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Bluetooth

bluetooth head-set, usb-stick
Bluetooth is used for high-frequency voice and data transfer over short distances. Bluetooth enables various devices to be connected wirelessly, e.g. a mobile phone to a hands-free headset or a laptop wittoh a printer or mouse. Bluetooth technology is simple, cheap and booming; new applications are reaching the market all the time.
Bluetooth devices are assigned to one of three power classes: 1, 2 and 3. The radiation energy emitted by class 2 and 3 Bluetooth devices is weak and limited in range.  Most of the Bluetooth applications used close to the body belong to one of these two power classes. Bluetooth transmitters in the most powerful class 1 can cause exposure to radiation similar to that emitted by a mobile phone if they are operated in the immediate vicinity of the body.
The radiation exposure caused by Bluetooth devices in all three power classes is below the international recommended levels. The information currently available does not suggest that this radiation represents an immediate threat to health.
Precautions are unnecessary for Bluetooth devices in the lower power classes 2 and 3. Some mobile phones that use Bluetooth to access the Internet have class 1 transmitters. It is advisable to switch off the Internet connection when making phone calls with such devices to reduce the additional exposure of the head to radiation.

Bluetooth hands-free headsets to minimise radiation from mobile phones
Nowadays Bluetooth hands-free headsets are also used as a precautionary measure to reduce exposure to radiation from mobile phones. Instead of the mobile phone, a Bluetooth transmitter that emits a far lower level of radiation is held to the ear during phone calls, thus reducing exposure of the head considerably.

Detailed information

Bluetooth (IEEE 802.15.1) is the first standard to be introduced for voice and data transfer over short distances (known as WPAN, or wireless personal area network). As Bluetooth transmitters are very small and cheap and use little power, a large number of devices already contain them, and new applications are being developed all the time.

Typical applications
  • Hands-free headsets for mobile or cordless phones
  • Cordless telephones for Internet telephony (Voice over IP)
  • Synchronisation of PDAs with each other and with a computer
  • Wireless connections for audio and video systems and MP3 players
  • Wireless connections between computers, printers, mice, digital cameras etc.
  • Connection to external car antennas for mobile phones
  • Patient monitoring in hospitals

1. Technical data

Frequency: 2.4-2.4835 GHz, Bluetooth uses the globally free and unlicensed ISM band; Wavelength: approx. 12.5 cm.

Transmission power
There are three power classes for various Bluetooth applications, each with a different range. The most common class is the weakest (Table 1).
The effective transmission power is usually lower than the maximum power as transmission is only ever strong enough for the receiving device to pick up the signal. The receiving device can measure the transmission power and request the transmitter to increase or reduce it if possible. This power regulation prolongs battery life and avoids interference with other Bluetooth networks.

Power class
Peak transmission power (mW)
Maximum transmission power (mW)
Minimum transmission power (mW)
Range (m)
1
100
76
1
100
2
2.5
1.9
0.25
40
3
1
0.8
 
10
Table 1: Power classes for Bluetooth transmitters

This means that the transmission power, and thus radiation exposure, are not constant. Power regulation is mandatory for power class 1 and optional for classes 2 and 3.

Operational architecture
There are various communication profiles for Bluetooth devices depending on the application. Each device only supports certain profiles. Two devices can only communicate via a common profile, and are thus only compatible if this is the case.
When Bluetooth devices with the same communication profile are in proximity, they automatically communicate with each other. Up to eight devices can be actively linked in a network known as a piconet. One device (known as the master) takes the lead and organises data transfer within the piconet; all the other devices are known as slaves.
Bluetooth frequency hoppingnew window
Bluetooth frequency hopping Figure 1 shows an example of a Bluetooth time slot structure. The master transmits on channels 12, 5, 55 and 70; the slaves transmit on channels 68, 1, 17 and 41. On channel 5, five time slots are combined to allow a larger volume of data to be transmitted; on channel 17, three time slots are combined. Individual time slots can be in use for different lengths of time depending on the volume of data being transmitted. Source of image [4]
79 non-overlapping transmission channels of 1 MHz bandwidth. Each device in the piconet uses the different transmission channels in the order determined by the master (known as the hopping sequence). The transmission channels in the piconet generally change up to 1600 times per second (known as frequency hopping), producing a pulsed transmission with a maximum pulse frequency of 1600 Hz (Figure 1). The master and slaves transmit and receive alternately.
The amount of time (known as a slot) during which a transmission channel is assigned to a master-slave pair is 0.625 ms. A device can transmit for 1, 3 or 5 time slots and then has to receive (Figure 1). If several time slots are combined, the pulse frequency drops to 533 Hz (for 3 time slots) or 320 Hz (for 5 time slots).
If no data transmission is taking place, the slaves generally do not transmit, and receive only sporadically. The master sends a beacon even if no data are being transmitted (once a second, for example) so that the slaves can synchronise with it.
Low-frequency fields emitted by Bluetooth devices
Since a Bluetooth device only consumes power while it is transmitting and receiving, the battery is switched on and off repeatedly. This produces low-frequency magnetic fields from around 1 Hz (beacon) up to several thousand Hz.

2. Measuring exposure

The basic unit for measuring exposure to high-frequency radiation is absorbed energy per time interval and bodyweight, expressed as the specific absorption rate (SAR) in watts per kilogram (W/kg).
SAR is measured for Bluetooth devices operated in proximity to the body (up to 13 cm). For devices operated further away from the body, the electrical field is also an important parameter.
A study commissioned by the FOPH [1, 2] measured the SAR and the electrical field of various applications:
  • Two different Bluetooth USB plug-in antennas in power classes 1 and 2 at the maximum data rate and maximum transmission power
  • A class 2 PDA (personal digital assistant)
  • Two different hands-free headsets in class 3 (only SAR)
E-Feld von Bluetooth USB-Stecker und PDA
Figure 3: Maximum electrical field (E field) as a function of distance for two Bluetooth USB plugs in different power classes and a PDA. The electrical field decreases very rapidly with increasing distance. The measurements were carried out at maximum transmission power [1, 2].
Bluetooth normally reduces the transmission power if the connection between the devices is good in order to save energy and avoid interference with other devices. This produces an even smaller E field and lower SAR values.

3. Effects on health

Based on the current state of knowledge and available exposure measurements, the high-frequency radiation emitted by Bluetooth networks is too weak to have an acute impact on health due to an increase in temperature following absorption.
Long-term and non-thermal effects have not been researched sufficiently. Available studies on the effects of high-frequency, low-dose EMF show that effects on health are not to be expected, and the very low transmission power of class 2 and 3 Bluetooth devices also makes health effects very unlikely.
Mobile phones with inbuilt class 1 Bluetooth transmitters are a special case. Radiation emitted by mobile phones is not currently known to have any effect on health, but the subject is being investigated at an international level. The FOPH has published tips on minimising radiation exposure from mobile phones; this information also applies to Bluetooth-enabled mobile phones.

4. Regulation in law

Bluetooth devices are telecommunications terminal devices and as such regulated by the Swiss regulation on telecommunications systems. This regulation lists technical standards which may be used to evaluate electromagnetic radiation. These standards are issued by the European Committee for Electrotechnical Standardization CENELEC and describe the procedures for measuring the radiation emitted by specific devices.

5. References

1. Kramer A. et al. Development of Procedures for the Assessment of Human Exposure to EMF from Wireless Devices in Home and Office Environments. 2005. IT'IS Report. See "Documents"
2. Kühn S et al. Development of Procedures for the EMF Exposure Evaluation from Wireless Devices in Home and Office Environments. Supplement 1: Close-to-Body and Base Station Wireless Data Communication Devices. 2006. IT'IS Report. See "Documents"
3. ICNIRP. Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz), Health Physics 74, (4): 494-522; 1998. See "Further information"
4. Forschungsvorhaben Bestimmung der Exposition bei Verwendung kabelloser Übermittlungsverfahren in Haushalt und Büro. Abschlussbericht. See "Further information"

Specialist staff: emf@bag.admin.ch
Last updated on: 30.01.2007

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