National Radiation Commission P.O.Box 743, Arusha, Tanzania.

Transcription

1 Preliminary Measurements of Electromagnetic Radiation Levels from a Radio Transmitter and Cellular Phone Base Stations Randomly Selected at Dar es Salaam City in Tanzania M. M. Nyaruba 1 and J. B. Ngatunga 2 1 Radiation Protection Services, 2 Nuclear Instrumentation Maintenance National Radiation Commission P.O.Box 743, Arusha, Tanzania nrctz@habari.co.tz Abstract. Cellular phones have become the most reliable means of communication in Tanzania after their introduction in the country five years ago. At present the providers of this service are forced to expand Cellular telephone networks due to high demand, a situation, which has resulted in a proliferation of base station towers in recent years. This situation has been accompanied by an increase in the level of community concerned about possible health effects from the EM radiation levels from antennae mounted on the base station towers. In attempting to solve this problem, we have measured the RF radiation levels from Cellular phone base stations and radio transmitters in the country in order to assess the levels of EM radiation the public is subjected to. The preliminary study comprised of 30 randomly selected base stations having antennae transmit power ranging from 56.5 to 63 dbm, majority of which are situated in densely populated areas in Dar Es Salaam city, and one radio transmitter having 100 kw forward power with 567 khz located near a primary school about 40 kilometres outside the city centre. The measurement was done using power density meter model 495 and a probe model 94, RAHAM, General Microwave Cooperation, USA, sensitive to 200 khz 40 GHz frequencies. The observed results ranged from to 1.25 mw/cm 2 for the areas around Cellular phone base stations and 0.6 to 6.5 mw/cm 2 (47.6 and V/m) at different locations of the primary school near the radio transmitter. Some of the values were higher than the recommended values in the International guidelines developed by ICNIRP 1998 (0.45 mw/cm 2 for 900 MHz, 0.9 mw/cm 2 for 1800 MHz, and 87 V/m for MHz) suggesting further study in this area to be undertaken. 1. Introduction Individual communications through cellular phones were introduced in Tanzania about five years ago. Up to three years after their introduction there was a high demand of this service, which led the providers to expand cellular telephone networks. Later on, the demand was slowed down by a public concern on possible health hazards (e.g. brain cancer) though the present knowledge does not suggest any evidence for a causal relationship between RF radiation from cellular phones and cancer [1]. Besides, should any overexposure to RF radiation likely to occur, the ICNIRP guidelines for limiting exposure to time varying electrical, magnetic and EMF (up to 300 GHz) [2] may act as reference to both countries with or without established regulations and guidelines. The World Health Organization has also recommended measures to be taken while waiting for the new emerging findings which, if followed, would minimize the risk of radiation hazard should be any [3]. However, in order to be able to prevent hazards associated with the use of non ionizing radiation, the first step is for countries to have legal mandate to control the use. Unfortunately, until 2003 there was no legislation governing the control of non ionizing radiation in 1

2 Tanzania, a situation that led the present available four cellular phone service providers to erect masts using unknown reference. However, the Legislation that has just passed by the parliament will empower the regulatory authority to audit, among other things, all the cellular phones base stations. We have measured the RF radiation from a radio transmitter and cellular phone base stations randomly selected in Dar es Salaam city to assess the levels that the public is subjected to. 2. Materials and Method Thirty Cellular phone masts randomly selected in populated area in Dar es Salaam city and one state owned radio transmitter having 100 kw forward powers at 567 khz were included in this study. Of the masts, six were sited near primary schools. The measurements were repeatedly done, first in May and secondly in December Thirteen masts, (13) belonged to Vodacom, nine (9) to Mobitel and seven (7) to Celtel cellular phone service providers. Except for the Vodacom, which for security reasons did not reveal the transmission power and heights where the antennas are installed, the rest had the antennas installed meters high. Vodacom uses German made Siemens brand antennas, while Mobitel and Celtel use Ericson (Ex- USA) and Alcatel (Ex- France), respectively. On the other hand, a radio transmitter, which was constructed in mid 1950 s about 40 kilometres from the City centre in a remote area where there was nobody residing, was renovated five years ago. In 2001, however, the indigenous people decided to move towards the transmitter and decided to settle near it consequently building a primary school 300 meters from the transmitter. The school, which had 500 pupils in May 2003 (girls and boys) aged 7-12 years and four teachers, is situated in the direction of transmission of the radio waves (FIG. 1) Apparently, when few villagers heard rumours that radio waves may cause biological hazards to humans, they decided to persuade the government to relocate the radio transmitter from the school area to somewhere else, a situation which still prevails. The measurement of RF radiation was done using a Radiation Hazard Meter (RAHAM) model 40, serial number , (General Microwave Cooperation, USA) (FIG. 2). The Meter is a battery-operated that can detect and measure potentially hazards EM radiation from RF and microwave source. It consists of a power density Meter model 495 serial number and a probe model 94 serial number The probe is sensitive to 200 khz 40 GHz frequencies with a full-scale reading of 20 mw/cm 2. It is isotropic with isotropy response db maximum for energy incident from any direction except through the handle. The RAHAM has a test source model 309 A, serial number for testing the response of the probe prior to applying it to the field. In order to obtain reliable and consistent data, measurements were done in locations that were direct to the directions of antennas, at a height of ~ 1.7 meters above ground, and within the radius of 200 meters around the base stations. The readings were recorded in power density, S, (mw/cm 2 ) which measures the strength of any incident EM wave. In spite of the normal operating procedure, the Meter can also be adjusted to average and peak operations depending on the RF field being measured and the purpose of the power-density measurement. Hence, in average mode the RF field can be monitored over specified time interval of 6 minutes, updated every 1.5 minutes whereas in peak mode the maximum power-density value can be recorded. The measured power density can easily be converted to the electric field strength through the following formula: - E = 61.4 S [4]; Where, E is the electric field strength (in V/m) in the organism at a point of interest and S is the power density (mw/cm 2 ) measured by the Meter 2

3 Hence, for S of 6.5 mw/cm 2, E = = V/m, and similar calculations were performed for the rest. The calculated E values were compared with the ICNIRP guidelines reference levels for general public exposed to time-varying electric field, which for the frequency range 150 khz 1 MHz is 87 V/m. A B C D F Broadcasting Antenna E Reflector Antenna I Control Panel H G FIG. 1. Salasala primary school built 300 metres from radio transmission area. The school has 500 pupils and is situated at the direction of transmission of the radio waves 3. Results Table 1 summarizes the measured power density and the calculated electric field strength at different locations near the primary school built 300 metres and in other areas from the radio transmitter. The normal values behind the classrooms, in front of the classrooms and at the school toilet (areas D, E and F) were 3.3 mw/cm 2 (111.5 V/m), 3.0 mw/cm 2 (106.4 V/m) and 4.0 mw/cm 2 (122.8 V/m), respectively. The electric field strength was higher than 87 V/m 3

4 recommended by the ICNIRP 1998 for 150 khz to 1 MHz for members of public. The values were also high at the residential areas 3.0 mw/cm 2 (106.4 V/m) to 6.5 mw/cm 2 (156.5 V/m). The average values recorded were not consistent hence not included in this table. Also table 2 presents the measurement of the power density near cellular phone base stations. The normal values ranges from to , to 0.07 and to mw/cm 2 for Vodacom, Mobitel and Celtel cellular phone service providers, respectively, whereas the average values over six minutes were mw/cm 2. The power density Meter can read up to 20 mw/cm 2, hence the peak values exceeded this. The high normal value of 1.25 mw/cm 2 was observed at 70 metres from one of the Vodacom base stations. A C B D FIG. 2. Radiation Hazard Meter (RAHAM) comprising of A: test source Model 309 A, B: Power density Meter model 495, C: Probe model 94 and D: an extension cable 4

5 Table 1: The measurement of power density at Salasala primary school built 30 metres at the direction of the radio waves, from a radio transmission area. The school has 500 pupils aged 7-12 yrs. Location Distance from the broadcasting antenna (metres) Measured power density, S, (mw/cm 2 ) Calculated electric field strength, E, (V/m) Normal Peak Normal Peak A (residential area) B (residential area) C (residential area) D (behind classrooms) E (front of classrooms) F (school toilet) G (residential area) H (outside control panel room) I (outside control panel room)

6 Table 2: Measurement of power density at different locations in populated areas near cellular phone base stations. The measurement were done for the three cellular phone service providers: - Vodacom (13 base stations), Mobitel (7 base stations) and Celtel (10 base stations) Provider & Location Number of Antenna Measurements Base Antenna Make (Metres) Stations Transmit Power Power Density (mw/cm 2 ) (n) (dbm) Normal Average Peak Unknown > Unknown >20 VODACOM Unknown >20 [Siemens] Unknown > Unknown > Unknown > Unknown >20 MOBITEL >20 [Ericson] > > > > > >20 CELTEL >20 [Alcatel] > > > >20 4. Discussion This study indicates high values of power density and the electric field strength measured 50 to 350 metres from the broadcasting antenna. The electric field strength values exceeded the reference levels of 87 V/m recommended in the ICNIRP 1998 guidelines for members of public for frequencies 0.15 to 1 MHz [2]. There is a probability that the pupils and some villagers are exposed to RF radiations from the facility without their knowledge. Such high levels of RF radiation in public areas may not be observed in developed countries due to strict adherence to the National and International standards. However, in countries like Tanzania where no legislation to control the use of non ionizing radiation do exist, such exposure to high levels of RF radiation to public may lead to long term biological effects if any. It is known that one of the protective measures to preclude unauthorized access to the areas where exposure limits to radiation are likely to be exceeded is by either fencing or putting barriers. However, during our study one side of this facility was found without being fenced leading to a free access to the transmitter. From the discussion with one of the guard revealed that the villagers were responsible for the removal of the original fences. This is dangerous, and continuation of this situation may increase the probability of RF exposure to children. 6

7 It is important to audit cellular phone base stations especially if the facilities are constructed without following National regulations and standards. Our preliminary data obtained from auditing thirty base stations shows that the normal and averaged RF radiation levels were below the levels recommended by the ICNIRP 1998 guidelines [3]. However, we observed a gradual increase in RF radiation levels when auditing the base stations cited outside the city centre as compared to those in city centre, suggesting the importance of auditing the entire base stations in the country. We think that a national RF level profile will increase public trust in the safety of the base stations. On the other hand, one can speculate that the increase in radiation levels may be due to the increase in transmission power. However, this was not proved since the information provided on the transmission powers of all base stations were not ascertained. This is the first study in this country and the present data obtained suggests a thorough work to be carried out while waiting for more worldwide findings. Reference 1. Moulder, J.E., Erdreich, L.S., Malyapa, R.S., Merrit, J., Pickard, W.F., Vijayalaxmi, Review Cell Phones and Cancer: What is the Evidence for a Connection. Radiation Research 151: (1999) 2. World Health Organization, Electromagnetic Field and Public Health: Mobile Telephones and Their Base Stations. Fact Sheet No. 193 (2000) 3. International Commission for Non-Ionizing Radiation Protection. Guidelines for limiting Exposure to Time Varying Electric, Magnetic and Electromagnetic Fields (up to 300 GHz). Health Phys., 74(4): (1998) 4. General Microwave, Radiation Hazard Measuring Systems: Operating and Service Manual, New York (1991) 7