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HPA Press Statement

Advice on risks from tritium

29 November 2007


The independent Advisory Group on Ionising Radiation has published a report* reviewing the risks of exposure to tritium, a radioactive isotope of hydrogen. Tritium is used in scientific and medical research and it also has various industrial applications. Following an extensive review of scientific evidence on the risks from exposure to tritium, the Advisory Group suggests that the International Commission on Radiological Protection (ICRP) should consider increasing its radiation weighting factor for tritium from 1 to 2. Radiation weighting factors are used to calculate doses and risks from radiation exposures.

Tritium is a radioactive isotope of hydrogen that decays by beta emissions with a maximum energy of 15.6 keV and a half-life of 12.3 years. It is formed through several processes, both natural and artificial, including nuclear fission and fusion power generation.

The Advisory Group has examined biological evidence from published laboratory experiments with tritium in cell cultures (in vitro) and from animal experiments (in vivo). They have also reviewed the published evidence for effects on human health (epidemiology). The evidence indicates that tritium has a larger impact on biological systems than gamma rays or x-rays and its relative biological effectiveness (RBE) is greater than 1. This is the basis for the Advisory Group's recommendation that the RBE value for tritium should be taken as 2 and its suggestion that ICRP should consider increasing its radiation weighting factor for tritium from 1 to 2.

Epidemiological data on risks associated with tritium exposure are not strong because most of the studies conducted worldwide involved small numbers of people. The report therefore recommends that consideration is given to an international collaborative epidemiological study of tritium exposed populations.

In reviewing biokinetic models for tritium the report notes a wide range of animal and human data support the current ICRP models for radiation exposure and that models for tritiated compounds are under development. The report concludes by welcoming the development of new tritium models by ICRP and recommends that they be adopted for routine dose assessments when available.

Professor Bryn Bridges, Chairman of the Advisory Group said, “ A lot of work has gone into this report and I hope the International Commission on Radiological Protection will consider our suggestion. Tritium is not highly radioactive but it can become widely dispersed in the environment and we felt a special review of the evidence was necessary.“

 

Press enquiries. Please contact the Agency's press office at Chilton who will arrange interviews with members of the Advisory Group; Tel 01235 822744 or 822745; E-mail chilton.pressoffice@hpa.org.uk

* Review of Risks from Tritium. Report of the independent Advisory Group on Ionising Radiation. Documents of the Health Protection Agency. Radiation, Chemical and Environmental Hazards, REC-4. November 2007. ISBN: 978-0-85951-610-5. Available to download from the Health Protection Agency website at:

http://www.hpa.org.uk/publications/PublicationDisplay.asp?PublicationID=0

Printed copy, £20.00 + 10% postage and packing, available from CRCE Information Office (Tel: 01235 822742/822603, email: chiltoninformationoffice@hpa.org.uk ).

 

Notes for Editors

 

  • The terms of reference of the Agency's Advisory Group on Ionising Radiation (AGIR) are “to review work on the biological and medical effects of ionising radiation relevant to human health in the occupational, public health, medical and environmental fields and advise on research priorities.”
  • AGIR reports to the sub-committee of the Board of the Agency that considers radiation, chemical and environmental hazards. The remit of AGIR is restricted to the provision of scientific judgements and does not include the development of specific recommendations relating to radiation protection policy. These are matters for the Agency and its Board.
  • The AGIR Tritium Internal Dosimetry sub-group was established following a recommendation from COMARE, in its response to the CERRIE report, that: ‘… the NRPB be asked to carry out a review with the widest possible consultation, of internal tritium dosimetry paying particular attention to tritiated water and organic compounds containing tritium' .
  • The report considers the properties and sources of tritium, its relative biological effectiveness (RBE), epidemiological studies of tritium exposed populations and the biokinetics of tritiated water and organically bound tritium. The report concludes that the weight of evidence suggests that the RBE for tritium is 1-2 compared to x-rays and 2-3 compared to gamma rays. Given the dependence of RBE values on the reference radiation, the sub group recommend the use of a standard high energy g -ray source such as 60 Co as a reference for RBE studies and that an RBE value of 2 is used for tritium epidemiological studies and retrospective dose assessments. Further, the report suggests that ICRP consider adopting a value of 2 for the tritium radiation weighting factor (w R ). The report deals with a number of other issues including the epidemiological studies of tritium- exposed populations, the biokinetic models used for estimating doses form tritium and the risks associated with tritium incorporated into DNA, particularly in female germ cells. Evidence to underpin the conclusions drawn was obtained from the scientific literature and from a consultation seminar where individuals holding specific views on aspects of tritium dosimetry and risk were invited to present.
  • Concerns have been expressed about possible genetic effects of tritium. There are routine liquid discharges of tritium from an industrial source into the Cardiff Bay area and calculations were undertaken for people consuming fish caught in the bay area, and specifically for flounder. These calculations indicate that the risk of hereditary effects due to such exposures is several orders of magnitude less than the 3-4% spontaneous (i.e., natural) incidence of hereditary effects. The Advisory Group report therefore concludes that specific measures to protect against possible hereditary effects from tritium are unwarranted.