Radon Risk in Spas?

For centuries or millennia spas have been important tools of medicine and preferred sites of social life. The features, the warm water with chemical contents (inorganic minerals, salts and organic components – the messages of the old past) build up the pleasant smell, touch, taste. These are the gifts of Nature. The analysis of the health effects is therefore, a proper topic for environmental medicine, too. The subjects of the present review is the significance of radon in the natural waters and consequently in the air of spas and whether the radon contributes to the healing effects in balneotherapy. Several international conferences have dealt with these topics in the recent decades like “Grundlagen der Radontherapie” (1979), “Radon in Bädern” (1993), “Radon Risk” (2006). Of course, the convening of conferences was stimulated by the increasing social interest in radiation protection in the seventies and since then (ICRP60, 1990). The radon being a radioactive noble gas and its decay products being radioactive metals evoked wide special research and debate as carcinogenic environmental factors especially when their concentrations enhanced technologically like in underground mines or in indoor surroundings of the population. In the context of spas and radon, the approaches to conclude whether they are beneficial or detrimental and risky might cover the following:
– whether the presence of radon in its concentrations relevant to the spas means quantifiable risk as derived from epidemiological data,
– whether the ionizing radiation due to the radon and its decay products in the relevant low doses means any biological effect on the cells and organism based on radiobiological observations,
– whether the radioactivity in health spas contribute to the healing of illnesses or wellness of the patients and visitors.

The answers concern public health, radiation hygiene and protection, radiation biology, balneology and last but not least the health tourism. Accordingly, the present review provides data – far from being complete – to guide the interested reader and stimulate further research and studies.

Radon is a colourless, odourless and tasteless gas produced by radioactive decay of uranium and thorium. There are two main isotopes of radon in nature: – ²²²Rn (T_1/2=3.82d) and its short-lived decay products: ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, ²¹⁴Po, ²¹⁰Pb, ²¹⁰Bi, ²¹⁰Po (uranium series), – ²²⁰Rn (T_1/2=55.6s, also called thoron) and its decay products: ²¹⁶Po, ²¹²Pb, ²¹²Bi, ²¹²Po, ²⁰⁸Tl (thorium series). Radon is found in most earth materials, in the atmosphere, in ground water and even in drinking water. It can enter building materials in a number of ways. Large quantities of radon are found in many homes due to the break and stones often have large radioactive backgrounds caused by radon. The radon content of outdoor air 1 metre above ground typically gives 4 to 15 Bq·m^–3. The average indoor air concentration of radon varies from location to location, depending upon the uranium and thorium content and physical characteristics of the soil, moisture, winds and building materials. In most countries the average indoor radon concentration is a few tens of Bq·m^–3, however, during the surveys hundreds and even thousands could be found. The International Commission on Radiological Protection (ICRP) therefore recommended action levels 200–600 Bq·m^–3 for homes and 500-1500 Bq·m^–3 for workplaces which correspond to annual doses of 3–10 mSv in either case (ICRP65, 1993).
     The radioactive decay chain is shown in Fig 1. (Darby, 2006). From health risk aspects mostly the short-lived progenies are important, i.e. those alpha-emitting nuclides which have short physical half-lives. The alpha particles in tissues have a path of a few tens of micrometers (Table 1; Hofmann and Steinhäusler, 1979). Accordingly, either nuclide is involved it is deposited in the bronchi or attached to the skin, irradiates the sensitive cell layers, i.e. bronchial epithelium and epidermal basal cells, respectively. When entering the blood stream by either way the nuclides easily deposit in the lipid material of cells and tissues irradiating the neighbouring cells. The cellular reactions depend on the nature of the cells. It has to be emphasized that the fraction of radon or decay products reaching other cells than the bronchial epithelium or skin epidermis is much less. Table 2 indicates these differences well (Pohl, 1979). Naturally, the less the local concentration is the less the absorbed dose is, and consequently the radiation risk, if any, as well.

Fig. 1. The radon decay chain (Darby, 2006)

The average activity-concentrations of radon in the human environment cover rather wide ranges. Table 3 presents some examples (Köteles, 1993). The concentrations in soil, water and earth gas might occur in five orders of magnitude range. Beside the natural occurrences there are technologically enhanced natural environmental conditions, where the radon concentrations might be even higher than above, like man-made structures, houses, mines. Like in any environmental element, the radon concentrations of mineral and thermal waters, consequently of the spas differ in rather wide range. Examples are given in Table 4.

     The air in various rooms of spas has also a wide range of activity-concentration of radon which is inhaled by visitors and workers. For example, the values in the mentioned spas are as follows:

From the radon concentration in air the effective dose values can be assessed (Dombóváry et al., 2006) by taking into consideration the activity-concentration (C_Rn in Bq·m^–3), the dose conversion factor (K) from the inhaled amount to sievert (7.9 · 10^–9 Sv · (Bq·m^–3)^–1 · h^–1), the equilibrium factor (f) of radon and decay products (usually 0.4) and the time length of staying in the respective room (T), i.e.

The radiation dose from drinking radon-containing water can be assessed similarly as follows.

The effective dose values can also be assessed for persons having a bath course. A few examples of results of such calculations are as follows: The radiation doses to cells and tissues from environmental radon activity-concentrations can also be assessed by the exposure, exposure rate, time length of exposure usually by models. An example is given (Maushart, 2005) suggesting that when the water source contains 3256 Bq·L^–1, then in the water of the bath-tub there is only 1800–2000 Bq·L^–1. Accordingly, a healing course (10 occasions, 20 minutes per each) results in 2 mGy to the surface of the skin, 30 µGy to the whole skin and a few µGy dose for the other tissues like blood. The latter contained under such conditions 4 Bq·L^–1, the exhaled air contained 2.5 Bq·L^–1. Beside the exposures from the water and air in the spa itself, the patients staying in the various rooms of the health centres might get even higher exposure than in the basins (Pohl, 1979).
     The committed effective dose assessment of visitors in Hévíz Lake spa in case of every day bathing is between 0.7–0.9 mSv per year (Committed effective dose is the effective dose to tissue over an integration time elapsed after an intake of radioactive substances). The patients’ effective dose in the hospital is not more than 102 µSv and in the health hotel 34 µSv (Dombóváry et al., 2006). These values are due to the radon activity-concentrations in air, i.e. between 340–625 Bq·m^–3 in various rooms.
     It is known since the beginning of such calculations that mostly the air radon concentration contributes to the radiation dose by inhalation and that the professional staff is exposed for longer time than the visitors and effective doses what they get are a few – 0.2 to 15 – mSv per year (Pohl, 1979; Mócsy, 2005).
     In a “mofetta” in Kovasna, the radiation burden for staying for 30 minutes with 11.1 Bq·L^–1 radon air concentration is appr. 200 µSv (Szabó, 1998). In Rudas spa the visitors’ committed effective doses are between 0.8–1.7 mSv per year (Szerbin, 1996).
     It is obvious that the excess radiation burden from spas is less than the annual natural background in which all persons live during the whole life. Table 5 summarizes the average radiation doses from natural sources.

The adverse health effects of exposure to radon are caused primarily by damage due to alpha-particles. The possible effects will depend on the exposure level. The main danger from high radon exposure is an increased risk of lung cancer. Radon as a noble gas is rapidly exhaled after being breathed in; however radon progenies combine with other molecules in the air and with particles of dust, aerosols or smoke, and readily deposit in the airways of the lung. While lodged there, the progenies emit ionizing radiation in the form of alpha particles, which can damage the cells lining the airways, i.e. bronchial epithelial cells where they could initiate cancer. In human population the epidemiological studies on thousands of uranium miners in different countries support this fact (UNSCEAR, 2000). But exposure to radon in houses can also lead to lung cancer (WHO, 2002). Lung cancer risk, however, is several times higher when radon exposure is combined with smoking. It is believed that the relationship between radon and risk of lung cancer is linear. In other words, doubling the exposure doubles the risk and halving the exposure halves the risk. Lung cancer risk from residential radon exposure is substantially lower since the exposure in homes is much lower than in mines, although the risk increases with radon concentration level and duration of exposure. For life-time exposure to radon of 20 Bq·m^–3 level at home the risk of lung cancer is estimated to be 0.3% (or 3 death in 1000 people). For comparison, risk of accidental death at home is 0.7% (or 7 in 1000) (WHO, 2002). A recent large meta-analytical study of epidemiological investigations on the risk of indoor radon concluded that the risk estimate for lung cancer increased by 16 per cent per 100 Bq·m^–3 when such concentration is involved in dwellings almost all the day and many years (Darby, 2006). In the risk assessments beside the concentration, the time-length also has to be considered. In spas the people stay for much less time than in their homes.
     It has been suggested that other effects of radon exposure include increased risk of non-malignant respiratory diseases but this is much less clearly established than the lung cancer risk. It is still not clear whether children are more sensitive to radon exposure. Studies on childhood leukaemia (the most common form of cancer in childhood) have not found clear evidence of risk associated with radon concentrations in homes.

The biological effects of ionizing radiation for radiation protection considerations are grouped into two categories according to their dose-effect relationships: the deterministic and the stochastic ones (Fig 2). The former ones are caused by high doses, the latter ones by low doses. The stochastic effects might occur following several tens of mSv. The probability of consequences increases with the dose and the relationship between dose and effect is assumed to be linear. Accordingly, not having a “threshold” dose a certain risk – albeit very small – can be attributed to any low dose.

Fig. 2. Schematic dose-response curves for the stochastic
and deterministic effects of ionizing radiation (Köteles, 1998)

     The model for assessing the detrimental health effects used for the stochastic effects is the linear-non-threshold “L-NT” one. In the low dose dilemma the problem raised is whether the use of the L-NT model is justified to attach any health risks to low doses as the risk of low-level exposure to ionizing radiation is uncertain and a single extrapolation from high-dose effects may not be wholly justified in all instances (Köteles, 1998; ICRP99, 2006). Based on epidemiological data of radiation-induced cancer occurrences, various authors agree that low dose is below 200 mGy as under this level the statistical evaluation of data becomes more and more uncertain (UNSCEAR 1994; Heidenreich et al., 1997; Tubiana, 2003). Accordingly, based on the frequency of cancer cases the extrapolation of risks from high doses to low ones is not justified.
     Among the low dose radiation-induced cellular alterations recently special interest has been focussed towards the hormesis and adaptive responses. These phenomena, i.e. inducing stimulatory or beneficial effects have more and more become the targets of research.

The term refers to a process whereby low doses of ionising radiation may result in beneficial or stimulatory effects. The underlying property is a physiological effect that cannot be anticipated by linear downward extrapolation from the toxic levels of exposure. There is a large body of literature, supported by statistically significant epidemiological studies, that speaks in favour of radiation hormesis, which have demonstrated that exposure to low level irradiation has apparently resulted in positive health effects (Kant et al., 2003; Mortazavi et al., 2006; Averbeck et al., 2006). The hormetic argument that whole body exposure to low level irradiation may actually decrease cancer risk is based primarily on the analysis of occupational and environmental data of various related studies (Kant and Chakarvarti, 2006). Reports exist on various epidemiological studies demonstrating a negative correlation of lung cancer risk with radon in dwellings, which shows that exposure to low level ionising alpha radiation has apparently resulted in positive health effects (Averbeck et al., 2006). Studies show that there is a protective effect of radon in the range of appr. 20–200 Bq·m^–3 (LáZázr et al., 2003; Kant and Chakarvarti, 2006). All these advocate the non-linearity of the dose-response curve and speak in favour of the protective effect of low-dose exposures and even indicate also that radiation hormesis exists. As possible explanation to this interesting phenomenon, it has to be considered that low doses of ionising radiation may induce or activate DNA repair functions, immune responses, anti-tumour defence systems, and detoxification mechanisms, the so-called adaptive responses with the result that there are fewer cancer deaths in individuals exposed to low level irradiation (Cameron, 1992; Pollycove and Feinendegen, 1999). Furthermore, the argument that all radiation is harmful even down to ambient levels is based upon extrapolation of data with high doses and with virtually no acceptable data with low doses. These scaled epidemiological extrapolations are invalid and the majority of the available data on low level irradiation suggest benefits (Luckey, 1999).

The term ‘adaptive response’ is used to refer to the possibility that a prior exposure to a small dose of radiation, which is variously referred to as the conditioning, adapting or priming dose, may mitigate the severity of the effect due to a subsequent high dose or challenging dose of ionising radiation. Studies have established the existence of an adaptive response to radiation in human lymphocytes, pre-irradiating cells with about 0.01 Gy of low LET radiation which protects them from a subsequent dose of about 1 Gy, as measured by a lower yield of chromosome aberrations, genetic mutations, cell transformation and resistance to cell death (UNSCEAR 1994; Köteles, 2004). The protection is probably mediated by newly synthesised enzymes involved in DNA repair or in antioxidant processes. Beside the experimental observations the existence of adaptive response was demonstrated in lymphocytes of human population (Bognár et al., 2007).

There is more and more information cumulating on the effects of radon at cell biological level. The gene activity of p53 cancer suppressor gene increases to double value at a triple concentration of natural radon in a Japanese region, as well as the superoxid dismutase (SOD) enzyme activity increases by 15 per cent (Ma et al., 1996; Yamaoka et al., 2005). Similarly the increase of this antioxidant enzyme activity was observed during balneotherapy and parallelly an other positive effect, i.e. decrease of serum lipids (Mitsunobu et al., 2003).
     Rather early, the stimulation of DNA repair was observed upon radon exposure (Altmann and Tuschl, 1978). Similar DNA repair was indicated in lymphocytes of people living in increased radon concentration and also the adaptive response reaction was provoked under 10 mSv “priming” dose (Masoomi et al., 2006).

Clinical aspects: radon as medicine

The spas evidently containing radon have been used with success for hundreds of years for special illnesses mainly in the pain therapy of chronic rheumatic illnesses (Table 6; Deetjen et al., 2004; Meara, 2006). Radon spas are wide-spread in Europe (Germany, Austria, Czech Republic, Hungary, Romania, Slovenia, etc). Clinical experience has shown that the long-lasting pain of the patients was considerably reduced with less analgetic pharmaceuticals. The reduction of anti-rheumatic drugs is important in the prevention of side effects like gastric bleeding, duodenal ulcer up to the perforation (Rühle, 2005). The cause behind seems to be a regulatory change of cellular reaction in the inflammatory tissues. This is evoked by alpha-radiation like following UV-B exposure or x-irradiation. The mechanism could be that the alpha irradiated dying cells release cytokines like TGFß which inhibits the inflammation. In the radon-treated patients with Morbus Bechterew the TGFß could be detected. Another speculation on the pain relief is, though not well understood, that the action potential of the pain neural network is increased in response to the internal radon radiation field (Lykken et al., 2005). Thus the chronic pain signals are ignored since they can not sum sufficiently to trigger the new activation threshold for the pain response of neural network system. The long-lasting effects are due to the fact that the radon within the organism is stored in the lipids of the brain, blood forming organs, lymphoid tissue. The lipid storage of radon allows it to deliver the short-lived radon decay products to the neural network system continuously over periods of several weeks to months. Further important observations concern the stimulation of endocrine system especially the secretion of adrenal and thyroid glands (Phaller et al., 1979; Nagy and Berhes, 2001; Nagy et al., 2006).

– Although in many sources of thermal and mineral waters the radon activity-concentrations are rather high, by the time they reach the consumer either as drinking water, or in spas the concentrations will be considerably lower.
– Radiation protection calculations suggest that due to the exposure levels the effective doses are in the range of the natural background.
– Epidemiological and cell biological studies indicate that the low level of irradiation causes less detriment than it would be expected from extrapolation from high doses.
– Low level irradiations might be beneficial through stimulating cellular and tissue reactions. This might be one of the effects of radon spas, too.
– The presence of radon in spas, accordingly, can not be considered as risky to health, just the opposite, more and more information cumulate on its positive health effects completing the other beneficial factors present in health spas.

The author highly acknowledges the interest and advice of Professor Attila Vértes, member of Hungarian Academy of Sciences.