Review of evidence
relating to the use of Wireless Networks
with particular reference to schools and educational
establishments
September 2009
Wi-Fi safety
The Scottish ICT Development Group asked at their June meeting that LTS should review and update the evidence in relations to the safe use of wireless networks in schools. This was in the light of several ‘scare stories’ in the media and where authorities might wish to inform schools and parents of the evidence from bodies with expertise and experience in this area. It should be noted that wireless networks have been used without ill effect in schools for over 10 years now. High frequency wireless signals from Television and Radio have covered the whole country for even longer periods.
Organisers for this report
1 Health - mainly electromagnetic field emissions
2 Security
3 Speed and reliability
Health Protection Agency
The Health Protection Agency is an independent UK organisation that was set up by the government in 2003 to protect the public from threats to their health from infectious diseases and environmental hazards. It does this by providing advice and information to the general public, to health professionals such as doctors and nurses, and to national and local government
It is the successor to the former National Radiological Protection Board (NRPB)
The summary about Wi-Fi networks is at
http://www.hpa.org.uk/HPA/Topics/Radiation/UnderstandingRadiation/1199451940308
The international guidelines on electromagnetic fields (EMF) are available from
http://www.icnirp.org/documents/emfgdl.pdf
There is no UK legislation specifically requiring compliance with any EMF protection guidelines. The Health and Safety Executive's requirements for safe working (qv) are at
http://www.hse.gov.uk/radiation/nonionising/electro.htm
The European Union's Radio and Telecommunications Terminal Equipment Directive (1999/5/EC) extends the objectives with respect to safety requirements contained in the Low Voltage Directive (73/23/EEC) in order to apply to all apparatus, with no voltage limit applying. The text of these directives can be obtained from
http://europa.eu.int/comm/enterprise/newapproach/standardization/harmstds/reflist.html
The HPA has made measurements of the power density of radio waves generally in and about the offices where WLANs are deployed and these have always been found to be very much below the guideline levels referred to above. The situation is rather more complicated for exposure within the first few centimetres of the transmitters, for example, for the situation where a laptop computer is placed on someone's lap with its transmitting antenna projecting on one side, or mounted inside the case. This is the situation where exposure would be highest and there is no practical assessment that can be rapidly performed to check levels with an installed system. Nevertheless, given the low powers, a problem with guideline compliance would not be expected
When a radio terminal is used close to the body, some of the radio energy penetrates into the body and it is absorbed in the tissues. The pattern of this absorption and the total amount of energy absorbed depend on parameters such as the frequency, output power, position of use, antenna type, etc and it is not easy to predict without a detailed investigation. The exposure is characterised by the specific absorption rate of energy (SAR) in the tissues and this will generally have a spatial peak value near the surface of the body and close to the radiating antenna
Guidelines expect SAR to be averaged over various different tissue-masses and time-periods before comparison with the basic restrictions. The most stringent basic restrictions for the situation where low power radio transmitters are used near to the body are those on localised SAR in the head, which involve an averaging mass of 10g and an averaging time of six minutes. For this situation, the ICNIRP guidelines for workers advise that SAR should not exceed 10W/kg. The ICNIRP guidelines … basic restriction for the general public is that SAR averaged in this way should not exceed 2W/kg
HPA's conclusion
On the basis of current evidence, the HPA does not consider there to be a problem with the safety of WLAN. If an explicit statement that exposures are within the ICNIRP guidelines is required, this would have to be obtained from the manufacturers; however, it could be argued that this is implicit in the CEmarking
HPA current research
On 12October 2007, the agency announced a programme of research into wireless local area networks (WLANs) and their use
"There is no scientific evidence to date that WiFi and WLANs adversely affect the health of the general population. The signals are very low power, typically 0.1watt (100milliwatts) in both the computer and the router (access point) and the results so far show exposures are well within ICNIRP guidelines. Given this, there is no particular reason why schools and others should not continue to use WiFi or other wireless networks. However there has not been extensive research into what people's exposures actually are to this new technology and that is why we are initiating this new programme of research and analyses. We have good scientific reasons to expect the results to be re-assuring and we will publish our findings" (Prof Pat Troop, Chief Executive, HPA)
NB ICNIRP is the International Commission on Non-Ionizing Radiation Protection
HPA's website (last reviewed in July 2009) says the following, in addition to repeating some of the above
• On the basis of the studies so far carried out in house, theHPA sees no reason why Wi-Fi should not continue to be used in schools. However with any new technology it is a sensible precautionary approach, as happened with mobile phones, to keep the situation under ongoing review so that parents and others can have as much reassurance as possible
• There is no consistent evidence to date that exposure to RF signals from Wi-Fi and WLANs adversely affect the health of the general population
• The signals from Wi-Fi are very low power, typically 0.1watt (100milliwatts) in both the computer and the mast (or router) and resulting exposures should be well within internationally-accepted guidelines
• The frequencies used are broadly the same as those from other RF applications such as FM radio, TV and mobile phones
• Based on current knowledge, RF exposures from Wi-Fi are likely to be lower than those from mobile phones
• On the basis of current scientific information, exposures from Wi-Fi equipment satisfy international guidelines. There is no consistent evidence of health effects from RF exposures below guideline levels and no reason why schools and others should not use Wi-Fi equipment
Results of HPA's further researches announced in October 2007
Followingthe announcement by the Board of HPAon 12 October 2007, a systematic programme of research into WLANs and their use started at theHPA Radiation Protection Division. At the start of the project, comprehensive Wi-Fi test facilities were set up at theHPA Chilton site and a review of technical standards and wireless equipment used in UK schools was carried out
Due to the popularity of laptops in classrooms and the likelihood that the majority of Wi-Fi exposure would come from these devices because they are generally nearer to children than the access points, it was decided that the experimental measurements would begin with laptops transmitting in the 2.4GHz frequency band. A total of 15laptops were chosen from among the most popular models usedin the education sector in the UK
The objective of the laboratory measurements was to establish the radiation pattern (ie the angular distribution of electric field strength around each laptop) during transmission and identify the angles at which the field was a maximum. The electric field strength at these angles was then measured as a function of distance
The results have so far shown that, for a given position, the field strength fluctuated between 2 (and sometimes 3) distinct levels because of the existence of several transmitting antennas within each laptop. Overall, similar radiation pattern measurements for all 15laptops have been observed with a minimum in the direction from the front of the laptop (towards the torso of the user). Generally, two angular maxima were observed that were symmetrically opposed across a vertical plane bisecting the screen and keyboard. All 15laptops tested had electric field strength values indicating they had output powers during transmission in the range 6-20mW. Taking into account the directional properties, the Equivalent Isotropically Radiated Power (EIRP) calculated for all laptops was in the range 17-57mW and well below the 100 mW (EIRP) limit set for Europe
These results are consistent with the HPA position that exposures to the radio waves from Wi-Fi equipment are not expected to exceed internationally-accepted guidelines and that they are less than from mobile phones. Further results will be published on theHPA website after they have been finalised
Further work
The remainder of the laboratory measurements includes the assessment of the electric field strength around access points operating at 2.4 GHz. Measurements will also be carried out on a selection of laptops and access points operating in the 5 GHz band
Further work will then involve the modelling of Wi-Fi equipment and its internal RF structures (antennas) in order to assess the localised specific energy absorption rates (SARs) in users, including children. In addition, measurements of radiated powers and transmit time proportions in schools are planned
The experimental results, together with information from other studies on radio signals and health, will then be used as the basis for a wider health risk review
Some of the preliminary results were reported in a June 2009 paper to the European BioElectromagnetics Association conference in Davos [http://bioem2009.org/session-9], from which the following is a selection
"People using wireless computer networks (Wi-Fi), or in proximity to the equipment, are exposed to the radio signals and will absorb some of the transmitted energy in their bodies. The output power of Wi-Fi equipment is restricted to a maximum of 100 mW at 2.4 GHz in Europe, and there is no expectation that exposures will exceed international guideline levels. Nevertheless, this rapidly developing technology is increasingly used in schools and, given the existing precautionary advice in many countries to discourage non-essential use of mobile phones by children…, it is important to quantify the exposure from Wi-Fi equipment, as used by children in schools"
"Discussions and information gathered at the start of the project showed that laptops are the most popular wireless devices used in schools, with IEEE 802.11g as the most widely utilised standard. For this work, a total of 14laptops were chosen from among the most popular models used by the education sector in UK. The objective of the laboratory measurements was to establish the radiation pattern (ie angular distribution of electric field strength around each laptop) and identify the angles at which the field is maximum. The electric field strength at these angles was then measured as a function of distance. The measurements were carried out within an anechoic chamber (3.7m x 2.4m x 2.4m), lined internally with radiofrequency absorber material, and with a purpose built manual positioning system. Dedicated software (LanTraffic) was used to generate and monitor the Wi-Fi signal from the laptops set to transmit at roughly 22Mbps, the maximum sustained rate that could be reliably maintained using the IEEE 802.11g standard. The screen of the laptops was opened to an angle of 115degrees for this work"
"An Agilent N9020A MXA signal analyser was used which has a bandwidth of 25MHz allowing the detection of the whole WLAN signal. This instrument captures individual Wi-Fi bursts in the time domain and demodulates them to identify the burst power, modulation scheme and many other parameters. For this work, the power of 50bursts was measured at each position and then analysed in terms of the statistical distribution. To establish the radiation pattern, the E-field strength at 1m distance from the laptop was measured by an ARC Seibersdorf miniature biconical antenna in horizontal and vertical polarisations for azimuth and elevation rotations in 30steps for the laptop on the manual positioning system (168positions in total). The measured E-field data were then analysed and the angles of maximum radiation were identified. The manual positioning system was then set up at these maximum angles and the E-field strength was measured in 10cm steps from 0.5m to 1.9m for each laptop"
Results
"Burst power measurements drifted by 15% in the first 30minutes after switch-on as opposed to less than 3% after 2hours of transmitting, emphasising the need for adequate equipment warm-up times. Furthermore the results showed that, for a given position, the power level fluctuated between 2 (and sometimes 3) distinct levels because of the use of switched diversity with several antennas within each laptop. Overall, the results showed similar radiation pattern measurements for all laptops, with a minimum in the direction of the front of the laptop (torso of the user). Generally, two angular maxima were observed that were symmetrically opposed across a vertical plane bisecting the screen and keyboard. The laptops had antennas mounted on the top left and top right corner behind their screens and each of these antennas would have been responsible for producing one of the maxima. The maximum E-field recorded at 1m varied from (719±14)mVm-1 to (1306±3)mVm-1. In terms of power density, all these values are well below the level that would be expected based on the 100 mW (EIRP) limit, as shown in Figure 1"
Figure 1. The calculated power density for all 14measured laptops. The error bar represents the repeatability of E-field measurements for 50samples (only the highest observed variation is presented)
Health and sustainable homes and communities conference: September 2009
Some of the findings of their research were outlined in more detail at Health Protection 2009 at the University of Warwick in September 2009
The slides presented by Simon Mann, one of the researchers involved in the project can be seen at
http://www.hpa-events.org.uk/HPA/media/uploaded/EVHPA/event_74/Mann_14.pdf
The conclusions from the research as summarised as follows
"Emitted powers were very low for the 15laptops
" • Integrated (total) radiated powers ranged from 6 to 19 mW
" • Effective powers ranged from 17 to 57 mW in the direction of maximum emission (within the regulatory maximum of 100 mW)