Cordless phones and their base stations have output powers much too low for exposures to exceed internationally accepted guidelines and HPA does not consider there are particular safety issues with their use.
Concern about the safety of mobile phones has led to questions about the safety of cordless phones, as used by the general public in their homes and in places of work. This information sheet summarises the technical and health-related aspects of cordless phones.
Cordless phones operate in a similar way to mobile phones, i.e. by using radio signals to communicate between a handset and a base station. However, there are some differences in the technical details of the radio signals and in the distances over which communications are carried out. In a system where there are multiple handsets and chargers, only one of the chargers is the base station.
The base station for cordless phones is usually a handset cradle (one of the charger units) and is not normally more than a few tens of metres away from the phone, whereas mobile phone base stations can be kilometres away from mobile phones. More power is required for radio communications over greater distances and so cordless phones should only need to emit a fraction of the power of a mobile phone.
All cordless phones have average output powers of around 10 milliwatts (10 mW, one hundredth of a watt) during conversations, although the instantaneous output power can vary over time due to the structure of the signal.
The signals from older analogue cordless phones are at a constant level of 10 mW, whereas the signals from modern digital cordless phones are in the form of 100 bursts every second, each of around 0.4 millisecond (ms) duration. The bursts are at a peak power level of 250 mW, but on average the phone only transmits for 1/25 of the time and so the average power is 10 mW.
The average output power of mobile phones can be up to 250 mW, but unlike cordless phones, mobile phones have adaptive power control and can reduce their output power in a series of steps to as low as around a mW, or even lower in the case of 3G phones. Hence, in circumstances where there is a very good communications link to the base station, e.g. when a mobile phone is right next to it, the instantaneous power from a mobile phone could actually be lower than from a cordless phone. However, recent research suggests that, in more typical call circumstances, mobile phones spend a large proportion of the time at their highest power levels and so average powers of mobile phones are appreciably higher than from cordless phones.
Cordless phones are held to the head like mobile phones and some of the radio waves they produce are incident on the head. The radio waves carry energy and, as with mobile phones, some of this energy is absorbed in the head and could produce temperature rises if too much were absorbed too quickly.
The energy absorption is measured in the quantity 'specific energy absorption rate' (SAR) which has the unit watt per kilogram (W kg -1) . SAR is closely related to the electric field strength produced inside the body tissues, but electric field strength outside the body, while more easily measured, is of less relevance.
Many expert groups and advisory bodies have reviewed the scientific information relating to health effects that might arise due to absorption of radio waves in the head. The former NRPB (now the HPA Radiation Protection Division) completed a formal review process in 2004 and advised that the exposure restrictions of the International Commission on Non-ionizing Radiation Protection (ICNIRP) should be adopted in the UK . These restrictions advise that SAR should not exceed 2 W kg -1 when averaged over any 10 g of tissue in the head and any 6 minute period for the general public.
Frequency of the radio waves is an important factor in determining the depth to which they penetrate the head. Higher frequency radio waves penetrate less distance into the head. The frequencies used by digital cordless phones are around 1900 MHz (megahertz), which is similar to those used by mobile phones (900, 1800 and 2100 MHz). Older cordless phone handsets used in the UK radiate at a lower frequency than mobile phones, either around 47 MHz or 39 MHz, depending on the system, meaning that there is greater penetration into the head.
However, SAR is the quantity of interest in evaluating exposures. On a theoretical basis, radio transmitters producing an average power of 20 mW or less cannot exceed the guideline exposure limit value (2 W kg -1), irrespective of how they are used. Hence, as all cordless phones have output powers around 10 mW, they should not be able to cause exposures above guidelines however they are used.
Measurements of the SAR values from cordless phones are not generally available, however it is possible to scale the range of typical values reported for mobile phones operating at full power (0.2-1.4 W kg -1). Assuming physical factors (such as the antenna types and positions in relation to the head) are similar to those with mobile phones, this suggests that DECT cordless phone SAR values would be expected to be in the range 0.008-0.06 W kg -1, at least 30 times below the ICNIRP guidelines.
Just as digital cordless phones send radio signals to the base station (one of the charger units) during calls, so the base station transmits radio signals back to the phones. The base station frequencies, signal structures and power levels are the same as those of phone handsets when a call is being made.
The base station is not located immediately adjacent to the head and consequently exposure levels produced are much lower than with the phones. One situation where a user may be particularly close to a base station would be if the main charger unit were placed on a bedside table. If such a base station were 30 cm from the head, this would be at least ten times further away than a phone handset and SAR values would be at least a hundred times lower, and thus thousands of times below the ICNIRP guidelines. Often the cordless phones in bedrooms are secondary units with a charger, and the main base station/charger is somewhere else in the house.
When no calls are being made, DECT base stations emit 0.08 ms pulses once every 10 ms at a peak power level of 250 mW. This implies the average output power is 2 mW which is five times lower than when a call is being made. SAR values would also be five times lower.
In summary, the technical standards for cordless phones and their base stations indicate that they have output powers much too low for people's exposures to exceed internationally accepted exposure guidelines and HPA does not consider there are particular problems with their safety.
Last reviewed: 4 September 2008