Typical Base Station Installation
Cellular wireless technology can now allow the delivery of voice, text, images, music, etc., and it relies on an extensive network of base station to do this (Valberg et al., 2007). However, base stations have generated much public concern. They need to be high up, so they are often located on towers or masts or rooftops. Towers, which are usually 75 to 250 feet tall, are particularly often in the news when local residents object to their location, because of health concerns or simply because of their appearance.

This section will attempt to address health concerns. First, some background is given about how base stations work. Next, the radiation from them is discussed. The principles behind their regulation are reviewed, and finally summaries from some independent reviews are presented.

How base stations work

When a user of a wireless phone makes a call, it is transmitted to the nearest base station, which receives and transmits radio signals in its area - thus acting as a two-way radio. The area covered by a base station is called a cell, which is why wireless phones are sometimes called cell or cellular phones. The largest cells are known as macrocells; smaller cells, particularly in urban areas, are called microcells or picocells. The number of cells varies in different areas, depending on the volume of use. Areas with a high volume of mobile phone use will have more cells. As the phone user moves around, the radio signal can be switched from one cell to another, maintaining a good connection. The call is connected with the local phone network and delivered by phone lines, using a ground based antenna, or when the call is going to another wireless phone, by radio signals. Each base station covers a limited radius. Beyond that, the power density (strength) is too small for a mobile phone to work. When this happens, the phone is automatically switched to a closer base station.

As you travel the signal is automatically switched to a closer base station (with a higher power density)

The power level generated by a cellular phone to maintain a good connection depends on the distance from the base station; the greater the distance, the more power is needed. Cellular phones automatically step down to the lowest power level that maintains communication with the base station. Since there may be interference from neighbouring channels in the cell, or from physical obstacles, the amount of power needed may vary within a single telephone call.

Radiation and power output of base stations.

Base stations produce radiofrequency radiation (RFR), which is non-ionizing. This means that it does not break chemical bonds and is very different from x-rays. For more information on this, see the FAQ # 5. Antennas from base stations transmit radio waves that travel as fast as the speed of light. A good analogy is the beam of light coming from a flashlight. As with the beam of the flashlight, antenna beam width can be limited in both the vertical and horizontal planes. The beam is tilted a few degrees in the vertical direction, so that it intersects the ground a considerable distance away from the antenna tower - usually between 50 and 200 metres from the foot of the mast. Base stations typically transmit using beam widths roughly seven degrees in the vertical plane in elevation, and about 120 degrees or one third of a circle, in the horizontal plane. Outside this small beam the radiation is typically 100 to 1000 times less. Usually towers have several antennas transmitting signals in different directions.

Base stations are typically placed close together and transmit around 60 watts of power

The total power output of a typical mobile phone base station depends on the type of antenna, the number of analog and digital channels that operate at a given time and at their maximum strength, and the antenna gain which gives the signals direction and strength. The power output is less when the base station is close to the phone user. Hence, base stations are being placed closer together, to make it simpler for a phone and a base station to communicate reliably. The power output is measured in watts, and is about 1,000 times less than that produced by a television transmitter. Maximum powers radiated from the antenna at the top of the mast are in the region of 60 watts. However, the situation is more complicated when there are multiple antennas at the same site. It is even more complicated when base stations are located at the same site as television or radio transmitters.

In summary, factors that influence how much RFR an individual may be exposed to include:

  • The power output, frequency and type of transmitter;
  • The distance the person is from the transmitter;
  • The location of the person with respect to the transmitted beam;
  • The type of antenna and the direction of the transmitted beam;
  • The presence of other structures near the person that may shield them or reflect the RF signals towards them;
  • The time spent in a particular area of the RF field
    (Royal Society of Canada, 1999).

It is obvious from the above that the amount of exposure that an individual experiences from a base station cannot be estimated simply by the distance from the transmitter.RF field exposures, when measured on the ground near a base station, have always been very low. In Canada a survey of five Vancouver schools, conducted in response to parental concerns about safety, showed levels of radiofrequency radiation many times below established safety limits. The maximum level at one school with a PCS antenna across the street was 1620 W/m, whereas the Canadian safety limit is 10 million W/m. At a second school with an analogue base station on the roof, the maximum level was 25,600 W/m. At a third, also with an analogue station nearby, the maximum level was 2,250 W/m. The safety limit for these two situations is 5.9 million W/m. Other studies in Ontario, Canada, in response to concerns about analogue base stations had even lower measurements - 10 W/m at one site, and 0.2 W/m at another.

In the UK, the NRPB took measurements at 118 locations of public access near base stations and found that the maximum exposure was 0.023% of their guidelines. Another report by the NRPB in September 2004 examined exposure to the public at 610 locations in the area of 10 microcell and picocell base stations. Exposure quotients were generally in the range 0.002 -2% of the ICNIRP public reference level (Cooper 2004, 2006). The maximum level found was 8.6% of the reference level. In 2001 the Radiocommunications Agency, a government agency in the UK, undertook an audit of mobile phone base stations with a focus on those sited on schools. The audit surveyed 101 sites and recorded emissions "typically many thousands of times below" the guidelines of the International Commission on Non-Ionizing Radiation Protection. In Australia, a survey of 60 base stations showed that the maximal exposure was 0.2% of the general public exposure limit (Henderson, 2006).

While these studies are reassuring, it is true that base stations give rise to whole body exposure 24 hours a day, for those who remain in the neighbourhood. More studies of health effects need to be done, but one of the major problems to be solved is how to measure exposure accurately, using meters or another measurement tool (Ahlbom and Feychting, 2003). Radon (2006) compared 2 types of personal dosimeters and found them to be in moderate agreement, but neither correlated well with self-reported exposures to base stations.

One situation where the RFR from a base station could harm an individual is when someone approaches within a few metres of an antenna. This can happen during maintenance work on the base station. In this situation, appropriate safety measures need to be taken. Similarly, RF absorption limits for the general public may be exceeded immediately adjacent to wireless local area network (WLAN) and Bluetooth transmitters (Valberg, 2007).



References:
Radiofrequency radiation in five Vancouver schools: exposure standards not exceeded. Thansandote A, et al. Can Med Assoc J 199;160:1311-2.
Royal Society of Canada Expert Panel, 1999, p. 24.
Industry Canada
Ahlbom A, Feychting M. Electromagnetic radiation. British Medical Bulletin 2003;68:157-165.
Cooper TG, Mann SM, Khalid M, Blackwell RP. Exposure of the general public to radio waves near microcell and picocell base stations for mobile telecommunications. NRPB Report, September 2004.
Cooper TG, Mann SM, Khalid M, Blackwell RP. Public exposure to radio waves near GSM microcell and picocell base stations. J. Radiol Prot 2006;26:199-211.
Henderson SI, Bangay MJ. Survey of RF exposure levels from mobile telephone base stations in Australia. Bioelectromagnetics 2006;27:73-76.

Regulation of base stations

Guidelines for maximum exposure of the public to RFR have been formed by various agencies. In Canada, the guidelines are called Safety Code 6, and they use principles similar to those adopted in other countries. They are based on the minimum level of RFR that has been shown to produce biological effects. A 50-fold safety factor is then applied. Hence, the maximum RFR allowed is 2% of the level associated with biological effects.

Each country has its own regulatory system. In Canada, Industry Canada requires that all operators of radiofrequency transmitters ensure that electromagnetic fields produced by installations do not exceed maximum levels contained in Safety Code 6. In the USA the Federal Communications Commission (FCC) performs this function, and in the UK it is the National Radiological Protection Board (NRPB). ). For further information about the EMF regulations in your country, see the web site of the World Health Organization International EMF Project. This is found at www.who.int/peh-emf . Follow the links to the EMF Standards Worldwide Database.

Independent reviews of the risks from base stations.

Independent expert groups around the world have stated that there is no risk to the general public from base stations. The World Health Organization, for example, stated: "None of the recent reviews have concluded that exposure to the radiofrequency fields from mobile phones or their base stations causes any adverse health consequence".

An Expert Panel of the Royal Society of Canada said

"it appears that exposure of the public to radiofrequency fields emitted from wireless telecommunication base station transmitters is of sufficiently low intensity that biological or adverse health effects are not anticipated".

The Independent Expert Group on Mobile Phones in the UK said

 

"there is no general risk to the health of people living near to base stations where the exposures are only small fractions of guidelines".

Current Research

Research on health effects of base stations has been scanty. A few studies have specifically examined this issue. These can be found in "Research - Epidemiology - Santini" and in "Research - Human Experiments - Zwamborn and Regel". Research on base stations effects is very difficult, because of the problems associated with measurement of radiation exposure, and the lack of appropriate control groups. Schuz and Mann studied exposure to RFR from base stations and concluded: "It would be inappropriate to conduct epidemiological investigations based on proximity to mobile phone base stations since this exposure metric is a poor surrogate for an individual's exposure to radiowaves". A detailed examination of the issues surrounding epidemiological studies of health effects of base stations has been carried out by a panel that was sponsored by Swiss agencies (Neubauer, March 2005).

Research projects on base stations and health effects are being conducted in some areas of the world at the present time. Perhaps the leader in this area is the UK, where the Mobile Telecommunications and Health Research Programme (MTHR) has sponsored several studies. These include investigation of cancer incidence in young children living close to base stations, and examination of the relationship between base stations and a variety of symptoms. For more, see Research Programs ".

References: Schuz J, Mann S (2000): A discussion of potential exposure metrics for use in epidemiological studies on human exposure to radiowaves from mobile phone base stations. Journal of Exposure Analysis and Environmental Epidemiology 10:600-605.

Study on the feasibility of epidemiological studies on health effects of mobile telephone base stations - final report. Neubauer G, Roosli M, Feychting M, Hamnerius Y, Kheifets L, Kuster N, Ruiz I, Schuz J, Uberbacher R, Wiart J. March, 2005.
www.mobile-research.ethz.ch/var/pub_neubauer_pref14.pdf


Valberg PA, van Deventer TE, Repacholi M (2007): Workgroup report: base stations and wireless networks - radiofrequency (RF) exposures and health consequences. Environ Health Perspect 115:416-424.



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