1. Why was the study done?
Brain tumors are uncommon but often cause serious disabilities.
Some types of brain tumors, particularly the most common type, glioma,
have poor survival rates.
Very little is known about the causes of malignant or benign
brain tumors. To pursue possible clues about causes from previous
studies and identify promising new leads, the National Cancer Institute
(NCI) began a comprehensive study of possible environmental and genetic
causes of malignant and benign brain tumors in 1994. The present
report focusing on cellular telephones is one part of the much larger study
and was intended to determine whether use of these devices increases the
risk of brain tumors.
2. Why were cell phones included in the NCI study?
The number of people using cellular telephones has increased
dramatically during the past decade and is likely to continue increasing
for the foreseeable future. According to the Cellular Telecommunications
Industry Association, there are currently about 107 million mobile phone
subscribers in the United States, increasing at a rate of about 2 million
per month (http://www.wow-com.com).
Use of hand-held cellular phones involves placing a small transmitter
which emits radio frequency radiation (microwave radiation) next to the
head. There has been widespread public concern that radiation from
cellular phones might cause tumors of the brain and nervous system.
Unlike ionizing radiation, which, at high doses, is known to cause brain
tumors, it is not known whether the non-ionizing radiation from cell phones
poses a cancer risk. Because so many people use cellular phones,
it is important to learn whether it does, and to provide reassurance if
it does not.
3. Who were the participants in the study?
The NCI study included 782 brain tumor cases and 799 controls
from three medical institutions: St. Joseph’s Hospital and Medical Center
in Phoenix; Brigham and Women’s Hospital in Boston; and Western Pennsylvania
Hospital in Pittsburgh. The study included brain tumor patients recently
diagnosed with glioma (489 cases), meningioma (197 cases) or acoustic neuroma
The controls are people who were admitted to the same hospitals
as the brain tumor cases for a variety of non-cancerous conditions.
Controls were matched with cases by hospital, sex, race, age, and distance
of residence from hospital. This type of study is known as a “case-control”
study. The study was restricted to adults who were age 18 or older
who received care at one of the participating hospitals, resided within
50 miles of the hospital, and could understand English or Spanish.
Data collection began in 1994 and was completed in 1998.
4. How was cellular phone use measured?
The extent of use of hand-held cellular phones was assessed by
interview. A card displaying photographs of examples of different
types of portable phones was shown, including hand-held, car, and transportable
cellular phones, plus cordless phones. Of these, only hand-held cellular
phones have elicited widespread concern as a possible cause of brain tumors
(see question 13).
To help ensure that study participants did not confuse cordless
phones with hand-held cellular phones, the interviewers described distinguishing
features, including that cellular phones have their own phone and billing
numbers, whereas cordless phones can be on the same number as other household
phones. Names of local cellular service providers were mentioned
by the interviewer, as these often were different from the regular (non-cellular)
Questions were asked about whether a person had used a hand-held
cellular phone never, one to five times, or more than five times in their
lifetimes. Persons who reported using a hand-held cellular phone
more than five times were asked about when they started and stopped using
the phone. Regular users were asked about their average minutes of
use per day and duration of regular use (regular use was defined as two
or more calls per week). The questionnaire also asked about which
hand was used to hold the phone when talking.
Information about specific make or model of phone was not collected.
Knowing the specific type of phone used does not tell you how much radiation
a person was exposed to, because the level of exposure to the brain depends
on a variety of factors, such as distance from the nearest cellular base
station (see question 11).
5. What were the major findings concerning use of hand-held cellular
There was no indication of higher brain tumor risk among persons
who had used
hand-held cellular phones compared to those who had not used
them. More importantly, there was no evidence of increasing risk
with increasing years of use or average minutes of use per day, nor did
brain tumors among cellular phone users tend to occur more often than expected
on the side of the head on which the person reported using their phone.
Specifically, there was no indication of increased risk associated with
use of a cell phone for one hour or more per day, for five or more years,
or for cumulative use of more than 100 hours. These findings pertain
to all three tumor types considered (glioma, meningioma, and acoustic neuroma).
6. What conclusions can be drawn from the results?
The results pertain primarily to patterns of cell phone use in
the United States during the early to middle 1990s. During the period
of this study, there was no evidence that use of hand-held cellular phones
caused tumors of the brain and nervous system. The findings suggest
that, if there was any increase in risk, it was small, particularly for
malignant tumors (glioma).
Caution should be used in interpreting the findings for several
reasons: (1) Widespread use of cellular telephones is a recent phenomenon,
and few people used cell phones before the 1990s in the United States.
This study included a small number of persons who reported using a cell
phone frequently for more than five years. If there is an increased
risk of brain tumors due to use of cellular phones that only appears after
five or more years, or only among very heavy users, it is probable that
this study would have failed to detect it. (2) This study was designed
to be large enough to detect an increased risk of all types of glioma combined.
The study was not large enough to detect an increased risk for different
subtypes of glioma. (3) Information about duration and frequency
of cellular phone use was obtained by interview and thus dependent on the
ability of study participants to recall this information accurately.
(4) Other factors influence the level of exposure of brain and nervous
system tissue in the head to radio frequency radiation from cellular phones,
in addition to amount of use. These include distance from the cellular
base station, local topography and vegetation, whether the phone is used
indoors or outdoors, design of the particular model of phone, and position
of the antenna and phone in relation to the head. (5) During the
time period covered by our study, the overwhelming majority of cellular
phones in use were analogue phones. Today, most cellular phones use
are based on digital, rather than analogue, technology.
Given these limitations, it would be premature to conclude that
use of hand-held cellular telephones does not cause tumors of the brain
and nervous system.
7. What have other studies shown about the relationship between use
of cellular telephones and brain tumors?
The literature is very limited. A recently published study
by Muscat et al involved 469 brain cancer (glioma) cases and an equal number
of controls. The researchers did not find an association between
use of hand-held cellular telephones and the risk of gliomas. A small
Swedish study by Hardell et al published in 1999 involving 233 brain tumor
cases and 466 controls did not find an association between the amount of
use of analogue or digital cell phones and risk of brain tumors.
The investigators reported an association between side of the head on which
the tumor occurred and side of phone use, but this was based on a small
number of brain tumor cases.
8. Are there other studies of cell phones and brain tumors in progress?
Yes. The largest of these is a multicenter, international
case-control study involving about 3,000 cases and 3,000 controls, coordinated
by the International Agency for Research on Cancer (IARC), based in Lyon,
France. Results are expected in several years. (http://www.iarc.fr).
A Danish study, which includes a cohort of 550,000 cellular phone
1982 to 1995, is expected to be published soon.
A $10 million program on cell phone research was recently announced
in the United Kingdom. (http://www.doh.gov.uk/ click on “News Desk”
for Dec. 8, 2000). Also, the U.S. Food and Drug Administration (FDA)
and the Cellular Telecommunications Industry Association (CTIA) have recently
signed a Cooperative Research and Development Agreement (CRADA).
FDA will provide scientific and technical guidance for studies that evaluate
the health effects of cellular phone use. (http://www.fda.gov/cdrh/ocd/wlessphonecrada.html)
9. Are there any other health risks associated with the use of cell
Yes. One study by Hayes et al in 1997 involving 980 people
with pacemakers reported that a cellular telephone held over the pacemaker
interfered with a substantial proportion of pacemakers, in some cases leading
to symptoms. However, holding the telephone against the ear caused
little interference; the main risk occurs when the telephone is held against
To date, the most significant health effect for cell phone use
is an increased risk of car accidents. A 1997 article in New England
Journal of Medicine showed that the risk of a collision was about four
times greater when the driver was using either a hand-free telephone or
holding the phone to the ear with one hand. There is also evidence
from a 1999 study in Journal of the American Medical Association (JAMA)
by Dreyer et al that the heaviest users have more than twice the mortality
rates from car accidents than the lightest users.
10. Might the cancer risk due to use of cellular phones be greater in
children than in adults?
The NCI study included only adults (age ?18 years at the time
of brain tumor diagnosis), and few children used cell phones prior to 1994,
so there are no data concerning possible risks among children. Certain
agents known to cause cancer of the brain and nervous system in rats (e.g.
ionizing radiation or certain chemicals) have the greatest effect when
administered very early in life, while the nervous system is developing.
However, the situation with cellular phones is somewhat different insofar
as exposure to non-ionizing radiation from cell phones has not been demonstrated
to cause brain tumors in humans or experimental animals of any age.
11. How do cellular phones work?
Cellular technology provides a two-way radio communications system
between a portable handset and the nearest base-station. A given
geographical region is divided into zones or cells, each of which is equipped
with a base station. The base station receives radio frequency signals
from cell phones and sends radio frequency signals to neighboring stations
and other cell phones. The base station is also connected to the
land-line telephone network. When a call is placed from
a cellular phone, a signal is sent from the antenna of the phone to the
base station antenna. The base station routes the voice signal though
a switching center, where the call can be transferred to another cellular
telephone, another base station, or to the local land-line telephone system.
The intensity of radio frequency exposure depends on the power
level of the signal which depends, in part, on the distance of the telephone
from the base station. The farther the telephone is from the base
station antenna, the higher the power level needed to maintain the connection.
In a rural area, the cell may extend over many miles and in urban areas
may cover only a fraction of a mile.
12. Did the study consider radiation exposure from cellular base station
No. Radiation levels drop off dramatically with increasing
distance from base station antennas. One would have to be very close
to the antennas to have much exposure. It is difficult to estimate
an individual’s exposure to radio frequency radiation from base stations
because many persons do not know if they are living, working, or driving
close to base station antennas, and most probably cannot estimate the distance
13. What level of radio frequency exposure is associated with the different
types of wireless phones (hand-held cellular phones, car cellular phones,
transportable cellular phones, and cordless phones)?
The closer the transmitter of the wireless telephone is to the
body, the greater the exposure. With hand-held cellular phones, the radio
transmitter is in the handset which is typically held against the side
of the head while the phone is in use. With car cellular phones the
antenna is mounted on the outside of the car some distance from the user.
With transportable cellular phones or “bag phones” the transmitter is with
the battery pack in a portable unit separate from the handset. Cordless
phones are not cellular phones. They have a base unit wired to the
land-line telephone service and typically operate at a lower frequency
and much lower power than other wireless phones. Lower power
is typical of cordless phones because the radio signal from the handset
only needs to reach the base unit in the home or office versus a distant
If hand-held cellular phones do not cause brain tumors, it would
be surprising if the other types of phones do. They involve much
lower radiation exposures to the brain.
14. What have been the changes
in cellular phone technology since the data were collected?
During the 1980s and early to middle 1990s, most cell phones in
the United States operated in the microwave frequency range between 800
and 900 MHz (megahertz: a million hertz; a hertz has a frequency
of one cycle per second). There has been a trend towards use of higher
frequencies. It is unknown whether cancer risk depends on the frequency.
There also has been a change from phones based on analogue signal transmission
to those based on digital signal transmission, and increased use of devices
that keep the radio transmitter farther from the head while the phone is
in use (hands-free accessories).
15. What is the difference between analogue and digital phones?
Analogue phones transmit a constant radio frequency signal and
encode conversations through modulation of that signal. Digital phones
transmit information through a series of pulses of radio frequency radiation
rather than through modulation of a constant radio discrete frequency signal.
At this point, there is little or no evidence that use of either
type of cellular phone increases the risk of cancer. Digital phones
typically operate at lower average power levels. Some people have
speculated that risk might be related to the nature of modulation of the
radio frequency signal rather than the power or rate of energy deposition
in tissue. However, there are no epidemiologic data to support this
16. What is the difference between radio frequency radiation and microwave
Radio frequency radiation covers a range of frequencies (from
about 10,000 Hz to 1,000,000,000,000 Hz or 1 million MHz) of which microwave
radiation is one type. Cellular telephones operate in the microwave
frequency range (about 800 MHz - 2,100 MHz). AM/FM radios, and VHF/UHF
TVs operate at lower radio frequencies than cellular phones, while microwave
ovens, radar, and satellite-stations operate at higher radio frequencies
than those used for cellular phones.
17. What are some of the differences between the types of brain tumors?
Primary brain tumors are tumors that arise in the brain, unlike
tumors that begin elsewhere in the body and then spread to the brain.
They are classified by the type of cell in which they develop. The
most common brain tumors are gliomas. Gliomas develop in the glial
cells which make up the soft, spongy tissue that supports the nerve cells
in the brain. There are several types of gliomas. One type,
astrocytoma, arises from small, star-shaped cells called astrocytes, and
can grow anywhere in the brain or spinal cord. In adults, astrocytomas
most often arise in the cerebrum, the largest part of the brain that fills
most of the upper skull. Glioblastoma is an especially malignant
form of astrocytoma. Gliomas are more common among men. When
people say “brain cancer,” they usually are referring to glioma.
Meningiomas are brain tumors that develop in the meninges, the
protective membrane covering the brain directly underneath the skull.
These tumors are usually benign and grow slowly. They occur most
often in women between 30 and 50 years old.
Schwannomas are benign tumors that develop in Schwann cells which
produce the myelin that protects the acoustic nerve. (Myelin is a
fatty substance that covers and protects nerves.) Acoustic neuromas
are a type of schwannoma that occur in the nerve between the brain and
the ear. They occur primarily in adults and affect women more often
Among adults, the most frequent types of brain tumors are nervous
system gliomas (especially astrocytic gliomas), meningiomas, acoustic
neuromas, and pituitary gland tumors. Less common types include lymphomas,
vascular system tumors, and pineal gland tumors.
18. How common are brain tumors and has the incidence of brain tumors
changed over time?
About 17,000 cases of brain cancers are diagnosed in the United
States each year. This comprises only 1 percent to 2 percent of the
total number of cases of cancer diagnosed each year.
Between 1990-1997, the over-all incidence rates for brain cancer
decreased by 1.4 percent (from 6.5 cases to 5.9 cases for every 100,000
persons in the United States). The mortality rates from
1990 to 1997 have also decreased slightly -- by 0.7 percent ( from
4.3 deaths to 4.1 for every 100,000 persons in the United
Looking at long-term trends for specific age groups, it appears
that among persons 70 or older, there was a higher incidence of brain cancer
in 1991 to 1995 compared to 1975 to 1979. This is in contrast to
persons under 70, for whom the rates were similar in both periods.
The most dramatic increase was in the 85 or older group, for which the
rates in the most recent five-year period were nearly three times higher
than in those in 1975 to 1979 (15.7 cases vs. 5.4 cases per 100,000 persons
in the United States). The higher rates probably are due, at least
in part, to improvements in the ability to diagnose and treat brain tumors
in elderly patients. The increased use of CT (computed tomography),
MRI (magnetic resonance imaging) and stereotactic biopsy procedures (more
precise methods for locating and diagnosing tumors) among physicians for
this population during the later time period correlates with the increased
incidence trends, and represents a more recent widespread tendency of physicians
to aggressively pursue diagnoses in older patients.
19. What other possible causes of brain tumors are being evaluated in
the NCI study?
Other factors under consideration include workplace exposures
to chemical agents and electromagnetic fields, dietary factors, family
history of tumors, genetic factors, home use of selected appliances, reproductive
history and hormonal exposures, viruses, medical and dental exposure to
ionizing radiation, and other aspects of medical history. Results
from these studies will be reported in future publications. For periodic
updates, refer to the NCI Web site: http://www.cancer.gov.
20. Who are the principal investigators for the NCI study?
The principal investigators at NCI are Peter Inskip, Sc.D. and
Martha Linet, M.D. of NCI’s Division of Cancer Epidemiology and Genetics
in Rockville, Md.
21. Where can I find out more about the NCI brain tumor study?
Inskip P.D., Tarone R.E., Hatch E.E., Wilcosky T.C., Shapiro W.R.,
Fine H.A., Black P.M., Loeffler J.S., Linet M.S. Cellular-
telephone use and brain tumors. N Engl J Med 2001;344:79-86. ( http://www.nejm.org/content/index.asp
Inskip P.D., Hatch E.E., Stewart P.A., Heineman E.F., Ziegler
R.G., Dosemeci M., et al. Study design for a case-control investigation
of cellular telephones and other risk factors for brain tumors in adults.
Radiat Prot Dosim 1999;86:45-52.
22. Where can I find out more about cell phones or brain tumors in general?
Hardell L., Näsman Å., Påhlson A., Hallquist
A., Hansson Mild K. Use of cellular telephones and the risk for brain
tumours: a case-control study. Int J Oncol
Cardis E., Kilkenny M. International case-control study
of adult brain, head and neck tumours: results of the feasibility study.
Radiat Prot Dosim 1999;83:179-83.
Dreyer N.A., Loughlin J.E., Rothman K.J.. Cause-specific
mortality in cellular telephone users (Research letter). JAMA 1999;17:1814-6.
Independent Expert Group on Mobile Phones. Mobile phones
and health. Chilton, Didcot (United Kingdom): National Radiological Protection
Board, 2000. Web site
Web site: http://www.iegmp.org.uk.
Inskip P.D., Linet M.S., Heineman E.F. Etiology of brain tumors
in adults. Epidemiol Rev 1995;17:382-414.
Moulder J.E., Erdreich L.S., Malyapa R.S., Merritt J., Pickard
W.F., Vijayalaxmi. Cell phones and cancer: what is the evidence for
a connection? Radiat Res 1999;151:513-31.
Muscat et al . Handheld cellular telephone use not associated
with risk of brain cancer. JAMA 2000; 284:3001-3007
Preston-Martin S., Mack W.J. Neoplasms of the nervous system.
In: Cancer Epidemiology and Prevention. Edited by Schottenfield D.
and Fraumeni JF. Oxford University Press: New York, 1996, pp.1231-81.
Rothman K.J. Epidemiological evidence on health risks of
cellular telephones. Lancet 2000;356:1837-1840.
Rothman K.J., Chou C.K., Morgan R., Balzano Q., Guy A.W., Funch
D.P., et al. Assessment of cellular telephone and other radio frequency
exposure for epidemiologic research. Epidemiology 1996;7:291-8.
National Radiological Protection Board (NRPB) Web site: http://www.nrpb.org.uk/index.html.
World Health Organization International Commission on Non-Ionizing
Radiation Protection (select Qs and As): http://www.who.int/emf.
Federal Communication Commission (FCC) RF Safety Program (select
“Information on Human Exposure to RF Fields from Cellular and PCS Radio
Cellular Telecommunications Industry Association (CTIA): http://www.wow-com.com.
Food and Drug Administration: http://www.fda.gov/cdrh/ocd/mobilphone.html.
Central Brain Tumor Registry: http://www.cbtrus.org/page2t.htm