Following the announcement by the Board of the HPA on 12 October 2007, a systematic programme of research has been undertaken to assess exposures of children from wireless computer networking equipment used in schools. At the start of the project, comprehensive test facilities were set up at the HPA site at Chilton and a review of technical standards and wireless equipment used in UK schools was carried out. The main objectives of the project were as follows.
The original announcement can be found in the archive of HPA press releases for 2007 (these are listed in chronological order).
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 access points, it was decided that the experimental measurements would begin with laptops and then continue with access points, the points of entry to the wired network. A total of 15 laptops and 12 access points were chosen from among the most popular models used in the education sector in the UK. Both 2.4 GHz and 5 GHz frequency bands were considered during the assessments.
The objectives of the laboratory measurements were to establish the radiation pattern (ie the angular distribution of electric field strength around each device under test) during transmission, identify the angles at which the field was at a maximum, and then measure the electric field strength at these angles as a function of distance.
A more detailed description of the experimental set-up and some early results were presented here Wi-Fi in schools (PDF, 276 KB).
The results of the laboratory measurements are published in the following paper - Peyman, A et al (2011). Assessment of exposure to electromagnetic fields from wireless computer networks (Wi-Fi) in schools; results of laboratory measurements. Health Physics, 100, Issue 6, 594-612 at http://journals.lww.com/health-physics/Abstract/2011/06000/Assessment_of_Exposure_To_Electromagnetic_Fields.4.aspx.
The results showed that for a given position, the field strength around laptops fluctuated between 2 (and sometimes 3) distinct levels because of the existence of several transmitting antennas within each laptop. The signal was rapidly switched from one antenna to another in order to counteract fading through a process known as switched diversity.
Overall, the radiation patterns of the 15 laptops had certain similarities. A minimum electric field strength was generally observed from the front of the laptop (towards the torso of the user) and 2 maximum readings of the electric field strength were generally observed symmetrically opposed across a vertical plane bisecting the screen and keyboard. Analysis of the laptop internal structures suggested that the maxima arose from antennas mounted behind the top left and top right corners of the laptop screen. The radiation patterns for access points were more symmetrical with generally higher power densities at a given distance.
The total radiated power emissions from the laptops were evaluated by summing the power flowing through a spherical surface enclosing the laptops. This was based on the measurements at 1 m for devices operating at 2.4 GHz and 1.5 m for devices operating at 5 GHz. These spherically-integrated radiated powers (IRP) ranged from 5 to 17 mW for 15 laptops in the 2.45 GHz band and ranged from 1 to 16 mW for 8 laptops in the 5 GHz band.
For practical reasons and because access points are generally wall-mounted with beams directed into the rooms, their powers were integrated over a hemisphere. These ranged from 3 to 28 mW for 12 access points at 2.4 GHz and from 3 to 29 mW for 6 access points at 5 GHz.
In addition to the spherical measurements of IRP, power densities were measured at distances of 0.5 m and greater from the Wi-Fi devices. The maximum power density values for the laptops and access points at 0.5 m were 22 mWm-2 and 87 mWm-2 respectively, decreasing to 4 mWm-2 and 18 mWm-2 at 1 m distance. Consistent with the low radiated powers, these power density values are well below the International Commission on Non-Ionizing Radiation Protection (ICNIRP) reference level of 10 Wm-2.
Where the fields are non-uniform it is also important to consider the absorption of radiofrequency energy in the body of a person near the devices. To address this, computer modelling techniques were used to assess the localised SARs arising from Wi-Fi equipment in models of adults and children. Using a sitting voxel model of a ten-year-old child, and considering an exposure scenario of a Wi-Fi device operating at 2.4 GHz with an output power of 100 mW and a duty factor of 1 (100%), the highest localised SAR value in the head and torso were calculated as 5.7 mW kg-1 and 14.4 mW kg-1 respectively. These values are significantly lower than the limit of 2 W kg-1 basic restriction advised by the ICNIRP for the general public. The reported SAR value in the head represents less than 1% of the SAR previously calculated in the head for a typical mobile phone exposure condition; see Findlay, R P, and Dimbylow, P J (2010). SAR in a child voxel phantom from exposure to wireless computer networks (Wi-Fi). Physics in Medicine and Biology, 55 (15) at http://iopscience.iop.org/0031-9155/55/15/N01).
It is also notable that a duty factor (the fraction of time the device is transmitting) of 1 (100%) is unlikely to be achievable in practice, especially for the 6-minute time-averaging period relevant to assessments in the context of the ICNIRP guidelines. The duty factor would have to be much less so that multiple users could access the system simultaneously. There are a range of technical factors implicated as well, including the quality of the air interface, signal strength, interference from other devices, modulation scheme and the performance of the hardware. For this reason, and in order to have a more realistic assessment of exposure, a series of on-site measurements have also been performed to record the proportion of the time that individual Wi-Fi computers transmit during typical school lessons. Two parallel methods have been used to record the transmission time for the Wi-Fi devices. The first was using a Wi-Fi packet capture system that allows all the network wireless communications to be examined individually. The second method involved a transmit time counter (developed in-house) which was attached to the laptops.
The results of the duty factor investigation are available in the following publication - Khalid, M et al (2011). Exposure to radio frequency electromagnetic fields from wireless computer networks: Duty factors of Wi-Fi devices operating in schools. Progress in Biophysics and Molecular Biology at http://www.ncbi.nlm.nih.gov/pubmed/21856328
The duty factor investigation shows that laptops and access points transmit for only small proportions of the time during typical lessons less than 1% of the time for laptops and less than 12% for access points. This means that the time-averaged exposure arising from the use of laptops in schools is even lower than those when laptops were measured under controlled conditions in the laboratory, and the SARs will also be smaller than those estimated above.
The data gathered during the project continue to reinforce the position adopted by the HPA at the beginning of the project that exposures are small in relation to the ICNIRP guidelines and less than those from mobile phones.
The outcome of the project is considered by the Advisory Group on Non-Ionising Radiation in its 2012 review Health effects from radiofrequency electromagnetic fields (Doc HPA, RCE-20).
Last reviewed: 26 November 2013