Characteristics of the application and its use
Base stations are required to enable mobile phone communication, including calls and data transfer. They consist of different electronic components and antennas and can be located on masts, on rooftops, or on the outside or inside of buildings. Base stations emit high frequency (HF) fields in the range from several hundred MHz to several GHz. The exact frequency bands used differ between technologies (GSM, UMTS, CDMA2000, 4G) and between countries.
HF fields allow the transport of large data volumes through vacuum and air, and to a certain extent through solid matter, at a very fast rate (up to the speed of light). Over large distances, the signals must be relayed by a communication network comprising base stations and often supported by a wired network.
The power of a base station varies (typically between 10 and 50 watts) depending on the area that needs to be covered and the number of calls processed. This is low compared to other transmitters such as radio and television, which usually work at power levels ranging from several kilowatts to several megawatts. In general, the field strength decreases very rapidly with distance from the source and can be calculated (as the inverse square of the distance). Personal exposure to HF fields from base stations is different to that from mobile phones. HF exposure from mobile phones is mostly limited in time and is largest in the head or in those parts of the body close to where the phone is held. By contrast, base stations are a source of continuous whole-body exposure. This exposure is less intense than from a mobile phone, but occurs whether a mobile phone is being used or not.
Assessments of personal exposure levels are most accurately achieved through onsite field measurements. Theoretical calculations are also common but are complicated by the many factors that influence the actual exposure such as the height, tilt and direction of antenna, absorption from trees and plants or reflections from buildings, as well as distance. Distance alone is not a reliable proxy for exposure from a base station.
HF effects on the body and health implications
High frequency fields have the ability to penetrate the human body (though the higher the frequency, the lower the depth of penetration), with the main effect of this being a rise in temperature in the exposed tissue. The human body can adjust to small temperature increases in the same way as it does when undertaking exercise and performing sporting activities. This is because the body can regulate its internal temperature. However, above a certain level (referred to as the threshold) depending on the duration, HF exposure and the accompanying temperature rise, can provoke serious health effects, such as heatstroke and tissue damage (burns).
A large number of studies have been undertaken on both acute and long-term effects from HF exposure typical of base stations. Research at these levels of exposure has provided no conclusive evidence of any related adverse health effects.
The ICNIRP guidelines give general recommendations regarding HF exposure. These provide limitations, expressed in terms of the Specific Absorption Rate (SAR), for the absorption of energy from HF fields. Distinct SAR values apply to whole-body exposure, which is typical from base stations, and for the head, which is relevant for exposures from mobile phones.
ICNIRP follows up the HF related scientific research and any new information relevant to health. A revision of the current ICNIRP guidelines on HF used in conjunction with base stations is underway.