Feature

Communicating Radiation

What are you really receiving from cellphone transmitters?

Martin Gledhill

Recent controversy over the siting of cellphone transmission stations has highlighted public fears and misunderstandings of radiation, in a debate which has often generated far more heat than light.

Four frequency bands are available for cellphone communications. Two of these are owned by Telecom, the third by BellSouth, and the fourth by the Australian telecommunications company Telstra. Both Telecom and BellSouth have large established networks covering both urban and rural areas, but Telstra have yet to announce any plans for using their portion of the spectrum.

From the point of view of radiation protection, the existing networks are broadly similar. Each transmission station is designed to provide coverage over a limited area, with limitations being set by the local topography and the power of the transmitters used. The transmitting antennas may be mounted on buildings or, where these are unavailable, on masts.

The Equipment

Depending on the type of coverage required, either panel antennas or dipoles are used. Both types shape the transmitted radiofrequency (RF) beam so that the majority of the power is emitted horizontally, with relatively minor amounts directed towards the ground in sidelobes. Whereas the dipole emits uniformly in all horizontal directions, the panel antennas transmit a fan-shaped beam, generally into a 120^o or 180^o sector. Separate antennas are needed for transmitting the signals to cellphones in the area, and for receiving transmissions back from the phones.

Transmission frequencies are in the range 800-950 MHz, in the UHF (ultra-high frequency) band of the radiofrequency spectrum. The power transmitted varies from site to site, but is normally in the range of 20-150 Watts. BellSouth sites transmit the same power at all times, while the Telecom system varies the output according to the number of cellphone calls being handled by the transmitter.

In addition to the antennas used to communicate with cellphones in the area, some of the sites are connected to other parts of the network through a microwave link. The dish antennas used emit a very low power (40-600 mW), highly directional beam. At the frequencies used, the link is very sensitive to the effects of any intervening structures, and a direct line of sight from one end of the link to the other is necessary, with no obstruction by buildings or trees.

Typical Exposures

The National Radiation Laboratory has been approached by councils, residents and both network operators to provide independent assessments of the likely exposures to people who live or pass close by cellphone sites. These assessments have been made both by calculation and by measurements around operational sites.

Exposure is quantified in terms of the power flux density -- the radiated energy passing through unit area at right angles to the direction of propagation. In SI units, this would be given in watts per square metre, but because of the low levels involved with cell sites, it is more common to use microwatts per square centimetre (mW/cm²). Invariably, maximum exposure levels which have been calculated or measured are around 5mW/cm². Typically, exposures are less than 1mW/cm², and around some sites it has not been possible to detect anything with the laboratory's monitoring instrument, which can measure signals down to about 0.01mW/cm².

The body can be thought of as an ellipsoidal bag of conducting fluid. Like any conductor, the body will absorb energy when exposed to RF radiation. The dose -- the amount of the RF energy actually absorbed by the body -- depends on a number of factors, such as the polarisation of the RF waves and their wavelength in relation to body height.

For adults, the maximum absorption occurs at a frequency of about 70 MHz (wavelength 4.3 metres), whereas for children the maximum is at about 2-300 MHz (wavelength 1.5-1 metres). At cellphone frequencies, absorption is a factor of five lower than the maximum, and approximately the same for both children and adults. In making safety assessments and in setting exposure standards, these differences are taken into account and worst case conditions assumed.

Biological Effects and Exposure Limits

Several thousand research papers have been published investigating the biological and health effects of exposure to RF radiation have been published over the years. These include studies of long and short term exposure of humans and other primates, other mammals such as rats and mice, as well as isolated cell cultures.

One indisputable effect, familiar to anyone with a microwave oven, is the warming of body tissue. At sufficiently low exposures this heating is negligible compared with the body's own metabolic processes, which at rest produce about one watt of heat per kilogram of body-mass. A variety of effects have been noted in humans and other animals once RF heating exceeds levels with which the body's natural cooling mechanisms can cope. The threshold for effects arising from such heating corresponds to increases in body temperature of about 1^oC.

Somewhat more controversial are so-called "athermal" effects which are reputed to occur at exposure levels too low to cause significant heating, and particularly when RF carriers are modulated at low frequencies. Effects reported have often involved ion movements across membranes in isolated tissue or cell cultures, with the effect only occurring at specific modulation frequencies and power flux densities. Some of these effects have been difficult to confirm, and there is little indication that they occur in vivo in animals or humans.

In recent years, the research database has been reviewed extensively and independently in the US and Britain, and by the World Health Organisation. These reviews have all concluded that any adverse health effects which have been noted were due primarily to thermal interactions (ie, any heating effect of the RF exposure on the body, however slight) or occurred at levels where heating would have occurred. Reports of athermal effects were not ignored, but they have not been shown to be indicative of potential health effects.

Exposure standards are intended to limit exposures to levels well below those at which adverse health effects would occur. In the case of the New Zealand Standard, adopted in 1990, the limit for the public is set at least 50 times below this level. At the frequencies used by cellphones, the limit is 200mW/cm², considerably greater than the levels found near cellphone transmitters. Maximum levels at these sites are generally from one to five thousand times lower than the threshold for health effects: typically they are less than one ten-thousandth.

Of course, it has not been proven that such exposures are safe. Experiments can prove whether there are harmful effects, and at what levels these effects occur. However, the absence of an effect does not prove safety as there are a large number of other effects which could be looked for, and an infinite variety of exposure conditions which could be tested. Nevertheless, the absence of a wide range of potentially harmful effects under particular exposure conditions can give good grounds for believing that any other effects are extremely unlikely.

Common Concerns

Objection

• Standards are based on thermal effects only: they ignore the research literature which suggests that athermal effects may occur.
• Cellphone technology is new and any effects of long term exposure may not show up for several years.
• At the frequencies used by cellphones, children absorb more of the RF energy than adults, and so may be more susceptible to any effects.

Fact

• Recent reviews and standards have not ignored the possibility of athermal effects: reports of such effects are cited in the reviews and the review committees have included members who have published such reports.
• Cellphone technology is a recent application of radio technology which has been around for at least 50 years. In the US in 1980, it was estimated that at least one million people were exposed to levels similar to those found around cellphone transmitters. No adverse effects have been noted in populations exposed to RF transmissions over periods of several years.
• The maximum absorption for adults occurs at about 70 MHz, and for children at 300 MHz. At cellphone frequencies, absorptions are similar. Nevertheless, exposure limits are based on the worst case absorption for both adults and children.

Martin Gledhill works at the National Radiation Laboratory in Christchurch.