ICNIRP Guidelines (International Commission for Non-Ionising Radiation Protection):
Current ICNIRP (1998) guidelines for general public exposure for 2.45GHz are:
Averaged whole body SAR 0.08 W/Kg (80 mW/Kg); localised to head and trunk SAR 2 W/Kg; for limbs SAR 4 W/Kg.
Electrical field strength 61 V/m.
Power flux density 10 W/m(2), which is 1 mW/cm(2).
Wi-Fi exposures and frequencies:
Wi-Fi-enabled computers have been reported to generally expose the user(s) to electrical field strengths of 0.1 - 6 V/m (Powerwatch, 2007). Typically Wi-Fi exposures range from 0.1 - 3 V/m (can go up to 6 V/m) for people in rooms with WLAN hubs. The higher exposures are likely to be when downloading files or sending E-mails. The Swiss Agency for the Environment, Forests and Landscape (Electrosmog in the Environment, 2005) describes maximum electrical field strengths measured from WLAN access points as 0.7 - 3 V/m at a distance of 1m and exposures from the wireless cards in computers to be 1.1 - 4.9 V/m at 50 cm.
The UK Health Protection Agency has found maximum field strengths of between 0.72 and 1.31 V/m at a distance of 1m from a single wireless laptop (2009). Exposures will be greater at 20-50 cm (the distance that children often sit from/lean towards a laptop). They will also increase if more than one wireless laptop is used in a classroom or if the access point emissions are also taken into account. The HPA intend to continue their measurements to see how the exposures are affected by the software and the work being done on the computers. They will also model how the radiation is absorbed in the body and go on to carry out a wider health risk review of the scientific literature.
The current ICNIRP exposure limit to protect against acute heating from microwaves is 61 V/m (at 2.45 GHz).
The Public Health Department of the Government of Salzburg (2002) recommend an outdoor exposure limit of 0.06 V/m (0.001 microW/cm(2), 1 nW/cm(2)) and an indoor limit of 0.02 V/m (0.0001 microW/cm(2), 0.1 nW/cm(2)) for radio frequency radiation, to protect against the damaging non-thermal effects of microwaves.
The Bio-Initiative report (2007) recommend 0.194 V/m (0.01 microW/cm(2), 10 nW/cm(2)) as the maximum indoor exposure limit for radio frequency radiation.
The Scientific Panel on Electromagnetic Health Risks (2010) recommend a provisional exposure guideline limit of 0.8 V/m (1.7 milliW/m(2)), going down to 0.25 V/m (0.17 milliW/m(2), 0.017 microW/cm(2), 17nW/cm(2)).
Wi-Fi, or WLANs transmit at 2.4 - 2.48GHz or 5.1 - 5.7GHz carrier wave frequencies, although other bands are likely in the future (e.g. 3.65 - 3.7GHz; 60GHz). Most Wi-Fi computers in schools use 2.45GHz (Becta School Survey 2007, IEEE 802.11g, 802.11b; a small number of schools use the 5GHz band, 802.11a).
The computer communicates using its wireless card with a base station (or access point), which is connected to the internet/intranet/local computer network. The access point is transmitting whilst it is switched on: when in standby mode the signal is pulsed with a frequency of 10 to 100Hz; during data transfer, both the access point and the communication card of the computer transmit with a pulse frequency of 10 to 250Hz (Swiss Federal Office for the Environment, 2005). When the computer is in use and Wi-Fi-enabled it will be transmitting signals, even if the access point is switched off. Therefore the wireless facility needs to be disabled in the computer if you do not want it to be transmitting.
For comparison, mobile phones transmit at carrier wave frequencies (cellular frequencies) 0.85, 0.9, 1.8, 1.9, or 2.1GHz (GSM, pulsed at 217Hz, each pulse 0.577 milli seconds in duration) or 0.8 - 2.7GHz (UMTS, pulsed at 100Hz). Further frequencies are likely in the future. DECT (digital enhanced cordless telecommunications) cordless phones transmit at 1.8 - 1.9GHz in Europe (pulsed at 100Hz; Swiss Federal Office for the Environment, 2005).
In addition to the carrier wave frequency, the pulse frequencies are relevant to the biological effects of wireless devices. Many processes in the body also have frequencies within these pulse ranges. For example, individual neurones (nerve cells) in the brain signal with electrical impulses/cycles (action potentials) with frequencies ranging from less than 1Hz up to around 350Hz (but could in theory reach higher frequencies). EEG (electroencephalogram) recordings on the surface of the scalp detect the synchronized activity of a large number of neurones, and these have frequencies of up to around 100Hz.
ICNIRP, 1998, Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields (up to 300 GHz). Health Physics 74 (4), 494-522, http://www.icnirp.de/PubEMF.htm (accessed Aug 2008).
Powerwatch, 2007, Philips, A., Public Exposure levels from WiFi systems 10/12/2007, http://www.powerwatch.org.uk/pdfs/20071210_wifi_signals.pdf (accessed Aug 2008).
Swiss Federal Office for the Environment, Reichenbach, A., 2005, Electrosmog in the Environment, http://www.bafu.admin.ch/publikationen/publikation/00686/index.html?lang=en&down (accessed January 2009).