Summary

General Exposure Levels

Wireless access points commonly have 2.4 GHz wLANs operating at 0.03 watts output power (5-6 GHz ones can use up to 20 times this).  In the classroom there would probably be one wLAN node (0.03 W) and, say, 20 laptops all at 0.03 W. However, they are only transmitting much power when actually transferring files.  So, let's say that we have the equivalent of one laptop operating absolutely continuously (actually the combined output of 20 may well be more that this), and that we are on average 2 metres from the antennas.  This seems reasonable based on the fact that there are 20 in the room in this example.  So E = (sq.root (30*0.03*2))/2 = 0.67 V/m equivalent continuous.  Now the ICNIRP guidance at 2.4 GHz is 61.5 V/m.  So the signal strength is about 1/100th of what is allowed.  Power is proportional to signal strength squared so that would be around 1/10000th of the ICNIRP power level.

A mobile phone next to head typically outputs 0.5% (1/200th) of the ICNIRP guidance.  In the 20 PC wLAN classroom example there would be something in the order of 0.01% (or 1/10000th) of ICNIRP guidance, about a 50-fold difference.

Therefore about 16 hours in the example classroom with 20 wLAN PCs, (approximately eight standard school days) would be equivalent to 20 minutes on a mobile phone running at typical power levels.

However, there are often a number of access points that are sending short bursts of data a few times per second, and the signal strength of these are all reasonably high.  If this is the case, the average exposure from any access point will be much higher than in the calculations above.

However, these figures are assuming that it is cumulative absorbed power that is being implicated in RF research, and that then implies a linear dose-response relationship model.  From the evidence that has found a risk, this seems unlikely to be the case.  Peak signal strength received may also be important, and people using WiFi enabled laptops would regularly be exposed to electric fields of 2 to 3 V/m.  Whilst this is far below ICNIRP, it is far above the levels where adverse health effects are being reported (~0.05 V/m).

Scientific Evidence

Firstly, it is very important to stress that there is currently (as of July 2007) nothing that we are aware of in the scientific literature that looks at WiFi. So initially the answer would be "none".  However, when anecdotal evidence of problems are being reported, it is prudent to do two things:  Firstly, prioritise research to be done looking specifically at effects from typical WiFi exposure, and secondly, to have a look at the literature published on exposures and technologies that may be relevant.

TV and Radio Transmitters use relatively similar frequencies, but are not digitally pulsed transmissions (AM and FM radio are entirely continuous wave and TV is almost entirely continuous wave).  There is a reasonable chance that, if humans react to TV and Radio transmissions, it may be very differently to the reaction to a digitally pulsed signal, so even though there is evidence of a possible risk^[1] these may not be relevant to WiFi.

Mobile Telecommunication systems (CDMA, GSM and 3G) are both closer in frequency and are also digitally pulsed information carrying signals - these are sufficiently close to WiFi that the research into phones and their base stations may be applicable to exposure from WiFi.

Firstly, typical exposure from a phone in use is likely to be far higher than from a typical WiFi laptop or access point due to the different in proximity to the device in question.  So whilst, again, there is research showing that there may be a risk^[2][3], this may not be relevant.

However, signal strength from a mobile phone base station where it reaches the ground (approximately 70 to 100 metres from the base station) is typically between 0.5 and 1.5 V/m, exactly the same as we measured in a WiFi classroom in a school in Norwich, and the same as found in the above calculations, and seems therefore to be very relevant.  A quick survey of the literature looking very specifically into mobile phone base station epidemiology finds some statistically significant health effects^{[4][5][6][7][8][9][10][11][12]}.  Many of these are summarised, with helpful graphs, etc, in our subscriber article "Radiofrequency EMFs and health risks".  There is, in fact, very little research looking at base stations that has failed to find an effect.  Also interesting is that many of the effects in the papers above (non-cancer effects) are those being reported in the anecdotal evidence from WiFi exposure.

In essence then, there is sufficient evidence to warrant some degree of precaution regarding WiFi until research has been done very specifically into its effects.  With dLAN systems and ordinary CAT5/6 wired networks offering better stability, bandwidth and security, there is simply no need for most homes, organisations and schools to switch to wireless networks, apart from the savings of the slight inconvenience in cables.

IT Practicalities

As briefly mentioned above, there are other serious drawbacks to using WiFi that are important considerations when implementing computer networks.  Firstly, the bandwidth is really not very good.  Modern wired network cables are running at 1 GBit/sec, and even older networks are running at 100 MBit/sec.  WiFi on the other hand typically performs at around 8-15 MBit/sec, even though the specs suggest it should be capable of about 54 MBit/sec.

On top of the speed restriction, WiFi is also susceptible to interference.  An access point is typically only designed to accept concurrent connections from a handful of laptops, else the system can easily start getting confused.  Likewise, it is easy to disable a network by blasting the area with higher powered 2.4 GHz radiation.  Then lastly, and from an IT point of view, is security.  Without technical knowledge, many access points come without having WEP encryption set up, and not doing so can leave your wireless network very vulnerable to intrusion from other wireless devices.  It is very easy to drive around a city residential area, and with the right hardware, find an unsecured wireless network adaptor and simply "hook in" to someone else network and internet connection.  Aside from the usage of their payed for bandwidth, this has a risk of letting them access files on other computers on the network, and also performing illegal activity whilst effectively framing the owner of the internet connection.  Again, wired networks are simply secure unless someone comes into your house and plugs a laptop of theirs into your router / switch.

Having said this, Wireless network does have the rather useful advantage of not having to cable up various rooms in your house (where you would intend on using a laptop) with networking sockets.  However, again, there is a more secure and stable alternative, at pretty much the same price: dLAN units allow you to use the buildings mains circuitry as an extension to your computer network, so all you have to do is plug one dLAN unit into the wall (and network to your router / wired network) and the whole building is networked.  Plug the other unit into any other socket on the same power circuitry, and voila, you have a new network connection.  Because the network traffic is still travelling down wires, and it doesn't get out beyond the house's consumer unit, the network is secure from those that do not have physical access to the house itself. It can also carry a higher bandwidth than a wireless network (typically 80 MBit/sec).

So there really is no need to use WiFi anyway, barring the convenience of not having to plug your laptop into anything at all (but for those that need a mains socket anyway, dLAN just uses one extra wire from the same socket as the laptop adaptor).  For schools we would recommend wiring up all the classrooms that need to have network access, preferably by putting the power and network cabling through the same trunking.  If done by IT staff, this is a lot cheaper than wireless networking equipment anyway.  If there is no-one capable, or allowed (perhaps on health and safety grounds), again dLAN is the next best option as this reduces the need for expensive sub-contracting work.

Other Resources

Dr. Magda Havas (Environmental and Resources department, Trent University, Ontario, Canada), has written an excellent precautionary paper, prepared for the Board of Supervisors, City and County of San Fransisco.  This 51 page document is available from here, and covers a wide range of literature and scientific findings on RF research, and presents them in a logical and progressive manner.

Concerned parents Jane Smith and Vanessa Spedding have recently fought a successful campaign to prevent WiFi being installed in Wigmore Primary School, Herefordshire.  Their campaign succeeded using a common sense approach combining precaution against the possible health effects, cost, and IT practicality.  They are happy with us presenting their letter, an accompanying letter from Prof. Olle Johansson, and a full implementation costing from an IT company that installs network systems.

BECTA have provided a very thorough albeit technical article on WiFi and wireless specifications.

References

[2] - Lönn S, Ahlbom A, Hall P, Feychting M (November 2004). "Mobile phone use and the risk of acoustic neuroma". Epidemiology 15(6):653-9 - [View Abstract]

[3] - Lennart Hardell, Kjell Hansson Mild, Michael Carlberg, and Fredrik Söderqvist (2006). "Tumour risk associated with use of cellular telephones or cordless desktop telephones". World J Surg Oncol. 2006; 4: 74 - [View Abstract]

[4] - Abdel-Rassoul G, El-Fateh OA, Salem MA, Michael A, Farahat F, El-Batanouny M, Salem E (March 2007). "Neurobehavioral effects among inhabitants around mobile phone base stations". Neurotoxicology. 28(2):434-40 - [View Abstract]

[5] - Yurekli AI, Ozkan M, Kalkan T, Saybasili H, Tuncel H, Atukeren P, Gumustas K, Seker S (2006). "GSM base station electromagnetic radiation and oxidative stress in rats". Electromagn Biol Med. ;25(3):177-88 - [View Abstract]

[6] - "Risk Evaluation of Potential Environmental Hazards From Low Frequency Electromagnetic Field Exposure Using Sensitive in vitro Methods". EU Programme, "Quality of Life and Management of Living Resources" - [View Foreword and Download Report]

[7] - Santini R, Santini P, Danze JM, Le Ruz P, Seigne M (July 2002). "Investigation on the health of people living near mobile telephone relay stations: I/Incidence according to distance and sex". Pathol Biol (Paris). 50(6):369-73 - [View Abstract]

[8] - Santini R, Santini P, Danze JM, Le Ruz P, Seigne M (September 2003). "Symptoms experienced by people in vicinity of base stations: II/ Incidences of age, duration of exposure, location of subjects in relation to the antennas and other electromagnetic factors". Pathol Biol (Paris). 51(7):412-5 - [View Abstract]

[9] - Balmori Alfonso (2005). "Possible Effects of Electromagnetic Fields from Phone Masts on a Population of White Stork (Ciconia ciconia)". Electromagnetic Biology and Medicine, 24: 109-119 - [View Summary and Download Report]

[10] - Wolf R, Wolf D (April 2004). "Increased incidence of cancer near a cell-phone transmitter station". International Journal of Cancer Prevention Vol1, No2, - [Download Report]

[11] - Horst Eger, Klaus Uwe Hagen, Birgitt Lucas, Peter Vogel, Helmut Voit (April 2004). "The Influence of Being Physically Near to a Cell Phone Transmission Mast on the Incidence of Cancer". Umwelt Medizin Gesellschaft 17 - [View Summary and Download Report]

[12] - Oberfeld Gerd, Navarro A. Enrique, Portoles Manuel, Maestu Ceferino, Gomez-Perretta Claudio (August 2004). "The Microwave Syndrome - Further aspects of a Spanish Study". Conference Proceedings - [View Summary and Download Report]