This manuscript was prepared as part of the Environmental
Epidemiology Planning Project of the Health Effects Institute, September
1990 - September 1992.
Introduction
This paper first discusses methods and data that pertain to occupational
exposures. It then reviews the literature on residential exposures and exposure
assessment, describing known sources of residential power-frequency fields
and the methods that have been used to assess residential exposures. This
paper also identifies and discusses current problems in occupational and
residential exposure assessment, with the twin goals of drawing conclusions
where possible and developing working hypotheses for future study. Finally,
this paper proposes areas where future research might prove of value.
Methods for Occupational Exposure Assessment
Job Titles
All occupational epidemiology studies to date have assessed exposure
by using job titles or categories of job titles. Wertheimer and Leeper (1)
mentioned in the very first epidemiology paper concerned with magnetic fields
that they had examined published data on occupations and cause of death
and had found an elevation in the cancer rate of electrical workers relative
to the general population. This category of workers included job titles
such as power station operators, linemen and servicemen, electricians, and
welders.
The first major study was reported by Milham (2), who stratified
deaths by occupation in the state of Washington for the period 1950 through
1979 and found that electrical workers tended to have higher than expected
mortality from leukemia. His classification of electrical workers was similar
to that used by Wertheimer and Leeper.
Perhaps because occupational studies like the two described above require
little field work and are, therefore, relatively inexpensive to perform,
a substantial number have been reported in the literature. Several reviews
of these studies have been published (3-6). Many of these
studies found elevated rates of certain cancers among individuals holding
electrical-worker job titles.
Because none of these studies reported exposure measurements, the connection
between electrical-worker job titles and elevated exposures to electric
and/or magnetic fields, while plausible, was unproven. Two occupational
exposure studies have been performed that deal with this question (7,8).
Occupational Exposure Measurements
Deadman et al. (7) had 20 workers, with six electric utility jobs
that were deemed to involve elevated exposure to power-frequency electric
and magnetic fields, wear personal-exposure meters for periods of 1 week.
This group consisted of 10 distribution linemen, three transmission substation
electricians, two transmission linemen, two cable splicers, two apparatus
mechanics, and one power plant worker. In addition, the authors had 16 electric-utility
office workers from two different buildings wear meters for 1-week periods.
The resulting data were divided into work, nonwork, and sleep periods. (During
sleeping, the meter was not worn but was placed near the bed.) Time-weighted-average
(TWA) exposures were calculated for each subject for these three periods.
The 20 electric utility workers studied by Deadman et al. would, in all
likelihood, be included in anyone's definition of electrical workers. Consistent
with this assignment, Deadman et al. found that these workers were exposed
more highly while at work (Table 1). However, the nonwork and sleep exposures
of the utility and office workers were the same (Table 1).
The results of Deadman et al. suggest that job titles might be a good
surrogate for electric and magnetic field exposures. However, these data
cover only a few highly exposed job titles within the much larger cohort
of electrical workers and, therefore, do not provide a very strong test
of this hypothesis.
Bowman et al. (8) measured spot electric and magnetic fields at
105 electric utility, aerospace, municipal government, motion picture theater,
and television repair work sites. Their survey included at least one worker
from every job title in Milham's 1982 electrical-worker category except
for aluminum workers and conductors and motormen on urban rail systems.
To provide a basis for comparison, electric and magnetic fields also were
measured at 181 sites in 18 residences. The electric- and magnetic-field
data are summarized in Tables 2 and 3, respectively.
The geometric mean electric and magnetic fields measured in the job sites
of electrical workers were 5 V/m (Table 2) and 0.5 µT (5 mG) (Table
3), respectively. The comparable numbers for the residential measurements
were 2.5 V/m and 0.06 µT (0.6 mG). The difference in electric-field
exposures was due entirely to utility jobs that involved work around high
voltages (overhead line and transmission and distribution substation workers).
Apparently, the job-title class electrical worker is not an effective surrogate
for electric-field exposure.
The difference between the occupational and residential magnetic fields
in Table 3 was reliable statistically. All of the electrical workers had
higher measured fields at their work sites except for electrical engineers
and technicians working in offices and calibration laboratories, radio dispatchers,
and microelectronic assemblers. Apparently, electrical workers, as a group,
are exposed somewhat consistently to elevated magnetic fields.
There are several large projects currently examining exposures that occur
in the telephone and electric utility industries. Consequently, it should
be possible in a few years to discuss much more intensively occupational
exposures to power-frequency electric and magnetic fields.
Sources of Residential Fields
The Electric Power Research Institute (EPRI) is executing a program to
identify and characterize residential and nonresidential sources of power-frequency
magnetic fields. This program started with a pilot study (9), and
it is continuing with the characterization of the fields in 1000 residences
selected randomly from a clustered sample of EPRI-member utilities. The
pilot study identified the following five classes of residential fields
sources: electric power transmission lines, electric power distribution
lines, ground currents, home wiring, and household appliances. These are
discussed in the next five sections.
Electric Power Transmission Lines
Electric power transmission lines operate at very high voltages (usually
>=50,000 volts, abbreviated 50 kilovolts or 50 kV) and may carry currents
of many hundreds of amperes. Thus, these lines can produce relatively strong
electric and magnetic fields. The exterior walls and roofs of most homes
are fairly effective shields for electric fields (10), but they have
little, if any, effect on the magnetic fields produced by power lines.
The magnetic field produced by a three-phase transmission line outside
its right-of-way, where most human exposure occurs, usually can be calculated
satisfactorly using the following formula (11):
where B is the field's resultant flux density in µT, I is
the current in amperes carried by each of the three phase conductors (these
currents almost are equal for transmission lines), R is the distance
in meters from the line to the point where the field is being calculated,
and Sij is the transverse distance in meters between the ith
and jth conductors. This formula is valid when R is substantially
larger than any of the Sij.
The most common transmission line configuration has all three conductors
arrayed in either a horizontal or a vertical plane. Equation 1 then simplifies
to
[2]
where s is the distance between adjacent conductors.
Figure 1 shows the fields produced 1 m above ground level by typical
115 kV (lower voltage) and 345 kV (higher voltage) transmission lines carrying
currents of 300 A. Magnetic flux densities are shown for various horizontal
distances from the lines. Note that fields >=0.1 µT (>=1 mG)
are produced up to about 70 m and 100 m from the 115 kV and 345 kV lines,
respectively.
Figure 1. Magnetic
fields produced by typical 115-kV and 345-kV transmission lines carrying
300 A of current in each phase conductor.
Electric Power Distribution Lines
Electric power generally is carried by electric power transmission lines
to receiving substations located within a few kilometers of the ultimate
consumers. At these substations, the voltage is reduced from transmission
to distribution levels (4-34 kV), and the power is distributed on primary
distribution lines to the immediate vicinity of the consumers. At this point,
distribution transformers further reduce the voltage to the level of ultimate
consumption (110-220 V for residential customers, 110-480 V for most commercial
customers). Power is carried from distribution transformers on secondary
distribution lines. Service drops to each customer are connected normally
to the secondary distribution lines. Some may originate directly from the
distribution transformer. While most primary and secondary distribution
in the United States is by overhead lines, it is common for new installations
to be underground.
Primary distribution lines can be either three-phase, two-phase, or single-phase.
The first two of these categories are subdivided further into those lines
with and without associated neutral conductors. Neutral conductors are operated
at zero voltage (but not zero current) by connecting them to the earth (usually
at many points) using ground rods or equivalent structures.
Because of their lower voltages, the conductors of distribution lines
are placed much closer together than the conductors of transmission lines.
Also, it is usual for distribution currents to be considerably less than
transmission-line currents. Consequently, Equation 1 predicts that distribution
lines will not, in most cases, produce magnetic fields much above ambient
levels in areas that normally would be occupied by people. However, in practice,
this is not always true because of the existence of net currents on some
distribution lines.
The net current, Inet, being carried by a power line
is defined to be the algebraic sum of all the conductor currents. (This
sum must be calculated taking into account both the magnitudes and phases
of the individual currents.) In principle, Inet, = 0 for
transmission and distribution lines. While this is nearly true for most
transmission lines, it can be far from correct for primary and secondary
distribution lines because of ground currents.
Net current is important because the magnetic field, Bnet,
produced by it depends on distance, R, from the line as 1/R
and is, therefore, spatially more persistent than the magnetic fields produced
by normal power-line currents. This is illustrated in Figure 2, which shows
the fields produced by a distribution line carrying a load current 100 A
and a net current of either 0 or 30 A.
Figure 2. Magnetic
field produced by typical primary distribution line carrying 100 A of load
current in each phase conductor and net currents of either 0 or 30 A.
Ground Currents
Ground and net currents produce spatially persistent fields. A point
where current frequently enters the ground is at the service entrance of
a residence because safety codes require that the neutral conductor be grounded
at this point. This ground may be to a rod driven into the earth, but it
is often to a metal water pipe. Often, the electrical service entrance is
at the rear of a home, and the water main is in front of the home, so ground
current in the water system must pass under the home. This current is not
compensated by any return current in the vicinity, so its magnetic field
is proportional to 1/R. Individuals in a home may be exposed to magnetic
fields from this source.
Wiring in a Home
Home wiring is not usually a significant source of magnetic-field exposure
because the two wires connecting to a household load (e.g., a light or appliance)
are located very close together and carry equal and opposite currents. However,
there are unusual wiring configurations where this is not true. Of those
known to the author, the most common are some three-way switch installations
and homes having two or more separated circuit breaker panels.
Three-way switches are used where it is desired to control a load from
multiple points. The most common application is probably lights that can
be turned on or off from either end of a hall or stairway. Figure 3 shows
two alternative ways that an installation could be made to control a light
from two different switches. In the upper diagram, the various wires are
routed in multiwire cables so that the net current in any of the cables
is zero. Consequently, the magnetic fields from the conductors in any cable
largely cancel, with the result that this installation would not be a significant
source of residential magnetic fields. A different installation--one requiring
less total wire--is shown in the lower panel of Figure 3. Here, a separate
wire is routed from each switch directly to the light, and the direct connections
between switches are made with a two-wire cable. However, this cable, and
the wires connecting to the light, will carry a net current--the entire
current required to energize the light. If these two elements are separated
significantly, the magnetic field in their vicinity could be significant.
(The author has studied a home where turning on a hall light raised the
field from about 0.01 µT to 0.5 µT.)
Figure 3. Alternative
methods to control an electric light with two different (3-way) switches.
The bottom installation could be a significant residential source of magnetic
fields.
As mentioned earlier, U.S. building codes require that the neutral bus
in the main circuit-breaker (or fuse) panel protecting a home's electrical
system be grounded. Some homes have multiple panels, usually because an
addition to the home required more electric power than could be supplied
by the original panel. Many electricians automatically will ground the neutral
bus in these subpanels, creating two routes for current flow between the
main and grounded subpanel, one through the neutral conductor connecting
the two panels, the other through the ground. In this way, local net currents
can be formed with the production of spatially more persistent fields, as
explained above.
Home Appliances
The magnetic fields produced by many home appliances can be quite strong
in their immediate vicinity, but these fields also are localized in space.
Figure 4 shows magnetic-field data from Gauger (12) for five electric
ranges (left graph) and three hand-held electric hair dryers. Note that
the fields produced by these appliances were all less than 0.1 µT
(1 mG) at distances from them exceeding 1 m. This is a characteristic of
the fields from most household appliances (12) because of their small
size, and because the magnetic fields produced by localized current sources
decay as 1/R3 when R is large (13).
Figure 4. Magnetic
fields produced by five electric ranges and three hand-held electric hair
dryers. Fields are shown as function of distances from surfaces of appliances.
At this time, the relative importance of home appliances as sources of
human exposure to magnetic fields is controversial. Some maintain that home
appliances are important, if not the dominant, sources of exposure of humans
to residential magnetic fields, while others argue that most appliance sources
are unimportant. Although this controversy continues, there is general agreement
that a few home appliances do contribute significantly to exposure. For
example, most electric blankets clearly lead to significant whole--or near
whole--body exposure because the distance between a user and an electric
blanket is small relative to the blanket's dimensions (so the 1/R3
law does not apply) and because blankets are used by many for the entire
nighttime period. (Recently, manufacturers have developed new blanket designs
that greatly reduce their magnetic fields.)
Methods of Residential Exposure Assessment
Most research related to the assessment of residential exposures to power-frequency
electric and magnetic fields has occurred in conjunction with on-going epidemiologic
studies. Table 4 is a list of epidemiologic publications from these studies
that present exposure-assessment data and techniques. Table 5 provides a
list of publications whose primary purpose is to report results related
to exposure assessment.
All residential assessments of exposure to power-frequency fields have
used one or more of the following techniques: questionnaire, wiring configuration
coding, theoretical estimation of fields produced by nearby electrical facilities,
spot electric- and magnetic-field measurements, electric- and magnetic-field
recordings at fixed locations covering periods of time from hours to days,
personal-exposure measurements, and geomagnetic-field measurements. The
exposure-assessment methods used by published residential studies are enumerated
in Tables 4 and 5.
Questionnaires
Questionnaires have been used in residential studies to assess exposure
to the power-frequency magnetic fields produced by electric blankets and
other home appliances. Typically, a case or control subject (or a relative
or care giver) would be questioned about their (or the subject's) pattern
of use of these sources.
Wiring Configuration Coding
The first method developed for exposure assessment was the wiring configuration
coding system of Wertheimer and Leeper (1,14). Originally
criticized by many, this method has stood the test of time. Research has
shown that wiring code is correlated with measured magnetic fields (but
not electric fields) in residences (15,16). The code now normally
in use was defined originally in Wertheimer and Leeper's 1982 paper (14).
The types of overhead electrical wiring that enter into the code are transmission
lines, three-phase primary distribution lines, and secondary distribution
lines.
Primary distribution lines are divided into thick and thin lines according
to whether or not their phase conductors are clearly larger in diameter
than the standard secondary wire used in the Denver, Colorado, area (14).
An alternative and more quantitative definition of thick and thin has been
developed in terms of the ampacities (i.e., current-carrying capacities)
of conductors used for primary distribution (15). This technique
is appropriate when the wire materials and gauges can be determined. Visual
discrimination of thick and thin conductors is the most subjective element
in wire coding.
Sections of secondary distribution lines are further categorized as being
first-spans or second-spans. A first-span secondary is that length of an
overhead secondary distribution line extending from the pole on which the
line's distribution transformer is located to an adjacent pole, which also
is carrying electric power to more than two residential loads or one or
more commercial loads. Secondaries not meeting this condition are called
second-span secondaries. (Sometimes, the term short first-span secondary
is used for a first-span not supplying sufficient load to be classed a first-span
secondary.)
Wire coding consists of identifying transmission and distribution lines
and measuring the distance of closest approach of each to the home being
coded. Table 6, then, can be used to code each structure, and the final
code for the home is taken as the highest of the codes for each of the lines.
There are four possible codes: very high current configuration (VHCC), ordinary
high current configuration (OHCC), ordinary low current configuration (OLCC),
and very low current configuration (VLCC).
The process of wire coding is illustrated in Figure 5, which shows a
schematic-plan view of a residence and the electrical wiring surrounding
it. A transmission line passes within 145 ft (44.2 m) of the home. According
to Table 6, this structure would be coded VLCC. The thin three-phase primary
line passing 80 ft (24.4 m) north of the home is coded OLCC. The single-phase
primary passing 40 ft south of the home is not coded at all in the Wertheimer-Leeper
system (only three-phase primaries are coded). The pole southeast of the
home has a transformer mounted on it that supplies a secondary line that
passes by the home. The segment of this secondary passing by the house carries
the power for three service drops. Consequently, this segment is a first-span
secondary and is coded OHCC. Because the highest structure code is OHCC,
the home is coded OHCC.
In practice, wire coding can sometimes be difficult. The most difficult
and time-cosuming part of wire coding is the identification of first-span
secondaries. The wires in a secondary often are bundled together, making
it difficult to see details from the ground, and the coder's view of these
wires often is obscured by trees. It also is sometimes difficult to determine
where one secondary ends and another starts. Nevertheless, with all these
difficulties, it is possible to train technicians to code reliably electrical
wiring using the Wertheimer-Leeper method. For example, Savitz et al. (17)
obtained 95% agreement between independent codings of homes made by trained
technicians.
Houses served by underground primary wiring were placed in the VLCC category
by Wertheimer and Leeper (14). Other researchers have chosen to treat
houses with underground wiring as a fifth category (17).
Because magnetic fields are produced by electric currents, the overt
purpose of wire coding is to discriminate between wiring configurations
that carry, on the average, different levels of current. As described earlier,
transmission lines are significant sources of magnetic fields, so their
treatment in the Wertheimer-Leeper wiring code seems reasonable. This conclusion
is not certain for primary and secondary distribution lines because, as
noted earlier, net currents on these lines often are the primary sources
of their magnetic fields, and net currents depend on the type of distribution
line (whether or not it has a neutral) and local grounding practices. It
may be that there is a statistical association between the total and net
currents carried by distribution lines, which could explain the apparent
ability of the Wertheimer-Leeper code to capture magnetic field levels produced
by distribution lines.
Theoretical Estimation
The strength of the electric and magnetic fields produced by electrical
facilities, such as power lines, transformers, and substations, depends
in a known way on the system voltage, current, and geometry. Thus, assuming
these parameters are known, one can calculate the electric and magnetic
field produced at any distance from a source. Several studies have used
this approach to assess magnetic-field exposure in residences located close
to power lines.
Fulton et al. (18) used a combined theoretical and empirical method
for their exposure assessment. They determined the closest distance, R,
of approach of every power line passing within 45.7 m (150 ft) of a house
under study. They placed the wires of each line into one of the following
four classes: high tension (i.e., belonging to a transmission line), large-gauge
(thick) primary, small-gauge (thin) primary, and secondary. They assigned
to these classes nominal field values based on data published in Wertheimer
and Leeper's original 1979 paper. They then weighted these nominal values
by the quantities 1/R2 to allow for different distances
between sources and the home under study, and they summed the weighted contributions
from all sources.
Tomenius (19) simply noted in his study whether there was a visible
electrical facility (6-200 kV high-voltage wires, substations, transformers,
electric railroads, and subways) within 150 m of each home. (The actual
epidemiological analysis performed by Tomenius defined exposure solely in
terms of proximity to electric power transmission lines.)
Myers et al. (20) measured the distances between homes occupied
by subjects of their study and all power lines (secondaries, primaries,
and transmission lines) located in their immediate vicinities. In conjunction
with the utilities operating these power lines, the authors estimated the
load currents in each line, assumed these currents were balanced (i.e.,
equal currents in all phase conductors of a line), and calculated the resulting
magnetic fields 1 m above ground at the center of each dwelling.
Spot Electric and Magnetic Field Measurements
A spot measurement is defined to be a measurement at a fixed location
(usually inside a residence) that occurs over a period of time less than
a few minutes. Survey meters customarily have been used for these measurements,
but some studies now in progress are using personal exposure meters for
this purpose. Savitz et al. (17) and Severson et al. (21)
used identical plastic fixtures to position Model 113 survey meters (Electric
Field Measurements Company, Lenox, MA) in three orthogonal directions to
measure the three vector components of electric field and magnetic flux
density.
Savitz et al. (17) and Severson et al. (21) jointly introduced
the notion of low- and high-power spot measurements. Low-power spot measurements
were made after electric power consumption in a home was reduced (by turning
off lights and appliances) to as low a level as practical. These measurements
were interpreted as being most reflective of magnetic fields produced by
sources outside the residence under study. Similarly, high-power measurements
were made after as many lights and appliances as possible were energized.
These latter measurements were thought to contain maximal contributions
from field sources inside the home.
Fixed-Site Magnetic-Field Recordings
Kaune et al. (15) made the first published fixed-site recordings
in 43 Seattle residences using a data acquisition system constructed for
this purpose. Three magnetic field sensors and one electric field sensor
were located at fixed positions in each home, and data from these sensors
were recorded on magnetic tape at 2-min intervals for a 24-hr period.
With the advent of small, battery-powered data acquisition systems, such
as the EMDEX family of meters, longer term measurements have become much
less intrusive and, thus, more practical. Tables 4 and 5 list studies that
have reported fixed-site recordings.
Personal Exposure Measurements
Although EMDEX and IREQ/Positron personal exposure meters have been available
for several years, only two studies that the author is aware of have published
residential personal exposure data (Table 5). In both of these, subjects
were asked to wear personal-exposure meters for periods from 24 hr (22)
to 7 days (7). There are currently several studies underway that
are collecting large amounts of personal exposure data.
Geomagnetic Field Measurements
Blackman et al. (23) published a paper reporting that a biological
response elicited in the laboratory by exposure to extremely low frequency
electric and magnetic fields was apparently also affected by the strength
of the static geomagnetic field (i.e., earth's magnetic field). These authors
found that the response occurred only when the frequency of the alternating
exposure fields lay in certain bands, and they showed that the frequencies
of these bands were dependent on the geomagnetic field (i.e., the static-magnetic
field, usually due largely to the earth's magnetic field). Because this
model has had considerable success in describing a variety of laboratory
results [summarized by Liboff et al. (24)], some have decided to
incorporate geomagnetic field measurements as part of their residential
exposure assessment protocol (25).
Current Issues in Exposure Assessment
This section discusses issues of current interest concerning the assessment
of human exposure to power-frequency electric and magnetic fields.
Figure 5. Schematic
plan view of residence and electric power transmission and distribution
wiring in its vicinity. Distances are not to scale.
Occupational Exposure Assessment
As related in the section "Job Titles," all published occupational
studies have used job titles as surrogates for electric- and/or magnetic-field
exposure. Separate research indicates that the exposures of electrical workers
are, in fact, elevated relative to those received in most other occupations
and at home (7,8). However, job titles, by themselves, must
be regarded as a crude measure of exposure. There are certainly exposure
differences within the general category of electrical workers or even within
workers holding the same job title, differences that could perhaps be exploited
to help detect the presence of confounders or dose-response effects.
What is needed in future occupational studies is a job-exposure matrix.
In its simplest form, the rows of this matrix would be labeled by job titles,
and a single column would contain exposure estimates for each job title.
A more complex matrix could contain several columns, each for a different
definition of exposure (i.e., a different exposure metric) or different
latency periods for disease onset after exposure. A complete job-exposure
matrix also should contain information about exposures unrelated to electric
and magnetic fields. In particular, because many jobs that fall within the
electrical workers category also involve the use of chemicals and possible
exposures to fumes, it is important that data on these factors be included
in the job-exposure matrix.
The construction of a complete job-exposure matrix is a daunting task.
Electric- and magnetic-field measurements of current exposure in the job
titles under study will be necessary in many, if not most, studies. In case-control
studies, the exposure of interest occurred in the past. Consequently, historical
changes in exposure patterns will have to be assessed during the construction
of the job-exposure matrix. Such historical changes may be more pronounced
for chemicals than for electric and magnetic fields.
Between-Study Variation of Spot and Fixed-Site Measurements
Magnetic field data from spot measurements and fixed-site recordings
are summarized in Table 7 for seven studies. The low-power and high-power
data (all from spot measurements) are from the Denver, Seattle, and Los
Angeles metropolitan areas in the United States and seem reasonably consistent.
However, the normal power data, consisting of spot measurements (8,26)
and fixed-site recordings (7,15,25) show considerable differences
between studies. For example, the geometric means measured by Deadman et
al. (7) are about three times larger than those measured by Kaune
et al. (15) and Bowman et al. (8). It is unknown if this difference
is attributable to geographical differences, measurement protocol differences,
or instrumentation differences. The data from Dlugosz et al. (26)
are much larger than the other normal power data. However, these measurements
were taken on the sidewalks outside homes and may reflect more strongly
sources under and above city streets.
Wiring Codes and Measured Electric and Magnetic Fields
The Wertheimer-Leeper wiring code was developed to provide a surrogate
measure of long-term historical exposure to power-frequency magnetic fields
that could be obtained without entry into residences (1,14).
Three studies have now been performed that report a statistically elevated
risk of cancer for children living in high-current-configuration homes.
Two of these studies (1,17) were performed in the Denver,
Colorado, area, with different groups of children. The third study was performed
in Los Angeles County (25). These findings have stimulated a strong
interest in wire codes and in various physical factors that might be associated
with wire codes.
Several published studies have found that wiring configuration is associated
statistically with magnetic fields measured in homes. Wertheimer and Leeper,
in both of their original studies (1,14), present magnetic-field
data, measured outside homes, that show an association between wire code
and magnetic-field levels. Kaune et al. (15) recorded electric- and
magnetic-field data for 24-hr periods in the bedrooms and family rooms of
43 homes in Seattle, Washington. These authors observed no relationship
between wire code and measured residential electric-field levels. However,
there was an association between wiring code and residential magnetic fields
(Fig. 6): Log-transformed averages of 24-hr mean magnetic fields were significantly
different for different codes, with the largest differences being between
the VLCC and OLCC taken as one group and the OHCC and VHCC as the other
group. However, this model left unexplained 79% of the total variation between
homes.
Figure 6. Scatter
plot of 24-hr-average house magnetic flux densities and Wertheimer-Leeper
wiring code. Data from Kaune et al. (15).
Barnes et al. (16) analyzed magnetic-field spot-measurement and
wiring-configuration data from the Savitz et al. (17) study and reached
a similar conclusion. These authors state:
The proportion of variance in fields explained by the wire codes, however,
is a rather modest 19%. In combination, these findings indicate that the
relationship between fields and wire codes is well beyond chance but that
the correlation is far from perfect.
In addition, London et al. (25) have reported recently that a
relationship between the Wertheimer-Leeper wiring code, spot, and 24-hr
magnetic-field measurements has been observed in Los Angeles County. This
is interesting because utility distribution practices are different in many
areas of Los Angeles County from those in the Seattle or Denver areas. In
particular, the grounding system for a distribution line in Seattle and
Denver is integrated along its entire length and typically might include
1000 to 2000 homes, whereas in Los Angeles, the grounding system for a secondary
distribution line (typically serving 1-10 homes) may be electrically isolated.
Thus, the Seattle, Denver, and Los Angeles results suggest that ground currents
may not be an important source of residential magnetic fields or, at least,
of that component of a residential field captured by the Wertheimer-Leeper
code.
There is evidence that wiring codes only are associated weakly with spot
measurements and fixed-site recordings of residential magnetic fields. It
is tempting to conclude that, for epidemiological purposes, wiring code
is a poor measure of magnetic-field exposure. While this conjecture ultimately
may be proven true, its validity is not certain at this time. For epidemiological
purposes, exposure generally is placed in categories (e.g., low and high)
and the definitive test of wiring configuration (or any other surrogate
measure of exposure) is its ability to place individual exposures in the
correct category. It is important to realize that the ability of a measure
to explain variability between homes is not the same as its ability to place
homes correctly in exposure categories.
Relative Effectiveness of Wire Codes and Spot Magnetic Fields
Several authors have discussed the possibility that wire codes are better
predictors of long-term historical exposure to magnetic fields than are
spot or 24-hr measurements of the present magnetic fields in a residence
(4,1,14). This notion is discussed in this section.
Wire codes seldom change over periods of months or years because utilities
seldom change their transmission and distribution systems. In fact, the
historical stability of wire codes is the reason most often advanced to
explain their hypothetical superiority in assessing historical magnetic-field
exposure. However, it seems that this property of wire codes will only be
of virtue if long-term magnetic-field exposure is, itself, historically
stable.
Assuming that long-term exposure is historically stable, we still need
to explain why spot (or 24-hr) magnetic-field measurements are not historically
stable indicators of human exposure to residential magnetic fields. There
seem to be three possibilities: a) Spot measurements exhibit such
large short-term variability that they are very poor indicators of mean
magnetic-field levels, whether in the present or the past. b) The
spatial variability of residential magnetic fields is so large that spot
or 24-hr measurements, even if temporally stable, could not be used to assess
present or past human exposure. c) Spot or 24-hr rmeasurements exhibit
much greater long-term variation than does personal magnetic-field exposure.
These three possibilities are discussed in the following paragraphs.
Short-term Temporal Variability of Spot Measurements. Figure
7 shows magnetic field records, covering approximately 24 hr, taken in the
bedrooms of two homes. These records, which consist of a large number of
spot measurements taken one after another, clearly show short-term, apparently
almost random, variation. Although the variability shown in this figure
seems large, several groups have found that spot measurements taken at different
times are strongly correlated.
Figure 7. Magnetic
fields measured in bedrooms of two homes during 24-hr periods.
Dlugosz et al. (26) made spot magnetic-field measurements during
seven successive evenings on 33 street corners in Buffalo, New York. The
intraclass correlation between these seven measurements was 0.94, indicating
a high degree of stability during the week. It should be noted that these
data, alone, are far from conclusive because they were measurements taken
outside homes where fields may be more stable.
Kaune et al. (27) recently obtained 24-hr EMDEX records in the
bedrooms of young children living in 29 homes. These records were regarded
as a series of spot measurements made every 15 min over 24-hr periods, and
the correlations between two spot measurements separated in time by varying
intervals were computed. Figure 8 shows the results of this analysis. Clearly,
there is considerable stability between spot measurements made at two times
separated by as little as 15 min or as much as 24 hr.
Delpizzo et al. (28) tested the ability of spot magnetic-field
measurements to correctly classify exposure. In this case, exposure was
defined in terms of the 24-hr average magnetic field measured in 40 homes.
Exposure was termed either high or low, depending on whether it was greater
than or less than 0.075 µT (0.75 mG). The authors then classified
exposure using a single spot measurement and found that this technique had
at least an 80% chance of classifying homes correctly. Furthermore, this
probability was not significantly increased as the number of spot measurements
used to estimate exposure was increased above one.
Figure 8. Correlation
between spot measurements made at two different times in bedrooms of 29
homes.
Thus, available data suggest that spot measurements may be rather stable
over periods up to one week in length.
Correlation of Spot and Personal Exposure Measurements. The
second possibility enumerated above to explain why wiring coding might be
a better estimator of historical long-term exposure than a spot or 24-hr
measurement is that the spatial variability of the magnetic fields in a
residence might be so great as to render a spot value ineffective as a measure
of residential human exposure. However, available data suggest that this
might not be the case.
Kaune et al. (27) found that a time-weighted average of a bedroom
spot (or 24-hr) measurement, a kitchen spot measurement, and a family-room
spot measurement were well correlated with the measured personal exposures
(measured using AMEX-3D meters) of 29 young children. (The correlation coefficient
between the log-transformed measured and predicted exposures was 0.8.) Wiring
code, on the other hand, was associated weakly with the measured exposures.
Kavet et al. (22) made the following measurements in 45 homes:
spot measurements in at least three rooms of each home, 24-hr fixed-site
bedroom measurements, and 24-hr personal exposure obtained by asking an
adult resident to wear an EMDEX meter. Thirty of the 45 subjects lived close
to transmission lines, so their data may not be representative. Limiting
analysis to those 15 who lived away from transmission lines, the correlation
between the measured at-home log-transformed exposures and the log-transformed
averages of the spot measurement taken in each home was 0.77. (The comparable
correlation for the entire sample of 45 homes was 0.76.)
Two exposure assessment studies, both with only small numbers of subjects,
do not provide a substantial basis on which to make any firm conclusions.
Nevertheless, if the trend continues--spot measurements predict contemporaneous
exposures better than wiring code--it will become progressively more difficult
to argue that wiring code is a better predictor of long-term magnetic-field
exposure than spot measurements.
Long-term Variation of Spot Measurements. The third possibility
introduced above to explain why wire codes might work better than spot measurements
to assess long-term historical exposure of people to residential magnetic
fields is that spot measurements might exhibit more long-term variability
than does exposure. This issue has been examined experimentally for the
first time by a recent study (29) in which a new set of measurements
were made during 1990 in 80 Denver-area homes that were part of the original
Savitz study (17). This study found correlations of 0.71 and 0.75
respectively, between their log-transformed low- and high-power spot measurements
and those made by Savitz et al. in 1985. This level of correlation was present
in both high-current configuration and low-current configuration strata.
Linear regression analysis showed that the slopes of the relationships between
the 1985 and 1990 low-power and high-power spot measurements were near 1.0.
Apparently, spot measurements in Denver are remarkably stable over a 5-year
period.
Let us now return to the original question: Are wiring codes or spot
measurements a better method of assessing long-term historical exposure
to power-frequency magnetic fields? First, evidence from three studies suggest
that short-term variability in spot measurements is not large enough to
render them ineffective estimators of TWA exposure. Second, evidence from
two studies suggest that spot measurements are as, or more, effective than
wire codes in assessing concurrent TWA exposure. Third, one study found
that spot measurements made in 80 Denver homes about five years apart are
correlated well. These results, while far from conclusive, seem to offer
evidence suggesting that spot measurements may be at least equivalent to,
if not superior to, wire codes as measures of historical TWA exposure to
residential magnetic fields.
At-Home and Away-From-Home Exposures
Two new studies have measured separately the residential and nonresidential
components of the total exposure of children and adults to power-frequency
magnetic fields. Kaune et al. (27) had 29 young children (ages 4
months through 8 years) wear AMEX-3D meters for 24-hr periods. Each child
was given two meters, one to be worn while at home, the other while away
from home. The cumulative exposure measured by each meter was divided by
the total time it was worn, yielding the TWA magnetic field to which it
was exposed. Table 8 presents geometric means and standard deviations summarizing
these two components of total exposure. Note that the geometric mean exposures
at home and away from home were both about 0.1 µT (1 mG), but that
the geometric standard deviations for these two exposures were very different,
with the at-home component being much more variable than the away-from-home
component. That is, most of the differences in exposure between subjects
occur during their time at home rather than when away from home.
Kavet et al. (22) measured residential and nonresidential exposures
for 45 adults. Geometric statistics summarizing the at-home and away-from-home
exposure fields for the 15 subjects who did not live close to transmission
lines also are given in Table 8. The same pattern is observed in these adult
data: The at-home component of exposure is more variable than the away-from-home
component.
The results discussed in the preceding two paragraphs were quite surprising.
They suggest the possibility that total time-weighted exposures of children
and adults can be categorized accurately by studying only their residences.
If valid, this would be a very important result; but at this time, it should
be regarded only as a working hypothesis. Considerable additional research
is needed to test this result among other populations.
Contribution of Home Appliances to Residential Exposures
Questions about the importance of home appliances to residential exposures
have been raised for years. It is well known that appliances such as hair
dryers, curling irons, and electric razors can deliver substantial short-term
partial-body exposures to their users. However, it is not clear that TWA
exposure is affected substantially by these sources. Delpizzo (30)
has performed a theoretical analysis of exposure to electric blankets, waterbed
heaters, and concrete slab heaters, and concludes that these sources can
make significant contributions to total exposure.
One way to examine this question is to compare magnetic fields measured
with a personal-exposure meter (which presumably captures appliance contributions)
to spot measurements (which are generally made to exclude appliance contributions).
This comparison can be performed using the data (Table 9) of Kavet et al.
(22). A t-test on log-transformed data confirms that the at-home
exposure values are significantly larger (p = 0.0004) than the spot
fields, suggesting the presence of significant appliance contributions to
personal exposure. However, many additional data are needed to confirm this
observation.
Exposure Metrics
An exposure metric is a function of an applied electric or magnetic field
that yields a value useful for predicting or describing a biological response
of interest. The simplest and most widely used metric is TWA exposure, in
this case, the average electric or magnetic field during the period of exposure.
But there are other possibilities. For example, exposure assessment for
radio-frequency electromagnetic fields commonly uses the square of the electric-
or, sometimes, of the magnetic-field strengths.
Past epidemiological studies, as well as most laboratory studies of electric
and magnetic fields, have used TWA-field strength as their explicit or implicit
measure of exposure. The validity of this approach is currently being questioned
because of several recent developments: a) Some biological responses
observed in the laboratory exhibit a complex dependence on intensity and
frequency of the exposure field (e.g., intensity and frequency windows)
as well as on the strength of the static magnetic field. b) Some
biological systems may be sensitive to a power-frequency magnetic field
only when its strength is abruptly changed (31). c) The use
of TWA magnetic-field exposure to explain the relationship between wire
code and childhood leukemia has not proven fruitful (25).
Because of the considerations listed in the preceding paragraph, some
effort has been devoted to identifying characteristics of residential or
occupational magnetic fields, other than TWA exposure, that might serve
as alternative exposure metrics. Presumably it would be desirable to identify
metrics that are not correlated strongly with TWA exposure, but this might
be a difficult goal to achieve. Armstrong et al. (32) calculated
correlations between a wide variety of electric- and magnetic-field exposure
indices and found, for nonwork exposures, that many were well correlated
with TWA exposure (Table 10).
One alternative that was not considered by Armstrong et al. (32),
and is discussed frequently, is exposure to temporally fluctuating magnetic
fields. This concept is illustrated in Figure 9, which shows actual 24-hr
magnetic-field recordings taken in two homes (27). In both cases
the TWA fields were about 0.36 µT (3.6 mG), but the short-term variability
of the field in the upper chart was clearly much greater than that in the
lower. It would not be difficult to invent a metric function to discriminate
between these two exposures.
Figure 9. Twenty-four-hour
records of magnetic fields in two homes. Time-weighted-average exposure
was 0.36 µT for both homes.
Research Recommendations
This section identifies research areas where progress can be made to
improve and clarify exposures and exposure-assessment methods related to
power-frequency electric and magnetic fields.
Development of Job-Exposure Matrices for Electrical
Workers
Because a number of studies have found elevations in the rate of mortality
from various cancers in electrical workers, it would be appropriate to develop
detailed job-exposure matrices for both electric-field, magnetic-field,
and chemical exposures received by members of this group. With such a matrix,
electrical-worker job titles that were exposed to fields could be separated
from those that were not, and confounding exposures could be evaluated.
Prediction of Historical Exposure
The ability of wiring codes and spot measurements to predict long-term
historical exposure needs to be thoroughly evaluated. In addition, techniques
need to be investigated that possibly could utilize available historical
information, such as residential billing records and utility loading data,
to sharpen historical residential-exposure estimates.
Alternate Exposures Associated with Wiring Codes
Because of the possibility that the Wertheimer-Leeper Code is detecting
some underlying factor that is unrelated to magnetic fields, an intensive
and multidisciplinary search for environmental correlates of wire codes
is needed.
Between-Studies Variability in Spot and Fixed-Site Magnetic Field
Measurements
Spot measurements and fixed-site recordings of residential magnetic fields
show considerable differences between studies, particularly for normal-power
measurements (Table 7). Research is needed to determine if these differences
are due to geographical, measurement protocol, or instrumentation differences.
In this context, the latter two possibilities are of particular concern
because they imply the existence of measurement errors that are not understood.
Residential and Nonresidential Exposures
As discussed earlier, there are data suggesting that the nonresidential
exposures of children, and perhaps adults, are considerably less variable
than residential exposures. This finding could be of great importance, but
it needs to be confirmed in different geographical areas with a variety
of different groups of adults and children.
Temporal Variability of Residential Exposure
No direct data exist on the variability or stability of residential exposure
over time periods greater than 24 hr. It was inferred from spot measurements
previously that exposure might be, in fact, stable over periods of years,
but this hypothesis needs to be tested with direct measurements.
Alternate Exposure Metrics
Biological hypotheses that include specification of the appropriate exposure
metrics need to be developed for testing in future epidemiological studies.
Although much of the rationale for a particular model must come from laboratory
research with in vivo and in vitro models, exposure-assessment research
may contribute by identifying metrics that are associated with wiring codes.
