Internet Info 1994 March/Internet Info CD-ROM (Walnut Creek) (March 1994).iso/answers/sci/powerlines-cancer-FAQ/part1

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From: [e-mail] (John Moulder)
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Subject: Powerlines and Cancer FAQs (1 of 4)
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Date: 25 Mar 1994 18:00:19 GMT
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Summary: Q&As on the connection between powerlines, electrical
  occupations and cancer.  Discussion of the biophysics of
  interactions with EM sources, summaries of the laboratory
  and human studies, information on standards, and references.
Keywords: powerlines, magnetic fields, cancer, EMF, non-ionizing
  radiation, FAQ
Xref: bloom-beacon.mit.edu sci.med.physics:1292 sci.answers:1011 news.answers:16847

Archive-name: powerlines-cancer-FAQ/part1
Last-modified: 1994/3/25
Version: 2.4

FAQs on Power-Frequency Fields and Cancer (part 1 of 4)

Current version available by anonymous FTP from "rtfm.mit.edu" Directory:
/pub/usenet-by-group/news.answers/powerlines-cancer-FAQ
Files: part1, part2, part3, etc. . . .

Revision notes:
v2.4 (23-Mar-94): Expanded to four parts.  Added table of contents.
Expanded and annotated bibliography.  Modified and expanded non-ionizing
bioeffects sections.  Expanded discussion of field reduction techniques.
Added sections on measurement techniques.  Expanded section on laboratory
studies and broke into multiple parts.  Added short section and references
on reproductive toxicity studies.

Table of Contents:
Part 1
1) Why is there a concern about powerlines and cancer?
2) What is the difference between the electromagnetic (EM) energy
associated with power lines and other forms of EM energy such as microwaves
or x-rays?
3) Why do different types of EM sources produce different biological
effects?
4) What is difference between EM radiation and EM fields?
5) Do power lines produce EM radiation?
6) How do ionizing EM sources cause biological effects?
7) How do RF, MW, visible light, and IR light sources cause biological
effects?
8) How do the power-frequency EM fields cause biological effects?
9) Do non-ionizing EM sources cause non-thermal as well as thermal effects?
10) What sort of power-frequency magnetic fields are common in residences
and workplaces?
11) Can power-frequency fields in homes and workplaces be reduced?
12) What is known about the relationship between powerline corridors and
cancer rates?
13) How big is the "cancer risk" associated with living next to a
powerline?
14) How close do you have to be to a power line to be considered exposed to
power-frequency magnetic fields?
15) What is known about the relationship between "electrical occupations"
and cancer rates?

Part 2
16) What do laboratory studies tell us about power-frequency fields and
cancer?
    16A) Are power-frequency fields genotoxic?
    16B) Are power-frequency magnetic fields cancer promoters?
    16C) Do power-frequency magnetic fields enhance the effects of other
    genotoxic agents?
17) How do laboratory studies of the effects of power-frequency fields on
cell growth, immune function, and melatonin relate to the question of
cancer risk?
18) Do power-frequency fields show any effects at all in laboratory
studies?
19) What about the new "Swedish" study showing a link between power lines
and cancer?
20) What criteria do scientists use to evaluate all the confusing and
contradictory laboratory and epidemiological studies of power-frequency
magnetic fields and cancer?
    20A) Criterion One: How strong is the association between exposure
    to power-frequency fields and the risk of cancer?
    20B) Criterion Two: How consistent are the studies of associations
    between exposure to power-frequency fields and the risk of cancer?
    20C) Criterion Three: Is there a dose-response relationship between
    exposure to power-frequency fields and the risk of cancer?
    20D) Criterion Four: Is there laboratory evidence for an association
    between exposure to power-frequency fields and the risk of cancer?
    20E) Criterion Five: Are there plausible biological mechanisms that
    suggest an association between exposure to power-frequency fields
    and the risk of cancer?
21) If exposure to power-frequency magnetic fields does not explain the
residential and occupations studies which show increased cancer incidence,
what other factors could?
    21A) Could problems with dose assessment affect the validity of the
    epidemiological studies of power lines and cancer?
    21B) Are there other cancer risk factors that could be causing a
    false association between exposure to power-frequency fields and
    cancer?
    21C) Could the epidemiological studies of power lines and cancer be
    biased by the methods used to select control groups?
    21D) Could analysis of the epidemiological studies of power lines
    and cancer be skewed by publication bias?
22) What is the strongest evidence for a connection between power-frequency
fields and cancer?
23) What is the strongest evidence against a connection between
power-frequency fields and cancer?
24) What studies are needed to resolve the cancer-EMF issue?
25) Is there any evidence that power-frequency fields could cause health
effects other than cancer.

Part 3
26) What are some good overview articles?
27) Are there exposure guidelines for power-frequency fields?
28) What effect do powerlines have on property values?
29) What equipment do you need to measure power-frequency magnetic fields?
30) How are power-frequency magnetic fields measured?

Annotated Bibliography
A) Recent Reviews of the Biological and Health Effects of Power-Frequency
Fields
B) Reviews of the Epidemiology of Exposure to Power-Frequency Fields
C) Epidemiology of Residential Exposure to Power-Frequency Fields
D) Epidemiology of Occupational Exposure to Power-Frequency Fields
E) Human Studies Related to Power-Frequency Exposure and Cancer
F) Biophysics and Dosimetry of Power-Frequency Fields

Part 4
G) Laboratory Studies of Power-Frequency Fields and Cancer
H) Laboratory Studies Indirectly Related to Power-Frequency Fields and
Cancer
J) Laboratory Studies of Power-Frequency Fields and Reproductive Toxicity
K) Reviews of Laboratory Studies of Power-Frequency Fields
L) Miscellaneous Studies
M) Regulations and Standards for Ionizing and Non-ionizing EM Sources.

-----

1) Why is there a concern about powerlines and cancer?

Most of the concern about power lines and cancer stems from epidemiological
studies of people living near powerlines, and epidemiological studies of
people working in "electrical occupations".  Some of these epidemiological
studies appear to show a relationship between exposure to power-frequency
magnetic fields and the incidence of cancer.  Laboratory studies have shown
little evidence of a link between power-frequency fields and cancer.

2) What is 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 (IR),
microwaves (MW), radiowaves (RF), and electromagnetic 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 wavelength 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 most of
the rest of the world is at 50 Hz.  The power-frequency fields are often
referred to as extremely low frequencies or ELF.  Broadcast AM radio has a
frequency of around one million Hz and a wavelength of around 1000 ft (300
m). Microwave ovens have a frequency of about 2.5 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) Why do different types of EM sources produce different biological
effects?

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

At the very high frequencies characteristic of UV light and X-rays, EM
particles (photons) have sufficient energy to break chemical bonds.  This
breaking of bonds is termed ionization, and this portion of the EM spectrum
is termed ionizing radiation. The well-known biological effects of X-rays
are associated with the ionization of molecules.  At lower frequencies,
such as those characteristic of visible light, RF, and MW, the photons do
not carry enough energy to break chemical bonds.  This portion of the EM
spectrum is termed the non-ionizing portion.  At RF and MW frequencies the
energy of a photon is very much (by a factor of thousands or more) below
those needed to disrupt chemical bonds. For this reason, there is no
analogy between the biological effects of ionizing and nonionizing EM
energy.

Non-ionizing EM sources can still produce biological effects.  One
mechanism is by inducing electrical currents in tissues, which cause
heating by moving ions and water molecules through the viscous medium in
which they exist.  The efficiency with which an EM source can induce
electrical currents, and thus produce heating, depends on the frequency of
the source, and the size and orientation of the object being heated.  At
frequencies below that used for broadcast AM radio, EM sources couple
poorly with the bodies of humans and animals, and thus are very inefficient
at inducing electrical currents and causing heating.

Thus in terms of potential biological effects the EM spectrum can be
divided into the three portions:
1) The ionizing portion, where direct chemical damage can occur (X-rays,
hard UV).
2) The portion of the non-ionizing spectrum in which the wavelength is
smaller than that of the body, where heating can occur (visible light, IR,
MW and RF).
3) The portion of the non-ionizing spectrum in which the wavelength is much
larger than that of the body, where heating seldom occurs (power
frequencies).

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

When dealing with fields from an EM source it is customary to distinguish
between fields (which do not transmit energy to infinity from the source)
and radiation (which does).  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 source
dominates.  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 a thousandth
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 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 far-field,
so that both field and radiation effects can be 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) Do power lines produce EM radiation?

The fields associated with transmission lines are purely near-field.  While
the lines theoretically might radiate some energy the efficiency of this is
so low that this effect can for all practical purposes be ignored.  To be
an effective radiation source an antenna must have a length comparable to
its wavelength.  Power-frequency sources are clearly too short compared to
their wavelength (3000 miles, 5000 km) to be effective radiation sources.

This is not to say that there is no loss of power during transmission.
There are many sources of loss in transmission lines that have nothing to
do with "radiation" (in the sense as it is used in EM theory).  Loss of
energy is a result of resistive heating, not "radiation".  This is in sharp
contrast to RF antennas, which "lose" energy to space by radiation.
Likewise, there are many ways of transmitting energy from point A to point
B that do not involve radiation.  Electrical circuits do it all the time.

The only "practical" exception to the statement that power-frequency fields
do not radiate is the use of ELF antennas to broadcast to submerged
submarines.  The US Navy runs a power-frequency antenna in Northern
Wisconsin and the Upper Peninsula of Michigan.  To overcome the inherent
inefficiency of the frequency, the antenna is several hundred kilometers in
length.

6) 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 inherited
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 [M3].

7) How do RF, MW, visible light, and IR light sources cause biological
effects?

A principal mechanism by which RF, MW, visible light, and IR light sources
cause biological effects is by heating (thermal effects).  This 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.  At the
whole-animal level, tissue injury and other thermally-induced effects can
be expected when the amount of power absorbed by the animal is similar to
or exceeds the amount of heat generated by normal body processes.  Some of
these thermal effects are very subtle, and do not represent biological
hazards.

It is possible to produce thermal effects even with very low levels of
absorbed power.  One example is the "microwave hearing" phenomenon; these
are auditory sensations that a person experiences when his head is exposed
to pulsed microwaves such as those produced by radar.  The ╥microwave
hearing╙ effects is a thermal effect, but it can be observed at very low
average power levels.

Since thermal effects are produced by heat, not by the electric or magnetic
fields directly, they can be produced by fields at many different
frequencies.  Well-accepted safety standards exist to prevent significant
thermal damage to persons exposed to MW and RFs [M2] and also for persons
exposed to lasers, IR and UV light [M4].

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

The electrical and magnetic fields associated with power-frequency fields
cannot break bonds because the energy per photon is too low.  The magnetic
field intensities to which people are exposed in residential settings and
in the vast majority of occupational settings cannot cause heating because
the induced electrical currents are too low. Thus the known mechanisms
through which ionizing radiation, MWs and RFs effect biological material
have no relevance for power-frequency fields.

The electrical fields associated with the power-frequency fields exist
whenever voltage is present, and regardless of whether current is flowing.
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.  If these currents are
sufficiently intense, they can cause heating, nerve excitation and other
effects [F4,K1].  At power frequencies, the body is poorly coupled to
external fields, and the induced currents are usually too small to produce
obvious effects.  Shocks, and other obvious effects usually require that
the body actually touch a conductive objects, allowing current to pass
directly into the body.

It requires a power-frequency magnetic field in excess of 5 Gauss (500
microT, see Question 10 for typical exposures) to induce electrical
currents of a magnitude similar to those that occur naturally in the body.
Well-accepted safety standards exist to protect persons from exposure to
power-frequency fields that would induce such currents (Question 27).

9) Do non-ionizing EM sources cause non-thermal as well as thermal effects?

One distinction that is often made in discussions of the biological effects
of non-ionizing EM sources is between "nonthermal" and "thermal" effects.
This refers to the mechanism for the effect, non-thermal effects being a
result of a direct interaction between the field and the organism, and
thermal effects being a result of heating.  Microwave burns are an obvious
thermal effect, and electrical shocks are an obvious nonthermal effect.
There are many reported biological effects (some of which have not been
reproduced) whose mechanisms are totally unknown, and one should be very
careful about drawing the distinction between "thermal" and "nonthermal"
mechanisms for such effects.

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

In the US magnetic fields are commonly measured in Gauss (G) or milliGauss
(mG), where 1,000 mG = 1G.  In the rest of the world, they are measured in
Tesla (T), were 10,000 G equals 1 T (1 G = 100 microT; 1 microT = 10 mG).
Power-frequency fields are measured with a calibrated gauss meter
(Questions 29 & 30).

Within the right-of-way (ROW) of a high-voltage (115-765 kV,
115,000-765,000 volt) transmission line, fields can approach 100 mG (0.1 G,
10 microT).  At the edge of a high-voltage transmission ROW, the field will
be 1-10 mG (0.1-1.0 microT).  Ten meters from a 12 kV (12,000 volt)
distribution line 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) from 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 cleaners, electric clocks, blenders, power
tools).  Appliance fields decrease very rapidly with distance. See
Theriault [F3] for further details.

Occupational exposures in excess of 1000 mG (100 microT) have been reported
(e.g., in arc welders and electrical cable splicers).  In "electrical"
occupations typical mean exposures range from 5 to 40 mG (0.5 to 4 microT).
 See Theriault [F3] for further details.

11) Can power-frequency fields in homes and workplaces be reduced?

There are engineering techniques that can be used to decrease the magnetic
fields produced by power lines, substations, transformers and even
household wiring and appliances.  Once the fields are produced, however,
shielding is very difficult.  Small area can be shielded by the use of Mu
metal, a nickel-iron-copper alloy with "high magnetic permeability and low
hysteresis losses".  Mu metal shields are very expensive, and limited to
small volumes.

Increasing the height of towers, and thus the height of the conductors
above the ground, will reduce the field intensity at the edge of the ROW.
The size, spacing and configuration of conductors can be modified to reduce
magnetic fields, but this approach is limited by electrical safety
considerations.  Placing multiple circuits on the same set of towers can
also lower the field intensity at the edge of the ROW, although it
generally requires higher towers.  Replacing lower voltage lines with
higher voltage ones can also lower the magnetic fields.

Burying transmission lines greatly reduces their magnetic fields.  The
reduction occurs because the underground lines use rubber, plastic or oil
for insulation rather than air.  This allows the conductors to be placed
much closer together and allows greater phase cancellation.  However,
placing high voltage lines underground is very expensive, adding costs that
are measured in hundreds of thousands of US dollars per mile.

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

Some studies have shown that children living near certain types of
powerlines (high current distribution lines and transmission lines) have
higher than average rates of leukemia [C1,C6,C10,C17], brain cancers
[C1,C6] and/or overall cancer [C5,C15].  The correlations are not strong,
and none of the studies have shown dose-response relationships.  When
power-frequency fields are actually measured, the correlation vanishes
[C6,C10,C17].  Several other studies have shown no correlations between
residence near power lines and risks of childhood leukemia
[C3,C5,C7,C8,C9,C14,C15], childhood brain cancer [C5,C8,C14,C15,C17], or
overall childhood cancer [C14,C17].  With one exception [C2] all studies of
correlations between adult cancer and residence near power lines have been
negative [C4,C8,C9,C11,C12,C16].

13) 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".  A RR of 1.0
means no effect, a RR of less the 1.0 means a decreased risk in exposed
groups, and a RR of greater than one means an increased risk in exposed
groups.  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 (see Olsen et al, [C15], Fig. 2 for an example of a
multiple-comparison adjustment).

An overview of the epidemiology requires that studies be combined using a
technique known as "meta-analysis".  Meta-analysis is not easy to do, since
the epidemiological studies of residential exposure use a wide variety of
methods for assessing "exposure".  Meta-analysis also gets out-of-date
rapidly in this field.  The following RRs (called summary RRs in
meta-analysis) for the residential exposure studies are adapted from
Hutchison [B4] and Doll et al [B5] by inclusion of the new European studies
(Question 19).  The confidence intervals should be viewed as measures of
the diversity of the data, rather than as strict tests of the statistical
significance of the data.
   childhood leukemia:     1.5 (0.8-3.0)  8 studies
   childhood brain cancer: 1.9 (0.9-3.0)  6 studies
   childhood lymphoma:     2.5 (0.3-40)   2 studies
   all childhood cancer:   1.5 (0.9-2.5)  5 studies
   adult leukemia:         1.1 (0.8-1.6)  3 studies
   adult brain cancer:     0.7 (0.4-1.3)  1 study
   all adult cancer:       1.1 (0.9-1.3)  3 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 [E6].

14) How close do you have to be to a power line to be considered exposed to
power-frequency magnetic fields?

The epidemiological studies that show a relationship between cancer and
powerlines do not provide any consistent guidance as to what distance or
exposure level is associated with increased cancer incidence.  The studies
have used a wide variety of techniques to measure exposure, and they differ
in the type of lines that are studied.  The US studies have been based
predominantly on neighborhood distribution lines, whereas the European
studies have been based strictly on high-voltage transmission lines and/or
transformers.

Field measurements: Several studies have measured power-frequency fields in
the residences [C6,C7,C10,C12,C17].  Both one-time (spot), peak, and
24-hour average measurement have been made; none of the studies using
measured fields have shown a relationship between exposure and cancer.

Proximity to lines: Several studies have used the distance from the power
line corridor to the residence as a measure of power-frequency fields
[C4,C5,C8,C9,C8,C9,C11,C12,C17].  When something we can measure (distance
to the line), is used as an index of what we really want to measure (the
magnetic field), it is called a surrogate (or proxy) measure.  With two
exception [C5,C17], studies that have used distance from power lines as a
surrogate measure of exposure have shown no significant relationship
between proximity to lines and the incidence of cancer.  The major
exception is a childhood leukemia study [C17] that showed a significant
increase in leukemia incidence for residence within 50 m (150 ft) of
high-voltage transmission lines.  This same study [C12,C17] showed no
elevation of child leukemia risks at 51-100 m (150-300 ft), and no increase
in childhood brain cancer, overall childhood cancer, or any types of adult
cancer at any distance.

Wire Codes: The original US powerline studies used a combination of the
type of wiring (distribution vs transmission, number and thickness of
wires) and the distance from the wiring to the residence as a surrogate
measure of exposure [C1,C2,C3,C6,C7,C10].  This technique is known as
"wirecoding".  Three studies using wirecodes [C1,C6,C10] have shown a
relationship between childhood cancer and "high-current configuration"
wirecodes.  Two of these studies [C6,C10] failed to show a significant
relationship between exposure and cancer when actual measurements were
made.  Wirecodes are stable over time [F5] and correlate with measured
fields, although the correlation is not very good [F1].  The wirecode
scheme was developed for the U.S., and does not appear to be readily
applicable elsewhere.

Calculated Historic Fields: The recent European studies have used utility
records and maps to calculate what fields would have been produced by power
lines in the past [C12,C14,C15,C17].  Typically, the calculated field at
the time of diagnosis or the average field for a number of years prior to
diagnosis are used as a measure of exposure (Question 19).  These
calculated exposures explicitly exclude contributions from other sources
such as distribution lines, household wiring, or appliances.  When the
field calculations are done for contemporary measured fields they correlate
reasonably well [C17].  Of course, there is no way to check the accuracy of
the calculated historic fields.

15) 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 cancer rates.  The original studies [D1,D2 were
only of leukemia.  Some later studies also implicated brain, lymphoma
and/or breast cancer [B1,B2,B3,B4,B5].  Most of the cautions listed for the
residential studies apply here also: many negative studies, weak
correlations, no dose-response relationships.  Additionally, these studies
are mostly based on job titles, not on measured exposures.

Meta-analysis of the occupational studies is even more difficult than the
residential studies.  First, a variety of epidemiological techniques are
used, and studies using different techniques should not really be combined.
 Second, a wide range of definitions of "electrical occupations" are used,
and very few studies actually measured exposure.  The following RRs
(Question 13) for the occupational exposure studies are adapted from
Hutchison [B4] and Davis et al [A2].  Again, the confidence intervals
should be viewed as measures of diversity rather than as tests of the
statistical significance.
leukemia:   1.15 (1.0-1.3)  28 studies
brain:      1.15 (1.0-1.4)  19 studies
lymphoma:   1.20 (0.9-1.5)   6 studies
all cancer: 1.00 (0.9-1.1)   8 studies

The above relative risks do not take into account more recent studies.  Two
recent European studies [D7,D9] have found excess leukemia in electrical
occupations, but no excess of other types of cancer (Question 19 for
details).

Two other new occupational exposure studies [D4,D5] shows small but
statistically significant increases in leukemia, but others [D3,D6,D8] do
not.  None of the new studies of electrical occupations show significant
elevation of any types of cancer other than leukemia (specifically brain
cancer or lymphoma)[D5,D7,D8,D9].  Adding these seven new studies raises
the summary RR for leukemia slightly, and lowers the summary RRs for brain
cancer and lymphomas to essentially one.

End: powerlines-cancer-FAQ/part1