 
 
Source: Paul Brodeur, "The Magnetic Field Menace," in "Macworld," July 
1990, pp. 136-145. Via Toxbase.
 
Various graphics appear in this magazine article. If you want a copy 
of this article, please contact Melinda Lawrence, Greenpeace USA, 4649 
Sunnyside Ave. N., Seattle, Washington 98103, (206) 632-4326; or via 
Environet (or via Greenlink, for Greenpeace staff).
 
======================================================================
 
THE MAGNETIC-FIELD MENACE
 
Computer monitors may pose a very real threat to users
 
     As the new decade begins, most Macintosh users and other inhabi-
tants of the vast computer community have become aware that serious 
questions are being raised about the potentially harmful health ef-
fects of electromagnetic emissions from display monitors.  However, 
the issue has been so shrouded by denial on the part of manufacturers 
and employers, and addressed with such incompetence by state and 
federal regulatory agencies, that computer users scarcely know what to 
think about it, who to turn to for reliable information, or how to 
protect themselves.  Meanwhile, industry, government, and the medical 
and scientific community are mounting belated attempts to study the 
problem and reach some consensus about how to deal with it.
     Since disease does not develop by consensus but by immutable laws 
of biology, it seems prudent to review what is known about the harmful 
biological effects of low-level electromagnetic emissions from display 
monitors, power lines, and other sources -- particularly magnetic-
field emissions, which have been linked for more than ten years to the 
development of cancer -- and to understand how this knowledge has been 
acquired and disseminated.  It also seems sensible to determine the 
strength of magnetic-field emissions from monitors -- something that 
has not been done with accuracy to date -- and to relate these emis-
sions, insofar as possible, with what is known about their potential 
for harm.
     For this reason, "Macworld" has undertaken to conduct careful 
measurements of the strength of the magnetic fields given off by 
monitors that are commonly used with the Macintosh.  The idea is to 
provide accurate readings so that Macintosh users can determine for 
themselves whether they wish to take protective measures in order to 
reduce their exposure to magnetic fields (see "At Arm's Length").
 
FIRST SUSPICIONS
     Radiation from computer terminals first became an issue in 1977, 
when officials of the National Institute for Occupational Safety and 
Health (NIOSH) measured emissions from several display monitors at the 
"New York Times," where two young copy editors had developed incipient 
cataracts after working on the machines for periods of a year or less.  
The NIOSH officials reported that the electric-field and magnetic-
field strengths of the VLF (very-low-frequency) radio-frequency radia-
tion being emitted were too weak to be detected by their instruments 
at a distance of 4 inches.  As it turned out, they were trying to 
measure the fields in terms of milliwatts per square meter, even 
though VLF and ELF (extremely-low-frequency) fields can't be accurate-
ly measured in this manner.
     Early in 1980, NIOSH officials measured VLF magnetic-field 
strengths of almost 9 milligauss (a gauss is a unit of strength of the 
magnetic field, and a milligauss is 1/1000 gauss) near the flyback 
transformers of several display monitors at newspapers in San Francis-
co and Oakland, California.  The NIOSH officials discounted the health 
hazard of these fields, claiming that "there is no occupational stand-
ard for this frequency and these frequencies have not been shown to 
cause biological injury."
     During the next two years, seven unusual clusters of birth de-
fects and miscarriages involving women who operated video-display 
terminals (VDTs) were reported in Canada and the United States.  
Instead of taking their own measurements of the machines in question, 
however, the health officials who investigated these cases relied on 
the flawed NIOSH reports and characterized each of the clusters as a 
chance occurrence.  By this time, the regulatory officials and comput-
er manufacturers of both nations seemed to be falling over one another 
in their haste to absolve computers of any blame.
     In March of 1981, the director of Canada's Radiation Protection 
Bureau declared that VDTs "carry no radiation hazard."  Similar claims 
were made before a congressional subcommittee by the U.S. Food and 
Drug Administration's Bureau of Radiological Health and by the direc-
tor of standards for IBM.  In October of that year, a senior scientist 
at the Bell Telephone Laboratories in Murray Hill, New Jersey, de-
clared that computer terminals "do not represent a health hazard from 
any radiation exposure caused by their use."  (At the time, there were 
well over 100,000 computer terminals in operation in the Bell 
systems.)
 
60HZ HAZARDS EXPOSED
     Unaccountably, no one in industry or government said a word about 
the pulsed 60Hz electric and magnetic fields that were being emitted 
by display monitors (see "Cathode-Ray Tubes Explained"), even though 
there were by then many studies in the medical literature to suggest 
that the 60Hz alternating-current fields given off by power lines 
might be hazardous to health.  Chief among these studies was one that 
had been published in March of 1979 in the highly respected "American 
Journal of Epidemiology" by epidemiologist Nancy Wertheimer and physi-
cist Ed Leeper, who live in Boulder, Colorado.  Wertheimer and Leeper 
had conducted an investigation showing that children in the Denver 
area who lived in homes near electric distribution wires carrying high 
current had died of cancer at twice the expected rate.  (Since magnet-
ic fields are produced by electric current, distribution wires carry-
ing high current produce relatively strong magnetic fields -- invisi-
ble lines of force that readily penetrate almost anything that happens 
to stand in their way, including the human body.)
     In their article, Wertheimer and Leeper pointed out that magnetic 
fields in homes near high-current wires might reach levels of 2 milli-
gauss or more "for hours or days at a time," and that if magnetic-
field exposure were responsible for the increased incidence of child-
hood cancer they had observed, the duration of exposure might be an 
important factor.  They also suggested that the magnetic fields from 
power lines might be promoting cancer in children by hindering the 
ability of the body's immune system to fight the disease.
     Instead of taking Wertheimer and Leeper's disturbing findings as 
a sign that the magnetic-field problem should be thoroughly investi-
gated, the electric-utilities industry tried to discredit their work.  
But in 1986 the association between magnetic fields from high-currency 
wires and childhood cancer was confirmed by a major study conducted 
under the auspices of the New York State Department of Health.  This 
investigation reported that "prolonged exposure to low-level magnetic 
fields may increase the risk of developing cancer in children."  
Earlier, a similar finding was announced by scientists studying child-
hood cancer in Sweden.  What should have been of profound concern to 
the manufacturers and users of display monitors was that the incidence 
of cancer in all three childhood studies was associated with 60Hz 
magnetic-field strengths of only 2 to 3 milligauss.
 
COMPUTER MONITORS IMPLICATED
     The fact that display monitors emit significant radiation in the 
form of pulsed ELF electric and magnetic fields did not come to light 
until October of 1982.  At that time, Dr. Karol Marha, a biophysicist 
at the Canadian Centre for Occupational Health and Safety (CCOHS) in 
Hamilton, Ontario, revealed that Canadian researchers had measured 
60Hz magnetic fields greater than 2 milligauss at distances of 12 
inches from two display monitors, and fields of approximately 1 milli-
gauss at a distance of 20 inches from several screens.  In 1983, CCOHS 
issued press releases carrying Marha's warning that there was scien-
tific evidence to suggest that pulsed electric and magnetic fields 
could be more harmful than nonpulsed fields, as well as his recommen-
dation that workplaces be redesigned so that VDT operators do not sit 
close to their display monitors or to neighboring monitors.
     Marha's recommendations were ignored by government health offi-
cials in Canada and the United States, who failed to appreciate the 
possible connection between the potential health hazard of alternat-
ing-current 60Hz power-line magnetic fields and that of the pulsed 
60Hz magnetic fields given off by display monitors.  Moreover, the 
CCOHS press releases were not picked up by any major newspaper in the 
United States or Canada.  A year later, the medical director of the 
"New York Times" told a congressional subcommittee that he was aware 
of "no medical evidence of serious VDT-related health effects."  By 
then, of course, newspapers everywhere had become highly dependent 
upon computer technology.
 
SUPPORTING EVIDENCE
     In July of 1982, shortly before Marha's announcement that 
display monitors were emitting potentially hazardous electric and 
magnetic fields, Dr. Samuel Milham, Jr., a physician and epidemi-
ologist for the Washington State Department of Social and Health 
Services, published a letter in the "New England Journal of Medicine" 
that furnished a new insight into the problem.  Milham had examined 
that data for 438,000 deaths occurring between 1950 and 1979 among 
workers in Washington State and had found that leukemia deaths were 
elevated in 10 out of 11 occupations involving exposure to electromag-
netic fields.  His pioneering study provided the starting point for 
some 20 subsequent investigations here and abroad, which showed that 
persons whose occupations require them to work in electromagnetic 
fields -- among them electricians, electrical engineers, and tele-
phone- and power-line workers -- die of leukemia and brain cancer at a 
much higher rate than other workers.
     For example, a 1984 study demonstrated that a significantly 
higher than expected number of Maryland men who had died from brain 
cancer had been employed in electrical occupations, and a 1988 study 
of men who had died of brain cancer in East Texas revealed that the 
risk for electric-utility workers was 13 times greater than that for 
workers who were not exposed to electromagnetic fields.
     Additional cause for concern came in November of 1989 with the 
announcement that a study conducted by epidemiologists at the Johns 
Hopkins University School of Hygiene and Public Health, in Baltimore, 
had found an elevated risk of all cancers among cable splicers working 
for the New York Telephone Company.  Indeed, the incidence of leukemia 
among these men, who often work close to power lines, we 7 times that 
of other workers in the company.  Moreover, measurements of their on-
the-job exposure showed that the mean level of the 60Hz alternating-
current magnetic-field strengths to which they had been subjected was 
only 4.3 milligauss.  Considering the fact that a pulsed ELF magnetic 
filed level of between 4 and 5 milligauss has been measured at a dis-
tance of 12 inches from the Apple 13-inch color monitor and from E-
Machine's Color-Page 15, this is a discomfiting finding, to say the 
least.
 
LABORATORY STUDIES CONCUR
     While epidemiologists were investigating the incidence of cancer 
among human beings exposed to low-level electromagnetic fields, other 
scientists were studying the effect of weak ELF fields on test ani-
mals.  Chief among them was Dr. W. Ross Adey, a clinical neurologist 
and neuroscientist, who was formerly the director of the Brain Re-
search Institute at the University of California at Los Angeles and is 
now associate chief of staff for research at the Jerry L. Pettis 
Memorial Veterans' Hospital, in Loma Linda, California.  During the 
1970s, Adey and his colleagues discovered that weak ELF electromagnet-
ic fields altered brain chemistry in living cats.  During the 1980s 
they found that low-level electromagnetic fields can interfere with 
the ability of T-lymphocyte cells -- the soldiers of the immune system 
-- to kill cancer cells, which suggests that these fields may be 
acting as cancer promoters by suppressing the immune system.
     In 1988, Adey and his associates demonstrated that weak 60Hz 
electric fields similar in strength to those that can be found i the 
tissue of a human being standing beneath a typical over-head high-
voltage power line (or, for that matter, in the tissue of someone 
standing very close to a display monitor) could increase the activity 
of an enzyme called ornithine decarboxylase, which is associated with 
cancer promotion.
     Back in 1980 and 1981, even as government health officials in the 
United States and Canada were denying any possible connection between 
electromagnetic emissions from display monitors and adverse pregnancy 
outcomes among women who worked with those machines, Spanish research-
ers were conducting experiments showing that when chicken eggs were 
exposed to weak pulsed ELF magnetic fields, nearly 80 percent of them 
developed abnormally, with malformations of the cephalic nervous 
system being particularly prevalent.  The adverse effect of pulsed 
magnetic fields upon the development of chick embryos was confirmed in 
1984 by scientists at the Swedish National Board of Occupational 
Safety and Health.
     Later that year, however, Professor Arthur W. Guy, director of 
the Bioelectromagnetic Research Laboratory at the University of Wash-
ington, in Seattle, who had been hired by IBM to review the literature 
on the biological effects of VDT emissions, pointed out that the weak 
magnetic-field pulses used by the Spanish researchers did not match 
the sawtooth shape of the pulses emitted by computer display monitors, 
and concluded that there was no valid evidence that monitor emissions 
posed any health hazard.
 
FURTHER INDICATIONS
     Early in 1986, Guy's criticism was addressed in a Swedish study 
conducted by Dr. Bernhard Tribukait, a professor of radiobiology in 
the Department of Radiobiology of the world-renowned Karolinska Insti-
tute, in Stockholm.  Together with a colleague, Tribukait discovered 
that the fetuses of mice exposed to weak pulsed fields with the same 
sawtooth shape as those given off by display monitors experienced more 
congenital malformations that did the fetuses of unexposed test ani-
mals.  (This finding was reported by Tom Brokaw on "NBC Nightly News," 
but went unmentioned by the "New York Times" and virtually every major 
daily newspaper in the United States.)
     In the spring of 1987, Dr. Hakon Frolen, of the Swedish Universi-
ty of Agricultural Sciences, in Uppsala, Sweden, reported that he and 
a colleague had found a significant increase in fetal deaths and fetal 
losses by resorption (a phenomenon similar to miscarriage in humans) 
among pregnant mice exposed to weak pulsed magnetic fields, compared 
with those occurring in nonexposed test animals.  In June, other 
Swedish scientists reported that radiation similar to that emitted by 
display monitors could cause genetic effects in exposed tissue sam-
ples.  An important aspect of all three Swedish studies was that the 
radiation exposure in each of them had been designed to mimic as 
closely as possible the sawtooth magnetic-field pulses emitted by 
VDTs.
     Further evidence that weak pulsed magnetic fields might be haz-
ardous to health came in the spring of 1988, when the combined results 
of a six-laboratory experiment conducted in the United States, Canada, 
Spain, and Sweden confirmed the earlier finding that such fields could 
indeed adversely affect the development of chick embryos.  Later that 
year, Frolen found that the fetuses of pregnant mice were most sensi-
tive to pulsed magnetic fields in the early stages of pregnancy, which 
was consistent with a similar observation by Canadian and Spanish 
researchers.
     At the second international VDT conference, which was held in 
Montreal in September of 1989, Frolen described a series of experi-
ments in which he delayed exposing pregnant mice to pulsed magnetic 
fields for up to nine days after conception.  The results were strik-
ing.  All of the mice that were exposed immediately after conception, 
or on the first, second, or fifth day after conception, had statisti-
cally increased rates of resorption.
     Louis Slesin, the editor and publisher of VDT NEWS -- a 
newsletter that reports six times a year on the biological effects of 
display monitors (see "Conspicuous Consumer," in this issue, for con-
tact information) -- has emphasized the importance of Frolen's find-
ings, pointing out that the lack of any effect after the ninth day 
following conception "clearly indicates that the pulsed magnetic 
fields -- not some as-yet-unrecognized factor -- are damaging the 
embryos."
 
INDUSTRY RESPONSES
     Meanwhile, the Coalition for Workplace Technology -- a powerful 
lobbying group set up by the Computer and Business Equipment Manufac-
turers Association (CBEMA) and strongly supported by IBM -- had been 
lobbying since 1984 in various state legislatures against laws de-
signed to protect the health of VDT workers.  Computer manufacturers 
continued to scoff at the idea that their devices might emit hazardous 
radiation.  One industry spokesperson, Charlotte Le Gates, the direc-
tor of communication for CBEMA, declared that for pregnant operators 
to ask to be transferred away from VDTs "is like asking to be trans-
ferred away from a light bulb."
     By using this simile repeatedly, computer manufacturers and their 
paid consultants in CBEMA and the Center for Office Technology have 
been unquestionably successful in allaying growing concern among 
computer users that the emissions from display monitors might be 
hazardous.  The comparison is specious and unscientific, however.  A 
light bulb emits no magnetic field whatsoever -- a fact that can 
easily be ascertained by holding a gauss meter (a device that measures 
the strength of a magnetic field) to an incandescent light bulb.  As 
the accompanying measurements taken by "Macworld" clearly show (see 
"Macworld Tests"), however, many display monitors DO emit magnetic 
fields that are as strong or even stronger than the magnetic-field 
levels that have been associated with the development of cancer in 
children and workers.
 
RISK ACKNOWLEDGED
     The accumulation of evidence suggesting that the electromagnetic 
fields given off by display monitors may be hazardous, together with 
the fact that there are now some 40 million computer terminals in the 
workplace, raises the question of why so few epidemiological studies 
have been conducted in the United States to determine whether monitor 
emissions are affecting the health of American users.  Astonishingly, 
only one major epidemiological study has so far been conducted in this 
country.  It was performed by researchers at the Northern California 
Kaiser Permanente Medical Care Program, in Oakland, who conducted a 
case-control study of 1583 pregnant women who had attended Kaiser 
Permanente obstetrics and gynecology clinics during 1981 and 1982.
     In an article entitled "The Risk of Miscarriage and Birth Defects 
among Women Who Use Visual Display Terminals During Pregnancy" 
("American Journal of Industrial Medicine," June 1988), Kaiser re-
searchers wrote that they had found that women who worked with VDTs 
for more than 20 hours a week experienced a risk of both early and 
late miscarriage that was 80 percent higher than the risk for women 
who performed similar work without using VDTs.  In their conclusion, 
the researchers stated, "Our case-control study provides the first 
epidemiological evidence based on substantial numbers of pregnant VDT 
operators to suggest that high usage of VDTs may increase the risk of 
miscarriage."
 
APPLE RESPONDS
     As might be expected, the results of the Kaiser Permanente study, 
together with the Swedish experiments demonstrating that the emissions 
from display monitors can adversely affect the fetuses of test ani-
mals, have prompted many computer users to write to computer manufac-
turers to ask whether their monitors are safe to use.  One such letter 
was sent on November 5, 1989, to John Sculley, chief executive officer 
of Apple Computer, by Professor Harris Barron, who taught electronic 
media in art-making at the Massachusetts College of Art in Boston for 
25 years.  In his letter, Barron told Sculley that he was writing on a 
Macintosh SE; that his young daughter-in-law, "an avid law school 
scholar, sits long hours at the terminal of her own SE"; and that "she 
and her computer-user husband intend to raise a family in the near 
future."  (As the reader will note in "Macworld Tests," MACWORLD has 
measured an ELF magnetic field of slightly more that 2 milligauss at a 
distance of 12 inches from the screen of the SE display monitor.)  
Barron then asked whether his daughter-in-law was at risk from the 
electromagnetic fields emitted by her monitor and told Sculley that 
"the results of any studies that Apple has made in this regard would 
be helpful."
     On December 6, 1989, Barron received an unsigned letter from 
the Apple Customer Relations Department, thanking him for his 
letter and informing him that some materials were enclosed for 
his perusal.  The enclosed material consisted of an article from the 
February 1984 issue of "Health Physics," which said that X-ray emis-
sions from VDTs posed no health problem; some 1984 recommendations by 
the European Computer Manufacturers Association on how to avoid ergo-
nomic problems from VDT use; a 1983 policy statement issued by the 
American Academy of Ophthalmology, which said that VDTs presented no 
hazard to vision; and some 1985 Apple safety data sheets about the 
testing of toner materials.
     On December 11, 1989, Barron wrote to Sculley to express disap-
pointment with Apple's response to his initial query.  "With your pro 
forma mailing, I am now armed with 1984 materials, data so antiquated 
that I would be embarrassed to use it, as would Apple in any of its 
public relations," Barron said.  "Reprints of ergonomic factors, 
ocular data, toner safety data, and the 'put-to-bed' X-ray issue 
totally ignored my one basic question on permanent harm from ELF 
magnetic-field VDT emissions."  In conclusion, Barron told Sculley 
that he intended to prepare a statement about his correspondence with 
Apple for circulation to his contacts in higher education, including 
the National Education Association.
 
FURTHER EQUIVOCATIONS
     On January 9, 1990, Barron received a reply to his second letter 
from David C. McGraw, Apple's newly appointed manager for corporate 
environmental health and safety.  McGraw apologized for the delay and 
confusion in getting back to Barron, and assured him that "the pro 
forma response to your initial letter dated 11/5/89 is not the way 
Apple wishes to respond to this important issue."  He went on to tell 
Barron that "Apple believes that no increased risk of adverse pregnan-
cy outcome due to VDT work has been demonstrated," and to point out 
that Apple's position in this regard "is supported by the American 
Medical Association, the American College of Obstetricians and Gyne-
cologists, the National Institute for Occupational Safety and Health 
(NIOSH), and the World Health Organization (WHO)."
     McGraw said that the Kaiser Permanente study "drew public atten-
tion because of what appeared to be an increase in miscarriages among 
women who use VDTs more than 20 hours per week," but that the re-
searchers who conducted it "were unable to determine the specific 
cause of the increased rate of miscarriages."  He then noted that 
"similar studies in Canada and Scandinavia have found no relationship 
between VDT work and adverse pregnancy outcome."  McGraw enclosed the 
results of a recent animal study that had been conducted for IBM and 
Ontario Hydro by researchers at the University of Toronto, who, unlike 
Drs. Frolen and Tribukait, had found that pulsed magnetic fields did 
not adversely affect the fetuses of test mice.  He also recommended 
that Barron read a compendium entitled LATEST STUDIES ON VDTs, pub-
lished in August 1989 by the Center for Office Technology.  (This is 
the new name of the Coalition for Workplace Technology of the Computer 
and Business Equipment Manufacturers Association, which had previously 
assured computer users that the emissions from a display terminal were 
no different than those from a light bulb.)
     In January of this year, McGraw sent Barron the names and resumes 
of three people whom he described as "experts in the field of biologi-
cal effects of electromagnetic radiation."  One was Edwin L. Carsten-
sen, a professor of electrical engineering at the University of Roch-
ester, who had been a paid consultant of the electric-utility industry 
for nearly 15 years and has testified for power companies in court 
cases on several occasions.  Another was Kenneth R. Foster, a profes-
sor in the Department of Bioengineering at the University of Pennsyl-
vania, who has not only discounted the possibility that low-level 
electromagnetic radiation can have adverse biological effects but has 
even suggested that restrictions be placed on further investigation of 
the problem.  The third was Eleanor R. Adair, a physiologist at the 
John Pierce Foundation, in New Haven, Connecticut, who, in spite of 
dozens of scientific studies published in leading scientific journals 
around the world demonstrating that weak pulsed electromagnetic fields 
given off by display monitors and low-level fields emitted by radar 
and other sources can cause adverse biological effects at field 
strengths far below those necessary to produce heat, has recently been 
quoted as saying that she has "never seen one bit of scientific evi-
dence -- and let me emphasize the word SCIENTIFIC -- that ELF or 
microwave radiation has any nonthermal biological effects."
 
ANSWERING CRITICS
     Macintosh and other computer users must now decide for themselves 
whether monitor manufacturers are dealing forthrightly with the issue 
of display monitor emissions.  It is clear that computer users are 
being asked by manufacturers to extend the presumption of benignity to 
the pulsed electric and magnetic fields given off by display monitors, 
even as scientists continue to investigate the apparent health hazard 
posed by these emissions.  One of the chief rationales behind this 
strategy is the belief that there is no "conclusive" proof that VDT 
emissions have any harmful effects on computer users.  Another is that 
no biological mechanism has yet been postulated to show exactly how 
pulsed magnetic fields might cause miscarriages and cancer.  In other 
words, if scientists can't explain how something is happening, it 
can't be happening.  Someone should remind the monitor manufacturers 
that scientists don't know exactly how inhaled asbestos fibers act to 
cause cancer; yet everyone knows that asbestos causes cancer, and only 
fools would willingly expose themselves to asbestos.
     As it happens, a model of how a 60Hz alternating-current magnetic 
field may cause or promote cancer has been provided by Dr. Harris 
Busch, an oncologist, who was chairman of the Department of Pharmacol-
ogy of the Baylor University College of Medicine in Houston for 25 
years and was also formerly an editor of the distinguished "American 
Journal of Cancer Research."  After explaining that a 6 60Hz alternat-
ing-current magnetic field vibrates to and fro 60 times a second, 
Busch points out that there will be a similar to-and-fro movement on 
the part of anything magnetic in such a field.  According to Busch, 
this means that "any kind of molecule that is in a person's brain, or 
in a person's body, is being twisted 60 times a second up and back."
     Recently, Dr. W. Ross Adey has made the point that in the case of 
weak electromagnetic fields given off by display monitors, the tissue 
responses can take account of the regularity of the repeating pulses 
and assume the rhythm of those pulses in a phenomenon called ENTRAIN-
MENT, which, in turn, can alter the normal activation of enzymes and 
cellular immune responses in ways consistent with the promotion of 
cancer.
     One does not need to be a medical doctor to appreciate that such 
electromagnetic phenomena, which have no counterpart in man's evolu-
tionary history, may well prove hazardous to health.
 
---------------------------------------------------------------------
 
Author:  PAUL BRODEUR, a staff writer at the "New Yorker" since 1958, 
specializes in medical and science writing.  The winner of many na-
tional awards for his reporting on the dangers of asbestos, the haz-
ards of enzymes in household detergents, the destruction of the ozone 
layer, and the effects of electromagnetic emissions, Brodeur's most 
recent book is "Currents of Death" (Simon and Schuster, 1989).
 
---------------------------------------------------------------------
 
[GRAPHIC ARTICLE INSERTS:]
 
 
CATHODE-RAY TUBES EXPLAINED
 
     Computer display monitors operate on much the same principle as 
television sets.  An evacuated glass tube containing an electron gun, 
called the cathode-ray tube (CRT) (A), produces a narrow electron beam 
(B); a step-up transformer known as the flyback transformer (C) then 
accelerates and directs the beam toward the front of the tube.  When 
the beam strikes the inner surface of the CRT screen, it interacts 
with a phosphor coating (D) on the face of the tube to generate a spot 
of visible light.
     To produce a screen image, the electron beam sweeps from left to 
right and from top to bottom in a series of raster line (E).  The 
movement of the electron beam is controlled by deflection coils (F) 
wound like a yoke around the neck of the CRT; electric current flowing 
through the coils produces magnetic fields that control the electron 
beam.  Increasing current in the horizontal-deflection coil forces the 
beam from left to right; a drop in current causes the beam to return 
to the left.  Meanwhile, an increase in the vertical-deflection coil's 
current aims the beam down a line.  This pulsing actions results in a 
sawtooth waveform (G).
     The horizontal-scan frequency for a typical computer monitor is 
generally between 10kHz and 30kHz, which falls in the very-low-fre-
quency (VLF) range.  Because most monitors operate at 60 to 75 frames 
per second, their vertical-scan frequency is between 60Hz and 75Hz, 
within the extremely-low-frequency (ELF) range.  Both electric and 
magnetic fields are generated in the ELF and VLF ranges.
     In addition, 60Hz alternating-current (AC) fields originate in 
the monitor's power transformer.  (60Hz AC current flows back and 
forth 60 times a second.)  Since the AC fields decay rapidly over 
distance, they can usually be measured only in the immediate vicinity 
of the power transformer. -- P.B.
 
 
ELECTROMAGNETIC SPECTRUM
     While all electromagnetic radiation, from the longest radio wave 
to the shortest gamma ray, travels at 186,000 miles per second -- the 
speed of light -- visible light makes up only a small portion of the 
spectrum.  AS the wavelength (shown in meters) increases, the frequen-
cy (shown in hertz, or cycles per second) decreases.  Display monitors 
give off several types of electromagnetic emissions; most of the 
emissions consist of pulsed radio-frequency (VLF) electric and magnet-
ic fields of between 15 and 20kHz and pulsed ELF electric and magnetic 
fields of 60Hz.  The ELF magnetic fields is the dominant waveform 
given off by VDTs. -- P.B.
 
 
MACWORLD TESTS
     To determine the strength of the ELF magnetic fields emitted 
by monitors regularly used with Macintosh computers, MACWORLD 
tested ten monitors in our labs.  Using the Holaday HI-3600-02 
ELF/Power Frequency EMF Survey Meter, we measured emissions at 4, 
12, 28 (arm's length), and 36 inches from the center of the 
front, back, left, right, top, and bottom of the monitors.  (For 
logistic reasons, we could not complete all the measurements from 
the bottom.)  While it is important to note that magnetic-field 
strengths may vary somewhat from monitor to monitor, even within 
a single product line, the overall test results do confirm that 
ELF magnetic-field emissions from monitors used with the Macin-
tosh are worrisome.
     The strongest emissions are at the sides and tops of the 
monitors -- over 70 milligauss (mG) 4 inches from the right side 
of the AppleColor High-Resolution RGB Monitor, for instance.  At 
the same distance from the front, emissions are over 22mG for the 
Apple monitor and the E-Machines ColorPage 15.  As detailed in 
the main article, levels much lower than these have been corre-
lated with cell mutation and cancer in humans.  At 28 inches 
(arm's length), however, the emissions from the front fall to 
below 1mG.
 
 
AT ARM'S LENGTH
     While ELF magnetic-field emissions of roughly 5 to 23 milli-
gauss (mG) were found at 4 inches from the front of monitors commonly 
used with the Macintosh, "Macworld" found that at 28 inches from the 
screen, all the monitors tested at less than 1mG.  (The ambient ELF 
magnetic-field emissions measured in the MACWORLD offices ranged from 
0.1 to 0.5 mG.)  Macintosh users wishing to reduce exposure to pulsed 
electromagnetic fields should position their display monitors at arm's 
length (with fingers extended)(A).
     Because magnetic fields emitted from the sides and backs of most 
monitors are considerably stronger than those given off from the 
front, users should consider maintaining a distance of at least 4 feet 
from the sides or back of any other monitor in the workplace (B).  
Keep in mind that magnetic-field emissions are not stopped by cubicle 
partitions, walls, lead aprons, or even the human body.
     Curiously, there are no standards for ELF magnetic-field emis-
sions, although several countries, Sweden and Canada among them, have 
developed standards for VLF magnetic-field emissions.  A number of 
vendors -- IBM, DEC, and Phillips, for instance -- market monitors for 
PCs that meet those standards.  For the past two years, Sigma Designs 
has supplied the European market with monitors for the Mac that meet 
the VLF standards, and American users can now special order these 
monochrome and gray-scale 15-, 19-, and 21-inch monitors.  Also, any 
monitor based on a technology other than a cathode-ray tube will have 
the advantage of not emitting the types of pulsed radiation associated 
with vertical-and horizontal-deflection coils.  For a discussion of 
various products that claim to mitigate monitor emissions, see "Con-
spicuous Consumer" in this issue.
     The controversy surrounding low-frequency electromagnetic emis-
sions will continue until further research is completed.  In the 
meantime, prudent avoidance -- sitting at arm's length from the front 
and 4 feet from the sides or back of a monitor -- is a sensible solu-
tion.  "Macworld" is committed to documenting any new developments as 
they relate to this issue. Stay tuned. --Suzanne Stefanac.
 
