Radon is a gas produced by the radioactive decay of uranium. Radon also decays producing alpha and beta particles along with gamma rays. All the particles can become trapped in the lungs where they can damage the cells, possibly causing lung cancer. The risk is greatly increased with smoking.

The concentration of radon in the open air is very small but the amount increases significantly when enclosed by a room or cave. Radon concentrations are measured in Bq m^-3 (Becquerels per cubic metre), where a Becquerel is the disintegration of 1 atom in one second. Some examples of radon concentrations in British caves are shown below.

Max Bq m^-3

From Radon Underground, BCA

Between 5th August and 15th November 2012, a detector was placed in each of Cueva-Cubio de la Reñada in La Vega, Matienzo and Cueva-Cuvío del Llanío, 5.2km to the north in Riaño. Corentium Canary detectors were placed in Cueva Asiul and Cueva-Cubio de la Reñada at new year 2013 - the results will appear in due course.

Discussion
   It has been recommended that the radon concentration in houses be less than 100 Bq m^-3 (A Review of Radon Doses). On that basis, one day in Reñada is equivalent to 18 days in a house with 100 Bq m^-3 and, in Llanío, equivalent to 40 days in the house. It follows that no more than 9 days per year should be spent in Cueva-Cuvío-del Llanío. However, there will probably be a high seasonal difference as air exchange is a lot greater in the summer than in winter. This should be tested as winter trips in the bottom Reñada entrance should be in fresher air, drawn in from outside.
    The results from Matienzo look similar to those from Britain but greater than any of those tested in Cantabria. (Analysis of the main factors affecting the evaluation of the radon dose in workplaces: The case of tourist caves).
    For the usual type of trips in these Matienzo caves lasting 4 to 10 hours it would appear there is no significant risk, or extra risk compared to caving in Britain. Most Matienzo Expedition cavers do more caving in the UK than Matienzo so the risk from general expedition caving does not seem to add to the overall risk. However, there will be extra exposure when certain sites are persistently excavated either by archaeologists or cavers and it may be worthwhile an individual wearing a dosimeter over, eg a four week period. Guides running trips through, eg Azpilicueta-Reñada on a regular basis may be at extra risk and may also come under work regulations.
    The radon content in these two caves is high enough to monitor seasonal changes by measuring over 2 - 3 weeks with dosimeters. It is likely that more comprehensive testing will now occur, possibly with the assistance of local institutions.

Juan Corrin, Phil Papard, Torben Redder. December 2012. Additions May, November 2014 Juan Corrin.

References and further reading
• Landauer Nordic. Results of the Matienzo Radon testing, 2012.
• BCA. Radon Underground. 2nd Edition, 2012
• Jing Chen. A Review of Radon Doses. Radiation Protection Management, Vol 22, No 4, 2005. Canada.
• Sainz et al. Analysis of the main factors affecting the evaluation of the radon dose in workplaces: The case of tourist caves. Journal of Hazardous Materials 145 (2007) 368-371.
• Smetanova I, et al. Radon monitoring in Domica Cave, Slovakia - preliminary results. (Date after Jan 2011; published by ?)
• from New Scientist 12^th September 1992 Radon in Caves. Belfry Bulletin 472, January 1994
• Sabroux JC et al. Radon Monitoring In A Geothermal Ice Cave Of Mt Erebus, Antartica. 1999. IX^th International Symposium On Vulcanospeleology.
• Cigna, A, A. 2005. Radon in caves. International Journal of Speleology, 34 (1-2), 1-18. Bologna (Italy). ISSN 0392-6672
• J. Lario et al 2005. Radon continuous monitoring in Altamira Cave (northern Spain) to assess user’s annual effective dose. Journal of Environmental Radioactivity 80 (2005) 161–174