CELL-PHONE TECHNOLOGY

Wireless phones which receive their signals from towers. A cell is typically the area (several
miles) around a tower in which a signal can be received.
Cell phones provide an incredible array of functions. Depending on the cell-phone model, you
can:
• Store contact information
• Make task or to-do lists
• Keep track of appointments and set reminders
• Use the built-in calculator for simple math
• Send or receive e-mail
• Get information (news, entertainment, stock quotes) from the internet
• Play games
• Watch TV
• Send text messages
• Integrate other devices such as PDAs, MP3 players and GPS receivers
A cell phone is a full-duplex device. That means that you use one frequency for talking and a
second, separate frequency for listening. Both people on the call can talk at once.
Division of a city into small cells allows extensive frequency reuse across a city, so that
millions of people can use cell phones simultaneously. Cell phones operate within cells, and they
can switch cells as they move around. Cells give cell phones incredible range. Someone using a
cell phone can drive hundreds of miles and maintain a conversation the entire time because of the
cellular approach. Each cell has a base station that consists of a tower and a small building
containing the radio equipment.

A single cell in an analog cell-phone system uses one-seventh of the available duplex voice
channels. That is, each cell is using one-seventh of the available channels so it has a unique set of
frequencies and there are no collisions:
• A cell-phone carrier typically gets 832 radio frequencies to use in a city.
• Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically
395 voice channels per carrier. (The other 42 frequencies are used for control channels)
Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people
can be talking on their cell phone at one time. Analog cellular systems are considered
first-generation mobile technology, or 1G. With digital transmission methods (2G), the number
of available channels increases. For example, a TDMA-based digital system (more on TDMA
later) can carry three times as many calls as an analog system, so each cell has about 168
channels available.
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths:
0.6 watts and 3 watts. The base station is also transmitting at low power. Low-power transmitters
have two advantages:
• The transmissions of a base station and the phones within its cell do not make it very
far outside that cell. Therefore, 2 different cells can reuse the same 56 frequencies.
Hence, the same frequencies can be reused extensively across the city.
• The power consumption of the cell phone, which is normally battery-operated, is
relatively low. Low power means small batteries, and this is what has made handheld
cellular phones possible.
The cellular approach requires a large number of base stations in a city of any size. A typical
large city can have hundreds of towers. But because so many people are using cell phones, costs
remain low per user. Each carrier in each city also runs one central office called the Mobile
Telephone Switching Office (MTSO). This office handles all of the phone connections to the
normal land-based phone system, and controls all of the base stations in the region.

All cell phones have special codes associated with them. These codes are used to identify the
phone, the phone's owner and the service provider. The various Cell Phone Codes used are as
follows:
1. Electronic Serial Number (ESN) : It is a unique 32-digit number programmed into the
phone when it is manufactured.
2. Mobile Identification Number (MIN) : A 10-digit number derived from the phones
number
3. System Identification Code (SID) : A unique 5-digit number that is assigned to each
carrier by the Federal Communications Commission (FCC).
ESN is a permanent part of the phone while both MIN and SID codes are programmed into
the phone when a service plan is purchased and the phone is activated.
Some of the necessary terminologies for cell-phone connection are described:
1. Mobile Telephone Switching Office (MTSO) : The switching office that all base
station cell sites connect to. It is a sophisticated computer that monitors all cellular calls,
keeps track of the location of all cellular-equipped vehicles traveling in the system,
arranges hand-offs, keeps track of billing information, etc. The MTSO in turn interfaces
to the PSTN by connection to a Control Office.
2. Public Switched Telephone Network (PSTN) : It is the network of the world's public
circuit-switched telephone networks, in much the same way that the Internet is the
network of the world's public IP-based packet-switched networks. Originally a network
of fixed-line analogue telephone systems, the PSTN is now almost entirely digital, and
now includes mobile as well as fixed telephones.
3.
If you have a cell phone, you turn it on and someone tries to call you. Here is what happens to
the call:
• When you first power up the phone, it listens for an SID on the control channel. The
control channel is a special frequency that the phone and base station use to talk to one
another about things like call set-up and channel changing. If the phone cannot find any
control channels to listen to, it knows it is out of range and displays a "no service"
message.
• When it receives the SID, the phone compares it to the SID programmed into the phone.
If the SIDs match, the phone knows that the cell it is communicating with is part of its
home system.
• Along with the SID, the phone also transmits a registration request, and the MTSO
keeps track of your phone's location in a database -- this way, the MTSO knows which
cell you are in when it wants to ring your phone.
• The MTSO gets the call, and it tries to find you. It looks in its database to see which
cell you are in.
• The MTSO picks a frequency pair that your phone will use in that cell to take the call.
• The MTSO communicates with your phone over the control channel to tell it which
frequencies to use, and once your phone and the tower switch on those frequencies, the
call is connected. Now, you are talking by two-way radio to a friend.
• As you move toward the edge of your cell, your cell's base station notes that your signal
strength is diminishing. Meanwhile, the base station in the cell you are moving toward
(which is listening and measuring signal strength on all frequencies, not just its own
one-seventh) sees your phone's signal strength increasing. The two base stations
coordinate with each other through the MTSO, and at some point, your phone gets a
signal on a control channel telling it to change frequencies. This hand off switches your
phone to the new cell.

If you're on the phone and you move from one cell to another -- but the cell you move into is
covered by another service provider, not yours. Instead of dropping the call, it'll actually be
handed off to the other service provider.
If the SID on the control channel does not match the SID programmed into your phone, then the
phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of
your home system, which then checks its database to confirm that the SID of the phone you are
using is valid. Your home system verifies your phone to the local MTSO, which then tracks your
phone as you move through its cells. All of this happens within seconds.
On most phones, the word "roam" will come up on your phone's screen when you leave your
provider's coverage area and enter another's. If you want to roam internationally, you'll need a
phone that will work both at home and abroad. Different countries use different cellular access
technologies.

ELECTROMAGNETIC INTERFERENCE

Most of us experience electromagnetic interference on a fairly regular basis. For example:
• If you put your cell phone down on your desk near the computer, you can hear loud
static in your computer's speakers every time the phone and the tower handshake. In the
same way, your car's stereo produces loud static whenever you make a call on your cell
phone.
• When you dial a number on your home's wireless phone, you can hear the number being
dialed through the baby monitor.
• It is not uncommon for a truck to go by and have its CB radio overwhelm the FM station
you am listening to.
• Most of us have come across motors that cause radio or TV static.



None of these things, technically, should be happening. For example, a truck's CB radio is not
transmitting on the FM radio bands, so your radio should never hear CB signals. However, all
transmitters have some tendency to transmit at lower power on harmonic side bands, and this is
how the FM radio picks up the CB. The same thing holds true for the wireless phone crossing
over to the baby monitor. In the case of the cell phone affecting the computer's speakers, the wire
to each speaker is acting like an antenna, and it picks up side bands in the audible range.
These are not dire problems -- they are just a nuisance. But notice how common they are. In an
airplane, the same phenomena can cause big trouble.
An airplane contains a number of radios for a variety of tasks. There is a radio that the pilots use
to talk to ground control and air traffic control (ATC). There is another radio that the plane uses
to disclose its position to ATC computers. There are radar units used for guidance and weather
detection, and so on. All of these radios are transmitting and receiving information at specific
frequencies. If someone were to turn on a cell phone, the cell phone would transmit with a great
deal of power (up to 3 watts). If it happens to create interference that overlaps with radio
frequencies the plane is using, then messages between people or computers may be garbled. If
one of the wires in the plane has damaged shielding, there is some possibility of the wire picking
up the phone's signals just like my computer's speakers do. That could create faulty messages
between pieces of equipment within the plane.

Many hospitals have installed wireless networks for equipment networking. For example, in
case of a heart monitor, the black antenna sticking out of the top of the monitor connects it back
to the nursing station via a wireless network. If you use your cell phone and it creates
interference, it can disrupt the transmissions between different pieces of equipment. That is true
even if you simply have the cell phone turned on -- the cell phone and tower handshake with
each other every couple of minutes, and your phone sends a burst of data during each handshake.
The prohibition on laptops and CD players during takeoff and landing is addressing the same
issue, but the concerns here might fall into the category of "better safe than sorry." A poorly
shielded laptop could transmit a fair amount of radio energy at its operating frequency, and this
could, theoretically, create a problem.

MULTI-BAND VS. MULTI-MODE CELL-PHONES

1. Multiple band - A phone that has multiple-band capability can switch frequencies. For
example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or
a 1900-MHz system. A quad-band GSM phone could use GSM service in the 850-MHz,
900-MHz, 1800-MHz or 1900-MHz band.



2. Multiple mode - In cell phones, "mode" refers to the type of transmission technology
used. So, a phone that supported AMPS and TDMA could switch back and forth as
needed. It's important that one of the modes is AMPS -- this gives you analog service if
you are in an area that doesn't have digital support.

3. Multiple band/Multiple mode - It allows you to switch between frequency bands and
transmission modes as needed.

Changing bands or modes is done automatically by phones that support these options. Usually
the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at
that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it
cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the
digital mode(s) first, then switch to analog.
You can find both dual-mode and tri-mode phones. The term "tri-mode" can be deceptive. It
may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well
as analog. In that case, it is a true tri-mode phone. But it can also mean that it supports one
digital technology in two bands and also offers analog support.

ANALOG CELL-PHONES (FIRST GENERATION)

In 1983, the analog cell-phone standard called AMPS (Advanced Mobile Phone System) was
approved by the FCC and first used in Chicago. AMPS uses a range of frequencies between 824
megahertz (MHz) and 894 MHz for analog cell phones. In order to encourage competition and
keep prices low, the U. S. government required the presence of two carriers in every market,
known as A and B carriers. One of the carriers was normally the local-exchange carrier (LEC),
a fancy way of saying the local phone company.
Carriers A and B are each assigned 832 frequencies: 790 for voice and 42 for data. A pair of
frequencies (one for transmit and one for receive) is used to create one channel. The frequencies
used in analog voice channels are typically 30 kHz wide -- 30 kHz was chosen as the standard
size because it gives you voice quality comparable to a wired telephone.


The transmit and receive frequencies of each voice channel are separated by 45 MHz to keep
them from interfering with each other. Each carrier has 395 voice channels, as well as 21 data
channels to use for housekeeping activities like registration and paging.
A version of AMPS known as Narrowband Advanced Mobile Phone Service (NAMPS)
incorporates some digital technology to allow the system to carry about three times as many
calls as the original version. Even though it uses digital technology, it is still considered analog.
AMPS and NAMPS only operate in the 800-MHz band and do not offer many of the features
common in digital cellular service, such as e-mail and Web browsing.

DIGITAL CELL-PHONES (SECOND GENERATION)

They use the same radio technology as analog phones, but they use it in a different way. Analog
systems do not fully utilize the signal between the phone and the cellular network -- analog
signals cannot be compressed and manipulated as easily as a true digital signal. Digital phones
convert your voice into binary information (1s and 0s) and then compress it. This compression
allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.
Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth
over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly
between the two to send digital information between the cell tower and the phone. Clever
modulation and encoding schemes are required to convert the analog information to digital,
compress it and convert it back again while maintaining an acceptable level of voice quality.
Hence, digital cell phones have to contain a lot of processing power.

INSIDE A CELL-PHONE

A basic digital cell phone contains just a few individual parts:

• A circuit board containing the brains of the phone
• An antenna
• An Liquid Crystal Display (LCD) screen
• A keyboard
• A microphone
• A speaker
• A battery
The circuit board is the heart of the system and contains several chips. The analog-to-digital and
digital-to-analog conversion chips translate the outgoing audio signal from analog to digital and


the incoming signal from digital back to analog. The digital signal processor (DSP) is a highly
customized processor designed to perform signal-manipulation calculations at high speed. The
microprocessor handles all the functions for the keyboard and display, deals with command and
control signaling with the base station and also coordinates the rest of the functions on the
board.The Read Only Memory (ROM) and Flash Memory chips provide storage for the phone's
operating system and customizable features, such as the phone directory. The Radio Frequency
(RF) and power section handles power management and recharging, and also deals with the
hundreds of FM channels. Finally, the RF amplifiers handle signals traveling to and from the
antenna.
The display has grown considerably in size as the number of features in cell phones have
increased. Most current phones offer built-in phone directories, calculators and games. And
many of the phones incorporate some type of PDA or Web browser. Some phones store certain
information, such as the SID and MIN codes, in internal Flash memory, while others use external
cards. Cell phones have tiny speakers and microphones.

CELL-PHONE TOWER

A cell-phone tower is typically a steel pole or lattice structure that rises hundreds of feet into the
air. The box houses the radio transmitters and receivers that let the tower communicate with
the phones. The radios transmitters and receivers connect with the antennae on the tower through
a set of thick cables. The tower and all of the cables and equipment at the base of the tower are
heavily grounded.



HOW VIBRATOR WORKS IN CELLPHONE

If you have a cell phone or a pager, then you know that having it ring in the middle of a movie or
performance is enough to get you killed in some cities. Vibrating devices that quietly replace the
ringer are therefore life-saving devices that are an important part of urban survival!
Figure below shows the inside of a small toy which vibrates heavily similar to a cellphone
device.

Inside the control unit is a small DC motor which drives the gear. Attached to the gear, there is a
small weight. This weight is about the size of a stack of 5 U.S. nickels, and it is mounted
off-center on the gear. When the motor spins the gear/weight combination (at 100 to 150 RPM),
the off-center mounting causes a strong vibration. Inside a cell phone or pager there is the same
sort of mechanism in a much smaller version.

COMMUNICATION TECHNOLOGIES IN SECOND GENERATION CELL-PHONES

There are four common technologies used by 2G cell-phone networks for transmitting
information:

1. Frequency Division Multiple Access (FDMA) : FDMA separates the spectrum into
distinct voice channels by splitting it into uniform chunks of bandwidth. Each call
sends its signal at a different frequency within the available band. FDMA is used mainly
for analog transmission.

 

2. Time Division Multiple Access (TDMA) : TDMA is the access method used by the
Electronics Industry Alliance and the Telecommunications Industry Association for
Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a
narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three
time slots. Each conversation gets the signal for one-third of the time. This is possible
because voice data that has been converted to digital information is compressed so that it
takes up significantly less transmission space. Therefore, TDMA has three times the
capacity of an analog system using the same number of channels. TDMA systems
operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

3. Code Division Multiple Access (CDMA) : CDMA takes an entirely different approach
from TDMA. CDMA, after digitizing data, spreads it out over the entire available
bandwidth. Multiple calls are overlaid on each other on the channel, with each assigned
a unique sequence code. CDMA is a form of spread spectrum, which simply means that
data is sent in small pieces over a number of the discrete frequencies available for use at
any time in the specified range.

4. Global System for Mobile Communication (GSM) : GSM implements TDMA in a
somewhat different and incompatible way from IS-136. GSM systems use encryption to
make phone calls more secure. GSM operates in the 900-MHz and 1800-MHz bands in
Europe and Asia and in the 850-MHz and 1900-MHz band in the United States. It is
used in digital cellular and PCS (Personal Communication Services)-based systems.
GSM is also the basis for Integrated Digital Enhanced Network (IDEN), a popular
system introduced by Motorola and used by Nextel.
PCS was designed from the ground up for greater user mobility. PCS has smaller cells and
therefore requires a larger number of antennas to cover a geographic area. PCS phones
use frequencies between 1.85 and 1.99 GHz (1850 MHz to 1990 MHz).
Technically, cellular systems in the United States operate in the 824-MHz to 894-MHz
frequency bands; PCS operates in the 1850-MHz to 1990-MHz bands. And while it is
based on TDMA, PCS has 200-kHz channel spacing and eight time slots instead of the
typical 30-kHz channel spacing and three time slots found in digital cellular.
GSM is the international standard in Europe, Australia and much of Asia and Africa. In
covered areas, cell-phone users can buy one phone that will work anywhere where the
standard is supported. To connect to the specific service providers in these different
countries, GSM users simply switch subscriber identification module (SIM) cards. SIM
cards are small removable disks that slip in and out of GSM cell phones. They store all the
connection data and identification numbers you need to access a particular wireless service
provider.

The 850MHz/1900-MHz GSM phones used in the United States are not compatible with
the international system. If you live in the United States and need to have cell-phone
access when you're overseas, you can either buy a tri-band or quad-band GSM phone and
use it both at home and when traveling or just buy a GSM 900MHz/1800MHz cell phone
for traveling.

COMMUNICATION TECHNOLOGIES IN THIRD GENERATION CELL-PHONES

3G technology is intended for the true multimedia cell phone -- typically called smartphones --
and features increased bandwidth and transfer rates to accommodate Web-based applications and
phone-based audio and video files. 3G comprises several cellular access technologies as follows:
1. CDMA2000 : based on 2-G Code Division Multiple Access
2. Wideband Code Division Multiple Access-UMTS (WCDMA-UMTS) : In W-CDMA
interface different users can simultaneously transmit at different data rates and data rates
can even vary in time. UMTS networks need to support all current second generation
services and numerous new applications and services.

3. Time Division Synchronous Code Division Multiple Access (TD-SCDMA) :
TD-SCDMA uses the Time Division Duplex (TDD) mode, which transmits uplink
traffic (traffic from the mobile terminal to the base station) and downlink traffic (traffic
from the base station to the terminal) in the same frame in different time slots. That
means that the uplink and downlink spectrum is assigned flexibly, dependent on the type
of information being transmitted. When asymmetrical data like e-mail and internet are
transmitted from the base station, more time slots are used for downlink than for uplink.
A symmetrical split in the unlink and down link takes place with symmetrical services
like telephony.

PROBLEMS WITH CELL-PHONES

1. Generally, non-repairable internal corrosion of parts results if you get the phone wet or
use wet hands to push the buttons. Consider a protective case. If the phone does get wet,
be sure it is totally dry before you switch it on so you can try to avoid damaging internal
parts.

2. Extreme heat in a car can damage the battery or the cell-phone electronics. Extreme cold
may cause a momentary loss of the screen display.

3. Analog cell phones suffer from a problem known as "cloning." A phone is "cloned"
when someone steals its ID numbers and is able to make fraudulent calls on the owner's
account. When your phone makes a call, it transmits the ESN and MIN to the network at
the beginning of the call. The MIN/ESN pair is a unique tag for your phone -- this is
how the phone company knows who to bill for the call. When your phone transmits its
MIN/ESN pair, it is possible for nefarious sorts to listen (with a scanner) and capture the
pair. With the right equipment, it is fairly easy to modify another phone so that it
contains your MIN/ESN pair, which allows the nefarious individual to make calls on
your account.

CELL PHONE JAMMERS

It's great to be able to call anyone at anytime. Unfortunately, restaurants, movie theaters,
concerts, shopping malls and churches all suffer from the spread of cell phones because not all
cell-phone users know when to stop talking. Who hasn't seethed through one side of a
conversation about an incredibly personal situation as the talker shares intimate details with his
friend as well as everyone else in the area?
Disrupting a cell phone is the same as jamming any other type of radio communciation. A cell
phone works by communicating with its service network through a cell tower or base station.
Cell towers divide a city into small areas, or cells. As a cell-phone user drives down the street,
the signal is handed from tower to tower.
A jamming device transmits on the same radio frequencies as the cell phone, disrupting the
communication between the phone and the cell-phone base station in the tower. It's a called a
denial-of-service attack. The jammer denies service of the radio spectrum to the cell-phone
users within range of the jamming device.

Jamming devices overpower the cell phone by transmitting a signal on the same frequency and at
a high enough power that the two signals collide and cancel each other out. Cell phones are
designed to add power if they experience low-level interference, so the jammer must recognize
and match the power increase from the phone.
Some jammers block only one of the frequencies used by cell phones, which has the effect of
blocking both. The phone is tricked into thinking there is no service because it can receive only
one of the frequencies. Less complex devices block only one group of frequencies, while
sophisticated jammers can block several types of networks at once to head off dual-mode or
tri-mode phones that automatically switch among different network types to find an open signal.
Some of the high-end devices block all frequencies at once, and others can be tuned to specific
frequencies.
Although different cellular systems process signals differently, all cell-phone networks use radio
signals that can be interrupted. GSM, used in digital cellular and PCS-based systems, operates in
the 900-MHz and 1800-MHz bands in Europe and Asia and in the 1900-MHz (sometimes
referred to as 1.9-GHz) band in the United States. Jammers can broadcast on any frequency and
are effective against AMPS, CDMA, TDMA, GSM, PCS, DCS,iDEN and Nextel systems.
Old-fashioned analog cell phones and today's digital devices are equally susceptible to jamming.
The actual range of the jammer depends on its power and the local environment, which may
include hills or walls of a building that block the jamming signal. Low-powered jammers block
calls in a range of about 30 feet (9 m). Higher-powered units create a cell-free zone as large as a
football field. Units used by law enforcement can shut down service up to 1 mile (1.6 km) from
the device.

Electronically speaking, cell-phone jammers are very basic devices. The simplest just have an
on/off switch and a light that indicates it's on. More complex devices have switches to activate
jamming at different frequencies. Components of a jammer include:
1. Antenna : Every jamming device has an antenna to send the signal. Some are
contained within an electrical cabinet. On stronger devices, antennas are external to
provide longer range and may be tuned for individual frequencies.
2. Circuitry : The main electronic components of a jammer are
a) Voltage-controlled oscillator - Generates the radio signal that will
interfere with the cell phone signa
b) Tuning circuit - Controls the frequency at which the jammer broadcasts its
signal by sending a particular voltage to the oscillator
c) Noise generator - Produces random electronic output in a specified
frequency range to jam the cell-phone network signal (part of the tuning
circuit)
d) RF amplification (gain stage) - Boosts the power of the radio frequency
output to high enough levels to jam a signal
1. Power Supply : Smaller jamming devices are battery operated. Some look like cell
phone and use cell-phone batteries. Stronger devices can be plugged into a standard
outlet or wired into a vehicle's electrical system.
During a hostage situation, police can control when and where a captor can make a phone call.
Police can block phone calls during a drug raid so suspects can't communicate outside the area.
Cell-phone jammers can be used in areas where radio transmissions are dangerous, (areas with a
potentially explosive atmosphere), such as chemical storage facilities or grain elevators. The
TRJ-89 jammer from Antenna System & Supplies Inc. carries its own electrical generator and
can block cellular communications in a 5-mile (8-km) radius.

Corporations use jammers to stop corporate espionage by blocking voice transmissions and
photo transmissions from camera phones. On the more questionable end of the legitimacy
spectrum, there are rumors that hotel chains install jammers to block guests' cell-phone usage
and force them to use in-room phones at high rates.

CELL PHONE RADIATIONS

There's a lot of talk in the news these days about whether or not cell phones emit enough
radiation to cause adverse health effects. The concern is that cell phones are often placed close to
or against the head during use, which puts the radiation in direct contact with the tissue in the
head. There's evidence supporting both sides of the argument.
When talking on a cell phone, a transmitter takes the sound of your voice and encodes it onto a
continuous sine wave.A sine wave is just a type of continuously varying wave that radiates out
from the antenna and fluctuates evenly through space. Sine waves are measured in terms of
frequency. Once the encoded sound has been placed on the sine wave, the transmitter sends the
signal to the antenna, which then sends the signal out.
Cell phones have low-power transmitters in them. A handheld cell phone operates on about 0.75
to 1 watt of power. The position of a transmitter inside a phone varies depending on the
manufacturer, but it is usually in close proximity to the phone's antenna. The radio waves that
send the encoded signal are made up of electromagnetic radiation propagated by the antenna.
The function of an antenna in any radio transmitter is to launch the radio waves into space; in the
case of cell phones, these waves are picked up by a receiver in the cell-phone tower.
When talking on a cell phone, most users place the phone against the head. In this position, there
is a good chance that some of the radiation will be absorbed by human tissue. All cell phones
emit some amount of electromagnetic radiation. Given the close proximity of the phone to the
head, it is possible for the radiation to cause some sort of harm to the user. What is being debated
in the scientific and political arenas is just how much radiation is considered unsafe, and if there
are any potential long-term effects of cell-phone radiation exposure.

There are two types of electromagnetic radiation:
1. Ionizing radiation - This type of radiation contains enough electromagnetic energy to
strip atoms and molecules from the tissue and alter chemical reactions in the body.
Gamma rays and X-rays are two forms of ionizing radiation. We know they cause
damage, which is why we wear a lead vest when X-rays are taken of our bodies.
2. Non-ionizing radiation - Non-ionizing radiation is typically safe. It causes some
heating effect, but usually not enough to cause any type of long-term damage to tissue.
Radio-frequency energy, visible light and microwave radiation are considered
non-ionizing.
On its Web site, the FDA states that "the available scientific evidence does not demonstrate any
adverse health effects associated with the use of mobile phones." However, that doesn't mean
that the potential for harm doesn't exist. Radiation can damage human tissue if it is exposed to
high levels of RF radiation, according to the FCC. RF radiation has the ability to heat human
tissue, much like the way microwave ovens heat food. Damage to tissue can be caused by
exposure to RF radiation because the body is not equipped to dissipate excessive amounts of
heat. The eyes are particularly vulnerable due to the lack of blood flow in that area.
The added concern with non-ionizing radiation, the type of radiation associated with cell phones,
is that it could have long-term effects. Although it may not immediately cause damage to tissue,
scientists are still unsure about whether prolonged exposure could create problems. This is an
especially sensitive issue today, because more people are using cell phones than ever before.
Here are a few illnesses and ailments that have potential links to cell-phone radiation:
1. Cancer
2. Brain Tumers
3. Alzheimers
4. Parkinsons
5. Fatigue
6. Headaches
Studies have only muddled the issue. As with most controversial topics, different studies have
different results. Some say that cell phones are linked to higher occurrences of cancer and other
ailments, while other studies report that cell-phone users have no higher rate of cancer than the
population as a whole. No study to date has provided conclusive evidence that cell phones can
cause any of these illnesses. However, there are ongoing studies that are examining the issue
more closely.
If you are worried about the potential hazards of cell-phone radiation, here are few ways to
reduce your risk:
1. Use a hands-free headset
2. Use a phone that places the antenna as far away from you as possible.

CELL PHONE VIRUSES

The first known cell-phone virus appeared in 2004 and didn't get very far. Cabir.A infected only
a small number of Bluetooth-enabled phones and carried out no malicious action -- a group of
malware developers created Cabir to prove it could be done. Their next step was to send it to
anti-virus researchers, who began the process of developing a solution to a problem that
promises to get a lot worse.
A cell-phone virus is basically the same thing as a computer virus -- an unwanted executable file
that "infects" a device and then copies itself to other devices. But whereas a computer virus or
worm spreads through e-mail attachments and Internet downloads, a cell-phone virus or worm
spreads via Internet downloads, MMS (multimedia messaging service) attachments and
bluetooth transfers. The most common type of cell-phone infection right now occurs when a cell
phone downloads an infected file from a PC or the Internet, but phone-to-phone viruses are on
the rise.


Current phone-to-phone viruses almost exclusively infect phones running the Symbian operating
system. The large number of proprietary operating systems in the cell-phone world is one of the
obstacles to mass infection. Cell-phone-virus writers have no Windows-level marketshare to
target, so any virus will only affect a small percentage of phones.
Infected files usually show up disguised as applications like games, security patches, add-on
functionalities and, of course, pornography and free stuff. Infected text messages sometimes steal
the subject line from a message you've received from a friend, which of course increases the
likelihood of your opening it -- but opening the message isn't enough to get infected. You have to
choose to open the message attachment and agree to install the program, which is another
obstacle to mass infection: To date, no reported phone-to-phone virus auto-installs. The
installation obstacles and the methods of spreading limit the amount of damage the current
generation of cell-phone virus can do.
Phones that can only make and receive calls are not at risk. Only smartphones with a Bluetooth
connection and data capabilities can receive a cell-phone virus. These viruses spread primarily in
three ways:

1. Internet downloads - The virus spreads the same way a traditional computer virus does.
The user downloads an infected file to the phone by way of a PC or the phone's own
Internet connection. This may include file-sharing downloads, applications available
from add-on sites (such as ringtones or games) and false security patches posted on the
Symbian Web site.

2. Bluetooth wireless connection - The virus spreads between phones by way of their
Bluetooth connection. The user receives a virus via Bluetooth when the phone is in
discoverable mode, meaning it can be seen by other Bluetooth-enabled phones. In this
case, the virus spreads like an airborne illness.

3. Multimedia Messaging Service - The virus is an attachment to an MMS text message.
As with computer viruses that arrive as e-mail attachments, the user must choose to open
the attachment and then install it in order for the virus to infect the phone. Typically, a
virus that spreads via MMS gets into the phone's contact list and sends itself to every
phone number stored there.

In all of these transfer methods, the user has to agree at least once (and usually twice) to run the
infected file. But cell-phone-virus writers get you to open and install their product the same way
computer-virus writers do: The virus is typically disguised as a game, security patch or other
desirable application.
The Commwarrior virus arrived on the scene in January 2005 and is the first cell-phone virus
to effectively spread through an entire company via Bluetooth. It replicates by way of both
Bluetooth and MMS. Once you receive and install the virus, it immediately starts looking for
other Bluetooth phones in the vicinity to infect. At the same time, the virus sends infected MMS
messages to every phone number in your address list. Commwarrior is probably one of the more
effective viruses to date because it uses two methods to replicate itself.
The first known cell-phone virus, Cabir, is entirely innocuous. All it does is sit in the phone and
try to spread itself. Other cell-phone viruses, however, are not as harmless.
A virus might access and/or delete all of the contact information and calendar entries in your
phone. It might send an infected MMS message to every number in your phone book -- and
MMS messages typically cost money to send, so you're actually paying to send a virus to all of
your friends, family members and business associates. On the worst-case-scenario end, it might
delete or lock up certain phone applications or crash your phone completely so it's useless. Some
reported viruses and their vital statistics are listed below.
The best way to protect yourself from cell-phone viruses is the same way you protect yourself
from computer viruses: Never open anything if you don't know what it is, haven't requested it or
have any suspicions whatsoever that it's not what it claims to be. That said, even the most
cautious person can still end up with an infected phone. Here are some steps you can take to
decrease your chances of installing a virus:

1. Turn off Bluetooth discoverable mode. Set your phone to "hidden" so other phones
can't detect it and send it the virus. You can do this on the Bluetooth options screen.

2. Check security updates to learn about filenames you should keep an eye out for. It's
not fool-proof -- the Commwarrior program generates random names for the infected
files it sends out, so users can't be warned not to open specific filenames -- but many
viruses can be easily identified by the filenames they carry.

3. Install some type of security software on your phone. Numerous companies are
developing security software for cell phones, some for free download, some for user
purchase and some intended for cell-phone service providers. The software may simply
detect and then remove the virus once it's received and installed, or it may protect your
phone from getting certain viruses in the first place. Symbian has developed an
anti-virus version of its operating system that only allows the phone's Bluetooth
connection to accept secure files.
Future possibilities include viruses that bug phones -- so someone can see every number you call
and listen to your conversations -- and viruses that steal financial information, which would be a
serious issue if smartphones end up being used as payment devices. Ultimately, more
connectivity means more exposure to viruses and faster spreading of infection. As smartphones
become more common and more complex, so will the viruses that target them.

Mobile phone use risk

What effects do mobile phones have on people’s health?

The issue

During recent years, the use of mobile phones has increased substantially and has been paralleled by a
growing concern about the effects on health attributed to exposure to the electromagnetic fields
produced by them and their base stations. Demonstrating that radiation causes adverse effects on
health would signal a widespread public health problem.






Findings


Mobile phones have been in extensive use for a relatively short period of time, and their technology
has progressively changed, from analogue to digital systems. Mobile phones and base stations emit
radio frequency or microwave radiation. Exposure to such a radiation could affect health directly. The
use of mobile phones also results in indirect effects, such as car accidents and interference with health
equipment.
Experimental research on the effects of radio-frequency radiation is very broad and heterogeneous. It
includes both studies of cell cultures and tissues (in vitro) and of laboratory animals (in vivo), as well
as of people (volunteers). On one hand, these studies focus on functional changes in the brain and the
resulting effects on cognition, and (to some extent) well-being – that is, the influence of exposure to
radiation on the head. On the other hand, these studies focus on the possibility of a relationship
between mobile phone use and carcinogenic processes, reproduction and development, the
cardiovascular system and longevity – that is, exposure of the whole body. These studies found very
small and reversible biological and physiological effects that do not necessary lead to diseases or
injuries. Also, the research findings on the changes at the molecular level associated with the
development of cancer are inconsistent and contradictory.
Epidemiological studies in general populations, such as communities, concentrate on a possible causal
relationship between mobile phone use and the occurrence of brain tumours, acoustic neuromas,
tumours of the salivary glands, and leukaemia and lymphomas. Although weak and inconclusive, most
of the evidence available does not suggest that there are adverse effects on health attributable to longterm
exposure to radio-frequency and microwave radiation from mobile phones. However, recent
studies have reported an increased risk of acoustic neuroma and some brain tumours in people who use
an analogue mobile phone for more than 10 years. Also, no data is available on the reproduction of
these effects when digital mobile phones are used. Finally, there is good evidence that the use of
mobile phones while driving translates into a substantially increased risk of an accidental collision.

Policy considerations

For the majority of tumours studied so far, a long latency period might exist, and the finding of any
link to the use of mobile phones is complex. Consequently, most of the published research cannot
elucidate the risk of long-term effects. If there is a risk, the current evidence suggests it is small.
Since there are still gaps in knowledge, continued research and better health risk analyses are needed.
Moreover, without scientifically recognized adverse effects on health, it is not possible to produce
evidence-based recommendations.
Therefore, a precautionary approach to the use of this communication technology should be adopted
until more scientific evidence on its effects on health becomes available. Such an approach includes
restricting exposure (according to existing guidelines and the European Union (EU) Directive) and
providing the public with information and options.

Introduction

In recent years, mobile telecommunication systems have grown significantly, to the point where more
than a sixth of the world’s population use mobile phones. By the end of 2004, more than a billion
subscribers across more than 200 countries were estimated to be using mobile phones (1, 2).
The development of mobile communications has moved rapidly. In the 1980s, first generation mobile
phones, using analogue technology, allowed the transmission of sound only. Digital transmission, and
the global system for mobile communication, started in 1991 and includes such new developments as
data and image transmission. Third generation mobile phones currently in the market offer additional
services to the users (such as fax, e-mail and Internet access). For both analogue and digital mobile
phones, the signals transmitted and received are in the form of waves in the radio frequency (RF)
(analogue) and microwave parts of the electromagnetic spectrum. RFs are non-ionizing radiation with,
wavelengths that range from 3 kHz to 300 MHz, and microwaves range from 300 MHz to 300 GHz1.
The frequencies that mobile phones and telecommunication networks use range from 900 MHz to
1.8 GHz and up to 2.1 GHz, although it should be noted that the wavelength of the different types of
mobile phones varies. This applies to both mobile phones and their base stations, which send and
receive calls.
People have welcomed the technology, as indicated by the widespread use of mobile phones, which
suggests that they do not perceive it as a potential health hazard. However, concerns about the possible
adverse effects on health, as a result of the exposure to RF and microwave electromagnetic fields, have
been expressed since the introduction of mobile phones.
Since the year 2000, several reports have reviewed relevant studies and summarized current
knowledge about mobile phones and health, particularly that related to the commonly accepted
carcinogenic effects of RF and microwave energy. The aim of this synthesis is to combine the
available epidemiological evidence, to learn whether exposure to RF and microwave radiation from
mobile phones and their base stations might affect health. This synthesis does not aim to analyse the
effect of other man-made sources of electromagnetic fields, nor does it aim to develop safety
standards. The WHO International EMF Project is currently producing this information (3).
Sources for this review
This synthesis assesses the clinical effects of day-to-day exposure to mobile phones in general
populations, such as communities – specifically, the impact on developing head and brain tumours
(benign and malignant). It also addresses other morbidity related outcomes and summarizes the
biological effects of RF and microwave radiation.
Two main sources of information were considered and reviewed. A search of scientific and biomedical
databases was performed until March 2006. Observational studies that assessed the effects of mobile
phones on general populations were included in the synthesis. Experimental studies that assessed
clinical effects were excluded, as were those of occupational settings. The quality of the studies was
assessed by using the grading system developed by the Scottish Intercollegiate Guidelines Network. Also, a review of documents and web sites of governments, health councils, radiological
protection boards, advisory and expert groups, and the like, was undertaken. Annex 1 gives details
about the literature search strategy.

Findings
Scientific literature
Biological and physiological effects
Experimental research on the biological effects of RF and microwave fields is very broad and includes
studies of volunteers, animals and in vitro, cell-based techniques (5, 6). The studies cover the effects
of RF and microwave radiation between 100 MHz and 60 GHz and focus both on the functional
changes in the brain (influence of exposure to RF and microwave fields on the head) and on
carcinogenic processes, reproduction and development, the cardiovascular system and longevity (as a
result of whole body exposure to RF and microwave fields).
The biological effects observed on the cardiovascular, endocrine and immune systems and on the
behaviour of animals studied seem to be thermal effects of acute exposure to RF and microwave
radiation, with increases of at least 1 °C or 2 °C in temperature needed to produce these effects. As to
the increased risk of developing cancer after exposure to RF or microwave fields, the evidence for
such an association is extremely weak. Since the radiation from mobile phones and signal stations
does not have enough energy to break chemical or molecular bonds directly, there is no basis in theory
to suggest that they can damage DNA. Moreover, a biological mechanism that explains any possible
carcinogenic effect from RF or microwave fields has yet to be identified. Because of the difficulties in
interpreting findings from laboratory studies, the hypothesis that RF or microwave radiation is harmful
and could have effects on health that have not yet been recognized cannot be rejected.
Indirect experimental results are difficult to extrapolate. In vitro experiments that show abnormal cell
proliferation, changes in cell membranes, and movement of ions and substances across membranes are
difficult to extrapolate to people. It is also difficult to extrapolate to people the observed effects on
cerebral functions that relate to the behaviour of rodents since, among other reasons, the whole brain
of these small animals is exposed to radiation whereas the brains of people who use mobile phones,
although being exposed, receive the highest exposure in the part closest to the handset. Moreover, the
thermal effects of radiation are unlikely to be seen in people, as the increase in the local temperature of
the brain induced by the microwaves generated by mobile phones is negligible (it has been estimated
to be up to 0.1 °C) (7). Finally, there is no evidence of non-thermal effects on human health.
It is important to distinguish between biological (or physiological) effects and psychological and
health effects. The demonstration of an RF or microwave radiation effect in experimental research
does not necessarily mean that such exposure will lead to harmful effects on human health. Human
bodies, with the aid of their immune, nervous or endocrine systems, can effectively resist some
external pressures, adapt to them and maintain the stability (homeostasis) disrupted by those changes.

Clinical effects

Within human population studies, epidemiological studies provide the most direct information on the
long-term effects on health of any potential harmful agent. To assess the adverse effects on health that
may result from the use of mobile phones, research with a specific focus on cancer has been carried
out. By the end of the 1990s, the number of studies was small and the works presented major
methodological limitations, the most outstanding one being the lack of enough people with an
exposure time long enough to accurately assess the potential adverse late effects on health of mobile
phone use.
The majority of those studies suggested the need for additional, high-quality research. As a result of
these recommendations, a series of multinational case-control studies, coordinated by the International
Agency for Research on Cancer (IARC), were set up after a detailed feasibility study was carried out
in 1998 and 1999.

Overall, these studies are named the INTERPHONE Study (8), and their primary objective is to assess
whether exposure to RF or microwave radiation from mobile phones is associated with a risk of
cancer. Priority is given to epidemiological studies of the relationship between the use of mobile
phones and the incidence of:
• brain tumours;
• salivary gland tumours, acoustic neuromas and other head and neck tumours; and
• leukaemia and lymphomas.
If the risk of developing a brain tumour exists at all, the wider use of mobile phones and the expected
number of people who will develop a brain tumour will be sufficient to detect a potential 1.5-fold
increase in risk 5–10 years from the start of use.
Participant countries, with the longest and highest use of mobile phones, are Australia, Canada,
Denmark, Finland, France, Germany, Israel, Italy, Japan, New Zealand, Norway, Sweden and the
United Kingdom. Initially, the study expected to find about 6000 cases of glioma and meningoma
(both benign and malignant), 1000 cases of acoustic neuroma, 600 cases of parotid gland tumour and
their respective controls. The first results of the INTERPHONE Study were available in 2004 (9, 10)
and, since then, four additional papers have been published (11–14). It should also be noted that these
studies evaluated the impact on health of exposure to RF and microwave radiation emitted by mobile
phones, and not by antennas and base stations.
With regard to brain tumours, most of the studies yielded negative results, although a few of them
suggested an increased risk for mobile phone users. Because of these results, it is not possible to
establish an association between the use of mobile phones and an increased risk of brain tumours.
With regard to acoustic neuroma – a rare, benign tumour on the auditory nerve – the studies available
reported inconsistent results, except for the most recent ones, which found an association between an
increase in the risk of this type of tumour and 10 years or more of mobile phone use; moreover, the
increased risk is confined to the side of the head where the phone was usually held. No indications of
an increased risk for less than 10 years of mobile phone use were found. Before definite conclusions
can be drawn, the results of these studies have to be confirmed by additional research.
This type of research, however, faces several problems. One is that long-time users first used analogue
phones, and then digital phones. No risk has been found for digital phone use only, but then the
follow-up time is shorter. Other methodological problems, such as recall bias, have been identified:
people, especially patients, might have a selective memory on the side of the head where the telephone
was used (15).
A number of clinical complaints related to the use of mobile phones are reported in the scientific
literature. They include headache, fatigue, sleep disorders, loss of memory, dizziness, feelings of heat
or tingling in the auricular (or auditory) area or in the head, vertigo, deafness and blurred vision. Very
few studies are available, and their results provide no evidence of an association between these
symptoms and the use of mobile phones. It should be noted, however, that these are general,
nonspecific symptoms that may be induced by a wide range of causes. Since they represent a problem
for those suffering, the cause should be elucidated.
In summary, the evidence available does not support the hypothesis that mobile phone use is
associated with an increased risk of malignant brain tumours, but an increase in the risk of acoustic
neuroma after 10 years or more of mobile phone use has been found. Therefore, it seems that neither
acoustic neuroma nor brain tumours are related to mobile phone use of less than 10 years.
Nevertheless, those studies were conducted with data from the time when only analogue mobile
phones had been in use for more than 10 years, and they cannot determine if the results would be similar after long-term use of digital mobile phones. Likewise, a carcinogenic effect after a very long
period of exposure would remain undetected.
The most important and clearly defined effect of mobile phones on health, and the only clearly
established risk from an epidemiological perspective, is motor vehicle accidents, which obviously are
not related to exposure to RF or microwave radiation. The results of some studies show that the use of
a mobile phone up to 10 minutes before a crash is associated with a fourfold increase in the risk of
having a collision that results in injury. The risk increases irrespective of whether or not a hands-free
phone is used (16, 17).
Table 2 describes the characteristics of the design of epidemiological studies and the outcomes of
these studies.
Reports (grey literature)
Since the year 2000, a significant number of reports and reviews on the connection between mobile
phone use and health have been issued by committees, institutions, expert groups and agencies of
worldwide prestige, to appraise relevant literature, draw up guidelines and make recommendations to
limit exposure to RF and microwave radiation.
The National Radiological Protection Board summarized (18) the information from several sources,
from the publication of the Stewart Report, in May 2000, to the end of 2004. The Board, an
independent body that is now part of the Health Protection Agency, has responsibility for advising
government departments and others in the United Kingdom on standards of protection for exposure to
ionizing and non-ionizing radiation, which includes electric and magnetic fields.
The Stewart Report (5) is a widely quoted review on mobile phones and health. The Government of
the United Kingdom commissioned the Independent Expert Group on Mobile Phones, which was
chaired by Sir William Stewart, to prepare the Report. The Report concluded that the balance of
evidence did not suggest that exposure below international guidelines could cause adverse effects on
health. However, it recommended that a precautionary approach – that is, limiting exposure to RF and
microwave radiation, planning the location and setting of base stations and encouraging a selective use
of mobile phones – be adopted until more detailed and scientifically robust information on any adverse
effects on health becomes available. Besides health issues, the Report offered advice on exposure
standards and planning to government, industry and others and on public information and consumer
choices. It also proposed setting up a research programme.

According to the National Radiological Protection Board summary report (18), most of the 26 reports
examined reached similar conclusions and made comparable recommendations. Overall, the reports
acknowledge that exposure to low-level RF and microwave fields may cause a variety of slight
biological effects on cells, animals or people, particularly on brain activity during sleep, but the
possibility of exposure causing adverse effects on health remains unproven. The reports also present
guidance on public policy to decision-makers and legislators, and some of them favour any form of
precautionary or prudent approach to reducing personal exposure to the fields produced by mobile
phones.
Specifically, most reports recommend limiting the use of mobile phones by children. This has been
recommended in the absence of explicit scientific data. However, ethical and practical concerns limit
or prevent experimental studies on children. Because of a much higher cumulative exposure than
today's adults when they were at the same age, children might be more vulnerable to any effects of RF
and microwave radiation. As long as adverse effects on health cannot be ruled out with some degree of
certainty, it appears to be appropriate to instruct children and their parents about a prudent use of
mobile phones. Moreover, in the absence of new scientific evidence, WHO is focusing attention on the
potential effects of exposure to electromagnetic fields on children (19). Finally, many reports agree
that the distraction caused by mobile phone use while driving represents a serious threat to health.

Table 3 contains a selection of national and international reports on mobile phone use and its effects
on health, along with the links to the corresponding web pages.

Discussion

Results among investigations were inconsistent, and these investigations indicated little or no
association between exposure to RF and microwave radiation and cancer. It should be noted that the
weak evidence on carcinogenicity obtained from several epidemiological studies applies only to the
type of cancer studied and to the time intervals observed between exposure and occurrence of disease.
Also, experience with cancer in people indicates that, in some cases, the period from first exposure to
the development of clinical cancer is seldom less than 20 years; moreover, latency periods
substantially shorter than 30 years cannot provide evidence for lack of carcinogenicity (20).
Progress and changes in mobile phone technology (such as analogue to digital signals) make it
difficult to assess exposure in the people studied. Furthermore, because the use of mobile phones is
relatively recent, it may be premature to conduct an exhaustive epidemiological assessment of its
impact on health. In the case of cancer, for example, the information available does not rule out the
possibility of an association between the use of mobile phones and the occurrence of this disease.
Thus, it is advisable to monitor the incidence of tumours supposedly associated with exposure to RF
and microwave radiation, to assess possible changes in trends.

Conclusions

The evidence available does not provide a clear pattern to support an association between exposure to
RF and microwave radiation from mobile phones and direct effects on health (such as increasing the
risk of cancer). However, the quality of this research and the relatively short-term data do not allow
ruling out adverse effects on health completely. In other words, the absence of evidence of detrimental
effects on health associated with mobile phone use is not evidence of absence of such effects. At the
moment, it is impossible to state that exposure to RF or microwave radiation (even below the
permitted levels) does not have adverse effects on the health of the general population. The current
evidence, however, does suggest that if there is a risk, it is small. Therefore, a precautionary approach
(as recommended by the EU (21)) to the use of this communication technology should be adopted
until more scientific evidence on effects on health becomes available.
Finally, evidence shows that the use of a mobile phone while driving translates into a significantly
increased risk of a traffic accident.

Annex 1. Sources of information and methods

Scientific and biomedical literature

The review was done after a bibliographic search (up to March 2006) of databases, using relevant key
words, selection (inclusion/exclusion) criteria and a grading scale.
Databases
The following databases were searched:
• MEDLINE
• EMBASE
• The Cochrane Library
• ENVIROLINE
• INSPEC
• PASCAL
• SCISEARCH.
Key words
The following keywords were used in the search: telephone, phone, cell phone, cellular phone, mobile
phone, cell telephone, cellular telephone, mobile telephone, antenna, station, hazards, risks, health
electromagnetic fields, radio waves, microwaves.
Selection criteria
The following inclusion and exclusion criteria were used in this synthesis:
• inclusion criteria: papers about the effects of RF from mobile phones and their base stations
on the general population published in English, French and Spanish.
• exclusion criteria: papers about the effects of RF from mobile phones and their base stations
in experimental studies and occupational settings; however, a summary of experimental
research findings is provided.
In an experiment, the exposition to the agent or putative cause is due because the investigator has
assigned the exposure to the subject in order to comply with a study protocol. Because the goals of the
study rather than the subject’s needs determine the exposure assignment, ethical constraints limit the
circumstances in which these types of studies are feasible. Experiments are ethically permissible only
when adherence to the scientific protocol does not conflict with the subject’s best interests.

Grading scale

The grading system used for the evidence is that developed by the Scottish Intercollegiate Guidelines
Network (SIGN) (4). Table 1 shows the SIGN levels of evidence for this system.
Grey literature
A review was undertaken of major documents and web sites of governments, health councils,
radiological protection boards, advisory and expert groups, and the like, since the year 2000.
Most of the epidemiological studies from which the reviewed evidence comes are case-control studies.
In case-control studies, subjects are selected according to their disease status (in the case of this synthesis, it was presence or absence of a brain tumour) and further classified according to their
exposure status (in the case of this synthesis, it was exposure to mobile phones). This type of design
provides mid- to low-level evidence, according to the grading system used (4), since the methodology
is less strong. Nevertheless, for the ethical reasons mentioned above, which relate to experimental
studies, cohorts – that is, studies in which subjects are classified according to their exposure status and
followed over time to ascertain disease incidence – and case-control studies are considered the best
designs to study potential risk factors for human health due to the inability to use intervention studies.


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