Our special guest at NAG's July meeting was Dr John Harrison, Head of the Dose Assessment Centre at the Health Protection Agency's Centre for Radiation, Chemicals, and Environmental Hazards.
Dr Harrison spoke about radiation risks at low doses, at a special evening organised in memory of Wendy MacLeod-Gilford, a former NAG member who took her own life recently.
Dr Harrison explained that the main source of scientific information about the health impacts of low level radiation is the United Nations Scientific Committee on the Effects of Atomic Radiation. Policy on radiation and health protection is determined internationally by the International Commission on Radiological Protection, and translated into safety standards by other United Nations organisations and the European Union.
Scientific information on radiation and health is drawn from epidemiological studies, experimental studies, and modelling work. As a result of such studies radiation is well understood, and we know that it can kill and cause cancer, and that sophisticated control systems are needed to limit the doses that humans receive. However, there is a gap in the scientific knowledge at low dose rates, which are below the statistical limits of epidemiological studies. The impacts of low level doses (below 100 mGy) have to date been established by modelling. Extrapolation from higher doses has conventionally assumed a linear relationship between dose and risk, but there are significant uncertainties as to whether this is indeed the case. The relationship may instead show a threshold effect, with a certain degree of cell damage required before cancer becomes apparent, or super-linearity as a result of a 'bystander effect' caused by the influence of neighbouring cells. The Health Protection Agency assumes that a linear relationship represents the best assumption of the relationship between risk and dose at low dose levels.
Our knowledge of the risks of cancer comes from survivors of the atomic bomb attacks on Hiroshima and Nagasaki in 1945, and from the UK's National Registry of Radiation Workers. Epidemiological studies in these groups show that there is a correlation between the risk of leukaemia and radiation exposure. The risks are greater for younger people and women, and a lower incidence of cancer is found among the working population because of the 'healthy worker effect'.
Information from US radium dial painters – mainly women who worked over the period 1915 – 1954 – has given us information about the impact of radionuclides inside the human body. Illnesses experienced by radium dial workers included bone sarcomas and head sinus carcinoma but not leukaemia. We have developed an understanding of the impacts of plutonium from studies of workers at the Mayak plutonium plant in Russia, where large numbers of people received high exposures to plutonium in the late 1940s and early 1950s. The Mayak workers suffered from lung fibrosis and liver and bone cancers as a result of exposure to plutonium, but leukaemia was only found to occur as a result of exposure to other external sources of radiation.
Good quality information exists on how plutonium behaves in the body as a result of autopsies and urinary studies. Plutonium is deposited principally in the skeleton, hence causing bone cancers, and also in the liver. Children are considered to be at particular risk as their bone marrow is more active. Within bones, plutonium is distributed in particular areas, with the bone surface being one target and the bone marrow another.
The average annual radiation dose to a UK citizen is 2.7 mSv. Of this the majority is from natural sources, with around 50% coming from natural radon gas. 16% is from artificial sources, with the largest – and growing – proportion of artificial exposure coming from medical sources. The Health Protection Agency believes that less than 0.1% of the total dose is derived from discharges from nuclear installations, and concludes that radon and medical exposures should be the highest priority for concern. However, these figures are based on averages: individuals living in different parts of the country with different lifestyles will experience different dose mixes.
Health protection regimes are based around setting a constraint and then choosing options which reduce radiation doses as far as possible below a maximum threshold limit. Constraints and reference level doses will depend upon the benefits arising from exposure to the radiation dose. Critical groups at risk from a radiation dose are identified by the Environment Agency and an appropriate dose is set for the group. For AWE Aldermaston the critical group is anglers on the River Thames and the maximum permissible dose is less than 0.005 mSv.
A high profile German government study has recently revived concerns about childhood leukaemias and low level radiation. The Kinderkrebs der Umgebung von Kernkraftwerken (KiKK) study showed an excess of leukaemias in children under five within five kilometers of German nuclear reactor plants. The Health Protection Agency currently takes the view that there is no need for a radical reappraisal of its approach to low level radiation as similar studies in the UK and France do not show an excess of leukaemias, and COMARE's 11th report, which studied leukaemia clusters, concluded that they occur naturally in space and time. Leukaemia excesses near Sellafield and Dounreay, where relatively large amounts of radiation are released to the environment, cannot be explained by radiation, and there is evidence that childhood leukaemias occur in clusters. An alternative theory as to the cause of childhood leukaemia clusters is provided by the Kinlen hypothesis, which suggests they are the result of population mixing and viral or other infections.
Plutonium 239, radon, and radium isotopes all pose cancer risks. Uranium 238 emits less radiation per unit mass, and therefore poses less of a radiation hazard, although it is a toxic metal. Uranium can be deposited in bones although not the liver, but is quite rapidly excreted and presents a relatively low risk to health. Tritium released to the environment rapidly oxidises to form water, which will be distributed through the body if ingested, but is a low energy beta particle emitter and again presents a lower risk. Research is underway to investigate populations at risk of exposure to tritium to provide a more extensive epidemiological picture.
A copy of Dr Harrison's presentation can be downloaded below.
We are very grateful to Penny Henrion, Chair of the Berkshire West Primary Care Trust, for chairing the evening's meeting for us. Members of the public are entitled to attend meetings of the Berkshire West PCT Board to ask questions about the issues raised at this meeting, or any other health issue.
NAG will be returning to the theme of radiation and health at our next meeting, which will be our Annual General Meeting in October. More details soon.
Information about radiation and health can be found on the following websites:
Information about the German KiKK study is available from NAG on request via our contact form.