American Journal of Epidemiology
© The Author 2013. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of
Public Health. All rights reserved. For permissions, please e-mail: [email protected].
Vol. 177, No. 9
DOI: 10.1093/aje/kws334
Advance Access publication:
April 9, 2013
Original Contribution
Residential Distance to High-voltage Power Lines and Risk of Neurodegenerative
Diseases: a Danish Population-based Case-Control Study
Patrizia Frei, Aslak Harbo Poulsen*, Gabor Mezei, Camilla Pedersen, Lise Cronberg Salem,
Christoffer Johansen, Martin Röösli, and Joachim Schüz
* Correspondence to Aslak Harbo Poulsen, Danish Cancer Society Research Center, Danish Cancer Society, Strandboulevarden, 2100
Copenhagen Ø, Denmark (e-mail: [email protected]).
Initially submitted March 22, 2012; accepted for publication July 23, 2012.
The aim of this study was to investigate the possible association between residential distance to high-voltage
power lines and neurodegenerative diseases, especially Alzheimer’s disease. A Swiss study previously found
increased risk of Alzheimer’s disease for people living within 50 m of a power line. A register-based case-control
study including all patients diagnosed with neurodegenerative diseases during the years 1994–2010 was conducted among the entire adult population of Denmark. Using conditional logistic regression models, hazard ratios
for ever living close to a power line in the time period 5–20 years before diagnosis were computed. The risks for
developing dementia, Parkinson’s disease, multiple sclerosis, and motor neuron disease were not increased in
persons living within close vicinity of a power line. The risk of Alzheimer’s disease was not increased for ever living
within 50 m of a power line (hazard ratio = 1.04, 95% confidence interval: 0.69, 1.56). No dose-response according
to number of years of living within 50 m of a power line was observed, but there were weak indications of an
increased risk for persons diagnosed by the age of 75 years. Overall, there was little support for an association
between neurodegenerative disease and living close to power lines.
dementia; environmental exposure; magnetic fields; neurodegenerative diseases
Abbreviations: CI, confidence interval; CPR, Central Population Register; ELF-MF, extremely low frequency magnetic field(s);
HR, hazard ratio; ICD-10, International Classification of Diseases, Tenth Revision.
Transmission of electric power is a prerequisite of modern
life and gives rise to extremely low frequency (ELF) electric
and magnetic fields (MF) in the vicinity of power lines.
ELF-MF have been classified as “possibly carcinogenic” by
the International Agency for Research on Cancer because of
limited evidence in humans in relation to childhood leukemia and inadequate evidence in experimental animal
studies, as well as a lack of a known biological mechanism
(1, 2). For noncancer endpoints, a 3- to 4-fold risk increase
for Alzheimer’s disease related to occupational exposure to
ELF-MF was suggested in an initial report by Sobel et al.
(3). Subsequent studies summarized in reviews and metaanalyses on occupational ELF-MF exposure (4–6) found
less consistent results, but indications of an increased risk of
Alzheimer’s disease remained, with reported risks on the
order of 1.5–2 for exposure levels of ≥0.1 µT to ≥0.5 µT
(4). A Swedish study suggested that the increased risk might
be restricted to cases with disease onset by age 75 years (7),
which was confirmed in 2 subsequent studies (8, 9), and the
risk was found to be higher in men than in women (8). For
other neurodegenerative diseases, amyotrophic lateral sclerosis, the most frequent motor neuron disease, has been
quite consistently linked with electrical occupations (5, 6).
Even if an association were found in workers, extrapolation
to the general public would remain difficult—for example,
due to occupational coexposure to other potential risk
factors, particularly electric shocks (6).
Recently, the first study analyzing a possible association
between residential ELF-MF exposure and neurodegenerative diseases was conducted in Switzerland (10). An
increased risk of Alzheimer’s disease was observed for
people living within 50 m of a high-voltage power line compared with people living at a distance of ≥600 m, and risks
were highest for people who had lived within 50 m of a
970
Am J Epidemiol. 2013;177(9):970–978
Power Lines and Neurodegenerative Diseases 971
power line for at least 15 years (hazard ratio (HR) = 2.00,
95% confidence interval (CI): 1.21, 3.33). However, the
study had to rely on information from death certificates, with
likely underreporting of neurodegenerative diseases. Furthermore, it was not possible to take into account full residential histories, due to reliance on census data that were
collected only every 10 years. Because of the intriguing
Swiss findings for Alzheimer’s disease, independent replication became a key research priority, as recommended by the
World Health Organization (2) and the Scientific Committee
on Emerging and Newly Identified Health Risks (11). Our
aim in this study was to investigate the association between
residential distance to power lines and risk of neurodegenerative diseases, particularly Alzheimer’s disease, in Denmark,
using data that allowed an improved study design in comparison with the original Swiss study.
MATERIALS AND METHODS
We conducted a register-based case-control study among
the entire adult population of Denmark (approximately 5.5
million inhabitants). Since 1968, the Central Population
Register (CPR) has assigned each Danish resident a personal
identification number (CPR number) at birth or on the date
of immigration, making it possible to track every resident in
and across all Danish registers.
Identification of cases and controls
Cases and controls were selected among all adult (aged
≥20 years) residents of Denmark (excluding Greenland and
the Faroe Islands) with a valid CPR number. Cases were
identified from the Danish Hospital Discharge Registry,
which was established by the Danish National Board of
Health in 1977 and in which more than 99% of all hospitalizations for somatic diseases are registered (12), including
outpatients from 1994 onwards. The dates of admission and
discharge and up to 20 diagnoses per discharge, according
to a modified Danish version of the International Classification of Diseases, Tenth Revision (ICD-10) (from 1994
onwards), are available for each patient. All persons hospitalized for the first time with a neurodegenerative disease
from January 1, 1994, to December 31, 2010, were defined
as cases. The following groups of neurodegenerative diseases were defined: Alzheimer’s disease (ICD-10 codes F00
and G30), vascular dementia (ICD-10 code F01), other
dementia (ICD-10 codes F02, F03, A81.0, B22.0, G10, and
G31), Parkinson’s disease (ICD-10 code G20), multiple
sclerosis (ICD-10 code G35), and motor neuron disease
(ICD-10 code G12). Any person with 2 or more of the above
diagnoses was considered a case for all registered diagnoses.
Cases with dementia (Alzheimer’s disease, vascular dementia, and other dementia) were included only if their first
diagnosis was made at the age of ≥65 years, because of differences in etiology between very-early-onset (≤65 years)
dementia cases and later-onset dementia cases and because
dementia diagnoses in the registers have been validated primarily in the elderly population (ages ≥65 years) so far (13).
For each case, 6 controls were randomly selected from the
CPR, individually matched by gender and date of birth (±30
Am J Epidemiol. 2013;177(9):970–978
days). The controls had to be free of the disease and alive at
the date of diagnosis of the matched case. Information on
marital and vital status for cases and controls was extracted
from the CPR.
Residential history and geocoding of addresses
The residential history of all cases and controls from 1968
onwards was obtained from the CPR. The data set included
addresses and all dates of moving in and out. Because of
poor address quality before 1974 (mainly due to missing
house numbers), we chose to consider address history only
up to 20 years before diagnosis. Information on geographical coordinates, placed within the house or, where applicable, the apartment (estimated deviation not more than
1–2 m), and number of floors in the building was obtained
from the Danish Building and Dwelling Register, which is
maintained by the Danish enterprise and construction
authority for 95.2% of the 2,044,842 relevant addresses.
Persons with a continuous geocoded address history dating
back to 20 years before diagnosis were included in the analysis (94.2% of eligible cases and 89.1% of eligible controls).
Socioeconomic variables
On the basis of a 100- × 100-m grid cell net of Denmark
maintained by Statistics Denmark, we obtained information
on socioeconomic indicators for all addresses. Income was
provided as average disposable income per household in the
grid cell, and information on education was based on the
person with the highest level of education in the household.
Location of power lines
For all current and historical overhead power lines operated with alternating current at a voltage level of ≥132 kV,
we obtained the geographical location, the date of entering
service and, where appropriate, the date of termination of
operation from all 7 Danish transmission companies responsible for these voltage levels. The majority of the power
lines were digitalized from orthophotos, and the accuracy of
these is estimated to be 3–5 m. For some historical lines, the
geographical coordinates were identified through land surveying. The locations of the power lines were validated
against current maps of power lines in Denmark and, for a
sample of lines, historical maps of power lines. The mapped
grid totaled 4,336 km of current and historical power lines
(23.1% 400 kV, 0.9% 220 kV, 46.3% 150 kV, and 29.8%
132 kV). For lines for which only the year of entering and
exiting service was available (77.5%), we set the date of
entering service to December 31 and the date of termination
to January 1 to avoid erroneously classifying people as
exposed. For each address, we determined the shortest distance to any of the power lines. Geographical data were processed in ArcGIS 9.3 (ESRI, Redlands, California).
Statistical analysis
Using conditional logistic regression, we calculated
hazard ratios and associated 95% confidence intervals for
972 Frei et al.
Table 1. Characteristics of Cases (at Diagnosis) With Neurodegenerative Disease and Controls in a Study of Residential Distance to Highvoltage Power Lines and Risk of Neurodegenerative Diseases, Denmark, 1994–2010
Diagnosis
Alzheimer’s Disease
No. of
Cases
Total no.
20,575
Female gender
13,248
Mean age, years (SD)
%
Vascular Dementia
No. of
Controls
%
113,217
64.4
81.0 (6.5)
74,072
No. of
Cases
%
10,207
65.4
81.1 (6.5)
5,606
Other Dementia
No. of
Controls
%
55,969
54.9
80.6 (6.7)
31,413
No. of
Cases
%
No. of
Controls
62.2
238,225
68,752
56.1
80.7 (6.7)
42,750
%
376,897
82.5 (7.0)
63.2
82.6 (6.9)
Marital statusa
Missing datab
675
3.3
3,176
2.8
463
4.5
1,924
3.4
4,042
5.9
15,228
4.0
Never married
4,522
22.0
25,561
22.6
2,135
20.9
11,455
20.5
17,064
24.8
88,672
23.5
Ever marriedc
9,433
45.8
62,352
55.1
4,217
41.3
29,499
52.7
28,810
41.9
190,330
50.5
Married at diagnosis
5,945
28.9
22,128
19.5
3,392
33.2
13,091
23.4
18,836
27.4
82,667
21.9
Disposable income
First quartile
6,898
33.5
32,299
28.5
3,548
34.8
15,564
27.8
26,202
38.1
111,179
29.5
Second and third
quartiles (plus
missing data)d
9,403
45.7
57,042
50.4
4,856
47.6
28,512
50.9
30,905
45.0
189,712
50.3
Fourth quartile
4,274
20.8
23,876
21.1
1,803
17.7
11,893
21.2
11,645
16.9
76,006
20.2
<23.5
6,852
33.3
41,239
36.4
3,674
36.0
20,072
35.9
26,694
38.8
138,347
36.7
≥23.5–<36.0 (plus
missing data)f
6,517
31.7
37,664
33.3
3,325
32.6
18,938
33.8
21,192
30.8
125,431
33.3
≥36.0
7,206
35.0
34,314
30.3
3,208
31.4
16,959
30.3
20,866
30.3
113,119
30.0
12,051
58.6
76,674
67.7
5,910
57.9
38,271
68.4
39,131
56.9
250,514
66.5
8,524
41.4
36,543
32.3
4,297
42.1
17,698
31.6
29,621
43.1
126,383
33.5
Urban
17,140
83.3
85,680
75.7
8,373
82.0
42,029
75.1
56,683
82.4
285,738
75.8
Rural
3,435
16.7
27,537
24.3
1,834
18.0
13,940
24.9
12,069
17.6
91,159
24.2
% in highest
educational levele
No. of floors in
residential building
1 floor (plus missing
data)g
>1 floor
Urbanity of residence
Table continues
persons who had, during the time period of 5–20 years
before diagnosis, ever lived within a certain distance (in categories: <50 m, 50–<200 m, or 200–<600 m) of a power
line, compared with those who never did (considering the
closest category for each person). We estimated crude
hazard ratios as well as hazard ratios adjusted for relevant
confounders, which were collected for the address at which
the participant lived at diagnosis (listed in Table 1). To
investigate a potential dose-response relationship, we further
categorized exposure according to time of residency within
50 m of a power line. For better comparability with the
Swiss study (10), we also performed all analyses for only
220- to 400-kV lines using the same reference category as
for the main analyses (≥600 m from all ≥132-kV lines). We
also repeated the analyses taking into account exposure
during the whole time period of ≤20 years before diagnosis.
For Alzheimer’s disease, we performed secondary analyses
by age at diagnosis (65–75 years vs. >75 years) and by
gender. Because of remarkable changes in clinical dementia
practice in the past 2 decades owing to major advances in
the understanding of the pathophysiology of the disease and
in diagnostics and treatment (14), we investigated whether
the risk was different for more recently diagnosed Alzheimer’s disease cases. The validity of registered dementia
diagnoses in 2003 was shown to be high (14) and thereby
suitable for register-based epidemiologic studies about
dementia. On the basis of this knowledge, we used the year
2003 as a cutoff. Data were analyzed using SAS 9.2 (SAS
Institute Inc., Cary, North Carolina).
RESULTS
The numbers and characteristics of cases and controls at
the date of diagnosis are shown by disease in Table 1. More
cases than controls were married at the time of diagnosis.
More cases with dementia or Parkinson diagnoses tended to
Am J Epidemiol. 2013;177(9):970–978
Power Lines and Neurodegenerative Diseases 973
Table 1. Continued
Diagnosis
Parkinson’s Disease
No. of
Cases
Total no.
%
16,925
Multiple Sclerosis
No. of
Controls
%
90,060
Female gender
7,452
Mean age, years (SD)
74.4 (10.1)
44.0
40,825
No. of
Cases
%
8,234
45.3
75.0 (9.6)
5,547
Motor Neuron Disease
No. of
Controls
%
34,535
67.4
44.7 (13.2)
23,562
No. of
Cases
%
2,990
68.2
46.5 (13.5)
1,375
No. of
Controls
%
14,996
46.0
64.9 (13.0)
7,239
48.3
66.2 (12.3)
Marital statusa
Missing datab
839
5.0
3,024
3.4
880
10.7
3,065
8.9
212
7.1
441
2.9
Never married
2,416
14.3
14,224
15.8
1,805
21.9
6,677
19.3
343
11.5
2,290
15.3
Ever marriedc
7,397
43.7
47,983
53.3
4,297
52.2
20,556
59.5
1,014
33.9
9,120
60.8
Married at diagnosis
6,273
37.1
24,829
27.6
1,252
15.2
4,237
12.3
1,421
47.5
3,145
21.0
Disposable income
First quartile
4,477
26.5
20,822
23.1
1,325
16.1
4,942
14.3
531
17.8
2,800
18.7
Second and third
quartiles (plus
missing data)d
8,400
49.6
46,772
51.9
4,215
51.2
17,678
51.2
1,554
52.0
7,665
51.1
Fourth quartile
4,048
23.9
22,466
24.9
2,694
32.7
11,915
34.5
905
30.3
4,531
30.2
<23.5
5,755
34.0
29,659
32.9
2,091
25.4
8,675
25.1
835
27.9
4,384
29.2
≥23.5–<36.0 (plus
missing data)f
5,685
33.6
31,216
34.7
2,875
34.9
12,060
34.9
1,026
34.3
5,225
34.8
≥36.0
5,485
32.4
29,185
32.4
3,268
39.7
13,800
40.0
1,129
37.8
5,387
35.9
11,498
67.9
64,754
71.9
6,047
73.4
26,492
76.7
2,232
74.6
11,399
76.0
5,427
32.1
25,306
28.1
2,187
26.6
8,043
23.3
758
25.4
3,597
24.0
Urban
12,695
75.0
65,411
72.6
5,950
72.3
23,952
69.4
2,145
71.7
10,654
71.0
Rural
4,230
25.0
24,649
27.4
2,284
27.7
10,583
30.6
845
28.3
4,342
29.0
% in highest
educational levele
No. of floors in
residential building
1 floor (plus missing
data)g
>1 floor
Urbanity of residence
Abbreviation: SD, standard deviation.
a
Being in a registered partnership is considered equivalent to being married.
b
4.2% of data were missing.
c
But not married at the time of diagnosis.
d
1% of data were missing.
e
Percentage of persons with more than 12 years of education, grouped into 3 equal-sized groups.
f
1% of data were missing.
g
5.1% of data were missing.
be in the lowest income group compared with their controls,
which was not observed for multiple sclerosis and motor
neuron disease. The distribution of educational levels was
similar for cases and controls, except for Alzheimer’s
disease, where the proportion of cases in the highest education group was higher than for the controls. Cases more
often lived in multistory apartment buildings and in urban
environments.
The risks of developing a neurodegenerative disease
according to distance from a 132- to 400-kV power line
are shown in Table 2. Overall, risks were not elevated for
people living closest (<50 m) to a power line. The risk of
Am J Epidemiol. 2013;177(9):970–978
Alzheimer’s disease for people living within 50 m of a
power line was 1.04 (95% CI: 0.69, 1.56). Results of analyses considering only 220-kV and 400-kV power lines were
based on smaller numbers but were similar. The adjusted
hazard ratio for Alzheimer’s disease for people living within
50 m of a 220- or 400-kV power line was 1.31 (95% CI:
0.50, 3.46; n = 5). There was no dose-response according to
the number of years spent living within 50 m of a power line
for any of the diseases (Table 3). For Alzheimer’s disease,
the highest risk estimate was observed in the middle group
(between 5 and 9 years), and the risk for those living within
50 m of a power line for 10 or more years was slightly
974 Frei et al.
decreased. Results remained virtually unchanged when we
used the exposure window of ≤20 years before diagnosis
(data not shown).
Subgroup analyses on Alzheimer’s disease
With regard to Alzheimer’s disease, there was no difference in risk estimates between men and women; adjusted
hazard ratios for living within 50 m of a power line were
0.99 (95% CI: 0.53, 1.85) for men and 1.07 (95% CI: 0.63,
1.83) for women. Table 4 presents secondary analyses by
age at diagnosis and calendar year of diagnosis. The risk for
persons diagnosed at ages 65–75 years was increased for
those living within 50 m of a power line (adjusted
HR = 1.92, 95% CI: 0.95, 3.87), while among those diagnosed at a later age, the hazard ratio was slightly decreased
(HR = 0.81, 95% CI: 0.48, 1.34). In addition, the risk was
slightly increased for cases living within 50 m of a power
line diagnosed in 2003 or later, but those diagnosed earlier
had a decreased hazard ratio (n = 2). Restricting analyses of
age at diagnosis to persons diagnosed in 2003 or later
yielded a significantly increased risk for those diagnosed by
the age of 75 years (HR = 2.59, 95% CI: 1.17, 5.76), based
on 9 cases, and a risk close to 1 for those diagnosed after the
age of 75 years (Table 4).
DISCUSSION
The results of our population-based study do not suggest
an increased risk of developing neurodegenerative diseases
from living within close vicinity of a high-voltage power
line. Overall, the risk of Alzheimer’s disease was not
increased. There were some weak indications from secondary analyses of an increased risk for persons diagnosed by
the age of 75 years among people living within 50 m of a
power line, which became statistically significant when data
were restricted to cases diagnosed in 2003 or later, but with
the overall finding of no association, other subgroups also
yielded somewhat decreased risk estimates.
Strengths and limitations
The current study was register-based, covering the entire
Danish population, and therefore eliminated the problem of
selection bias. Although slightly more cases than controls
had a full address history up to 20 years before diagnosis
(94.2% vs. 89.1%), the exposure distribution for the cases
and controls for which we had partial address information
was similar to the distribution for cases and controls with
complete address histories (cases: 0.14% vs. 0.15% had ever
lived within 50 m of a power line; controls: 0.24% vs.
0.17%). We had individual information on relevant confounders such as socioeconomic variables and very accurate
address information for determination of the geographical
coordinates.
Our study provides improvements over the Swiss study
(10) in several respects. Our results were based on hospitalization data rather than information from death certificates,
where underreporting of neurodegenerative diseases can be
expected, especially for dementia diagnoses (15, 16). In
Denmark, it is legally possible to obtain reimbursement for
medication expenses only if the drug has been prescribed by
a neurologist; therefore, we would expect that only a few
patients with these diagnoses were not registered in the
Danish Hospital Discharge Registry. Additionally, we had
continuous information on the residential history of all study
participants, while census data collected every 10 years were
used for the Swiss study. This allowed us to calculate cumulative exposure more precisely. We were able to take into
account a large time window for exposure (up to 20 years
before diagnosis). By doing this, we allowed for the fact that
the pathophysiological processes, especially for dementia
diseases, begin years before diagnosis. It has been suggested
that there is a temporal lag of about a decade between the
deposition of amyloid β and the clinical manifestation of
Alzheimer’s disease (17). Usually, cognitive symptoms
exist for a few years in dementia patients before admission
to a hospital (13); therefore, we excluded the time period of
5 years before diagnosis. Conducting the analyses using the
time frame ≤20 years before diagnosis yielded virtually the
same results. We put a lot of effort into identifying all
current and historical power lines with voltage levels of
≥132 kV.
While the validity of Parkinson’s disease diagnoses (18)
and dementia diagnoses (in the elderly population of
Denmark (evaluated in the year 2003 (13))) was found to be
quite high, the validity of dementia subtypes was shown to
be less reliable (13): 33% of Alzheimer’s disease cases in
2003 were misclassified as dementia without specification.
Nevertheless, for 81% of all patients recorded as having Alzheimer’s disease, the recorded diagnosis was correct. Thus,
the majority of our identified Alzheimer’s disease cases
would have been true cases of Alzheimer’s disease, which is
reassuring for the results. For vascular dementia, however,
only 18.5% of 27 cases recorded as vascular dementia were
confirmed as vascular dementia, with the rest being mainly
dementia without specification (48.1%) and Alzheimer’s
disease (22.2%); therefore, the results for vascular dementia
must be interpreted with caution.
A limitation of our study was the potential for exposure
misclassification. Distance to a power line is only a crude
proxy for exposure to magnetic fields (19), as the magnetic
field generated by power lines is determined by other
factors, including load characteristics and placement of
phases. However, a study in Denmark using personal measurement devices found that the magnetic fields in residences near power lines (defined as <100 m from 400-kV
lines, <50 m from 132-/150-kV lines, and <25 m from 50-/
60-kV lines) were markedly higher than those in houses
farther away (geometric mean: 0.29 µT vs. 0.04 µT) (20).
Other potential sources of ELF-MF from power lines not
considered in this study were underground transmission
cables, overhead power lines with lower voltage (≤132 kV),
and transformer stations. In Denmark, most of the urban
Copenhagen area is served by underground cables. In most
situations, the field generated by an underground cable only
exceeds background levels within a few meters of the cable,
limiting the number of potentially exposed residences (21).
Therefore, although there will be some highly exposed residences among persons classified as living beyond 600 m
Am J Epidemiol. 2013;177(9):970–978
Power Lines and Neurodegenerative Diseases 975
Table 2. Hazard Ratios for Neurodegenerative Disease Among Persons Who Had Ever Lived Within a Certain
Distance of 132- to 400-kV Power Lines in the Time Span 5–20 Years Before Diagnosis, Denmark, 1994–2010
Diagnosis and Distance
From 132- to 400-kV
Power Line, m
No. of
Casesa
No. of
Controls
Adjustedb
Crude
HR
95% CI
HR
95% CI
Alzheimer’s disease
0–<50
28
165
0.92
0.62, 1.37
1.04
0.69, 1.56
50–<200
184
1,178
0.85
0.73, 0.99
0.95
0.81, 1.12
200–<600
907
5,181
0.95
0.89, 1.03
1.05
0.98, 1.13
19,456
106,693
≥600
1
Referent
1
Referent
Vascular dementia
0–<50
11
82
0.75
0.40, 1.40
0.80
0.43, 1.52
50–<200
92
596
0.83
0.67, 1.04
0.94
0.75, 1.17
0.89
0.80, 0.99
0.99
0.89, 1.11
200–<600
≥600
420
2,558
9,684
52,733
1
Referent
1
Referent
Other dementia
0–<50
50–<200
200–<600
87
583
0.82
0.65, 1.02
0.93
0.74, 1.17
591
3,770
0.85
0.78, 0.93
0.97
0.89, 1.06
0.88
0.84, 0.92
0.97
0.93, 1.01
2,635
16,237
65,439
356,307
35
179
1.03
0.71, 1.48
1.14
0.79, 1.64
50–<200
207
1,067
1.01
0.87, 1.17
1.07
0.92, 1.25
200–<600
819
4,650
0.92
0.85, 1.00
0.97
0.90, 1.05
15,864
84,164
27
106
1.05
0.69, 1.61
1.03
0.67, 1.58
≥600
1
Referent
1
Referent
Parkinson’s disease
0–<50
≥600
1
Referent
1
Referent
Multiple sclerosis
0–<50
50–<200
200
775
1.05
0.89, 1.23
1.06
0.90, 1.24
200–<600
787
3,135
1.02
0.94, 1.11
1.03
0.95, 1.12
7,220
30,519
≥600
1
Referent
1
Referent
Motor neuron disease
0–<50
7
38
0.94
0.42, 2.10
0.80
0.34, 1.89
50–<200
45
227
0.96
0.69, 1.33
0.94
0.66, 1.32
200–<600
184
897
0.98
0.83, 1.16
0.97
0.81, 1.16
2,754
13,834
≥600
1
Referent
1
Referent
Abbreviation: CI, confidence interval; HR, hazard ratio.
a
Persons diagnosed with a neurodegenerative disease in Denmark during 1994–2010.
b
Adjusted for disposable income, education, urbanization category, number of floors in the residential building,
and marital status.
from a power line, they are likely to constitute a very small
percentage of the total reference population. We were not
able to investigate occupational exposures, which could
have constituted an additional source of ELF-MF exposure
for some of the study participants.
Comparison with previous studies and interpretation
Except for the Swiss study (10), most studies that have
investigated the relationship between exposure to ELF-MF
Am J Epidemiol. 2013;177(9):970–978
and neurodegenerative diseases have been conducted in occupational settings. The direct comparison of residential studies
with occupational studies is generally limited because of
potential occupational coexposures and different exposure patterns. Exposures in occupational settings are generally greater
than those in the general population (4, 6). In addition, while
exposure levels in residential settings can be expected to be
fairly constant, they might be more variable in occupational
settings. Since no established mechanism by which magnetic
fields could produce neurodegenerative diseases has been
identified so far (2, 22), it is unclear which aspect of exposure
976 Frei et al.
Table 3. Hazard Ratios for Neurodegenerative Disease According to Cumulative Duration of Residency Within 50 m of 132- to 400-kV Power
Lines in the Time Span 5–20 Years Before Diagnosis, Denmark, 1994–2010
Diagnosis and Cumulative
Duration of Residency
Within 50 m of a 132- to
400-kV Power Line, years
Adjustedb
Crude
No. of
Casesa
No. of
Controls
≥10
11
5–9
<5
HR
95% CI
HR
95% CI
95
0.60
0.32, 1.13
0.71
0.38, 1.33
10
31
1.64
0.80, 3.36
1.79
0.87, 3.68
7
39
0.98
0.44, 2.19
1.08
0.48, 2.45
19,456
106,693
≥10
7
40
1.01
0.45, 2.26
1.14
0.50, 2.57
5–9
2
13
0.81
0.18, 3.60
0.96
0.21, 4.29
<5
2
29
0.39
0.09, 1.63
0.39
0.09, 1.63
9,684
52,733
≥10
44
317
0.76
0.56, 1.05
0.90
0.66, 1.25
5–9
18
122
0.81
0.49, 1.32
0.87
0.53, 1.43
0.93
0.61, 1.42
1.01
0.66, 1.56
Alzheimer’s disease
Always lived ≥600 m away
1
Referent
1
Referent
Vascular dementia
Always lived ≥600 m away
1
Referent
1
Referent
Other dementia
<5
25
144
65,439
356,307
≥10
15
105
0.76
0.44, 1.31
0.86
0.50, 1.49
5–9
8
28
1.37
0.62, 3.02
1.40
0.63, 3.12
1.44
0.75, 2.75
1.61
0.84, 3.08
Always lived ≥600 m away
1
Referent
1
Referent
Parkinson’s disease
<5
12
46
15,864
84,164
2
20
0.43
0.10, 1.86
0.43
0.10, 1.86
5–9
3
23
0.47
0.14, 1.58
0.45
0.13, 1.51
<5
22
63
1.56
0.94, 2.59
1.54
0.93, 2.56
7,220
30,519
≥10
3
20
0.81
0.24, 2.74
0.51
0.14, 1.86
5–9
1
6
0.87
0.10, 7.22
1.52
0.18, 13.10
<5
3
12
1.19
0.33, 4.22
1.14
0.30, 4.30
2,754
13,834
Always lived ≥600 m away
1
Referent
1
Referent
Multiple sclerosis
≥10
Always lived ≥600 m away
1
Referent
1
Referent
Motor neuron disease
Always lived ≥600 m away
1
Referent
1
Referent
Abbreviations: CI, confidence interval; HR, hazard ratio.
a
Persons diagnosed with a neurodegenerative disease in Denmark during 1994–2010.
b
Adjusted for disposable income, education, urbanization category, number of floors in the residential building, and marital status.
is relevant for health, if any, and peak exposures might (for
example) be more influential than average exposures.
There was little evidence of an association between magnetic field exposure and Parkinson’s disease in occupational
settings and in the Swiss residential study (5, 6, 10), which
corresponds to our findings. Regarding motor neuron disease,
in the Swiss study (10) and a very recent Brazilian study (23),
no increased risks were observed, but the statistical power of
these studies was very limited because of few cases. The
occupational studies, however, found a more consistent link
between electrical occupations and motor neuron disease (5,
6). Confounding by electric shock has been discussed as a
potential explanation for the findings due to weaker evidence
of an association when magnetic field levels were measured
(6), but the evidence is controversial (9). A recent Danish
study did not find an increased risk of amyotrophic lateral
sclerosis for persons who had experienced an electric shock,
but the statistical power was low in that study as well (24).
With regard to Alzheimer’s disease or dementia, the results
of previous occupational studies are inconsistent but generally
indicate increased risks for stronger magnetic fields (4–6).
Associations for Alzheimer’s disease were found at levels of
≥0.1 µT to ≥0.5 µT (4). Residential exposures within 50 m of
a power line can be on the same order of magnitude: The
Am J Epidemiol. 2013;177(9):970–978
Power Lines and Neurodegenerative Diseases 977
Table 4. Hazard Ratiosa for Diagnosis of Alzheimer’s Disease Among Persons Living Within 50 m of a 132- to 400-kV Power Line in the Time
Span 5–20 Years Before Diagnosis, According to Age at Diagnosis and Year of Diagnosis, Denmark, 1994–2010
Year of Diagnosis
Age at Diagnosis,
years
≥2003
<2003
All Years
No. of
Casesb
No. of
Controls
HR
95% CI
65–75
2
12
0.93
0.21, 4.17
9
23
2.08
0.96, 4.50
11
35
1.70
0.86, 3.35
>75
0
34
—c
—
17
96
0.96
0.57, 1.60
17
130
0.71
0.43, 1.18
All ages
2
46
0.24
0.06, 0.99
26
119
1.18
0.77, 1.80
28
165
0.92
0.62, 1.37
65–75
2
12
0.85
0.18, 3.93
9
23
2.59
1.17, 5.76
11
35
1.92
0.95, 3.87
>75
0
34
—c
—
17
96
1.09
0.65, 1.83
17
130
0.81
0.48, 1.34
All ages
2
46
0.26
0.06, 1.07
26
119
1.35
0.88, 2.08
28
165
1.04
0.69, 1.56
No. of
Cases
No. of
Controls
HR
95% CI
No. of
Cases
No. of
Controls
HR
95% CI
Crude
estimate
Adjusted
estimated
Abbreviations: CI, confidence interval; HR, hazard ratio.
Reference category: always having lived ≥600 m from a 132- to 400-kV power line.
b
Persons diagnosed with Alzheimer’s disease in Denmark during 1994–2010.
c
No risk calculation was possible because of missing cases.
d
Adjusted for disposable income, education, urbanization category, number of floors in the residential building, and marital status.
a
Danish company energinet.dk (Fredericia, Denmark), the
owner of the electricity infrastructure in Denmark, has calculated magnetic field levels for the most common pylons with
average load currents in Denmark: For a 2-system 400-kV
line, calculated magnetic fields at a distance of 50 m from the
line were between 0.3 µT and 0.9 µT for optimal phase configurations and up to 1.1 µT for other configurations, and for a
132-kV line, they were approximately 0.1 µT with optimal
phase configurations and 0.3 µT with other configurations (21).
Potential work-related coexposures, such as exposure to solvents, pesticides, and lead, were frequently discussed as potential confounders for dementia (4, 5).
In addition, Garcia et al. (4) found indications of publication bias, with larger studies on Alzheimer’s disease showing
a smaller degree of association than smaller studies. Unlike
suggestions in some studies (8, 25), we have not observed
risk differences between men and women for Alzheimer’s
disease. Regarding latency of the effects, the results of some
occupational studies indicate a rather late-acting influence in
the disease process, while other studies have found effects
for accumulated exposure throughout life (4, 7, 9, 25). The
Swiss study found increased risk estimates for persons living
within 50 m of a 220- to 380-kV power line for at least 15
years (HR = 2.00, 95% CI: 1.21, 3.33), indicating a cumulative effect (10). In the present study, we did not observe
such a risk increase for people living close to power lines for
at least 10 years. As suggested by occupational studies in
Sweden (7–9), we found indications that age of onset might
play a role. The proportion of patients with relatively early
Alzheimer’s disease onset might have been increased in the
Swiss study, since underreporting of dementia diagnoses on
death certificates increases with the age of the deceased
(15, 16), which could be a potential explanation for the risk
increase found in the Swiss study.
Am J Epidemiol. 2013;177(9):970–978
Conclusions
Overall, we did not observe an increased risk of neurodegenerative diseases among persons living close to power lines.
Our study therefore confirms the findings of a previous study
(10) of no association for Parkinson’s disease, multiple sclerosis, motor neuron disease, vascular dementia, and other types
of dementia. Furthermore, it does not confirm the finding of
an increased risk of Alzheimer’s disease. Given the high
quality and representativeness of our study, this is reassuring,
since together the studies provide little evidence that living
close to a power line increases the risk of any neurodegenerative disease. We found some weak suggestions that ELF-MF
exposure might increase the risk of Alzheimer’s disease when
diagnosed by the age of 75 years, although those subgroup
analyses must be interpreted with care given the overall
finding of no association. If the latter observed association
were causal, ELF-MF exposure would explain 5 (0.02%) of
the Alzheimer’s disease cases in our study population.
ACKNOWLEDGMENTS
Author affiliations: Danish Cancer Society Research Center,
Danish Cancer Society, Copenhagen, Denmark (Patrizia Frei,
Aslak Harbo Poulsen, Camilla Pedersen, Christoffer Johansen);
Environment Department, Electric Power Research Institute,
Palo Alto, California (Gabor Mezei); Memory Disorders
Research Group, Department of Neurology, Copenhagen University Hospital, Rigshospitalet, Denmark (Lise Cronberg
Salem); Department of Epidemiology and Public Health,
Swiss Tropical and Public Health Institute, University of
Basel, Basel, Switzerland (Patrizia Frei, Martin Röösli); and
978 Frei et al.
Section of Environment and Radiation, International Agency
for Research on Cancer, Lyon, France (Joachim Schüz).
This work was supported by a fellowship for prospective
researchers awarded by the Swiss National Science Foundation (stipend PBBSP3-133396) to P.F. A.H.P. was supported
by a stipend for doctoral students from the Danish Graduate
School in Public Health Science. All other authors contributed to this work on the basis of their respective core budget
positions. Additional funding was provided by the Electric
Power Research Institute to the Danish Cancer Society
Research Center (contract EP-P38793/C17252).
We thank Rikke Baastrup for support with the address
data, Nick Martinussen for help in the preparation of the
data, the power-line companies for providing data on the
locations of their power lines, Kasper Grue Understrup of
the Danish National Board of Health for providing data on
cases and controls, and Henrik Hobel of DM Partner A/S
(Copenhagen, Denmark) for providing geocodes. Special
thanks go to Martin Hertach for supporting our analyses
involving ArcGIS with his excellent expertise.
Conflict of interest: none declared.
11.
12.
13.
14.
15.
16.
17.
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