Radio Frequency Fields and Users
This resource explains the technical, health and planning issues for the use of technology that emits radio frequency radiation in the City.
Background
An increasing amount of modern communications use radio waves, formed from radio frequency electric and magnetic fields. This includes AM and FM radio, television, remote-control devices (such as garage door openers), radiotelephones, walkie-talkies, and cellphones. Unlike systems such as conventional phones, there is no need for a physical link (such as a wire or fibre optic cable) between the sending and receiving point, hence the old fashioned term for radio was the ‘wireless’.
All these systems need transmitters, from which the radio waves are transmitted to the receiving equipment. While “conventional” radio and TV transmitters are familiar, newer technology, particularly that needed for cellphones, has given rise to concern in some areas.
Technicalities
Transmitters operate at different frequencies, depending on the application. There are national and international rules and conventions, which allocate frequencies for different purposes (e.g. for TV and radio transmissions, medical and industrial applications). This ensures that suitable frequencies are available for each purpose, and different users do not interfere with each other.
FM and AM radio and TV transmitters operate at comparatively high powers (several kilowatts). Cellsites (particularly in urban areas) and radiotelephones operate at moderate to low powers. Microwave systems used for point-to-point communications operate at low power.
Simple antennae, such as the ones used on a cordless phone or for some mobile radios, transmit equally in all directions. But it is often best to transmit more energy in some directions than others. For example, a TV transmitter may use directional antennae. These will aim the radio frequency signals towards the TV audience, rather than wasting signals on areas where there are no TVs. The TV station will get the same coverage - and use less power - than they would in broadcasting the transmission equally in all directions.
Exposure
Exposure to radio frequency fields produced by a source depends on a number of factors including:-
- Distance: Strength of the fields decreases with increasing distance from the source, just as sounds (also carried by waves) are quieter as you get farther from the source. Generally exposures decrease with the inverse square of the distance; someone 2km from a source receives one quarter the exposure of someone only 1km from the source.
- Transmitter power: In any given situation, stronger transmitters produce higher exposures. If a transmitter power doubles, exposures around the transmitter double.
- Directionality of antennae: Antennae can be designed so radio signals which would otherwise be transmitted in directions not required for achieving desired coverage (e.g. straight down towards the ground, or up to the sky) are redirected towards areas needed. Because less of the signal is wasted, a lower power transmitter can be used rather than if a simple, nondirectional antenna had been used. The amount by which the antenna increases the amount of power transmitted in any direction, compared to the amount, which would go in that direction if a simple antenna were used, is referred to as antenna gain.
- Height of the antenna above the ground: Increasing the height of an antenna above the ground is another means of increasing the distance for the antenna, and exposures in any given direction will decrease.
- Local terrain: The terrain can cause exposures to vary considerably from what they would be if the ground were flat. Many types of transmitter – for example, for FM radio, TV, cellsites, microwave communication systems, ideally require a direct line of sight to pick up the signal. Trees, buildings or intervening ridgelines markedly reduce exposures.
Exposures to radio frequency fields are usually described in terms of power flux density and are measured in microwatts per square centimetre (mW/cm2 ). Power flux density is the amount of radio frequency energy passing through a given area.
The amount of radio frequency power absorbed in the body depends not only on the power flux density, but also on the frequency of the signal. To a radio signal the body acts like a receiving antenna (albeit a poor one). For a given power flux density, the body absorbs more power at frequencies around 100MHz than at higher or lower frequencies.
Because exposures depend on several factors, it is not always obvious what the exposures will be in any situation. For example, a major TV or FM radio transmitter is quite powerful, so at first it might appear that exposures around it might be quite high. But the antennae on such transmitters are usually mounted quite high above the ground and direct most of the signal away horizontally, and comparatively little is directed steeply down towards the ground around the base of the transmitter.
In practice then, exposures are quite low. In comparison, cellphones are fairly weak transmitters, but because they are held right next to the head, exposures can be relatively high.
Health effects
It is not possible to prove anything absolutely safe, if “absolutely safe” means no possibility of harm to anyone. No matter how many scientific studies and observations are carried out to investigate possible health effects associated with an exposure, it will still be possible to conceive of circumstances where harm might occur. For example this might be because of unusually high exposures or because of the (theoretical) possibility of the existence of individuals who are extraordinarily sensitive to exposure.
Modern society wants to experience the benefits of technologies, and many of these entail new exposures for which we can have no assurance of absolute safety.
In the end society has to decide whether particular technologies and associated exposures are acceptable. It is not up to scientists alone to make the risk-benefit judgements. Where scientists can help is in supplying information that ensures such judgements are reached with the use of the fullest information possible.
The best scientists can offer is evidence of a high level of consistency in the results of a wide range of studies of various types showing no evidence of likely health effects. Only rarely do studies of any one form of exposure produce uniformly reassuring results on a wide range of different types of test. Often some tests will show no evidence of harm, whereas others of a different type may suggest adverse effects are indeed possible.
The extensive research literature investigating health effects of radio frequency fields has been reviewed many times. These reviews have concluded that at frequencies above about 1MHz, radio frequency energy is converted to heat inside the body.
In most situations, such as in publicly accessible areas around radio transmitters, this heating is negligible compared with heat produced by the body’s own metabolism, generally less than 0.0005 W/kg compared with 1-4 W/kg from metabolism.
There is a fairly wide consensus exposures to relatively weak fields appear unlikely to cause short or long term health effects, some scientists disagree. A great deal of research has already been carried out to find out if there are effects caused by low-level exposures. More consistent and replicable results are required before this data can be incorporated into a health risk assessment.
Exposure Standards
Scientific consensus on the health effects of radio frequency fields has been developed over the past thirty years. It is based on reviews of all the relevant research. Such reviews have also formed the basis for developing exposure standards that will limit exposures to levels which will ensure a safe and healthy living or working environment.
Standards must be based on a review of the relevant research by people who can make a critical assessment of the findings. The exposure levels, which are harmful, or are considered likely to be harmful to human health, must be determined. Exposure limits are set below such levels, and normally include a safety margin. The relevant New Zealand Standard is NZS 2772.1 (1999) which is based upon the findings of the International Commission on Non Ionising Radiation Protection (ICNIRP).
Typical maximum exposures around transmitters are plotted against the ICNIRP/NZS guidelines in the figure below. These are maximum exposures: levels in most areas near transmitters, especially close to all but AM transmitters, are normally at the low end of the range indicated or even lower.
Christchurch City Council City Plan Rules regarding the siting of RF transmitting devices are found in the Utilities section of Chapter 9 on General City rules.
Other sources of information
Relevant information such as committee reports and newspaper articles can be sourced from the Aotearoa New Zealand Centre at the Central City Library, Gloucester Street - 941 7923 and the Christchurch Environment Centre, 64 Kilmore Street - 379 2257. Librarians at the Central City Library are available to assist with research projects, as are staff at the Christchurch Environment Centre.
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