Ionisation Chamber Smoke Detectors

Ionisation chamber smoke detectors (ICSDs) contain a small americium-241 radioactive source that emits both gamma radiation and alpha particles. ICSDs are constructed to an international safety standard drawn up by the Nuclear Energy Agency of the Organisation for Economic Co-Operation and Development, and this standard specifies the maximum source size to be used and details a series of stringent tests which ICSDs should pass. Compliance with this standard ensures that detectors do not represent a significant risk to users, both during normal use and following accidental damage. This standard has been reviewed and updated by NRPB.

One of the requirements of the standard is that the radiation dose rates close to the external surfaces of the detector be very low, and the small activity of the radioactive americium-241 source in modern ICSDs guarantees that this is the case. NRPB has carried out detailed measurements on most, if not all, of the ICSDs now available in the UK and has found that the radiation dose rates close to their surfaces are extremely low. NRPB has also estimated that the maximum annual dose to a user of an ICSD is likely to be approximately 0.1 microsieverts. This value can be compared with the average annual radiation dose to the UK population of 2600 microsieverts. The potential dose to users in the event of mechanical damage to the source resulting in the release of radioactivity has also been calculated, and again this is very low, being in the order of 80 microsieverts.

The potential doses to firefighters from the release of americium-241 in the event of a fire has been assessed, and it has been concluded that for limited numbers of ICSDs the doses will be very low. The emergency services are well briefed on the hazards of ionising radiation and will wear respiratory protection in any situation where there is the potential for significant dose by inhalation, for example, fire in a factory where large numbers of ICSDs are manufactured and/or stored.

The source construction and activity of the americium-241 source in an ICSD minimises the radiation doses to persons arising from uncontrolled disposal. The NRPB calculations on unrestricted disposal of ICSDs have shown that the potential radiation doses to persons living close to a disposal site are extremely low, less than 1 microsievert per year. It follows that the disposal of modern ICSDs in domestic waste is a straightforward and cost effective method of disposal, and an Exemption Order ¹ issued by Her Majesty's Inspectorate of Pollution (now part of the Environment Agency) permits the disposal of ICSDs by this pathway.

The NEA and NRPB standards require the ionisation chamber of each ICSD to bear a label incorporating the trefoil symbol and the word 'radioactive'. This label must be clearly visible when the ICSD is opened, for example, for battery replacement. ICSDs from other countries may have additional statements on the labels forbidding the disposal of the detector in domestic waste. As stated above, this prohibition does not apply in the UK.

In view of the very low radiological risk and the substantial benefits in terms of life-saving potential associated with ICSDs, NRPB considers this particular risk of radioactivity to be extremely acceptable.

1 Atomic Energy and Radioactive Substances, The Radioactive Substances (Smoke Detectors) Exemption Order, 1980

Last reviewed: 2 October 2009