Mobile Telephony and Health Exposures from Mobile Phones

Exposures from Mobile Phones

Mobile phones are designed to transmit radio waves in all directions because base stations could be in any direction with respect to phone users. This means that a proportion of the radio waves they produce is directed towards the user's body.

The radio waves are mainly radiated from the antenna of a mobile phone, although leakage onto the phone body shell does occur. The antenna is sometimes visible as a projection on the top surface of the phone, although some phones have conformal antennas mounted inside their main plastic case.

The radio waves that are directed towards the head of the phone user penetrate into the body tissues for a few cm and tend to be absorbed. In being absorbed, they give up their energy to the body tissues and this adds to the energy being produced by the body's metabolism.

Up to a point, the body is able to accommodate extra energy being absorbed in its tissues, but beyond this point, temperature rises or thermoregulatory responses can occur. Protection guidelines advise restrictions on energy absorption in tissues designed to ensure that such effects are small enough not to pose a hazard.

Calculations have shown that the maximum temperature rise produced in the head due to absorption of energy in the radio waves from a mobile phone is around 0.1ºC. Although the IEGMP did not identify any adverse effects on health at this level of exposure, there is no comparable situation where large numbers of people are exposed. It is for this reason that research related to exposure to RF from mobile phones is currently being carried out.

Mobile Phone SARs

The quantity that is used to describe absorption of radio waves in the head is the Specific Absorption Rate (SAR) of the energy. ICNIRP has advised that this should not exceed 2 W kg ^-1 (watts per kilogram) when averaged over any 10 gram of contiguous tissue and over any 6 minute period in the head. This advice has been accepted by the mobile phone industry and all phones sold in the UK should produce a SAR below this level.

Until recently, there was no clear consensus over the best way to measure the SAR produced by a mobile phone. Consequently, results reported in one laboratory were not necessarily comparable with results reported in another laboratory. In July 2001, the European Committee for Electrotechnical Standardisation (CENELEC) published a technical standard detailing how to make SAR measurements and industry is now publishing SAR values for phones.

The Mobile Manufacturers Forum has produced a note explaining industry's approach to the reporting of SAR values, which can be obtained by following the link onwards to "wireless devices". The website has links through which consumers can find the SAR values for their phones. If a phone is not listed, it will be necessary to ask the manufacturer for the information.

There are many sites on the Internet that give information about how SAR is measured.

Maximum Output Powers from Phones

The output powers of phones are set in the relevant technical standards to ensure that mobile phones will work when they are used with any network. The peak output powers of GSM phones operating at 900 MHz and 1800 MHz are 2 W (watts) and 1 W respectively.

GSM mobile phones transmit their radio signals as 217 bursts of information every second. There is one burst every 4.6 ms (thousandth of a second) and each burst is 577 µs (millionths of a second) in duration. This means that, on average, they transmit for 1/8 of the time and their average output power is 8 times less than their peak output power.

Exposure guidelines, such as those published by ICNIRP, require exposures to be averaged over 6 minutes for comparison with their basic restrictions and it is more relevant to consider the average output power than the peak output power from phones. In this respect, GSM phones transmitting at 900 MHz and 1800 MHz have maximum time-averaged output powers of 0.25 W and 0.125 W respectively.

Exposure during Normal Use

There are several reasons why the SAR derived from a technical standard methodology represents a pessimistic estimate of the SAR typically received during a call. The SAR values quoted for mobile phones assume that a mobile phone is transmitting at its maximum possible power for a period of 6 minutes.

A key feature of mobile phone technology is that a mobile phone does not operate with a fixed output power level when a call is made. The maximum power output from a GSM mobile phone is around 2 W peak, but this can reduce in a sequence of 15 steps down to around 2 mW during calls, a power reduction factor of 1000.

The power level that a mobile phone operates at during a call depends on the quality of the radio link to the base station. If the link is good, a low output power level will be used, whereas if the link is poor, a higher output level will be used. A typical situation where a good link to a base station would occur is outdoors at a location where there is a clear view of the base station antennas. Poorer links would be obtained if a mobile phone is used indoors, or at a location where there are physical obstructions such as buildings or hills between the phone and the base station.

SAR is proportional to output power. So, if a mobile phone has a good link to the base station and reduces its output power by a factor of 1000 from the maximum, the SAR in the head would also be reduced by a factor of 1000.

In order to compare SAR with the ICNIRP basic restriction, it is first necessary to average SAR over a period of 6 minutes. For example, a 3 minute call during which the SAR was 1 W kg ^-1 followed by no use of the phone for the next 3 minutes, the 6 minute averaged SAR would be 0.5 W kg ^-1. This would be the correct figure to compare with the 2 W kg ^-1 ICNIRP basic restriction.

Factors affecting Exposure

It is not possible to show that reducing an exposure within the ICNIRP guidelines gives any specific health benefit. Nevertheless, IEGMP felt that people buying mobile phones should have the information to enable them to choose to reduce their exposure if they so wished. The issues identified by IEGMP to aid personal choice included information on mobile phone SAR assessments, the use of approved hands-free kits, and the number and duration of calls made.

• Phone SAR
  The value of SAR is obtained using an approved compliance testing procedure under constant high power and as such is the only practical option for comparison at the point of sale. In practice the measured SAR will not be the same as that to the user as the phone's power output may be on average much lower in normal use. If the phone is receiving a strong signal from a particular base station, it will require less power to communicate. This will reduce SAR in proportion to the reduction in the phone's output power.
• Distance from the head
  The antenna is the main source of the radio waves that produce SAR in the body. Moving the phone away from the head, for example by using a hands free kit, will reduce the localised SAR in the head but may increase the localised SAR in other parts of the body.
• Frequency of use and call duration
  Using a mobile phone less will give rise to lower exposure
  

Last reviewed: 4 September 2008