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All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only.This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional.These do not have to be declared unless overriding a default.Check that current character isn't whitespace.VDTVDT (video display terminal, or sometimes visual display terminal) is a term used, especially in ergonomic studies, for the computer display .In some computers, the display is packaged in a separate unit called a monitor .ELF ) radiation that emanates from VDTs.Do you believe using a computer can hurt you?What conclusions can be drawn?Radiation occurs in many differ ent 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.VDTs, it is absorbed by the thick glass of the screen before it can be emitted.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.VDT are all far below the established safety thresholds.Almost all video display terminals emit both VLF and ELF fields.However, these studies are extremely controversial.VLF controversy and to suggest some ways to manage the alleged risk while waiting for new findings.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 horizontal and vertical movement of the electron beam is controlled by pulses of electric current passing through deflection coils.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.Higher resolution monitors require even faster scanning rates to ensure a constant image.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.The specific ELF and VLF fields emitted by any given monitor are determined by the scan rates for that monitor, generally between 15 and 70 kHz.This transformer increases the voltage to around 10 to 25 kilovolts; the voltage requirement increases for color or oversize monitors.In particular, the rapid rise and very rapid fall of current in the deflection coils gives them a distinctive sawtooth shape.The highest levels are generally at the side and rear surface of the VDT, decreasing with distance from the unit.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 frequently referred to in the literature as VLF magnetic fields, while those generated by the vertical scan mechanism are referred to as ELF magnetic fields.They are not blocked by walls or partitions, so in some cases fields from other people's VDT may reach your work area.There are relatively few printed reports of electric field measurements around a VDT.Most recent measures have focussed 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 0.However, other recent measures have distinguished clear variations in field intensity at different points around VDTs and between different models of VDT.For example, Infoworld (Copeland, 1990) reported VLF magnetic field measurements at 80 different points for 12 different monitors.At a distance of 50 cm (about 20 inches), four monitors in that group showed very low emissions (less than .Gauss for almost all the measurement points), one showed high emissions (more than .These measurements were generally lower (the highest recorded field at a distance of 50 cm was 22.Monitors with relatively low emissions showed ranges of 2.ELF magnetic fields also vary among monitors and at different points near the same monitor.Macworld has published ELF magnetic field measurements for more than 30 monitors (Brodeur, 1990 and O'Connor, 1991).The measurements show wide variety among models at distances of 4 or 12 inches from the unit, but all measures dropped dramatically at distances of 28 and 36 inches.VLF fields from a VDT.The American Conference of Governmental Industrial Hygienists (ACGIH) has issued recommendations for industrial workers based on the frequency of the field.These recommendations apply to industrial hygiene and were not specifically developed with VDT use in mind.However, some areas around the country and some countries abroad have introduced legislation covering specific aspects of VDT use.The San Francisco Board of Supervisors approved legislation in December, 1990 requiring employers with 15 or more workers to comply with certain safety measures related to VDT use.VDT for every two hours of keyboard work (Updegrove, 1991; Steere, 1991; Damore, 1991).The state of Maine adopted a law in July, 1989 requiring employers with 25 or more VDTs at one location to provide both oral and written education and safety training.VDTs to provide written education and safety training.Beyond that, some Maine employers have further guidelines for VDT use.For example, as part of a collective bargaining agreement the University of Maine System allows pregnant VDT operators to request reassignment to other work not involving regular VDT use.VDT time in their daily work schedule in an effort to reduce potential eyestrain, muscle aches, or other problems (Sauda, 1991a and 1991b).Now look back at the measurements cited on page 4 in light of these recommendations.Newer models, however, may be shielded to reduce VLF electric fields (Chandler, 1991b).In the later set of tests involving different monitors and different measurement techniques (Carlson, 1991), none of the monitors exceeded the Swedish guidelines.Swedish VLF guidelines, although no specific measures are given.In this same set of monitors, 13 of the 20 are listed as meeting the ELF standards.However, at 28 inches (several inches further from the unit), two units had fields greater than the Swedish limit, and several others were close (within .In each case, the higher intensity fields were at the side of the unit.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.VDTs, and increased cancer risk in persons exposed to ELF fields.In order to evaluate the implications of these studies, let's consider some of them more closely.In later experiments, Ubeda et al.Gauss, similar to those produced by VDTs) had more congenital malformation in fetuses, as well as significant increases in fetal death and loss by resorption among pregnant mice exposed to weak pulsed magnetic fields.As in all animal research, questions remain about the generalizability of these results to other species.In particular, ELF fields decrease the outward flow of calcium from the cells.Although it is difficult to determine what effects seen at the cellular level may imply for the organism as a whole, it is important to note that calcium flow across the cell membrane affects things like muscle contraction, egg fertilization, and cell division.ELF fields do not appear to disrupt DNA structure or cause mutations like ionizing radiation and chemicals can.Again, it is difficult to infer the effect on the organism from these effects at the cellular level, but it is important to note that ELF fields similar to those produced by VDTs (and previously believed to have no biological effect) do indeed produce physiological changes.However, they may promote the growth of tumors already initiated.Epidemiological studies of abnormal reproductive outcome and increased cancer incidence are frequently cited in discussions of computers and health.As in any epidemiological study, these results are based on large groups of people, and are not predictive for any individual.Epidemiological studies in which the reproductive outcomes of women using VDTs are compared to those of women not using VDTs offer mixed results.It is also difficult to separate the efects of VDT use from other environmental factors and working conditions.Although the study has been criticized for relying on recall (the women reported their VDT use from memory), it is considered by many as the strongest study to date.However, this study has been criticized on two counts.This exclusion may be meaningful, since the risk of miscarriage decreases rapidly with gestational age, and since animal studies suggest that early stages of development may be particularly sensitive to electromagnetic signals.Secondly, while the overall VLF and ELF measurements near VDTs were higher than those surrounding the equipment used in the control groups, there was no difference between the two groups in the strength of the ELF field at abdominal level.Thus, the study could not determine whether fetal ELF exposure is associated with miscarriage.The primary author of the NIOSH study has confirmed this point and suggested that researchers concerned about ELF exposure look at other populations than the ones in her study (Branscum, 1991b).The other set of epidemiological studies relevant to this topic are those examining cancer incidence in populations exposed to elevated VLF and ELF levels.Included in this set are studies reporting increased cancer incidence in populations such as electrical workers, ham radio operators, and children raised in houses near high tension electrical lines.These studies involve ELF exposure from sources other than VDTs, but they are often cited in conjunction with the VDT question and so will be mentioned here.They observe that the fact that ELF fields have been implicated as a promoter of cancer may be an important point.What conclusions can be drawn?But it does not provide a basis for asserting that there is a significant risk.Others take the opposite approach, denying that any risks exist at all.Clearly, more research is needed, and it is in fact being carried out.The electromagnetic fields generated by many VDTs are strongest at the back and sides of the unit, and all the fields decrease rapidly with distance.Short term exposure to VDTs is generally not considered a problem.Some computers, laptop models in particular, do not use cathode ray tubes.These suggestions can be characterized as prudent avoidance: strategies which can reduce your exposure to ELF fields without introducing extreme costs or inconvenience (Nair et al, 1989; Morgan and Nair, 1990).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.ELF magnetic emissions (O'Connor, 1991).Other models use an additional series of wire coils to generate a magnetic field that cancels out the fields generated by the deflection coils.Swedish standard for ELF magnetic fields, although they do meet the VLF standards.Some of these products are of dubious worth.Note: Pregnant women should avoid lead aprons for daily use regardless of manufacturers' claims.Other products have some merit, although not necessarily all that their manufacturers claim.If you are considering purchasing one of these shields, keep these results in mind.But if you are worried about magnetic fields, these shields will not reduce your exposure.This review offers very technical information in a format that most interested lay readers can understand.Includes a comprehensive literature review and bibliography.Electromagnetic Fields: The Biological Evidence.Flying Blind: The Making of EMF Policy.Part 3: Managing the risks, by Granger Morgan and Indira Nair.Currents of Death: Power lines, computer terminals, and the attempt to cover up their threat to your health.If you read Brodeur's work, you should also read some contrasting points of view.Buyer Beware: Radiation finds its niche as a marketing tool.Annals of Radiation: The hazards of electromagnetic fields.VLF emissions from monitors: You be the judge of safety.July 18, 1991, page 105.Studies provide inconclusive findings about dangers of monitor emissions.Week, July 8, 1991, page 108.San Francisco law takes first step on safety issue.January 28, 1991, page 19.Embryological changes induced by weak, extremely low frequency electromagnetic fields.Electromagnetic Fields: The jury's still out.Proceedings of the National Academic of Sciences.Very Low Frequency Fields Near VDTs and an Example of Their Removal.Hamilton, Ontario: Canadian Centre for Occupational Health and Safety.Morgan, Granger and Nair, Indira.Electromagnetic Fields: The jury's still out.Background Paper prepared for U.Maine's VDT Operator Safety Law.Posted to the Bitnet SAFETY list, March 25, 1991.Video display terminals and the risk of spontaneous abortion.Pulse shape of magnetic fields influences chick embryogenesis.IN THE APRIL 2006 ISSUE OF INSIDE KUNG FU!!OPEN TO *ALL* MARTIAL ARTISTS !!!REGISTER EARLY* FOR SPECIAL DISCOUNTS!LESSONS VIA WEB BROADCAST, CLICK HERE FOR INFO!!WELCOME NEW STUDENTS AT THE VDT ACADEMY!
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