Suzy B Software 2/Suzy B Software CD-ROM 2 (1994).iso/nasa/emf_faq/emf_faq.txt

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Power-Frequency Fields and Cancer (J. Moulder, v5, 27-Aug-93)

Notice:  This FAQ sheet may be redistributed as long at remains correctly
attributed.  If it is edited or condensed prior to redistribution, please add
a note to that effect.

Revision notes:
v4 (24-Aug-93):  Q3 revised extensively, and minor changes made in Q5, Q6 and
Q7 to take a quantum approach to the answers.  Gauss to Tesla conversion
error corrected in Q8.

v5: (27-Aug-93): All units given in SI units as well as American units.
Dosimetry section (Q7) expanded and referenced.  Three new section added: Q15
making best argument for and against a EMF-cancer connection; Q16 discussing
potential confounders; Q17 addressing what types of studies are still needed
Base-line cancer risk data added to Q10.


1) Why is there a concern about power lines and cancer?

Most of the concern about power lines and cancer stems from epidemiological
studies of people living near distribution and transmission lines, and
epidemiological studies of people working in "electrical occupations".  Some
of these epidemiological studies appear to show a relationship between
exposure to power-frequency fields and the incidence of cancer.  Laboratory
studies have shown no link between power-frequency fields and cancer.

2) What's the difference between the electromagnetic [EM] energy associated
with power lines and other forms of EM energy such as microwaves or x-rays?

X-rays, ultraviolet (UV) light, visible light, infrared light, microwaves
(MW), radiowaves (RF), and magnetic fields from electrical power systems are
all parts of the EM spectrum.   The parts of the EM spectrum are
characterized by their frequency or wavelength.  The frequency and wave
length are related, and as the frequency rises the wavelength gets shorter.
The frequency is the rate at which the EM field changes direction and is
usually given in Hertz (Hz), where one Hz is one cycle per second.

Power-frequency fields in the US vary 60 times per second, so they are 60 Hz
fields, and have a wavelength of 3000 miles (5000 km).  Power in much of the
rest of the world is at 50 Hz.  Broadcast AM radio has a frequency of around
one million Hz and wavelengths of around 1000 ft (300 m). Microwave ovens
have a frequency of about 2 billion Hz, and a wavelength of about 5 inches
(12 cm).  X-rays and UV light have frequencies of millions of billions of Hz,
and wavelengths of less than a thousandth of an inch (10 nm or less).

3)  What differences are there in the biological effects of these different
portions of the EM spectrum?

The interaction of biological material with an EM source depends on the
frequency of the source.  We usually talk about the electromagnetic spectrum
as though it produced waves of energy.  This is not strictly correct, because
sometimes electromagnetic energy acts like particles rather than waves; this
is particularly true at high frequencies.  This double nature of the
electromagnetic spectrum is referred to as "wave-particle duality".  The
particle nature of electromagnetic energy is important because it is the
energy per particle (or photons, as these particles are called) that
determines what biological effects electromagnetic energy will have.

At the very high frequencies characteristic of UV light and X-rays,
electromagnetic particles (photons) have sufficient energy to break chemical
bonds.  This breaking of bonds is termed ionization, and this portion of the
electromagnetic spectrum is termed ionizing radiation.  At lower frequencies,
such as those characteristic of visible light, radiowaves, and microwaves,
the photons don't carry enough energy to break chemical bonds; but they do
carry enough energy to cause molecules to vibrate, causing heating. These are
called thermal effects, and this portion of the electromagnetic spectrum is
termed the thermal, non-ionizing portion.  Below the frequencies used in
commercial broadcast radio (such as the 60 Hz frequencies generated in the
production and distribution of electricity), the photons have insufficient
energy to cause heating, and this portion of the electromagnetic spectrum is
termed the non-thermal, non-ionizing portion.

4)  What is difference between EM radiation and EM fields?

In general, EM sources produce both radiant energy (radiation) and non-
radiant energy (fields).  Radiated energy exists apart from its source,
travels away from the source, and continues to exist even if the source is
turned off.  Non-radiant energy is not projected away into space, and it
ceases to exist when the energy source is turned off.  When a person or
object is more than several wavelengths from an EM source, a condition called
far-field, the radiation component of the EM fields dominates.  In the far-
field the electrical and magnetic components are closely related.  When a
person or object is less than one wavelength from an EM source, a condition
called near-field, the field effect dominates, and the electrical and
magnetic components are unrelated.

For ionizing frequencies where the wavelengths are less than thousandths of
an inch (less than 10 nm), human exposure is entirely in the far-field, and
only the radiation from the EM source is relevant to possible health effects.
For MW and RF, where the wavelengths are in inches to a few thousand feet (a
few cm to a km), human exposure can be in both the near- and the far-field,
so that both field and radiation effects are relevant.  For power-frequency
fields, where the wavelength is thousands of miles (thousands of km), human
exposure is always in the near-field, and only the field component is
relevant to possible health effects.

5)  How do ionizing EM sources cause biological effects?

Ionizing EM radiation carries sufficient energy per photon to break chemical
bonds.  In particular, ionizing radiation is capable of breaking bonds in the
genetic material of the cell, the DNA.  Severe damage to DNA can kill cells,
resulting in tissue damage or death.  Lesser damage to DNA can result in
permanent changes in the cells which may lead to cancer.  If these changes
occur in reproductive cells, they can lead to inheritable changes, a
phenomena called mutation.  All of the known hazards from exposure to the
ionizing portion of the EM spectrum are the result of the breaking of
chemical bonds in DNA.  For frequencies below that of UV light, DNA damage
does not occur because the photons do not have enough energy to break
chemical bonds.   Well-accepted safety standards exist to prevent significant
damage to the genetic material of persons exposed to ionizing EM radiation.

6)  How do the thermal non-ionizing EM sources cause biological effects?

Visible light, MW, and RF can cause molecules to vibrate, causing heating.
This molecular heating can kill cells.  If enough cells are killed, burns and
other forms of long-term, and possibly permanent tissue damage can occur.
Cells which are not killed by heating gradually return to normal after the
heating ceases; permanent non-lethal cellular damage is not known to occur.
All of the known hazards from exposure to the thermal non-ionizing portion of
the EM spectrum are the result of heating.  For frequencies below about the
middle of the AM broadcast spectrum, this heating does not occur, because the
photons do not have enough energy to cause molecular vibrations.

The molecular vibration caused by MW is how and why a MW oven works -
exposure of the food to the microwaves causes water molecules to vibrate and
get hot.  MW and RF penetrate and heat best when the size of the object is
close to the wavelength.   For the 2450 MHz (2.45 billion Hz) used in
microwave ovens the wavelength is 5 inches (12 cm), a good match for most of
what we cook.

7)  How do the power-frequency EM fields cause biological effects?

The electrical and magnetic fields associated with power-frequency fields
cannot break bonds or cause molecular heating, because the energy per photon
is too low. Thus the known mechanisms through which ionizing radiation, MW
and RF effect biological material have no relevance for power-frequency
fields.

The electrical fields associated with the power-frequency fields exist
whenever voltage is present.  These electrical fields have very little
ability to penetrate buildings or even skin.  The magnetic fields associated
with power-frequency fields exist only when current is flowing.  These
magnetic fields are difficult to shield, and easily penetrate buildings and
people.   Because power-frequency electrical fields do not penetrate, any
biological effects from routine exposure to power-frequency fields must be
due to the magnetic component of the field.

Exposure of people to power-frequency magnetic fields results in the
induction of electrical currents in the body.  These currents are similar to
naturally-occurring currents.  It requires a power-frequency magnetic field
in excess of 5 Gauss (500 mT, see Q8 for typical exposures) to cause
electrical currents of a magnitude similar to those that occur naturally in
the body.   Electrical currents that are above those that occur naturally in
the body can cause noticeable effects, including direct nerve stimulation.
Well-accepted safety standards exist to protect persons from exposure to
power-frequency fields that would induce such currents (see Q16).

8)  What sort of power-frequency magnetic fields are common in residences and
workplaces?

In the US magnetic fields are commonly measured in Gauss (G).  In the rest of
the world, they are measured in Tesla (T), were 10,000 Gauss equals 1 Tesla.
Within the right-of-way (ROW) of a high voltage transmission line, fields can
approach 100 mG (0.1 G, 10 microT).  At the edge of a high-voltage ROW, the
field will be 1-10 mG (0.1-1.0 microT).  Ten meters from a 12 kV distribution
line will be fields will be 2-10 mG (0.2-1.0 microT).  Actual fields depend
on voltage, design and current.

Fields within residences vary from over 1000 mG (100 microT) a few inches
(cm) certain appliances to less than 0.2 mG (0.02 microT) in the center of
some rooms.  Appliances that have the highest fields are those with high
currents (e.g., toasters, electric blankets) or high-speed electric motors
(e.g., vacuum cleaner, electric clock, blender).  Appliance fields decrease
very rapidly with distance. See ref. 24 for further details.

Occupational exposures in excess of 100 mGauss (10 microT) have been reported
(e.g., in arc welders and electrical cable splicers).  In "electrical"
occupations mean exposures range from 5 to 40 mG (0.5 to 4 microT).  See ref.
24 for further details.

9)  What is known about the relationship between powerline corridors and
cancer rates?

Some studies have shown that children (but not adults) living near certain
types of powerlines (high current distribution lines and transmission lines)
have higher than average rates of leukemia and brain cancers (Refs 1-3).  The
correlation is not strong, and none of the studies have shown dose-response
curves.  When power-frequency fields are actually measured, the correlation
vanishes (not surprising, since the major source of power-frequency fields
within most dwellings is inside the house).  Several other studies have shown
no correlations (Refs 4-6).

10)  How big is the "cancer risk" associated with living next to a powerline?

The excess cancer found in epidemiological studies is usually quantified in a
number called the relative risk (RR).  This is the risk of an "exposed"
person getting cancer divided by the risk of an "unexposed" person getting
cancer.  Since no one is unexposed to power-frequency fields, the comparison
is actually "high exposure" versus "low exposure".  Relative risks are
generally given with 95% confidence intervals.  These 95% confidence
intervals are almost never adjusted for multiple comparisons even when
multiple types of cancer and multiple indices of exposure are studied.

Taken together, using a technique known as "meta-analysis", the relative
risks for the residential exposure studies are (adapted from ref. 7):
   childhood leukemia: 1.3 (0.8 - 2.1)  5 studies
   childhood brain cancer:  2.4 (1.7 - 3.5)  3 studies
   adult leukemia:  1.1 (0.9-1.4)  2 studies
   all adult cancer:  1.2 (0.8-1.6)  2 studies

As a base-line for comparison the age-adjusted cancer incidence rate for
adults in the United States is 3 per 1,000 per year for all cancer (that is,
0.3% of the population gets cancer in a given year),and 1 per 10,000 per year
for leukemia (ref. 26).

11)  What is known about the relationship between "electrical occupations"
and cancer rates?

Several studies have shown that people who work in electrical occupations
have higher than average leukemia, lymphoma, and brain cancer rates (refs 8-
10).  Most of the cautions listed for the residential studies apply here
also: many negative studies, weak correlations, no dose-response curves.
Additionally, these studies are mostly based on job titles, not on measured
exposures.

Taken together, using a technique known as "meta-analysis", the relative
risks for the occupational exposure studies are (adapted from ref. 7):
leukemia: 1.1 (1.0-1.2)  24 studies
brain: 1.2 (1.0-1.5)  16 studies
lymphoma: 1.2 (0.9-1.5)  6 studies
all cancer:  1.0 (0.9-1.1)  8 studies

12)  What do laboratory studies tell us about power-frequency fields and
cancer?

Power-frequency fields show none of the classic signs of being carcinogens -
they do not cause DNA damage or chromosome breaks, and they are not mutagenic
(refs 11-15).  No studies have shown that animals exposed to power-frequency
fields have increased cancer rates.  On the other hand, numerous studies have
reported that power-frequency fields do have "effects", particularly at high
field strength (refs 16, 17).  Even among the scientists who believe that
there may be a connection between power-frequency fields and cancer, there is
no consensus as to possible mechanisms (refs 16, 18).

There are agents that influence the development of cancer without directly
damaging the genetic material.  It has been suggested that power-frequency
EMFs could either promote cancer or influence the progression of cancer.   A
promoter is an agent that increases the cancer risk in an animals already
exposed to a genotoxic carcinogen.  A progression effect would be one that
increased the growth rate of an existing tumor.  Promotion studies of power-
frequency fields have been uniformly negative (refs  14, 19-21).  Studies of
progression have been mixed: 75% show no effect on tumor growth, while the
rest are about equally mixed between studies showing increased growth and
studies showing decreased growth (refs 11, 15, 20-22).

13)  What about the new "Swedish" study showing a link between power lines
and cancer?

There are new residential and occupational studies from both Sweden and
Denmark.  None have been published in full, but translations of the
preliminary reports have been circulated.

- Fleychting & Ahlbom [Magnetic fields and cancer in people residing near
Swedish high voltage powerlines].   A case-control study of everyone who
lived within 300 meters of high-voltage powerlines between '60 and '85.  For
children all types of tumors were analyzed, for adults only leukemia and
brain tumors were studied.  "Exposure" was assessed by spot measurements,
calculated retrospective assessments, and distance from powerlines.  No
increased overall cancer risk was found for either children or adults.  An
increased risk for leukemia (but not other cancers) was found in children for
*calculated* fields at the time of diagnosis.  No significantly elevated
cancer risks were found for measured fields or proximity to powerlines.

- Olsen and Nielson [Electromagnetic fields from high-power electricity
transmission systems and the risk of childhood cancer].  Case-control study
based on all childhood leukemia, brain tumors and lymphomas diagnosed in
Denmark between '68 and '86.  "Exposure" was assessed on the basis of
calculated fields over the period from conception to diagnosis.  No overall
increase in cancer risk was found, but the risk of lymphoma was elevated.  No
increase in childhood leukemia or brain cancer was found.

- Guenel et al.  [Cancer incidence among Danish persons who have been exposed
to magnetic fields at work].  Case-control study based on all cancer in
actively employed Danes between '70 and '87 who were 20-64 years old in 1970.
Each occupation-industry combination was coded on the basis of supposed 50-Hz
magnetic field exposure.  No significant increases in risk were seen for
breast cancer, malignant lymphomas or brain tumors.  Leukemia incidence was
elevated among men in the highest "exposure" category; women in similar
exposure categories showed no excess risk.

-Floderus et al [Occupational exposure to EM fields in relation to leukemia
and brain tumors].  Case-control study of leukemia and brain tumors of men,
20-64 years of age in '80.  "Exposure" calculations were based on the job
held longest during the 10-year period prior to diagnosis.  Many measurements
were taken using a person whose job was most similar to that of the person in
the study. About two thirds of the subjects in the study could be assessed in
this manner.  A significantly elevated risk was found for leukemia, but not
for brain cancer.

14)  How do scientists evaluate all the confusing and contradictory
laboratory and epidemiological studies of power-frequency magnetic fields and
cancer?

There are certain widely accepted criteria that are weighed when assessing
such groups of studies.   These are the Hill criteria (ref. 23).

- First, what is the *strength of the association* between exposure and risk;
is there a clear risk associated with exposure?  A strong association is one
with a RR (see Q9) of 5 or more.  Tobacco smoking, for example, shows a RR
for lung cancer of 10-30 times that of non-smokers.

Most of the positive power-frequency studies have RRs of less than two.  The
leukemia studies as a group have RRs of about 1.2, while the brain cancer
studies as a group have RRs of about 2.  This is only a weak association.

- Second, are there many *consistent studies* indicating the same risk; do
most studies show about the same risk for the same disease?   Using the same
example, essentially all studies of smoking and cancer showed an increased
risk for lung and head-and-neck cancers.

Many power-frequency studies show statistically significant risks for some
types of cancers and some types of exposures, but many do not.  Even the
positive studies are inconsistent with each other.  For example, while a new
Swedish study shows an increased risk for childhood leukemia for one measure
of exposure, it contradicts prior studies that showed a risk for brain
cancers, and a parallel Danish study shows a risk for childhood lymphomas,
but not for leukemia.  Many of the studies are internally inconsistent.  For
example, where the Swedish study shows an increased risk for childhood
leukemia, it shows no overall increase in childhood cancer, implying that the
rates of other types of cancer are decreased.  In summary, few studies show
the same positive result, so that the consistency is quite weak.

- Third, is there evidence for a *dose-response relationship*; does risk
increase when the exposure increases?  Again, the more a person smokes, the
higher the risk of lung cancer.

No power-frequency exposure studies have shown a dose-response relationship
between measured fields and cancer rates, or between distances from
transmission lines and cancer rates.  The lack of a relationship between
exposure and increased cancer risk is a major reason why many scientists are
skeptical about the significance of the epidemiology.

- Fourth, is there *laboratory evidence* suggesting that there is a risk
associated with such exposure?  Epidemiological associations are greatly
strengthened when we have laboratory evidence for a risk.  When the US
Surgeon General first stated that smoking caused lung cancer, the laboratory
evidence was ambiguous.  We knew that cigarette smoke and tobacco contained
carcinogens, but no one had been able to make lab animals get cancer by
smoking.

Power-frequency fields show none of the effects on cells, tissues or animals
that point towards their being a cause of cancer, or to their contributing to
cancer.

-Fifth, are there *plausible biological mechanisms* that suggest that there
should be a risk?  If we understand how something causes disease, it is much
easier to interpret ambiguous epidemiology.  With smoking, for example, the
fact that there were known cancer-causing agents in tobacco made it very easy
to believe the epidemiology.

From what we know of power-frequency fields and their effects on biological
systems we have no reason to even suspect that they pose a risk to people at
the exposure levels associated with the generation and distribution of
electricity.

- Overall the evidence for a connection between power frequency fields and
cancer is at most weak, because of the weakness and inconsistencies in the
epidemiological studies, combined with the lack of a dose-response
relationship in the human studies, and the negative laboratory studies.

15)  If power-frequency fields don't explain the positive residential and
occupations studies, what could?

There are basically three factors that can result in false associations in
epidemiological studies.  These are:
a)  Inadequate dose assessment - if power-frequency fields are associated
with cancer, we do not know what aspect of the field is involved.  At a
minimum, risk could be related to the peak field, the average field, of the
rate of change of the field.  If we don't know who is really exposed, and who
is not, we will usually (but now always) underestimate the true risk.
b)  Confounders - power lines (or electrical occupations) might be associated
with a cancer risk other than magnetic fields.  Many confounders of the
powerline studies have been suggested: PCBs, herbicides, traffic density,
socioeconomic class.  The first two are unlikely.  PCB leakage is rare, and
PCB exposure has been linked to lymphomas, not leukemia or brain cancer.
Herbicide spraying would not effect distribution systems in urban areas
(where 3 of  4 positive childhood cancer studies have been done).  Traffic
density may be a real confounder (see ref. 28).  Socioeconomic class may be
an issue in both the residential and occupational studies, as socioeconomic
class is clearly associate with cancer risk, and "exposed" and "unexposed"
groups in many studies may be of different socioeconomic classes (see ref. 29
for a discussion of some of these issues)
c)  Publication bias - it is a known that positive studies are more likely to
be published than negative studies.  This can severely bias meta-analysis
studies such as those discussed in Q10 and Q11.  Publication bias will always
increase apparent risks.  This is a bigger potential problem for the
occupational studies than the residential ones.  It is also a clear problem
for laboratory studies - it is much easier to publish studies that report
effects than studies that report no effects (such is human nature!).

16)  What is the strongest evidence for and against a connection between
power-frequency fields and cancer?

The best evidence for a connection between cancer and power-frequency fields
is probably:
a)  The four epidemiological studies that show a correlation between
childhood cancer and proximity to high-current wiring (refs 1-3 plus the
Fleychting & Ahlbom study described in Q13).
b)  The epidemiological studies that show a significant correlation between
work in electrical occupations and cancer, particularly leukemia and brain
cancer (refs 8-10).
c)  The lab studies that how that power-frequency fields do produce
bioeffects.  The most interesting of the lab studies are probably the ones
showing increased transcription of oncogenes at fields of 1-5 Gauss (100 -
500 microT) (see ref. 17 and 18).

The best evidence that there is not a connection between cancer and power-
frequency fields is probably:
a) Application of the Hill criteria (Q14) to the entire body of
epidemiological and laboratory studies (refs 24 and 27)
b) The fact that all studies of genotoxicity and promotion have been negative
(Q12).
c) Adair's (ref. 25) biophysical analysis that indicates that "any biological
effects of weak [less than 500 mG, 50 microT] ELF fields  on the cellular
level must be found outside of the scope of conventional physics"
d) Jackson's (ref. 26) epidemiological analysis that shows that childhood and
adult leukemia rates in the US have been stable over a period of time when
per capita power consumption in the US has risen by a factor of five.

17)  What studies are needed to resolve the cancer-EMF issue?

In the epidemiological area, I don't think that more of the same types of
studies will resolve anything.  Studies showing a dose-response relationship
between measured fields and cancer incidence rates would clearly affect our
thinking, as would studies identifying confounders in the residential and
occupational studies.

In the laboratory area, I don't think that more genotoxicity and promotion
studies will be very useful, except possibly in the area of cell
transformation. Long-term rodent exposure studies (the standard test for
carcinogenicity) would have a major impact if they were positive, but if they
were negative it wouldn't change very many minds.  Further studies of some of
the known bioeffects would be useful, but only if they identified mechanisms
or if they established the conditions under which the effects occur (e.g.,
thresholds, dose-response relationships, frequency-dependence, optimal wave-
forms).

18)  What are some good overview articles?

A very good review of the area has just been published by Oak Ridge
Associated Universities.  It is titled "Health Effects of Low-Frequency
Electric and Magnetic Fields".   It costs $25 and is available from National
Technical Information Service (ARAU 92/F-8) and the US Government Printing
Office (029-000-00443-9).  If you're in the U.K., a good review is: R Doll et
al, Electromagnetic Fields and the Risk of Cancer, National Radiation
Protection Board, Chilton, 1992.  Two other good review are Theriault (ref.
24) and Bates (ref. 27).

19)  Are there exposure standards for power-frequency fields?

Yes, a number of governmental and professional organizations have developed
exposure standards.  These standards are based on keeping the body currents
induced by power-frequency EM fields to a level below the naturally occurring
fields.  The most generally relevant are:

- Guidance as to restriction on exposures to time varying EM fields and the
1988 recommendations on the International Non-Ionizing Radiation Committee,
National Radiation Protection Board, Chilton, 1989.
  50/60 Hz E-field: ~10 kV/m (freq. dependent)
  50/60 Hz H-field: 1630 A/m, 2 mT (20 G)

- Sub-radiofrequency (30 KHz and below) magnetic fields, In: Documentation of
the threshold limit values, American Committee of Government and Industrial
Hygienists, pp. 55-64,1992.
   At 60 Hz:  1 mT (10 G); 0.1 mT (1 G) for pacemaker wearers

- HP Jammet et al:  Interim guidelines on limits of exposure to 50/60 Hz
electric and magnetic fields.  Health Physics 58:113-122, 1990.
  *H-field (rms)
     24 hr general public: 0.1 mT = 1 G
     Short-term general public: 1 mT = 10 G
     Occupational continuous: 0.5 mT = 5 G
     Occupational short-term: 5 mT = 50 G
  *E-field (rms)
     24 hr general public: 5 kV/m
     Short-term general public: 10 kV/m
     Occupational continuous: 10 kV/m
     Occupational short-term: 30 kV/m



-----------------------
References:

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cancer. Amer J Epidemiol 109:273-284, 1979.
2) DA Savitz et al: Case-control study of childhood cancer and exposure to 60-
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Results Cancer Res. 120:166-180; 1990.
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frequency electromagnetic fields, Phys Rev A 43:1039-1048, 1991.
26) J.D. Jackson: Are the stray 60-Hz electromagnetic fields associated with
the distribution and use of electric power a significant cause of cancer?
Proc Nat Acad Sci USA 89:3508-3510, 1992.
27) MN Bates: Extremely low frequency electromagnetic fields and cancer: the
epidemiologic evidence, Environ Health Perspec 95:147-156, 1991.
28) DA Savitz & L Feingold:  Association of childhood leukemia with
residential traffic density.  Scan J Work Environ Health 15:360-363, 1989.
29) JM Peters et al:  Exposure to residential electric and magnetic fields
and risk of childhood leukemia.  Rad Res 133:131-132, 1993.



John Moulder ([e-mail])          Voice: 414-266-4670
Radiation Biology Group                      FAX: [phone]
Medical College of Wisconsin, Milwaukee