A European Atlas of Natural Radiations including harmonized radon maps of the European Union. What do we have, what do we know, quo vadimus?
G. Dubois and P. Bossew
European Commission – DG Joint Research Centre, Institute for Environment and Sustainability.Via Fermi 1, TP441, 21021 Ispra (VA), Italy.
Email: ;
Abstract:
Radon is a naturally occurring radioactive gas. It is known to be in average, by far, the main contributor to the exposure from natural background radiations received by the population and is also considered to be the main leading cause of lung cancer second to smoking. The last observation has stimulated most European countries to adopt a number of regulations and to identify radon-prone areas. During the years 2004-2005, the European Commissionconducted a survey to assess the means and methods used by national authorities to describe radon levels in their countries. The results show clearly that the variety of means and methods to measure and report radon levels is very large. It is the purpose of this paper to summarize the findings of this survey and discuss the reporting issue considering the need for a harmonised strategy between European countries to describe radon levels in our environment.
1. Introduction
Radioactivity, fromartificial or natural origins, can be detected in all environmental compartments and can thus be measured in the water we drink, in the food we ingest, in the air we breathe. A gross worldwide average annual effective dose is estimated to be 2.8millisievert (mSv)inyear 2000 [1]. As shown in Figure 1, natural radiation is by far the main contributor to the total dose since less than 15% of the radiation we receive is man-made, from which 98% come from medical activities (diagnostics and therapy). Obviously, the relative contributionsfrom the various sources are subject to very high variability:people who undergo radiotherapy will be exposed mainly to artificial sources.
Subsequent to its publication of the European atlas of 137Cs deposition following the Chernobyl accident [2], the Institute for Environment and Sustainability (IES) of the Directorate General Joint Research Centre (DG JRC) of the European Commission (EC) has started to explore the possibility of preparing a European atlas of natural radiation. Such an atlas would both help to familiarize the public with its environment which is naturally radioactive and to highlight the regions with elevated levels of natural radiation.
Because of radon’s high relevance in the radiation dose budget (Rn222isotope with a half-life of 3.8 days, 220Rn, 55 seconds, and respective progenies) – it is considered to be the main contributor to the total dose (~1 mSv/y in Figure 1) -, it has been the first variable considered for mapping in the frame of the preparation of an atlas of natural radiations. A recent study [3] has shown that radon in homes causes about 20,000 lung cancer deaths in the European Union (EU) each year. This is about 9% of the total lung cancer deaths in the EU and about 2% of cancer deaths overall.
To further identify regions that are susceptible to high radon levels, most European countries have organized large sampling campaigns, mainly by performing indoor and soil-gas measurements.
Figure 1.Pie chart showing the relative contributions from natural and man-made sources of radiation to the estimated gross worldwide average annual effective dose.Derived from [1].
Radon is produced by the radioactive decay of uranium and thorium which areelements that are naturally present in the earth’s crust. Radon is transported through the pores of top soil into the atmosphere mainly by diffusion and advection driven by pressure differences, and its concentration in indoor and outdoor air thus depends mainly on geological factors, soil texture, permeability, soil water content, and the pressure difference between the gas in the soil and at the surface. Relatively heavier than air, radon shows low concentrations outdoor (the mean annual concentration outdoor is on the order of 10 Bq/m3) but tends to be trapped in basements and the lower floors of buildings. For the same reasons, it is often found at high concentrations in mining galleries and caves.
Exposure to radon in dwellings is also the subject of Commission Recommendation of 21 February 1990 on the protection of the public against indoor exposure to radon (90/143/Euratom) [4]. Indoor radon concentration levels of 200 and 400 Becquerel per cubic metre (Bq/m3) are the reference concentrations above which mitigation measures should be taken in new and old buildings, respectively, to reduce exposure to radon. This can be done by improving the ventilation of the basements and/or by reducing the permeability of the foundations of the house to the gas.
2. Radon surveys in Europe
Recently, the European Commission conducted a survey in Europeto assess the means and methods used by the national authorities to describe radon levels in their countries [5]. A questionnaire was sent to national authorities and contact points for radon in 31 European countries. With a very few exceptions, all countries reported to have carried out large scale radon surveys and provided estimates of the average concentrations indoor(see Table 1).Even if various sampling strategies (systematic, preferential or random) were adopted and various types of buildings (e.g. public places, hospitals, schools, multifamily and/or single family houses)targeted, one can consider that, overall, most radon-prone areas have been identified throughout Europe, with a lower resolution in the Balkans.
Country / Estimated indoor annual mean levels (Bq/m3) / % dwellings> 200 Bq/m3 and <400 Bq/m3 / % dwellings
> 400 Bq/m3
Austria / 97 / 8 / 4
Belgium / 48 / 1.7 / 0.3
Croatia / 68 / 5.4 / 1.8
Cyprus / 19 / 0 / 0
CzechRepublic / 140 / 10 - 15 / 2 - 3
Denmark / 53 / 2.7 / 0.2
Estonia / 60 / 2 – 2.5 / 0.3 – 0.5
Finland / 120 / 8.7 / 3.6
France / 63 / 6.5 / 2
Germany / 50 / 2.5 / < 1
Greece / 55 / 2 / 1.1
Hungary / NA / 5.1 / 0.8
Ireland / 89 / 6 / 1.5
Italy / 70 / 3.2 / 0.9
Lithuania / 55 / 2.5 / 0.3
Luxembourg / 115 / NA / 3
Malta / 40 / 0 / 0
Netherlands / 23 / 0.3 / 0
Norway / 89 / 6 / 3
Poland / 49 / 1.6 / 0.4
Romania / 45 / NA / NA
Serbia-Montenegro* / 144 / 18 / 4
Slovakia / 108 / 14 / 11
Slovenia / 87 / 5.5 / 2
Spain / 90 / 4 / 2
Sweden / 108 / 6 - 7 / 3 - 4
Switzerland / 77 / 10 / 7
United Kingdom / 20 / 0.4 / 0.1
*Province of Vojvodina only
NA: Not Available
Table 1.Some statistics for European radon surveys taken from [5]. Because radon-prone areas are usually sampled more than others, overall statistics are usually biased and not always meaningful. Hence, the data presented above are based on estimations derived from statistics and/or models and one should be careful when using such information.
3. Mapping Radon
Two main strategies for delineating geographically the exposure to radon have been encountered in the survey:
1)radon-prone areas can be identified by means of geological maps and soil-gas measurements;
2)Rn concentration in indoor air, in some cases standardized to defined conditions, like certain types of buildings or rooms..
In many cases, the surveys have been based on a two step strategy of a soil-gas measurement campaign followed by indoor measurements. A total of more than 1.5 million dwellings have been investigated up to 2005 and measurement campaigns are still ongoing in most countries. While soil-gas measurements are not expected to fluctuate strongly over short distances (faults, geothermal activities and different rock types can however have a strong local impact), indoor radon levels are known to potentially presentsmall-scale fluctuations that can be of a factor 100 between two neighbouring rooms, depending on the design of the house, the construction material used, the insulation used in the house and the living habits of the inhabitants. As a result of such fluctuations, mapping indoor levels by means of regression techniques is everything but straightforward. For this last reason, most countries reported the results of their surveys by means of maps showing averaged values at various administrative levels. The variety of the means used in the European countries to report radon levels has been summarized in [5] by means of a “collage” of the European radon maps published by the national authorities (Figure 2).
4. Harmonized maps at the European level
Figure 2 is a self-speaking figure underlining the variety in the mapping methods adopted (average values on grids or administrative boundaries, isolines derived from spatial interpolation), in the chosen resolution (municipality level up to regional level), as well as in the number of isoline levels used to describe the various concentrations (low, medium and high concentration levels differ). Unless each single measurement is reported graphically, any map will involve some averaging of the data collected that will consequently smooth-out the hot-spots which inevitably occur.Even the choice of the colours and the resolution are largely based on a complex mixture of political and scientific decisions that are needed to find the best balance between an information that is detailed enough to be useful but also generalized enough to facilitate the identification of the general “radon patterns”. It is not only that different maps serve different purposes but, as it is the general case with maps derived from environmental data, each map is proper to its authors given the many, often arbitrary, decisions that are taken in the processing phase.
Figure 2.“Collage” of the European radon maps published by the national authorities. Colours and levels have not been harmonized in the figure. Taken from [5].
In addition to the mapping issue, one ought not to forget that the information processed can also be extremely different between countries: the sampling strategies diverge between countries and so is the bias in the overall statistics of the measurements. Looking further into the problem, one will also realise that measurements have been made using various types of detectors and for different time intervals. Some countries measure mainly in winter (winter time is usually preferred because exposure is highest due to lesser ventilation) only to estimate an average maximum concentration while others do measurements for a whole year. Even the actually measured physical quantities are not always identical: are the measurements referring to222Rn, or 220+222Rn, or Rn progenies? What equilibrium factor is assumed? As a result of this variety in the physical variables that can monitored, in the measurement techniques, in the sampled compartments and periods, direct comparison between estimated levels measured in the different countries should be made with caution. A legitimatequestion is thus if all European countries are dealing with the same variable? More provocatively, one may ask what is radon?
While most countries adopted a national monitoring strategy, minimizing so the heterogeneity of the tools and methods used to estimate radon levels, a number of countries had surveys organised at the regional level, complicating so the comparison between the results of the survey. Worth to be mentioned, Sweden which has had one of the largest radon survey in Europe has no maps at the country level, as the responsibility for monitoring and mapping radon lies in the hands of each municipality.
Putting aside the complex debate on whether soil-gas measurements used for radon risk mapping or indoor measurements should be made for identifying areas prone to radon and better assess exposure of the population, we here advocate a concerted approach at the European level for allowing the comparison of the data and maps. Such harmonisation should not only help everyone concerned, citizens and decision-makers alike, to assess better this natural threat to our health, but also to familiarize the population with the fact that their environment is naturally radioactive.
Because most measurements are available only to the national authorities, the preparation of harmonised radon maps at the European level is not feasible by a small group of experts but we still hope all authorities will find a common denominator in their data sets and agree on means to process the data, statistically or even using a modelling approach, in a way that comparability of the information shown will be guaranteed.
Acknowledgements
This European radon survey was possible only thanks to the excellent collaboration of all the national authorities and contact persons whose names are indicated in the report. Note that the report and the data are regularly updated on the European Forum on Radon Mapping [ ].
References
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[4] Commission Recommendation of 21 February 1990 on the protection of the public against indoor exposure to radon (90/143/Euratom). Official Journal of the European Union, OJ L-80 of 27/03/90, page 26.
[5] Dubois G. (2005). An overview of radon surveys in Europe. EUR 21892 EN. 168 pp.Office for Official Publications of the European Communities, Luxembourg.
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