Bioelectromagnetics 13:287301 (1992)

Residential Exposure to 60-Hz Magnetic Fields From Appliances D.L. Mader and S.B. Peralta Ontario Hydro, Research Division, Toronto, Ontario, Canada A model has been developed that permits assessment of residential exposure to 60-Hz magnetic fields emitted by appliances. It is based on volume- and time-averaging of magnetic-dipole fields. The model enables the contribution of appliances in the total residential exposure to be compared with that of other sources in any residence under study. Calculations based on measurements reported in the literature on 98 appliances revealed that appliances are not a significant source of whole-body exposure, but that they may be the dominant source of exposure of the body’s extremities. 0 1992 Wiley-Liss, Inc. Key words: ELF, magnetic field, exposure assessment

INTRODUCTION

The majority of studies that attempt to link 60-Hzmagnetic fields to the development of various forms of cancer in human beings lack a method for direct assessment of magnetic-field exposure levels [IARC, 19901. Instead, other indices such as wiring-configuration codes [Wertheimer and Leeper, 1979, 19821 or spot measurements [Savitz et al., 19881 have been used with varying degrees of statistical correlation with actual magnetic-field densities. A more accurate assessment of exposure can be gained by understanding the nature of the sources of residential and occupational magnetic fields. The sources of residential magnetic fields can be divided into four general categories: high-voltage transmission lines, distribution lines, building wiring, and appliances. Major advances have been made recently in the modeling of currentcarrying conductors in and around occupational and residential sites [Hayashi et al., 1989; Mader et al., 19901. These models have been successful in predicting actual magnetic-field densities due to transmission and distribution facilities and household wiring. Careful experimental measurements of the magnetic fields around appliances [Gauger, 19851 have also been made, which have verified that to a good approximation appliances act like point-source magnetic dipoles, with the field intensity falling off as the inverse cube of the distance from the centre of the appliance. However, controversy exists over the relative contribution of appliances to the Received for review June 11, 1991; revision received November 9, 1991. Address reprint requests to D.L.Mader, Ontario Hydro, Research Division, 800 Kipling Avenue KR 128, Toronto, Ontario M8Z 5S4, Canada. 0 1992 Wiley-Liss, Inc.

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a common Away from appilances

-

Next to appliances

P

Electric blankets Edge of right-of-way (distribution lines)

Within right-of-way (distribution lines) Edge of right-of-way (HV transmission lines) Within right-of-way (HV transmission lines) Office

0 rare

7

I

-

High field workplace

0.01

I

w / / / / 2 2

I

0.1

1

10

100

1000

60-Hz Magnetic Field Strength CT)

Fig. 1. Sources of ambient exposure to 60-Hzmagnetic fields.

total residential field [Leonard et al., 19901. Figure 1 shows much variability in densities of fields from various sources and at various distances from the sources. Although power lines and the grounding circuits in houses produce an average residential field on the order of 0.1 pT (1 mG), it is difficult to assess the contribution of appliances. This difficulty is due to three factors: the great variation of field strength with distance, the unknown distance of the subject from the appliance, and the duty cycle of the appliance (how often an appliance is ‘‘on” and “off”). In this paper we seek to remove the variability in Figure 1 for appliances as sources, by using averaging techniques that take into account both spatial and temporal factors. When considering proximity, even fields from small appliances can be important. For example, although the field 30 cm from an electric oven can be as great as 0.5 pT, at 3 cm a hair dryer produces a flux as great as 2,000 pT [Harvey, 19881. Awareness of these fields is important even in epidemiological work. In one instance, a study by Severson et al. [1988] indicated a low correlation of adult cancer with electromagnetic fields. By including the use of electric blankets in a re-analysis of the same data, Wertheimer and Leeper [1989] altered the results enough to indicate an increased risk with greater exposure. In another, admittedly speculative study, Delpizzo [1990] reconciled some of these apparent discrepancies by defining “significant household exposure levels. ” In this paper we formulate a simple model that permits the calculation of magnetic-field exposure levels from appliances, enabling assessment of their contribution to the total residential field. First, the appliance is replaced by a magnetic dipole source. The average spatial field is then obtained by averaging over the volume between two concentric spherical shells defined by the appliance’s dimensions and

Magnetic Fields From Appliances

289

X

Fig. 2. Geometry for a circular current-carrying loop.

the position of the subject with respect to the appliance. Finally, the time-weighted exposure level is obtained by considering the duty cycle of the appliance. Detailed calculations on the ensemble of appliances considered by Gauger [ 19851 are used to illustrate application of the model, and to provide order-of-magnitude estimates for exposure levels from specific appliances. THEORETICAL BACKGROUND Magnetic Dipole Fields An appliance may be treated as a current distribution localized to a spatial region that is small compared with the dimensions of the house. In most appliances the source of a magnetic field is a motor, a transformer, or a resistive heating element that may be approximated by a single, circular, conducting loop of radius a, carrying a current I (Fig. 2). In spherical coordinates (r,O,+), the radial and angular vector components of the magnetic field, in SI units, at a point (r,O), with r >> a, are given by Paris and Hurd [1969]

where p0 = 4 7 ~X H/m is the permeability of free space. Equations 1 and 2 show that the magnetic field at a large distance away from the current loop is dipolar in character. The magnetic dipole moment m of the loop is identified as m = pJa’14, and the magnitude of the magnetic field vector is given by

(BI

m =

- (1

3

+ 3cos2e)ln

(3)

The maximum field for a given distance r occurs when 9 = 0 or T ,on the central axis of the loop; along this line the field magnitude is 2m/r3. In general it can be shown that, at great distances, the field due to a localized current distribution is a

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Fig. 3. Spherical model for finding spatial average of magnetic field.

dipole field. Furthermore, for any current distribution reducible to a plane-current loop, the magnetic moment will be proportional to the product of I and the total loop area, regardless of the shape of the loop. From equation 3 we can see that the magnetic-field strength increases as l/? as a person approaches the appliance. This dependence is the reason that the fields in the immediate vicinity of an appliance may be orders of magnitude higher than those produced by power lines.

Average Fields Equation 3 defines the magnetic-field strength due to a local current source as observed by a subject at a fixed distance r from the source. In the use of appliances, the distance from a subject to the appliance usually vanes throughout the day-one may sit closer or further away from a television set, for example. It is therefore useful to describe appliance fields in terms of the volumetric average

Here JdV is the exposure volume defined by r, and r2, the distances defining the closest and furthest approach of the subject to the source, respectively, as shown in Figure 3. With 5 = cos 8, equation 4 becomes [CRC, 19801

=

Jry

dr

J1d5 SZndc$

(3C2

0

-1

4Tr

-03 3

-

+

1)”2 -

4)

4.14 m In (r2/r1)

4-4

(5)

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291

TABLE 1. Typical Appliance Usage Patterns, and Associated Duty Cycle Appliance Range Oven Dishwasher Refrigerator Clothes washer Clothes dryer Microwave Disposal Television Vacuum cleaner Coffee maker Toaster Crockpot Portable heater Portable fan Fluorescent fixture Fluorescent desk lamp Hair dryer Shaver Iron Can opener Mixer Blender Saw Drill

Wattage

kW-hrs/mo

Mday

Fd

12,500 12,500 1,300 300 500 4,800 1,450 450 200 800 900 1,150

100 100 18 100 8 80 22 2 30 4 6 4

0.263 0.263 0.455 10.965 0.526 0.548 0.499 0.146 4.934 0.164 0.219 0.114 0.083 0.987 1.144 3.947 3.947 0.132 0.083 0.395 0.188 0.263 0.084 0.120 0.110

0.0109 0.0109 0.0190 0.4569 0.0219 0.0228 0.0208 0.0061 0.2056 0.0069 0.0091 0.0048 0.0035 0.041 1 0.0477 0.1645 0.1645 0.0055 0.0035 0.0164 0.0078 0.01 10 0.0035 0.0035 0.0046

-

,ooo

1

115 100 50 1,000

-

1,ooo 175 125 390 275 300

-

30 4 12

6 4 12 1 1 1 1 1

The Duty Factor

The magnetic field as defined by equation 5 describes the average field experienced by a subject moving in the vicinity of an appliance. The total exposure level will also be dependent on how often the appliance is “on” or “off”; obviously, a subject experiences no magnetic field exposure from an appliance that is turned off. Table 1 shows some typical daily usages (in hours) of appliances, calculated from average wattages and the number of kilowatt-hours of electricity consumed per month [EnerMark, 19901. The product of the average magnetic field and the daily usage yields one possible metric for exposure level, expressed in pT-hours. To enable comparison between magnetic fields of appliances and those produced by other sources-transmission and distribution lines and grounding circuitsthe daily usage (in hours) may be divided by 24 h to yield a unit-less quantity we call the duty-factor F,. For example, a refrigerator with a daily usage of 12 h has a duty factor of 50% and a television with a daily usage of 8 hours has a duty factor of 33%. Multiplying the volume average field by this factor yields a spatially and temporally averaged value B,

= Fd

Equations 5 and 6 form the basis for our model for calculating exposure levels due to 60-Hz magnetic fields of appliances. Similar calculations may be made for powers , which may be useful if the

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Mader and Peralta DISHWASHERS

loo0 100

............0...........

10

F 3 m

1

0.1

0.01

0.03

1

0.1

3

d (m) 1000

* im - -m - -

CLOTHES WASHERS 100

F 3 rn

-

IOB ............o........... B lC

10

1

0.1 0.01

0.03

0.1

1000

1

TOASTERS

100

s

m

1

d (m)

-

-D

--

3

134 131

...........+.....-... I X

10

1

0.1

0.01

0.03

1

0.1

3

d (m) Fig. 4. Magnetic-flux density as a function of distance from the appliance surface. Symbols (A, 0,V) represent data from Gauger [1985]. Lines (solid, dashed, and dotted) represent the corresponding theoretical fit assuming a magnetic-dipole source. a: Dishwashers: Type 8A refers to Gauger’s Figure 8, Model A appliance. b: Clothes washers: Type 10A refers to Gauger’s Figure 10, Model A appliance. c: Toasters: Type 13A refers to Gauger’s Figure 13, Model A appliance. d: Irons: Type 15A refers to Gauger’s Figure 15, Model A appliance. e: Electric can openers: Type 16A refers to Gauger’s Figure 16, Model A appliance.

-

Magnetic Fields From Appliances 1000

I

IRONS

100

5 m

-............0...........

-

-

-0

293

I51 1s1

15c

10

1

0.1

0.01

1

0.1

0.03

d (m)

10000

- -p - -

1000

3

1611 16)

........... ........... I6C

100

F

3 m

10 1

0.1

0.01 0.1

0.03

1

3

d (m)

Fig. 4 , e .

response is best described by a combination of such powers. For example, can be calculated as



=

2m2

[(

1

(4 - 4

) (;

I);

-

(7)

DETAILED CALCULATIONS Magnetic Dipole Strengths

To illustrate the model for 60-Hz magnetic fields as outlined above, we apply it to the ensemble of appliances surveyed by Gauger [1985]. In this work, maximum magnetic field levels as a function of distance from an appliance’s surface were reported for 98 different appliances of 25 basic types; at least three different appliances were measured for each device type. Although the survey is far from comprehensive, the large sample size ensures the accuracy of any observed trends in magnetic-field behavior. Other works on appliances exist [Silva et al., 19891, but most

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TABLE 2. Calculated Magnetic Dipole Strengths and Dimensions for Appliances*

M Appliance Range

Oven

Dishwasher

Refrigerator

Clothes washer

Clothes dryer

Microwave

Disposal

Television

Vacuum cleaner

Coffee maker

Toaster

Crockpot

Type 4A 4B 4c 4D 4cs 5A 5B 5c 8A 8B 8C 9A 9B 9c 9C2 1OA 10B 1OC 11A 11A2 11B 11c 1IC2 6A 6B 6C 6D 7A 7B 7c 24A 24B 24c 24D 24E 19A 19B 19C 19D 19E 12A 12B 12c 13A 13B 13C 14A 14B 14C

tm3 PT]

tml

X

0.14416 0.05668 0.02310 0.04721 0.01856 0.04319 0.05642 0.02922 0.38970 0.20593 0.13578 0.17480 0.04294 0.00380 0.00225 0.20993 0.05519 0.03025 0.01844 0.01577 0.15 194 0.06416 0.04912 0.65742 0.78400 0.19325 0.34525 0.07296 0.03998 0.03375 0.07742 0.21732 0.22772 0.10831 0.00225 1.45966 0.08261 0.17479 0.11228 0.05705 0.00381 0.00466 0.02993 0.03925 0.01 150 0.00284 0.00573 0.00953 0.00571

0.059 0.049 0.097 0.134 0.096 0.169 0.241 0.281 0.216 0.300 0.310 0.475 0.367 0.156 0.147 0.142 0.147 0.298 0.103 0.138 0.629 0.482 0.5 18 0.130 0.153 0.079 0.154 0.034 0.022 0.045 0.094 0.188 0.204 0.176 0.073 0.107 0.027 0.046 0.044 0.031 0.021 0.053 0.248 0.098 0.070 0.043 0.055 0.102 0.119

0.067 0.096 0.113 0.102 0.303 0.090 0.028 0.048 0.059 0.090

0.020 0.098 0.109 0.108 0.250 0.044

0.084 0.076 0.044

0.124 0.042 0.019 0.052 0.050 0.084 0.21 1 0.088 0.066 0.079 0.095 0.068 0.075 0.048 0.036 0.096 0.148 0.224 0.107 0.030 0.240 0.107 0.080 0.112 0.072 0.073 0.059 0.135 0.054 0.060 (continued)

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TABLE 2. Calculated Magnetic Dipole Strengths and Dimensions for Appliances.* (Continued)

M Appliance Portable heater

Portable fan

Fluorescent fixture

Fluorescent desk lamp

Hair dryer

Shaver

Iron

Can opener

Mixer

Blender

Saw

Drill

Type 20A 20B 2oc 20D 21A 21B 21c 21D 21E 25A 25B 25c 25D 26A 26B 26C 26D 22A 22B 22c 22D 22E 23A 23B 23C 23D 23E 15A 15B 15c 16A 16B 16C 17A 17B 17C 18A 18B 18C 18D 27A 27B 27C 27D 27E 28A 28B 28C 28D

*Type designates Gauger’s labels, and

[m3 PT] 0.14022 0.11973 0.25341 0.00879 0.01633 0.43304 0.00260 0.00887 0.00130 0.00584 0.22354 0.01629 0.04916 0.03703 0.03749 0.01524 0.23453 0.22821 0.2331 1 0.00423 0.00322 0.00041 0.30987 0.2 1362 0.05746 0.05034 0.00248 0.01506 0.00498 0.00862 0.86146 0.08389 0.52974 0.02879 0.35189 0.01821 0.06551 0.09260 0.04727 0.03078 1.02152 0.22383 0.03563 0.09054 0.02526 0.08076 0.12841 0.11386 0.07379

x the deviation from equation 8.

rap,,

[ml 0.070 0.066 0.106 0.079 0.041 0.213 0.052 0.107 0.058 0.001 0.109 0.055 0.120 0.019 0.029 0.014 0.169 0.017 0.046 0.028 0.030 0.013 0.027 0.021 0.022 0.020 0.023 0.049 0.036 0.080 0.039 0.004 0.050 0.004 0.048 0.045 0.048 0.069 0.050 0.078 0.068 0.035 0.036 0.029 0.017 0.018 0.031 0.031 0.027

X 0.074 0.042 0.040 0.135 0.338 0.088 0.217 0.114 0.042 0.128 0.091 0.050 0.080 0.142 0.062 0.094 0.089 0.086 0.080 0.019 0.014 0.161 0.040

0.096 0.091 0.085 0.052 0.047 0.096 0.155 0.079 0.227 0.070 0.061 0.176 0.057 0.068 0.068 0.084 0.047 0.096 0.072 0.191 0.071 0.073 0.056 0.059 0.032 0.051

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Mader and Peralta

report only single-spot readings. Gauger’s distance-dependent data, however, are ideal for our application. Gauger’s maximum-field data may be curve-fitted to the dipole source equation, equation 3 with 8 = 0,

Here M is two times the magnetic-dipole strength m. The appliance dimension rappl (the distance from the centre of the internal source to the appliance surface) and M are used as fitting parameters. Here, d is the distance from the appliance’s surface to the point where the field is measured. Figure 4 shows some typical data and the fit to equation 8 for several appliances, as determined by separate Simplex and NewtonRaphson least-square algorithms. In all cases the fit was found to be excellent. The results obtained for all the appliances in Gauger’s ensemble are summarized in Table 2, which lists the original appliance-type designation, dipole strengths, appliance dimensions, and the relative deviation from Equation 8, defined by

where Bi are the measured magnetic field strengths at distances di, Bdipolethe corresponding theoretical fields from equation 8, N the number of data points, and N, the number of fitting parameters (in this case, two). x2 is the quantity minimized in the least-squares program to obtain the best fit. Spatial Average To obtain the spatially averaged field we must now define effective volumes for each type of appliance over which exposures occur. Proximity to most appliances ranges over moderate distances. As a typical example, we take the closest approach to the appliance’s surface (di) to be 30 cm and the greatest distance (df) to be equal to half the width of a typical home, 3.05 m [Cumberland, 19911. A subset of appliances, however, is usually operated at close range. Such appliances are portable, usually small, devices such as hair dryers and shavers. For convenience we shall refer to such devices as close-range appliances, and take their operating distance to range from 4 = 3 cm to d, = 30 cm. From these definitions we see that, while all appliances contribute to whole-body exposure, close-range appliances are the primary source of exposure of body extremities, such as the hands and face. For each appliance type, average values for M and rap,] were obtained from Table 2. Using the values rl = rap,] + di and r2 = rap,,] + d,, as defined above for each type of appliance, we calculated values for the spatially averaged fields and as defined by equations 5 and 7, respectively; the values are shown in Table 3. Since higher powers of the field are outside the scope of this study, we refer only to in the remainder of this work.

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TABLE 3. Spatial Averages and for Appliances Appliance (30 to 305 cm distance) Range Oven Dishwasher Refrigerator Clothes washer Clothes dryer Microwave Disposal Television Vacuum cleaner Coffee maker Toaster Crockpot Portable heater Portable fan Fluorescent fixture Fluorescent desk lamp (3 to 30 cm distance) Hair dryer Shaver Iron Can opener Mixer Blender Saw

Drill

[pTl

[pT2]

0.008742 0.005512 0.029878 0.006789 0.013103 0.006668 0.071030 0.007900 0.017793 0.059604 0.001885 0.002754 0.001060 0.019873 0.013830 0.011349 0.012692

0.00 1020 0.000218 0.005510 0.000274 0.001400 0.000206 0.054685 0.001131 0.003162 0.058006 0.000043 0.o0o111

12.02996 16.93913 0.67153 60.55654 16.21525 5.5546 32.46812 12.71828

880.481 1998.490 0.787 19724.710 1373.707 81.6598 4805.048 984.121

O.ooOo15 0.005472 0.002461 0.001875 0.002524

Time-Weighted Average The duty factor Fd for each appliance type may be obtained from Table 1. This , the spatially and value, multiplied by the spatially averaged field , yields B temporally averaged 60-Hz magnetic field level associated with each appliance. The results for whole-body exposure are summarized in Table 4, while Table 5 summarizes the results for exposure of body extremities due to close-range appliances.

DISCUSSION The results of applying our model for residential exposure to 60-Hz magnetic fields due to appliances, in the particular case of Gauger’s ensemble, are summarized in Figure 5 . A convenient way to interpret the quantities derived above is to relate the non-time-weighted, spatially averaged quantity with peak exposure to appliance magnetic fields, and to relate the time-weighted, spatially averaged quantity,B with long-term, chronic exposure. The health sciences community has not yet established whether peak or chronic exposure correlates with any health effects. However,

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Mader and Peralta TABLE 4. Whole-Body Exposures (Averaged From 30 to 305 cm From the Surface of the Appliance)

Range Oven Dishwasher Refrigerator Clothes washer Clothes dryer Microwave Disposal Television Vacuum cleaner Coffee maker Toaster Crockpot Portable heater Portable fan Fluorescent fixture Fluorescent desk lamp Hair dryer Shaver Iron Can opener Mixer Blender Saw Drill

0.008742 0.005512 0.029878 0.006789 0.013103 0.006668 0.071030 0.007900 0.017793 0.059604 0.001885 0.002754 0.001060 0.019873 0.013830 0.011349 0.012692 0.O 15306 0.020788 0.001421 0.079767 0.021541 0.009207 0.044956 0.016181

Total whole-body exposure

0.000095

o.ooO06o 0.000566 0.003102 0.000287 0.000152 0.001477 0.000048 0.003658 0.000408 O.ooOo17 O.ooOo13 0.000003 0.000817 0.000659 0.001866 0.002087 0.000083 O.ooOo72 O.ooOo23 0.000624 0.000236 0.000032 0.000156 0.ooOo74 0.016615

TABLE 5. Extremity Exposures From Close-Range Appliances (Averaged From 3 to 30 cm From the Surface of the Appliance) Appliance Hair dryer Shaver Iron Can opener Mixer Blender Saw Drill Total extremity exposure

[FTI 12.02996 16.93913 0.67153 60.55654 16.21525 5.55468 32.46812 12.71828

B, [PTI 0.065953 0.058816 0.011045 0.474281 0.177798 0.019521 0.112736 0.058106 0.978256

if we take the values calculated in Tables 4 and 5 to be typical, we may make several inferences. For individual appliances, chronic whole-body exposures to 0.001 pT are possible; these levels are considerably lower than typical exposures from power lines or grounding circuits. Taken as a group, the total chronic whole-body exposure is approximately 0.02 pT, still an order of magnitude lower than from other sources. For whole-body exposures, peak levels are similarly low.

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ALL APPLIANCES total whole-body exposure 0.02 JJT exposure volume

exposure volume

CLOSE-RANGE APPLIANCES total extremity exposure 1 JJT

Fig. 5. Application of spherical model (Fig. 3) to Gauger’s ensemble of appliances, for whole-body and extremity exposure.

The contribution of close-range appliances is similarly negligible when assessing whole-body exposure. For exposure to body extremities, however, the situation is different. In this case, close-range appliances constitute individual sources that may be large. Taken together, these appliances contribute a total chronic exposure to body extremities of about 1 pT. This is an order of magnitude higher than the typical residential time-weighted exposure of 0.1 pT due to other sources [Mader et al., 19901. Furthermore, in contrast to the case with whole-body exposures, peak levels of 100 pT or higher may be found for body extremities. These general trends persist even if we allow the operating range {di, &} for each appliance in our model to vary. We should note that appliances cause intense exposure of only a small fraction of the body’s tissues. This should be taken into account in the final assessment of any possible risks due to close-range appliances, and may even mitigate the effect of elevated exposures. For example, the risk due to ionizing radiation depends on the total exposure of the body’s bone marrow. Because only a small fraction of the total marrow is found in the hands, health physics regulations permit the hands to be exposed to higher levels of radiation than those of the whole body. Further work should focus on an experimental assessment of close-range appliances. It may be necessary to re-design existing instrumentation to accommodate

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measurements at body extremities. Dosimeters are usually worn on the waist, and are therefore indicative only of whole-body exposure. They will not be sensitive to the exposure levels at extremities. We should note, furthermore, a caveat with respect to measurements of magnetic fields from appliances. In a recent evaluation of instrumentation [IEEE, 19911 considerable variability was found in measurements of appliances. This variability was much more than for measurements on transmission lines. Typically, measurements differed by a factor of 3. However, differences to a factor of 11 were found when one operator used different instruments, and differences to a factor of 16 were found among several operators using the same instrument. In addition there are other complicating factors in characterizing appliance fields. The probe size may be inconvenient, especially when dealing with small appliances. Movement of the probe or vibration of the appliance may make precise measurements difficult. The increased level of harmonics and the nonuniformity of appliance fields may also complicate measurements. All these factors should be addressed in the design of any experimental assessment of appliance fields. Clearly, more work must be done to provide some basis for repeatability in appliance measurements. The model we have presented above is generally valid for any appliance or any localized piece of electrical equipment. The model can therefore be extended to cover occupational exposure levels due to most electrical equipment, including power tools, computer systems, and even large transformers and motors. The actual fields and exposure levels will differ from the values for household appliances because of the variations in dipole strength, the longer duty cycles, and differences in exposure vjy’vrnes associated with electrical equipment used in occupational situations. However, the same relationship between extremity and whole-body exposure noted above should still apply. CONCLUSIONS We present a method for assessing residential exposure to 60-Hz magnetic fields from appliances. We model appliances as point-dipole sources and then form a spatial average of magnetic field strength by averaging over an exposure volume defined as lying between two concentric spheres centred on the dipole source. An average over time is introduced through multiplication of the spatial average by the fraction of the time that the appliance is on. The model is in general applicable to any electrical appliance, and thus may be used in the assessment of both residential and occupational exposure levels. For close-range appliances such as hand-held tools, the method predicts that exposure to body extremities is much larger than whole-body exposure. As an example, the method was applied to Gauger’s data on residential appliances. For this ensemble, at least, we conclude that appliances do not contribute significantly to whole-body exposure, but they may be the dominant source of exposure to the body’s extremities. ACKNOWLEDGMENTS

We thank Mr. Bill Jones for providing us with material on appliance duty cycles, Dr. Stuart Harvey for useful discussions and for material relating to the

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variability of appliance field measurements, and Dr. Dave Agnew and Dr. Keith Donnelly for sharing their insights on personal dosimetry and residential fields. REFERENCES CRC (1980): “Handbook of Chemistry and Physics,” 60th ed. Boca Raton, FL: CRC Press, A-63: Integral 273. Cumberland S (1991): Personal communication. Toronto, Ontario: Royal Le Page Real Estate Services. Delpizzo V (1990): A model to assess personal exposure to ELF magnetic fields from common household sources. Bioelectromagnetics 11: 139-147. EnerMark (1990): Electricity and how you use it. Toronto, Ontario: Enermark. Data Sheet 984-0430:6 Rev. 5-90. Gauger JR (1985): Household appliance magnetic field survey. IEEE Trans Power App Syst PAS104:2436-2444. Harvey SM (1988): Evaluation of residential magnetic field sources. Ontario Hydro Research Report No. E88-17-K. Hayashi N, Isaka K,Yokoi Y (1989): ELF electromagnetic environment in power substations. Bioelectromagnetics 1051-64. IEEE Magnetic Fields Task F o r c e d l s e n R, Bracken D, Chartier V, Dovan T,Jaffa K,Misakian M, Stewart J (1991): An evaluation of instrumentation used to measure ac power system magnetic fields. IEEE Trans Power Deliv 6:373-383. International Agency for Research on Cancer Ad Hoc Working Group (1990): Extremely low-frequency electric and magnetic fields and risk of human cancer. Bioelectromagnetics 11:91-99. Leonard A, Neutra R, Yost M, Lee G (1990): “Electric and Magnetic Fields: Measurements and Possible Effects on Human Health. ” Berkeley, CA: Special Epidemiological Studies Program, California Dept. of Health Services. Mader DL, Barrow DA, Donnelly KE, Scheer RR, Sherar MD (1990): A simple model for calculating residential 60-Hz magnetic fields. Bioelectromagnetics 11:283-296. Paris DT, Hurd FK (1969): “Basic Electromagnetic Theory.” New York McGraw-Hill. Savitz DA, Wachtel H, Barnes FA, John EM, Tvirdik JD (1988): Case-control study of childhood cancer and exposure to 60 Hz magnetic fields. Am J Epidemiol 128:21-38. Severson RK, Stevens RG, Kaune WT, Thomas DB, Heuser L, Davis S, Sever LE (1988): Acute non-lymphocytic leukemia and residential exposure to power frequency electromagnetic fields. Am J Epidemiol 128:lO-20. Silva M, Hummon N, Rutter D, Hooper C (1989): Power frequency magnetic fields in the home. IEEE Trans Power Deliv. 4:465-477. Wertheimer NW, Leeper E (1979): Electrical wiring configurations and childhood cancer. Am J Epidemiol 109:273-284. Wertheimer NW, Leeper E (1982): Adult cancer related to electrical wires near the home. Int J Epidemiol 11~345-355. Wertheimer NW, Leeper E (1989): Re: Acute non-lymphocytic leukemia and residential exposure to power frequency electromagnetic fields. Am J Epidemiol 130:423-425.