Radon is a radioactive noble gas. It forms in the ground as a decay product of uranium and radium, has no smell, no colour, no taste, and is the second leading cause of lung cancer after smoking, and the leading cause among non-smokers (WHO 2009). The only way to know the level in one's own home is to measure.
The same uranium-lead decay physics that geochronologists use to date rocks lies behind the question of how much radon accumulates in living spaces. This text summarises what is documented about radon in Austria.
What radon is and why it is dangerous
Radon itself is largely breathed out again. What is dangerous are its short-lived decay products, above all isotopes of polonium, lead and bismuth. They attach to dust particles, reach the lung on inhalation and irradiate the sensitive bronchial tissue with alpha radiation, the biologically most effective radiation type over short range. The International Agency for Research on Cancer (IARC/WHO) classifies radon as a Group 1 carcinogen, that is, carcinogenic to humans (IARC 2012). How the noble gas becomes a radiation dose in the tissue is set out in the fold-out.
For experts, or those who want to become one: The decay chain, why the short-lived progeny matter, and how the lung dose arises
Radon-222 is a link in the uranium-238 decay series: via radium-226 the gaseous radon forms, escapes from the grain structure of the ground and accumulates indoors. Radon itself has a half-life of 3.8 days and, as a noble gas, is chemically unreactive, so it is largely exhaled again. What matters are its short-lived progeny polonium-218, lead-214, bismuth-214 and polonium-214, which follow one another within minutes to hours. Unlike the gas, these are electrically charged metal atoms that attach to aerosols and dust, are inhaled with them, and lodge in the bronchial epithelium.
There, polonium-218 and polonium-214 emit alpha particles as they decay. Alpha radiation has a range of only a few cell layers in tissue, but deposits its energy very densely over that short path, which is why it causes considerably more biological damage per unit energy than X-rays or beta radiation. It is precisely this dense ionisation of the cell nuclei in the epithelium that drives the lung-cancer risk, not the gas as such. How closely the progeny are in radioactive equilibrium with the radon is described by the equilibrium factor F; in the Austrian Radon Project it averaged about 0.5 (Friedmann 2005). The radiation dose per radon concentration is therefore not a natural constant but depends on the aerosol content and ventilation of the room.
How large the risk is
How large the risk is, is shown consistently by three large pooled analyses of case-control studies from Europe, North America and China: the increase in lung-cancer risk lies in the order of 10 to 16 percent per 100 Bq/m³ (becquerel per cubic metre) of additional radon concentration in the room air (Darby et al. 2005; Krewski et al. 2005; Lubin et al. 2004); the three individual values and their derivation are in the fold-out. The relationship is linear and shows no discernible threshold over the observed range, so there is no value below which the risk is demonstrably zero. In absolute terms this remains modest for non-smokers: at a lifelong radon concentration of 0, 100 and 400 Bq/m³, the risk of developing lung cancer by age 75 is, for lifelong non-smokers, about 0.4, 0.5 and 0.7 percent (Darby et al. 2005). For smokers it is about twenty-five times higher, so at the same concentrations about 10, 12 and 16 percent (Darby et al. 2005): the proportional radon effect is similar for both, but on the far higher baseline risk of smokers it weighs incomparably more heavily. Radon and smoking together are the truly dangerous combination. The derivation of the figures is in the fold-out.
For experts, or those who want to become one: The three pooled analyses, the measurement-error correction and the smoking-radon interaction
The most reliable evidence comes from the pooled reanalysis of individual data from residential case-control studies using long-term track-etch detectors. Europe (Darby et al. 2005, 13 studies, over 7,000 cases): plus 16 percent per 100 Bq/m³ after correction for the random measurement uncertainty of the radon determination; the uncorrected, directly observed slope is about 8 percent. North America (Krewski et al. 2005, 7 studies, 3,662 cases / 4,966 controls): odds ratio 1.11 per 100 Bq/m³ (95% confidence interval 1.00 to 1.28). China (Lubin et al. 2004, 1,050 cases / 1,996 controls): odds ratio 1.13 (1.01 to 1.36). All three agree with the downward extrapolation from the miner cohorts (about 1.12).
Two points for context. First, the measurement-error correction: because a single residential measurement reflects the long-term exposure only imprecisely, this random error dilutes the observed relationship; correcting for it raises the estimate. In both poolings the risk estimate grew accordingly when it was restricted to the subjects with the most accurate long-term dosimetry (Krewski et al. 2005; Lubin et al. 2004). The 16 percent are therefore not a contradiction to the 11 and 13 percent, but the measurement-error-corrected version of the same order of magnitude. Second, smoking: Krewski et al. found the proportional radon effect practically equal in never- and ever-smokers (excess odds ratio 0.10 each). The often-heard talk of a "multiple" refers to the absolute risk, which is far higher in smokers because of their high baseline risk, not to a disproportionate relative effect.
Why Austria is affected
Radon comes from the geological subsurface. Rocks with elevated uranium content release more radon: the granites and gneisses of the Bohemian Massif in the north, but also the carbonates of the Northern Calcareous Alps, whose fissures and karst channels conduct the gas. Large parts of Austria lie on such subsurface, and this is reflected in the measured values. In the current national survey (ÖNRAP 2, 2013 to 2019) the radon concentration in Austrian interiors averages around 110 Bq/m³, with individual dwellings well above that (Gruber et al. 2021), and thus clearly above the global average of about 40 Bq/m³ (UNSCEAR 2000). About 11 percent of Austrian households exceed the reference value of 300 Bq/m³ (Gruber et al. 2021). The burden is strongly regional: in Upper Austria and Tyrol the geometric mean is 130 and 127 Bq/m³ respectively and about 17 percent of dwellings are above the reference value, while in Vienna, Vorarlberg and Burgenland it is a fraction of that.
The gas enters the house through the ground. In winter the stack effect operates: warm air rises and escapes on the upper floors, a slight negative pressure forms in the basement, and radon-laden soil air is drawn in through cracks in the foundation, pipe penetrations and leaky cellar walls. This is why basement and ground-floor rooms are most affected, and why the values are highest during the heating season. How the survey is designed and what else is known about the distribution is in the fold-out.
For experts, or those who want to become one: The Austrian Radon Project, the geology and the building factors
The current data basis is ÖNRAP 2 (Gruber et al. 2021): about 25,000 dwellings measured, selected via a geographic 2-by-2-kilometre grid rather than population-weighted as in the predecessor project, measured with passive track-etch detectors (CR-39) over six months, half in winter and half in summer. The geometric mean is 109 Bq/m³ with a geometric standard deviation of 2.29; the distribution is log-normal, which is why the arithmetic mean, at 166 Bq/m³, lies considerably higher. The first Radon Project (Friedmann 2005, about 40,000 measurements 1992 to 2001) had determined an inhabitant-weighted arithmetic mean of 99 Bq/m³; because of the different study design, however, no direct time trend can be derived from it.
Geology explains about 20 to 25 percent of the total variation (Gruber et al. 2021). Highest are the values over the granitoids and migmatites of the Bohemian Massif and over the karstified limestones and dolomites of the Northern Calcareous Alps, lowest over siliciclastic sediments, phyllites and the Flysch. Building factors add to this: older buildings, rooms in direct ground contact and dwellings without a continuous basement show higher values, upper floors lower ones. This is precisely why the map is an orientation and not a substitute for measuring the individual house: the geological subsurface is only one of several factors.
The radon map: orientation, not diagnosis
The AGES provides an interactive radon map for all of Austria (geogis.ages.at). It is based on the radon potential, a combination of geological data and actual measured values, and assigns each municipality a classification: green for low, yellow for medium, orange to red for high potential.
What matters is what the map does not show. It provides an average per municipality, not the value for an individual house. Local geology varies within a municipality; one house may stand on granite, the neighbouring house 200 metres away on gravel. The construction is entirely absent: the age and condition of the floor slab, cracks, pipe penetrations, cellar use. Ventilation behaviour also plays a part. In practice we regularly see values above 300 Bq/m³ in municipalities classified green and low values in red ones. The map gives an orientation, not a diagnosis.
Regardless of the map colour, a measurement is sensible for old buildings with inhabited cellar rooms, for houses on a slope or on granite subsoil, before a house purchase, and everywhere people spend many hours a day in near-ground rooms, that is, in kindergartens, schools and care homes.
Measuring correctly: how long, where, how many
A snapshot is worthless, because the radon concentration fluctuates strongly within a few days depending on weather, air pressure and ventilation. The reliable method is the long-term measurement with passive track-etch detectors, small capsules without power or maintenance. Such detectors are sent by post by accredited measurement services and the Austrian radon competence centre; one sets them up, sends them in after the measurement period and receives the result.
- How long: at least three months, ideally during the heating season (October to March); the official surveys measure over six to twelve months. Whoever is below the reference value during the heating season is below it in the annual mean as well. Electronic short-term measurements over 24 to 72 hours are suitable only as a first screening, not as a substitute.
- Where: in the most-used living room on the ground or basement floor, not in the kitchen or bathroom, which are ventilated a lot. In the room at about 1 to 1.5 metres height, at least 20 cm from the wall, not at the window, not on the heater, not behind furniture.
- How many: at least one in the most-used room. For a complete picture two to three, for instance basement, ground floor and bedroom.
What the values mean
Measurement is in becquerel per cubic metre (Bq/m³), that is, in decays per second per cubic metre of air.
- Below 100 Bq/m³: unremarkable, no need to act.
- 100 to 300 Bq/m³: simple measures sensible, such as regular ventilation and sealing obvious entry paths in the cellar.
- Above 300 Bq/m³: remediation measures recommended. 300 Bq/m³ is the reference value of the Radon Protection Ordinance.
Because the relationship is linear and without a discernible threshold, a value just below 300 does not mean "safe": less is always better. The reference value is an action value, not a line between harmless and harmful.
The legal framework
Radon is regulated in Austria by the Radiation Protection Act 2020 (Strahlenschutzgesetz, BGBl. I No. 50/2020) and the Radon Protection Ordinance (Radonschutzverordnung, RnV, BGBl. II No. 470/2020) based on it, both implementing the EU basic safety standards (Directive 2013/59/Euratom). The ordinance sets a reference value of 300 Bq/m³ in the annual mean, both in living spaces of residential buildings and at workplaces. This is not a limit value in the legal sense, but the value above which measures are foreseen.
The ordinance distinguishes two area types. 104 municipalities whose predicted mean radon concentration exceeds the reference value are designated radon protection areas; there, a measurement obligation applies to workplaces in cellar and ground-floor rooms. On a separate, broader municipal list the ordinance defines radon precaution areas, in which radon-precaution measures are prescribed for newly erected buildings, in practice above all a radon-tight design of the floor slab.
What can be done
Radon is one of the most controllable pollutant problems in a building: the measurement is simple and unambiguous, and the countermeasures work. At moderately elevated values, better cellar ventilation and the sealing of cracks and pipe penetrations often already help. The most effective active measure is the radon extraction system, a negative-pressure ventilation beneath the floor slab that intercepts the gas before it enters the house; suction points, fan power and the tightness of the floor slab must be matched to one another and the success measured afterwards. For new builds in radon precaution areas, the radon-tight floor slab is standard anyway.
Sources
- AGES: Interactive radon map of Austria (radon potential per municipality). Austrian Agency for Health and Food Safety. geogis.ages.at
- BEIR VI (1999): Health Effects of Exposure to Radon. Committee on Health Risks of Exposure to Radon, National Research Council, Washington. Risk model from the miner cohorts. nap.nationalacademies.org
- Darby, S., Hill, D., Auvinen, A. et al. (2005): Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. British Medical Journal 330:223. About 16% per 100 Bq/m³ (measurement-error-corrected); absolute risk to age 75 for non-smokers 0.4/0.5/0.7% and smokers about 25 times higher at 0/100/400 Bq/m³. doi.org/10.1136/bmj.38308.477650.63
- Friedmann, H. (2005): Final Results of the Austrian Radon Project (ÖNRAP 1). Health Physics 89(4):339–348. Arithmetic mean 99 Bq/m³, median 61, about 40,000 measurements 1992–2001. doi.org/10.1097/01.HP.0000167228.18113.27
- Gruber, V., Baumann, S., Wurm, G., Ringer, W. & Alber, O. (2021): The new Austrian indoor radon survey (ÖNRAP 2, 2013–2019): Design, implementation, results. Journal of Environmental Radioactivity 233:106618. Geometric mean 109 Bq/m³, arithmetic mean 166, about 11% of dwellings above 300 Bq/m³. doi.org/10.1016/j.jenvrad.2021.106618
- IARC (2012): Radionuclides, incl. Radon-222 (Internalized alpha-particle emitting radionuclides). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol. 100D. Radon as Group 1 (carcinogenic to humans). ncbi.nlm.nih.gov/books/NBK304363
- Krewski, D., Lubin, J. H., Zielinski, J. M. et al. (2005): Residential Radon and Risk of Lung Cancer: A Combined Analysis of 7 North American Case-Control Studies. Epidemiology 16(2):137–148. Odds ratio 1.11 (95% CI 1.00–1.28) per 100 Bq/m³. doi.org/10.1097/01.ede.0000152522.80261.e3
- Lubin, J. H., Wang, Z. Y., Boice, J. D. et al. (2004): Risk of lung cancer and residential radon in China: pooled results of two studies. International Journal of Cancer 109(1):132–137. Odds ratio 1.13 (1.01–1.36) per 100 Bq/m³. doi.org/10.1002/ijc.11683
- RnV (2020): Radon Protection Ordinance (Radonschutzverordnung), BGBl. II No. 470/2020, based on the Radiation Protection Act 2020 (BGBl. I No. 50/2020), implementing Directive 2013/59/Euratom. Reference value 300 Bq/m³ in the annual mean; 104 radon protection areas; radon precaution areas. ris.bka.gv.at
- UNSCEAR (2000): Sources and Effects of Ionizing Radiation, Annex B (Exposures from natural radiation sources). United Nations Scientific Committee on the Effects of Atomic Radiation. Global average indoor radon concentration about 40 Bq/m³. unscear.org
- WHO (2009): WHO Handbook on Indoor Radon: A Public Health Perspective. World Health Organization, Geneva. Radon as the second leading cause of lung cancer after smoking. ncbi.nlm.nih.gov/books/NBK143219
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