now look back at the measurements citied on page 4 in light of these recommendations. Marha’s 1983 Measures of VLF electric fields do not extend to a distance of 50 cm, but the fields strength at 40 cm is still quite high relative to the guidelines. Newer models, however, may be shielded to reduce VLF electric fields (Chandler, 1991 b).

In the first set of monitors tested by Infoworld (Copeland 1990), VLF magnetic fields exceeding .25 mG were present in 9 out of 12 cases, including two monitors marketed as “low-emission.” In the later set of tests involving different monitors and different measurement techniques (Carlson, 1991), none of the monitors exceeded the Swedish guidelines.

Are these ELF/VLF Fields harmful?
This question is the source of a great deal of debate. Some studies indicate that even weak VLF or ELF fields can cause miscarriage, birth defects, or cancer. However, these studies D and their implications D are very controversial.

Three types of studies are commonly cited as cause for concern with regard to low frequency no ionizing radiation:

Standards

The U.S. government has no exposure or emission standards for magnetic fields from video display terminals (VDTs.).The Swedish government issued guidelines recommending that VDTs purchased by the government produce extremely low frequency (ELF) magnetic fields of no more than 2mG at a distance of 30 centimeters (11.8 inches) from the VDT surface. The standard is based on what is technologically achievable, not on medical or epidemiological research according to the Swedish Board for Technical Accreditation (SWEDAC) which issued the standard.

Computers

Computers are a complicated subject. know this: EMFs radiate from all sides of the computer. Thus, you must not only be concerned with sitting in front of the monitor but also if you are sitting near a computer or if a computer is operating in a nearby room.

The Swedish safety standard, effective 711/90,specifies a maximum of 0.25mG at 50 cm from the display. Many US manufactured computers have EMFs of 5-100 mG at distance. And know this too: the screens placed over monitors do NOT block EMFs. Not even a lead screen will block ELF and VLF magnetic fields.

Space does not permit a more thorough discussion of computers. If you use a computer, it is important that you

Measure your EMF exposure with a Gauss meter and review the literature concerning the health impacts of computer use.
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Are there exposure standards for magnetic fields?
Currently there are no federal or state health-based exposure standards for magnetic fields. This is dur to fact that there is inadequate scientific evidence to develop a health –based standard. References to safe/unsafe magnetic field levels in studies are not health-based standards; they are arbitrary exposure cut off points used by researchers, and they provide no scientific basis to evaluate or estimate potential health risks.

What about special equipment?
A number of products have been introduced specifically in response to the controversy over radiation fields around VDTs. While some of these products are technically sound,

others are simply marketing ploys.

Low-emission monitors

Several manufactures have introduced low-emission monitors to the U.S market. In some cases, these monitors were already being produced for the stricter European market; in others they were developed specifically in response to the controversy over VDT emissions.

Since magnetic fields are more difficult to block than electric fields, they have posed a greater problem for manufactures of low-emission monitors. A couple of techniques can be used to decrease ELF magnetic emissions (O’Connor,1991).In some models, the winding pattern of the deflection coils has been switched to a design (called a “saddle-saddle coil”) in which all the windings are inside a ferrite band. MacWorld’s lab tests cited by O’Connor confirm that units using this design emit weaker ELF fields than units using the older (“saddle-steroidal”) design.

Other models use an additional series of wire coils to generate a magnetic field that cancels out the fields generated by the deflection coils. In the MacWorld tests, a prototype monitor using canceling coils showed lower ELF magnetic fields than any other monitor tested (O’Connor,1991).

If you are interested in purchasing a low- emission monitor, try to determine whether the monitor meets the Swedish guidelines for both VLF and ELF electric and magnetic fields. Ask specifically for information about ELF magnetic fields since they are harder to attenuate. ( A few models marketed as low-emission do not meet the Swedish standard for ELF Magnetic fields, although they do meet the VLF standards.) For independent measures of specific units, check computer journals for testing lab reports like cited here.

Radiation shields and other devices

In addition to new low-emission monitors, some manufacturers have introduced products that claim to reduce electromagnetic exposure from existing VDTs.

Some of these products are of dubious worth. One company markets a five pound lead apron advertised as a shield against x-rays and extremely low frequency electromagnetic fields during VDT use. According to MacWeek, it provides no shielding at all of magnetic fields (Branscum, 1991a). (Note: Pregnant woman should avoid lead aprons for daily use regardless of manufacturers’ claims. According to Branscum, the Canadian Centre for Occupational Health and Safety warns that the use of such aprons is inappropriate and possible hazardous.)