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Chronic exposure in the spotlight

Research. To determine the long-term effects of chronic low-dose irradiation and contamination, IRSN experts are investigating epidemiological fields, and are also doing experimental and clinical research.

It is not easy to conduct scientific studies on the effects of chronic exposure to low doses. The effects are not very visible and the possible health impact can only be observed over the long term. Moreover, the studies are difficult to set up, either because they require vast cohorts [1] to be followed up over decades, or because experimental research is expensive and tricky to carry out. Nevertheless, this work is essential in order to find out and prevent risks to man.

Studying the exposed populations 

Nowadays no one can imagine doing without medical imaging. At each X-ray or scan, the body is subjected to ionizing radiation. Does this exposure present a risk? To find out, a study on 30,000 children, under the age of 5 at the time of their first test involving radiation, was started in January 2009 in collaboration with 18 French hospitals.

“Knowing the tests each child has undergone, you calculate the doses received. Then, with the national register of paediatric cancers and leukaemias, you can identify the children who develop cancers,” explains Marie-Odile Bernier, a researcher at IRSN in charge of the study. “It is very important to work on the follow-up of exposed children as they are more sensitive to radiation than adults. However, the results will not be available for several years!” adds Dr. Hervé Briss, radiopaediatrician in the radiodiagnostic department at the Curie Institute in Paris and scientific secretary of the paediatric and prenatal imaging company taking part in this study.

Children are also the subject of an environmental exposure study: “The issue of leukaemias in children living near nuclear power plants goes back to the 1980s,” says Dominique Laurier, the head of the Institute’s epidemiological laboratory. In 2008, IRSN analysed nearly 200 scientific studies on this subject. Conclusion: no increase in the risk of leukaemias in 0-14 year-olds living near a plant. “Except that none of these studies provides information on the exposure of these children or on their medical history,” he emphasizes.

In collaboration with Inserm [French National Institute of Health and Medical Research], Dominique Laurier’s team will be studying children living near nuclear sites. “We will evaluate their exposure to natural radioactivity and cross-check it against the national registers of child leukaemias.” For Pierre Barbey, a biologist and member of the ACRO [French Western Radioactivity Monitoring Association], “this study will provide answers but will also show that there are still many uncertainties.”

The link between lung cancer and radon is well studied in relation to environmental exposure. This radioactive gas naturally present in the atmosphere may be found in large quantities in certain houses. To quantify this risk, IRSN studied “600 people with lung cancer and 1,200 controls [people of the same age, not suffering from this disease, Ed.] who all live in Brittany and the Centre region, where exposure to radon is greater than in the rest of France,” explains Margot Tirmarche, the scientific coordination and evaluation director and IRSN and member of international scientific committees such as the International Commission on Radiological Protection (ICRP). The conclusion is that there is a clear link between radon in houses and lung cancer. “We now have to convince the general public that good ventilation and sometimes the intervention of housing professionals are necessary,” she says.

 

A laboratory dedicated to the health effects of low exposure to radiation

At IRSN, about fifteen people are conducting studies on the health effects in man of chronic exposure to low doses. They are monitoring very diverse populations in order to study all types of exposure: environmental, occupational or medical.  

These specialists work at Lepid, the epidemiology laboratory. Set up at the beginning of the 1990s, it is located in Fontenay-aux-Roses. Lepid is currently interested in the link between the dose received and the risk of occurrence of effects, the variation of the risk from one individual to another and non-cancerous pathologies.

  

 

Workers are not left behind

“There may be exposure to low doses during maintenance operations in nuclear power plants,” explains Camille Metz, a researcher on the effects of external radiation at the Institute. “This is measured by the individual dosimeter worn by the workers.” The researchers possess annual surveys of CEA employees since 1946, EDF since 1961 and Areva since 1976, in other words over 70,000 people! These data are cross-checked against death certificates issued in France since 1968.

“It is observed that the cumulated average dose of exposure throughout the professional career of these people is 20 mSv, which is very low. At CEA and Areva, 50% of workers have an undetectable dose. To detect the risks linked to such low doses, this has to be cross-checked against other cohorts [1]. So in about fifteen countries, i.e. 400,000 workers, a link has been shown between occupational exposure and risk of death from cancer. By continuing these studies, we will know whether the current recommendations need to be updated,” summarizes Camille Metz. The longer the duration of follow-up, the more relevant the results of these studies will be.

Determining the effects on a living organism

To understand the mechanisms linked to chronic contamination, researchers on the Envirhom program mainly study rodents whose drinking water contains uranium. “Nine months of daily contamination in a rat is equivalent to twenty years in a man in relation to his length of life,” specifies Isabelle Dublineau, the head of the experimental radiotoxicology laboratory at IRSN.

“Thanks to experimental studies, we control all the living and contamination parameters of the animal. This enables us to be very precise about the quantity ingested, and to ensure no interference with the effects of other pollutants. We are interested in the localization of uranium in the organism and the physiological responses. What changes are observed, for example, in the cardiovascular system, digestive tract, immunity or brain functions? How do the observed effects vary in relation to the doses, duration of exposure or type of uranium – depleted, natural or enriched?”
 
While bones and kidneys are the main target organs under acute exposure, the results of Envirhom have shown that the central nervous system, liver and (female) reproductive organs may also be affected. “We observe a change in major metabolisms [2], and also cognitive functions (i.e. the central nervous system, Ed.) and reproductive functions [3],” explains Isabelle Dublineau. “A low-level chronic contamination by uranium may therefore have non-cancerous effects on several organs. This contamination, in the quantities ingested, leads to adaptive biological effects [4], which do not cause pathologies.”
 

“For the first time, we have shown that chronic contamination by uranium disrupts the waking-sleeping cycle of rats. It increases their level of anxiety and memory disorders. With equivalent contamination from depleted uranium, the general condition is unchanged,” explains Philippe Lestaevel, a specialist in the central nervous system at IRSN. “We have also ascertained changes in the cardiovascular system after contamination with caesium 137, but this does not necessarily mean that there is a pathology.”

 

Envirhom, a program studies the mechanisms of action of uranium

The Envirhom experimental studies program was launched in 2001 by IRSN to permit a better evaluation of the risks to both the environment and human health associated with chronic exposure to radionuclides. Between 20 and 25 people per year work on the health aspect.

“It is a question of mimicking, in animals, what can happen in a population subjected to chronic contamination,” explains Isabelle Dublineau, the head of the Envirhom program. “We are carrying out descriptive studies to obtain new knowledge and more targeted studies on the mechanisms of action of uranium.”

 

Follow-up of populations of contaminated regions

The Chernobyl nuclear accident which occurred on 26 April 1986 remains to this day the largest catastrophe in the civil nuclear industry. The exposure of the population led to health consequences such as an increase in incidence of thyroid cancers in children who were under 15 years old at the time of the accident and living in the contaminated regions of Russia, Belorussia and Ukraine. Some studies conducted in Belorussia suggest that children living in the regions that were contaminated, especially by caesium 137, presented unusual non-cancerous pathologies, such as cataracts, anaemias or cardiac rhythm disorders.

In April 2005, IRSN decided to launch a research program called Épice [Evaluation of pathologies caused by caesium contamination]. It follows up children living in the contaminated Russian regions to evaluate their contamination from caesium 137. It is also intended to review non-cancerous pathologies, in particular cardiac arrhythmias and cataracts. It will therefore be possible to confirm or contradict the existence of a cause-and-effect link. Several preparatory phases have already been carried out, and have confirmed the feasibility of a vast epidemiological study. The study on cardiac arrhythmias was started in May 2009. It concerns 18,000 children, of whom half live in non-contaminated regions. The first results are expected in 2013.

 

Notes:

1-  Cohort : set of individuals followed up chronologically, from a given initial point in time, in an epidemiological study. A cohort forms a homogeneous group chosen for the study of a pathology. (Larousse medical).
2- The metabolism of cholesterol, xenobiotics (substances foreign to the organism such as drugs), steroidal hormones, iron, vitamin D. For example, uranium disrupts vitamin D metabolism which can lead to an increase in bone fragility.
3- Studies have shown a reduction in quality of female reproductive cells (oocytes) with no change in ovulation intensity.
4- The term adaptive biological effects is used when disturbances are ascertained without leading to a pathology or threat to health; the organism adapts to the entry of the radionuclide.

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