Electromagnetic Radiation from Microwave Ovens
Zeyad O.I. Alhekail
EE. Dept., King Suad University
P.O. Box 800, Riyadh 11421, Saudi Arabia
Email: [email protected]
Short title: Microwave Ovens
1
ABSTRACT
Electromagnetic radiation from microwave ovens in Saudi Arabia was
investigated by means of field measurement survey. The survey was carried
out for 106 ovens used in household and restaurants in Riyadh city. Ovens
were between 1 month and 14 years old with operating power ranging from 0.5
to 4.4 kW.
One oven was found to leak more than the 5mW/cm2 limit
specified in the standard. 15 other ovens were found to leak 1mW/cm2 or
higher with the rest of the ovens leaking less than that. Based on the survey
result, previous studies and the fast decay of radiated power density with
distance from the oven; the conclusion was that user exposure to RF radiation
from microwave ovens is much less than the general public exposure limit set
by most international standards at 2450MHz, i.e. 1 mW/cm2. And, that
detrimental health effect is unlikely result of exposure to radiation from
microwave ovens.
2
1. Introduction:
In the last decade, usage of microwave ovens in Saudi Arabia has increased
significantly. It became a familiar appliance available for customer use at fast
food restaurants and school cafeterias.
As a result, public concern about
possible harmful effect of microwave ovens became an issue.
In particular,
the effect of electromagnetic (EM) radiation (leakage) from microwave ovens
has been raised.
Public concerns were supported by the lack of adopted
emission standard for the widely available microwave ovens and the lack of
any regulation that ensure its proper use and maintenance. In addition to that,
until now, there is no adopted EM exposure standard in Saudi Arabia. That
initiated the need for field assessment of EM leakage from microwave ovens
used in Saudi Arabia.
Visits to households and restaurants were carried out and data from 106 ovens
were collected. The data were then statistically analyzed and compared to
established international standards. In the following section, brief background
information and previous studies are presented followed by discussion of the
procedure. After that, the results are presented along with some statistical
analysis and discussion followed by conclusions.
2. Background:
Most microwave ovens operate at 2450 MHz with operating power ranging
from .5 to 2 kW, industrial type ovens operating at higher power are also
available (Osepchuk 1978, Gandhi 1990). The US code of federal regulation
3
(CFR) 21 part 1030, specifies the maximum amount of leakage (emission)
from microwave ovens at distances of 5 cm from the oven to be 1 mW/cm2
before the oven is sold and 5 mW/cm2 throughout its operating life (FDA
1992). Similar standards are used in other countries, some countries also
require that microwave ovens are checked every three years. In addition to
maximum leakage limit, the above mentioned US code requires labels to be
placed on microwave ovens to warn against operation with a) Object caught in
door, b) Door that does not close properly, c) Damaged door, hinge, latch, or
sealing surface. Upon application by a manufacturer, FDA may grant an
exemption from one or more of the warning labels.
In terms of exposure limits, most international standards such as ICNIRP
(ICNIRP 1998), limit general public exposure to RF power to 1 mW/cm2 at
2450MHz. These standards are designed to protect against thermal effect of
microwave power. Note that although the 1mW/cm2 limit is below the
maximum permissible emission of 5 mW/cm2, the expected exposure for
microwave oven user is far below the 1 mW/cm2 limit because of the diverging
nature of the microwave radiation. Also note that exposure due to leakage
from microwave ovens is relatively low when compared to thermally harmful
power densities.
Several studies concerning leakage from microwave ovens have been
conducted.
One study (Skotte 1981) surveyed microwave ovens used in
restaurants and cafeterias and found that for most of the large ovens leakage is
4
in the range between 0.2 to 2 mW/cm2. Some ovens were found to radiate
more than the specified limit and that was attributed to oven age and the lack
of cleaning and proper maintenance. A thorough review of microwave ovens
safety, standards, leakage nature, and customers usage was conducted in the
United States (Osepchuk 1978).
The conclusion was that leakage from
microwave ovens is less than the specified safe limit at points very close to the
oven and, that the EM field’s intensity is very low at the distance where the
user usually stands. Another study for EM emission from microwave ovens
was conducted in Germany (Matthes 1992), in which a free oven check was
offered to the Munich area households and 130 ovens were brought in for the
check. Oven power ranged between 350W and 1200W and the age of the
ovens were between .5 and 18 years. Measured leakage from all 130 ovens
was less than 1mW/cm2. Statistical analysis suggested that 50% of operating
ovens emit less than 0.062 mW/cm2 and that the probability of emitting more
than 5 mW/cm2 is less than 0.05%. Furthermore, it was indicated that there
was no clear dependence of measured leakage and parameters such as oven
operating power, age or manufacturer.
3. Measurement Procedure
Measurements were carried out by visiting some restaurants and households in
Riyadh city. This ensured that measurements were done in the oven-working
environment, i.e., performing the measurements under actual cleanliness
levels. Also, that gives the opportunity to gather other useful data. The oven
5
selection was done in a way that ensured coverage of different sectors such as,
food chains, individually operated restaurants and school cafeterias.
The survey was conducted using a Narda 8201 electromagnetic leakage
monitor along with a Narda 8221 isotropic probe. This system accuracy is ±
0.75 dB and it was calibrated by the manufacturer prior to its use. It was also
checked against other instruments for verification.
Measurement of oven
leakage was performed as described in (CFR) 21 part 1030, by placing a 275ml
cup of tap water inside the oven and operating it at maximum power. The
measuring device probe was slowly moved around the outside surface of the
oven taking readings of emission level at 5cm from the oven. The probe was
moved around to cover all possible radiation points of the oven, i.e. door
edges, screen and ventilation points. Readings were logged in the specially
designed data collection sheet which also included information about the oven
such as manufacturer, country of origin, operating power, age, number of
users, daily use, location of the oven relative to the public, user awareness and
physical condition of the oven.
4. Results and discussion
The survey included 106 ovens from 24 different manufacturers, oven
operating power was between 0.5 and 4.4 kW. The ovens were between 1
month and 14 years old. One oven was found to leak more than the 5 mW/cm2
limit specified in the standard. 15 other ovens were found to leak 1 mW/cm2or
higher with the rest of the ovens leaking less than 1 mW/cm2. The average
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leakage level was 0.41 mW/cm2. The maximum measured leakage from each
oven against its operating power is shown in Figure 1. It can be seen that there
is no apparent correlation between measured leakage and operating power.
Figure 2 shows maximum measured leakage plotted against oven age. In this
figure one can observe correlation between oven age and measured leakage.
Note that correlation with age does not necessarily mean aging effect only; it
could also be attributed to manufacturers efforts to produce ovens with less
leakage compared to older ones.
In order to analyze the data from a statistical point of view, lognormal
distribution was found to give an adequate fit with -1.85 mean and 1.393
standard deviation. The measured data frequency is shown in Figure 3 along
with the lognormal expected frequency. Using the lognormal distribution, the
95% observation probability interval is found to be 0.01 to 2.44 mW/cm2.
Also, 50% of the ovens are expected to leak less than 0.157 mW/cm2 and, the
probability of finding an oven that leaks more than 5 mW/cm2 is 0.6%. This is
a relatively high probability, when compared to the one found by (Matthes
1992) where leakage measurement were performed on ovens brought in for
cost-free check. Note that, in this study, leakage measurement was carried out
at the oven-working environment under actual cleanliness levels and surveyed
ovens were randomly selected and included many restaurant ovens.
As
mentioned earlier, this study found several ovens leaking more than 1 mW/cm2
while (Matthes 1992) reported that all checked ovens were found to leak less
than 1 mW/cm2.
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In addition to figures 1 and 2, survey data can be presented in several ways;
tables 1 through 5 show the distribution of the surveyed ovens in relation to
user type, age, country of origin, manufacturer, and operating power. These
tables present the different characteristics of the surveyed sample along with
the average of the maximum measured leakage from ovens in each group. The
tables also present Pearson’s correlation factors between leakage level and
oven age and oven operating power.
Table 1 shows that ovens used in
restaurants tend to leak more than the ones used at households. This table also
shows that the surveyed ovens at households are newer and have less operating
power. Table 2 shows the above mentioned data grouped by age in addition to
the maximum measured leakage from each group.
Table 3 shows that microwave ovens in Saudi Arabia mainly come from three
countries, i.e. United Stats, Japan and Korea. with the Korean ovens having
the least average leakage, possibly attributed to less average age for these
ovens compared to ovens from the other two countries. Table 4 shows that
some manufacturers ovens tend to leak more than the others. Also, correlation
between measured leakage and age is apparent in most entries of table 3 and 4
and all entries of table 5, which is consistent with the overall correlation with
age mentioned above.
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4.1. Survey observations:
Some of the survey observations are the following:
1. No warning labels in the local language were fixed on any of the
surveyed ovens.
2. Low awareness level among many restaurant users of the oven nature
and that proper safe usage is required.
3. One of the surveyed ovens had a broken door glass, however, it was still
in place and the mesh was intact, no abnormal leakage was detected.
4. One oven was being operated by plugging and unplugging the power
cord, i.e., the oven circuit was on all times (with closed door), indicating
possible failure in timer unit. That brings the possibility of other unsafe
operations of the oven.
5. The maximum leakage for ovens with a door movable handle was
detected next to the handle. Also, these ovens were found to leak more.
This information was not included in the statistical analysis because of
missing data, i.e. not all ovens were labeled to be with or without a
handle. It is worth mentioning that most of the ovens leaking at
1mW/cm2 and above were noted to have a movable handle.
4.2. Forced leakage
In order to simulate the leakage that could occur due to a small particle being
caught up in the door seal, a small piece of paper about 2mm thick, 1cm wide
and 2cm long is inserted at the door seal of one oven. Leakage is then
9
measured using the same meter that was used in the survey, at different
distances from the inserted piece of paper. Measurement is carried out 5 times
and Figure 4 shows the average measured power density, decreasing with
distance from the oven along with the theoretically expected 1/r2 decrease of
the radiated power density from the oven (Osepchuk 1978). This demonstrates
the fast drop of power density with increasing distance from the oven. Note
that this oven operating power is 1250W and its normal leakage is 0.05
mW/cm2.
5. Conclusions
All surveyed ovens, except for one, were found to comply with the 5 mW/cm2
leakage limits at 5 cm from the oven, one oven was found to leak 6 mW/cm2.
Statistical analysis suggests that with 95% probability an oven will be found to
leak between 0.01 and 2.44 mW/cm2 and that 50% of the ovens are expected to
leak less than 0.157 mW/cm2. Also, the probability of an oven leaking more
than 5 mW/cm2 is 0.6%.
Correlation was observed between measured leakage and oven age. And, no
apparent correlation was found between measured leakage and operating
power.
In view of the radiated power density decrease with distance; user exposure is
much less than the specified general public exposure limit specified in most
international standards, i.e. 1 mW/cm2. Therefore, detrimental health effects
from the surveyed microwave ovens are not expected. And, since Riyadh city
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ovens represent all ovens in Saudi Arabia and assuming that surveyed ovens
represent microwave ovens in Riyadh city; then, based on this study findings
and previous studies in other countries, one can conclude that leakage from
microwave ovens used in Saudi Arabia does not expose surrounding people to
power densities more than the specified limit for the general public exposure, if
properly operated. However, it is recommended that one of the established
standards is adopted by the Saudi Arabian Standards Organization (SASO).
Aknowledgment
This work was supported by King AbdulAziz City for Science and Technology
(KACST) under grant # AT-15-54.
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References:
FDA 1992. 21 CFR Ch. I (4-1-92 Edition), PART 1030, Performance
standard for microwave and Radio frequency emitting products.
Paragraph 1030.10 Microwave ovens, pp. 496-499, Food and Drug
Administration.
Gandhi, OM P. 1990. Biological Effects and Medical Applications of
Electromagnetic Energy, Prentice Hall, New Jersey.
ICNIRP 1998. Guidelines for limiting exposure to time-varying electric,
magnetic, and electromagnetic fields(up to 300 GHz), Health physics,
Volume 74, No. 4, p:494-522, (1998).
Matthes, R. 1992. Radiation emission from microwave ovens, Journal of
Radiological Protection Vol. 12 No 3 p. 167-172.
Osepchuk J. M. 1978. A Review of Microwave Oven Safety, Journal of
Microwave Power, 13(1).
Skotte J . (1981). Undersoglese af mikrobolgeovne I storkokkener. (In
Danish) Arbejdstilsynet, Denmark, Report No. 6. (As referenced in Gandhi
1990, p. 67).
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List of tables:
Table 1: Survey data grouped by type of use.
Table 2: Survey data grouped by oven age.
Table 3: Survey data grouped by country of origin.
Table 4: Survey data grouped by manufacturer.
Table 5: Survey data grouped by operating power.
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Table 1: Survey data grouped by type of use.
Type of use
# of
samples
Household
Restaurants
Avg.
operating
power (Watt)
26
80
Average age
(months)
938
1361
Correlation
with age
Avg.
measured
leakage
(mW/cm2)
36.1
43.2
0.098
0.509
Correlation with
operating power
-0.15
0.65
0.18
-.18
Table 2: Survey data grouped by oven age.
Age in
years
# of samples
Total
<1
1–2
2–3
3–4
4–5
5–6
6–7
7–8
8–9
9 – 10
10 – 11
12 – 13
13 – 14
Homes
21
15
21
6
17
4
2
2
9
3
4
1
1
Restaurants
8
1
6
3
3
2
1
0
0
0
1
0
1
13
14
15
3
14
2
1
2
9
3
3
1
0
Avg.
operating
power
(Watt)
Maximum
measured
leakage
(mW/cm2)
1221
1456
1459
1903
1071
925
1125
1225
1083
800
700
950
1250
Avg.
measured
leakage
(mW/cm2)
0.4
0.8
0.4
0.6
0.7
0.4
0.3
2.0
3.0
1.0
6.0
2.0
0.05
Correlation
with operating
power
0.12
0.23
0.14
0.22
0.18
0.23
0.25
2.0
1.38
0.83
1.88
2.0
0.05
0.61
-0.22
0.35
0.83
0.33
0.42
-1.0
*
-0.07
-0.87
*
*
*
*Not computed because of low count or fixed value of one of the correlation variables.
Table 3: Survey data grouped by country of origin.
Country of
origin
# of samples
Total
FRANCE
JAPAN
Korea
USA
Canada
Swiss
Homes
1
33
14
53
3
2
0
10
5
10
1
0
Restaurants
1
23
9
43
2
2
Average
operating
power
(Watt)
850
1296
1115
1183
867
4400
Average
age
(months)
4
38
26.4
49.7
34.7
15
*Not computed because of low count or fixed value of one of the correlation variables.
14
Average
measured
leakage
(mW/cm2)
0.04
0.35
0.1
0.55
0.190
0.300
Correlation
with age
Correlation
with
operating
power
0.68
-0.22
0.62
0.61
-1
0.07
-0.1
-0.25
-0.67
*
Table 4: Survey data grouped by manufacturer.
Manufacturer
# of samples
Total
M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
M11
M12
M13
M14
M15
M16
M17
M18
M19
M20
M21
M22
M23
M24
Homes
1
8
1
1
6
1
3
9
2
1
26
1
8
3
11
4
2
1
2
2
5
5
2
1
Restaurants
1
0
0
1
0
1
1
0
1
0
0
0
3
2
7
0
2
0
2
0
4
0
0
1
Average
operating
power
(Watt)
Average
age
(months)
900
1468
1400
1250
1358
800
670
1824
900
700
1238
850
1125
1150
1081
1275
1000
600
825
4400
900
1000
925
1250
36
45
96
158
11.7
30
48
16.5
78
96
46.6
4
19.1
66
41.6
51.8
7.5
8
3.5
15
24
101.4
73
36
0
8
1
0
6
0
2
9
1
1
26
1
5
1
4
4
0
1
0
2
1
5
2
0
Average
measured
leakage
(mW/cm2)
Correlation
with age
Correlation
with
operating
power
*
0.89
*
*
0.95
*
*
0.86
*
*
0.62
*
-0.14
0.94
0.30
0.99
*
*
1
*
0.78
-0.65
1
*
*
-0.6
*
*
0.47
*
.99
.87
*
*
-.35
*
0.68
0.39
0.16
-0.99
*
*
1
*
-0.049
0.76
1
*
0.150
0.17
1.0
0.05
0.163
0.060
0.113
0.232
0.265
1.0
0.6
0.040
0.08
0.35
0.355
0.52
0.195
0.150
0.160
0.300
0.064
1.7
1.01
0.02
*Not computed because of low count or fixed value of one of the correlation variables.
Table 5: Survey data grouped by operating power.
Operating
power
500-800
800-1000
1000-1200
1200-1400
1400-1600
1600-1800
1800-2000
2000-2500
2500-4500
# of samples
Total
Homes
18
25
4
21
13
18
2
1
4
Average
operating
power
(Watt)
Restaurants
4
15
2
4
0
1
0
0
0
14
10
2
17
13
17
2
1
4
Average
age
(months)
679
876
1025
1264
1484
1605
1800
2250
3630
*Not computed because of low count or fixed value of one of the correlation variables.
15
68.33
37.76
26.25
50.1
37.5
23.1
12
36
25.5
Average
measured
leakage
(mW/cm2)
0.73
0.27
0.273
0.6
0.25
0.252
0.225
0.150
0.400
Correlation
with age
0.51
0.79
0.22
0.58
0.70
0.35
*
*
0.35
List of figures:
Figure 1: Maximum measured leakage from surveyed ovens verses oven
operating power in watt.
Figure 2: Maximum measured leakage from surveyed ovens verses oven age in
months.
Figure 3: Frequency of measured leakage along with lognormal expected
frequency.
Figure 4: Measured power density, due to forced leakage, at different distances
from the oven. 1/r2 theoretically expected decrease is also shown.
Note that instrument accuracy is ±0.75 dB.
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Leakage in mw per sq cm
0
1000
2000
3000
4000
5000
10
1
0.1
0.01
0.001
Oven operating power in watt
Leakage in mw per sq cm
Figure 1: Maximum measured leakage from surveyed ovens verses oven
operating power in watt.
6
5
4
3
2
1
0
0
20
40
60
80
100
120
140
160
Oven age in months
Figure 2: Maximum measured leakage from surveyed ovens verses oven age in
months.
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Number of Ovens
100
90
80
70
60
50
40
30
20
10
0
Measured
Computed Fit
0.25
0.75
1.25
1.75
2.25
2.75
3.25
3.75
4.25
4.75
5.25
5.75
Leakage in mw/ sq. cm
Figure 3: Frequency of measured leakage along with lognormal expected
frequency.
Power Density in mw
per sq cm
0.5
2
0.4
Theoretical value (1/r )
0.3
Measured value
0.2
0.1
0
5
10
15
20
Distance from oven in cm
25
30
.
Figure 4: Measured power density, due to forced leakage, at different distances
from the oven. 1/r2 theoretically expected decrease is also shown. Note that
instrument accuracy is ±0.75 dB.
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