Abstract: As the experts
notes, the use of the mobile phones contributes far more to microwave exposure
than the presence of base stations. But though, the phone masts remain
unpopular and it is not recommendable
to errect the base station masts near
schools, hospitals, in national parks and areas. Let us examine the real
situation in vicinity of mobile phone masts.
Keywords: base station, health
risk, athermal effects
I INTRODUCTION
Mobile communication network have operated in various forms for more than 50 years. However, with the introduction of cellular mobile telephone the use of mobile telephone services has expanded rapidly. Consequently, mobile phone base station have become a common sight around cities and along highways. But just now it has been raised fear if exposure to radiation from the base station may have an adverse effect on human health.
II WHAT KIND OF RADIATION DO BASE
STATION PRODUCE
Base station produce radiofrequency (RF) radiation mostly of 800 to 1800 MHz and it is concerned as a low-powered radio antenne that communicate with users’ handsets. But, RF radiation in the environment is also produced in varyng amounts by radio and television towers, CB and UHF aerials, pager services, as well as cordless phones and some remote controlled devices. Even the sun and the human body contribute to the amount of radiation in the environment. Now it is important the question about the amount of the each contribution, especially that by the mobile phone base station.
The design and installation of the antenna determines the distribution pattern of the RF radiation transmitted from the base station. The RF radiation is normaly directed at or just below the
horizon. Although in general the exposure level decreases with increasing distance, this is notnecessarily the case in areas close to the basestation. The exposure level immediately below a base station can be much lower than the level a few hundred meters away in the direction of the aerials. The presence of trees, hills, buildings and other structures can distort the RF radiation causing variations in its strength.
Scientists have known for a long time about the ability of RF radiation to cause heating, which can lead to severe health effects on the body such as fatique, reduced mental concentration and cataracts, if exposed to very high levels. These effects are known as thermal effects, some of which can be created by subjecting a person to a warm environment. But the Radiatiuon Protection Standard specifies limits on public exposure to RF from various sources, including mobile phone base stations. The level at which these limits are set is much lower than the levels at which any heating effects can occur, [1]. All base stations must comply with this Standard, except at the distance close to the station.
III CONCERNS FOR HEALTH
Several important consideration must be kept in mind when evaluting possible health effects of RF fields. One is the frequency of operation. It is important not to confuse such RF fields with ionizing radiation (x-rays or gamma rays). Unlike ionizing radiation, RF fields cannot cause ionization or radioactivity in the body.
RF fields penetrate exposed tissues to depths that depend on the frequency. In case of mobile phones it is up to a centimeter. RF energy is absorbed in the body and produces heat, but the body’s normal thermoregulatory processes carry this heat away. All established health effects of RF exposure are yet related to heating, and no study has shown adverse health effects at exposure levels below international standard limits. The intensity of radiation from mobile phones and base station is too low to produce significant heating. Most health concerns possible athermal effects arising from interference with the body’s natural electrical activity. Only few studies that have examined the result of short-term whole body exposure to RF fields at levels far higher than those associated with wireless communications address the consequences to the health hazard, [ 2].
IV EXPOSURE LEVELS
Mobile phone handsets and base stations present quite different exposure situations. RF exposure to a user of mobile phone is far higher than to a person living near a cellular base station. However, apart from infrequent signals used to maintain links with nearby base stations, the handset transmits RF energy only while a call is being made, whereas base stations are continuously transmitting signals. Both measurements and calculations show that RF signal levels in aresa of public access from base stations are far below international guidelines, typically by factor of 100 or more,[3][,4],[5]. RF exposure levels to a user from mobile handsets are considerably larger but below international guidelines.
Mobile phone handsets are low-powered RF transmitters, emitting maximum powers in the range of 0.2 to 0.6 watts. For example, “ walkie talkie” transmitter emits 10 watts or more. The RF field strength ( and hence exposure, too) falls off rapidly with distance from the handset. Therefore, the RF exposure to a user of mobile phone located 10s centimeters from the head ( using a “ hands free” appliance) is far lower than to a user who places the headset against the head. RF exposure to nearby people are very low, too, [1].
Base stations transmit power levels a few watts to 100 watts or more ( that is the same power as an average light bulb), depending on the size of the region or kind of the desined cell. Base station antennae are typically about 20-30 cm in width and a metre in length, mounted on buildings or towers at a height of from 15 to 50 meters above ground. These antennae emit RF beams that are typically very narrow in the vertical direction but quite broad in the horizontal direction. Because of the narrow vertical spread of the beam, the RF field intensity increases slightly as one moves away from the base station and then decreases at greater distances from the antenna. Typically within 2-5 meters of some antennae mounted on rooftops, fences keep people away from places where the RF fields exceed exposure limits. Since antennae direct their power outward, and do not radiate significant amounts of energy from their back surfaces or towards the top or bottom of the antenna, the levels of RF energy inside or to the sides of the building are normallly very low,[1].
Other RF sources in the community like paging and other communications antenae such as those by fire, police and emergency services, operate at similar power levels as cellular base stations, and often at similar frequencies. In many urban areas television and radio broadcast antennae commonly transmit higher RF levels than do mobile stations.
V MEASURED DATA OF BASE STATIONS
RADIATION
According to the measurement done by Australian Radiation Laboratory, [3],4],[5], which measured the exposure levels of RF electromagnetic energy radiated from mobile telephone base stations as a part of the other radiation in environment, the results are like shows the Fig. 1.
All measurements were made with Tetronix spectrum analyser. This equipment functions as a sophisticated radio receiver, which allows each received radio signal to be analysed, allowing the accurate measurement of magnitude and frequency. Recording of data from spectrum analyser was performed by a lap-top personal computer which has a communication card connection with the analyser. PC controls the operation of the analyser and record all relevant data. Signals measured by the analyser over the bands of interest were received by the following antennae: loop antennae (0.01-30 MHz), bi-conical antennae (20-320 MHz), log periodic antenna (300-1000 MHz) and magnetic base vehicle roof mount antennae (870-960 MHz).
The spectrum analyser measures the level of received signal in the power unit dBm. Calculation of field strength and hence power density requires a knowlwde of the receiving antennae properties and system losses. Power density is scommonly expresed in the unit of microwatt per centimetre squared (mW/ cm2) nd is calculated using the electric field strength and assumes far field conditions where the wave impedance is 377 ohms, If was possible, measurements were made in locatios hat maintain direct line-of-sight with known RF sources, at a height of cca 1.7 m above ground, and antennae were positioned in open areas away from likely sources of reflections so as obtain maximum signal strength for the particular frequency band being measured. [3].
Figure 1. Environmental RF EME Levels, [ 3]
Most of the RF signals are present throughout the day and come from a variety of communication sources like FM radio, AM radio, teleision and other broadcast services. The dominant signal came from AM radio transmitter antenae located in the region. The AM radio signals contribute in general more than 99% of the total RF radiation level of 0.0612 microwats per centimetre squered (mW/ cm2), while the sum of all base station was 0.000376 or 0.61% of the total, [3]. RF radiations have significant variations over short distances due to reflections from buildings and the ground. Hence the ratio of signals levels from all sources can be expected to change as a consequence of local conditions, but AM radio signals will still contribute the greatest proportion. It is important to know tahat AM radio signal have long wave length and are poorely absorbed by the human bod
The telephone activity of the base station varies over 24 hour period and at maximum level of activity, the RF radiation levels would be doubled. But neither this fact change radicaly situation in relation with obeying the Standards. These results show that the whole radiation in that area is far below the public exposure limit of 200 microwat per centimetre squered recommended by the Aystralian Standard.
VI DIMENSIONS OF HAZARD
Antennas operating at 800-1800 MHz will produce power densities in excess of guidelines, but only in the immediate vicinity. However, very little number of the population could be exposed to these fields. Power densities decline very rapidly and since the transmitters are usually mounted on rooftops (15 m) or on towers (25 m) , people at ground (at least 15 m from the antennae) typically receive exposures that are hundreds of times lower than the limits recommended by ICNIRP, [6]. As the experts notes, the use of the mobile phones contributes far more to microwave exposure than the presence of base stations. But though, the phone masts remain unpopular and it is not recommendable to errect the base station masts near schools, hospitals, in national parks and areas of natural beauty. Esspecially near the schools because up to 50% more radiation is absorbed in children as compared to the adults, [6].
CONCLUSION
None of the recent reviews have concluded that exposure to the RF fields from mobile phones or their base stations causes any adverse health consequences. However, there are gaps in knowledge that have been identified for further research to better assess health risks. WHO is also conducting RF research. A large epidemiology study is being co-ordinated in over 10 countries by International Agency for Research on Cancer (IARC), to identify if there are links between use of mobile phones and head and neck cancers. The study is anticipated to be completed in 2003.
In the meantime, WHO gives some recommendation like these: strict obeying the international standards, simple protective measures (fences or barrieres for base stations), consultations with the community in siting base stations, providing enough information, etc. Though, there are many scientists who got some results about the health hazard. The newer, digital, ones are likely to have more biological effects than the older, analogues, ones. Although existing evidence does not yet conclusively prove that there are any long-term adverse health implications, it is necessary to advise people to use the mobile phone as little as possible.
REFERENCES
[ 1 ] Fact Sheet N 193, Revised June 200, Mobile
Telephone and Their Base Stations
[ 2 ] Philips, Alasdair, Mobile Phone Adverse
Health Concerns, EMFacts Consultancy
[ 3 ] Measurement and Analysis of RF EME Levels
from Telestra Mobile Telephone Base Station
[ 4 ] Measurement and Analysis of RF EME Levels
from Telestra Mobile Telephone Base Station,
ARPANSA, Carey Park Bunbury, Western
[ 5 ] Measurement and Analysis of RF EME
Levels from Telestra Mobile Telephone Base
Station,ARPANSA, Engadine, NSW Australia
[ 6 ] Berclay, Christopher, Mobile Phones and
Mobile Phone Masts, SCIENCE AND
ENVIRONMENT, Research Paper 01/111,
11. Dec. 2001.
e-mail: prastalo@etfbl.net