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from Chernobyl must be based on the experience of other populations exposed to radiation and followed up for many decades. Such predictions are uncertain as the applicability of risk estimates from other populations with different genetic and environmental backgrounds is unclear. They do, however, provide an idea of the order of magnitude of the likely impact of the accident; among the nearly six million persons in the most exposed populations (liquidators, evacuees, residents of strict control zones and residents of contaminated areas of Belarus, the Russian Federation and Ukraine), predictions currently available are of the order of 9000 to 10 000 deaths from cancers and leukaemia over life. In the next years, careful studies of selected populations are needed in order to study the real effect of the accident and compare it to predictions.

1.INTRODUCTION

Over the last two decades, there have been many reports of increased incidence of cancer and of other health effects attributed to the Chernobyl accident. Within the framework of the United Nations Chernobyl Forum, the WHO convened expert groups to review published studies on the health consequences of the accident with the aim of assessing the impact of the accident, identifying gaps in knowledge and making recommendations concerning health care programmes in Belarus, the Russian Federation and Ukraine [1].

The basis for the evaluation was the comprehensive review conducted by the UNSCEAR and published in 2000. This was updated with a review of the peer-reviewed scientific literature published since then, presentations at scientific meetings, and recent reports and statistics provided by national authorities.

The expert groups provided a consensus opinion and recommendations for the topics below:

—Doses received from the accident;

—Thyroid cancer and other thyroid pathologies;

—Leukaemia;

—Solid cancers other than thyroid;

—Non-cancer and non-thyroid health effects;

—Medical and health care programmes in Belarus, the Russian Federation and Ukraine.

The current paper addresses the first issues — doses, thyroid diseases, leukaemia and other cancers, and the main long term effects expected as a

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result of radiation exposure [2, 3]. The other issues are the topic of a separate presentation in this session [4].

2.METHODOLOGICAL CONSIDERATIONS IN INTERPRETING REPORTS OF HEALTH EFFECTS AFTER CHERNOBYL

There are a number of limitations in what the epidemiological studies of the radiological consequences of the Chernobyl accident can establish. Indeed, to be informative to evaluate the effect of radiation exposure following Chernobyl, studies must fulfil several important criteria:

—They must cover very large numbers of exposed subjects;

—The follow-up must be complete and non-selective and precise; and

—Accurate individual dose estimates (or markers of exposure) must be available.

In particular, the feasibility and the quality of epidemiological studies largely depend on the existence and the quality of basic population-based registries, and on the feasibility of linking information on a single individual from different data sources. These requirements are particularly important in the context of the Chernobyl accident, where most people received relatively small doses of radiation and the resulting health effects are, therefore, expected to be relatively small and, hence, difficult to identify against background incidences more heavily influenced by other risk factors such as tobacco smoking, alcohol consumption, diet, lifestyle and some occupational exposures.

Although the first of these criteria is clearly fulfilled, given the large number of persons residing in contaminated areas, the absence of high quality disease registries in many of the contaminated regions at the time of the accident, recent changes in the longevity of the populations in the affected countries (both in contaminated and uncontaminated regions) and the absence of individual dose estimates for the majority of exposed persons makes the conduct of informative epidemiological studies — and the interpretation of published reports — very difficult.

One of the notable exceptions to the low doses received by Chernobyl populations is the dose to the thyroid, which, for a relatively large number of children, reached high levels in the most contaminated territories. These high thyroid exposures, which were mainly due to the consumption of milk contaminated with radioactive isotopes of iodine, were delivered within a few weeks after the accident and are the cause of extensive studies of thyroid cancer among the affected populations.

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The expert groups reviewed many reports of associations in which the numbers of cases and/or controls were too small to determine whether radiation was the cause of the health outcome. Further, results of a number of studies were presented to the expert group without the necessary supporting information to allow a judgement of the scientific merits of the findings.

In addition, most of the health effects considered have a wide variety of risk factors which, if not adequately taken into account, can significantly bias the results of the studies. For example, it is well known that smoking and alcohol consumption are responsible for large increases in the mortality and morbidity from many diseases, including various cancers [5], cardiovascular disease and, for tobacco, chronic respiratory diseases. In addition, radiation or accident-related stress may lead people to smoke more, which in turn can lead to more cancer and cardiovascular disease, without radiation having had any direct effect.

3.RADIATION DOSES FROM THE ACCIDENT

The main populations exposed to radiation from the Chernobyl accident are the:

(a)Liquidators: also referred to as ‘cleanup workers’. They include persons who participated in the cleanup of the accident (cleanup of the reactor, construction of the sarcophagus, decontamination, building of roads, destruction and burial of contaminated buildings, forests and equipment), as well as many others, including physicians, teachers, cooks, interpreters who worked in the contaminated territories. About 240 000 liquidators took part in 1986–87 in major mitigation activities specifically at the reactor and within the 30 km zone surrounding the reactor. This includes the early reactor workers and the ‘emergency’ cleanup workers who intervened in the first hours or days after the accident. Large numbers of liquidators also worked outside the 30 km zone, where they received lower doses. Residual mitigation activities continued on a relatively large scale until 1990. All together, about 600 000 persons (civilian and military) have received special certificates confirming their status as liquidators, according to laws promulgated in Belarus, the Russian Federation and Ukraine.

(b)Inhabitants who were evacuated or relocated from contaminated areas:

Massive releases of radioactive materials into the atmosphere brought about the evacuation of about 116 000 people from areas surrounding the reactor during 1986, and the relocation, after 1986, of about

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220 000 people from, what were at this time, three independent republics of the former Soviet Union: Belarus, the Russian Federation and Ukraine.

(c)Inhabitants of contaminated areas who were not evacuated: Many persons continue to live in vast territories of those three republics that were contaminated. The population of those areas, from which no relocation was required, amounts to about five million people.

Table 1 presents a summary of the number of persons exposed and the levels of doses received in these population groups. Residents of contaminated areas have been divided into residents of the strict control zones and residents of less contaminated areas.

3.1. Doses to liquidators

The liquidators, either emergency or cleanup workers, were subjected mainly to external exposure with gamma and beta radiation during their work on-site. Most of the information on the doses received by liquidators is in the Chernobyl State Registries of Belarus, the Russian Federation and Ukraine. The doses recorded in the registries range up to more than 500 mGy, with an average of more than 100 mGy for the 1986–87 liquidators who worked on the reactor site and in the 30 km zone.

It is estimated that early reactor and emergency workers received, on 26 April 1986, much higher doses of a few Gy up to 16 Gy; 28 of them died within the first four months due to acute radiation sickness.

TABLE 1. ESTIMATES OF MEAN EFFECTIVE DOSES FOR POPULATION GROUPS OF INTEREST [1]

*Strict control zones — included in the areas with deposition density >37 kBq/m2.

**For the period 1986–2005.

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The dosimetric information available from the Chernobyl State Registries for liquidators is subject to controversy as the personal dosimeters in use in the early days after the accident were too few and generally too sensitive. For the purpose of epidemiological studies, these dose estimates need to be supplemented using other information and verified using other methods. The best method currently available to estimate the doses received by the liquidators is a time and motion assessment, called RADRUE [6]. One of the main advantages of this method is that it can be applied to any worker.

3.2. Doses to the general public

The general public was exposed to radioactive materials externally from the radioactive cloud and later from radionuclides deposited in the soil and other surfaces, and internally, from inhalation during the cloud’s passage and from re-suspended materials, and consumption of contaminated food and water.

As the major health effect reported after the accident was an elevated thyroid cancer incidence in children and adolescents, much attention has been paid to estimating radiation dose to the thyroid.

A wide range of thyroid doses was received by the inhabitants of the contaminated areas of Belarus, the Russian Federation and Ukraine. Doses varied with age at exposure, level of ground contamination, milk consumption rate, and origin of the milk that was consumed. Reported individual thyroid doses varied up to about a few tens of Gy, while average doses are in the range of a few tens of milligray to a few Gy, depending on the age and area where people lived and were exposed (Table 2).

The main basis for the estimation of thyroid doses resulting from the intake of 131I are the results of 350 000 measurements of exposure rate performed using radiation detectors placed against the neck of residents of Belarus, Ukraine and the Russian Federation within a few weeks following the accident [2, 11–13]. Intake of stable iodine tablets during the first 6–30 h after the accident reduced the thyroid dose of the residents of Pripyat by a factor of six on average [7, 14]. Pripyat was the largest city close to the Chernobyl nuclear power plant and residents were evacuated the day after the accident.

Although the intake of 131I is the most important contributor to the thyroid dose, there are other components in the thyroid doses resulting from the Chernobyl accident that contribute to a few percent of the thyroid dose. These are: internal dose from intakes of short lived radioiodines (132I, 133I and 135I) and of short lived radiotelluriums (131mTe and 132Te); external irradiation from radionuclides deposited on the ground and other materials; and internal irradiation resulting from the intake of radionuclides other than radioiodines (essentially 134Cs and 137Cs). The thyroid doses from intake of radionuclides

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TABLE 2. ESTIMATES OF THYROID DOSES [2, 7–10; ZVONOVA, PERSONAL COMMUNICATION 2005]

other than 131I, and from external radiation only represent a small percentage of the thyroid dose due to 131I for most individuals.

The effective dose estimates for individuals in the general population accumulated over the 20 years following the accident (1986–2005) range from a few mSv to some hundred mSv depending on location, age and behaviour. These doses are mainly due to external exposure from a mixture of deposited radionuclides as well as to internal exposure from intake of 134Cs and 137Cs. As indicated in Table 1, the mean effective dose accumulated up to 2005 among residents in the strict control zones (with 137Cs deposition density of 555 kBq/m2 or more) is of the order of 50 mSv (40 mSv up to 1995 [2]) while in less contaminated areas it is of the order of 10 mSv (8 mSv up to 1995 [2]). For comparison, the average effective dose from natural background radiation to an average person is about 2.4 mSv/a. So during an entire life, each of us accumulates a dose of 100–200 mSv.

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