A multifactorial assessment of carcinogenic risks of radon for the population residing in a Russian radon hazard zone

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1 A multfactoral assessment of carcnogenc rsks of radon for the populaton resdng n a Russan radon hazard zone Vladmr L. Lezhnn 1, Evgeny V. Polzk 2, Vladmr S. Kazantsev 2, Mkhal V. Zhukovsky 1, Olga A. Pakholkna 3 Summary Background: Results of numerous epdemologc studes of carcnogenc effects of ndoor radon conducted n dfferent countres n the past 40 years reman controversal. To assess the contrbuton of the resdental radon exposure n the development of lung cancer n the populaton of the Russan regon wth a hgh radon hazard we conducted a cancer epdemology study based on a multfactoral analyss. Methods: The study was conducted n the town of Lermontov stuated n the area wth hgh background radon concentratons and lung cancer rates of the Caucasan Mneral Water Regon of Russa. Hgh ndoor radon levels were found n the houses of urban resdents, mostly employed by the mnng and chemcal enterprse. The cohort conssted of 122 lung cancer cases and 208 controls. Each of 330 study partcpants was characterzed by a set of 23 ndces reflectng known lung cancer rsk factors. We also collected data on occupatonal and resdental radon exposure of all subjects. Results: The analyss of a combned effect of 23 dfferent lung cancer rsk factors based on pattern recognton methods showed that the contrbuton of the non-occupatonal radon exposure was only about 2% whereas that of the occupatonal radon exposure equaled 15%. Concluson: Our fndngs showed that the effect of the resdental radon exposure on the lung cancer rate was tmes weaker than the effects of the man rsk factors such as smokng, occupatonal hazards, chronc lung dseases, socal and household factors, etc., although for the populaton of Lermontov ths factor was 2-3 tmes stronger than that found n the Ural towns of Russa. Key words: Lung Neoplasms; Rsk Factors; Russa; Ar Polluton, Indoor; Occupatonal Exposure Arch Oncol 2011;19(1-2):3-8. UDC: : :615.07(470) DOI: /AOO L 1 Insttute of Industral Ecology, Ural Branch of the Russan Academy of Scences, Ekaternburg, Russan Federaton, 2 Scentfc and Practcal Center Uralmedsoceconomproblem, Ekaternburg, Russan Federaton, 3 Tobolsk Bologcal Staton of the Russan Academy of Scences, Tobolsk, Russan Federaton Correspondence to: Vladmr L. Lezhnn, Insttute of Industral Ecology, UB RAS, Offce 202, 20, Sofa Kovalevskaya Street, Ekaternburg, , Russan Federaton lezhnn@ecko.uran.ru Receved: Provsonally accepted: Accepted: INTRODUCTION Carcnogenc rsks of the radoactve radon and ts decay products are extremely mportant but, at the same tme, poorly studed. Epdemologc studes of uranum and other mners showed that the occupatonal exposure to hgh radon concentratons posed the rsk of lung cancer. Based on epdemologc and expermental data, the experts of the Internatonal Agency for Research on Cancer stated that radon and ts decay products were carcnogenc to humans (1). Specal attenton s pad to ths problem due to hgh concentratons of ths radoactve gas n resdental and publc buldngs that pose the carcnogenc rsk for a sgnfcant part of populaton rather than occupatonal hazards for small groups of mners (2-4). Yet, results of numerous rsk assessment studes are nconsstent. Stdley and Samet analyzed 15 radaton epdemology studes, and noted that only seven of them demonstrated the correlaton between radon and cancer; sx studes showed no relatonshp at all and two studes even revealed a negatve correlaton between ndoor radon concentratons and lung cancer (5). Ten years later Krewsk et al. reported smlar fndngs when they summarzed the results of 20 epdemologc studes conducted n dfferent countres (6). Many specalsts beleve that contradctory results of these studes are the result of method errors made by the researchers, and ths s one of the man reasons for the remanng uncertanty n the ssue of carcnogenc rsks of the resdental radon exposure (7-10). Our own experence n applyng multple factor technques of the epdemologc analyss gves grounds to consder ths pont of vew well-founded. Results of our earler radaton epdemology studes based on the multple factor approach gave evdence that the effect of ndoor radon concentratons < 100 Bq/ m 3, observed n two towns of the Sverdlovsk Regon, Russan Federaton, was extremely low on the ncdence of lung cancer. Yet, when the same data were fed n the radaton rsk model BEIR VI, whch s now consdered to be most precse n the carcnogenc rsk assessment of radon exposure, the results appeared to be several tmes hgher (2). Addtonal arguments were necessary to confrm ths concluson and the queston about the extent of carcnogenc rsks of envronmental radon exposures that were hgher than those found n the study areas of the Sverdlovsk Regon remaned open. In order to elmnate exstng uncertantes we conducted addtonal radaton epdemology studes n one of the Russan regons wth a hgh background radon hazard; we used the multple factor analyss. Materals and methods The study was conducted n the town of Lermontov stuated n the North Caucasus of the Russan Federaton (the Caucasan Mneral Water Regon). At the tme of the study the sze of populaton n the town was 24,898 people. Lermontov s characterzed by hgh cancer ncdence rates and a hgh ntensty of radon emssons from sols n resdental areas. The man sources of the hgh background radaton are elements of natural radonucldes such as uranum-238 and thorum-232 and decay products of radon and thorum found n buldng materals and sol under resdental 2011, Oncology Insttute of Vojvodna, Sremska Kamenca 3

2 and publc buldngs. A mnng and chemcal enterprse Almaz n the vcnty of the town also contrbutes to the background radaton snce talngs and dumps of radoactve rocks have been left after closng of the enterprse. As a result, a unque for Russa radaton epdemology stuaton has formed n the town of Lermontov n the past 50 years causng medcal and socal tenson among the resdents and requrng a scentfcally founded concluson about ts carcnogenc rsks. At the same tme, whle developng the latter t was necessary to take nto account the fact that from 1950 to1990s a sgnfcant part of the town populaton worked at the Almaz, the frst Russan uranum mne and mll, and was occupatonally exposed to hgh levels of radaton related to radon and ts daughters. The radaton hygene expert examnaton of the study area and of the type of ts resdental development enabled us to select four urban sectors wth account for the followng features. The frst sector represented the oldest low-storey (1, 2 and 3-storey) part of the town constructed before The dstnctve feature of the old resdental development was the use of local buldng materals and wastes of uranum mnes wth a hgher level of natural radonucldes compared to other parts of the town. Besdes, many resdental buldngs had no basements and stood mmedately on the ground. The second sector ncluded the part of the town developed after 1970 wth modern mult-storey (5- and 9-storey) buldngs constructed of envronmentally frendly materals wth low concentratons of natural radonucldes brought from other regons of Russa. The radaton control was performed durng the development. All buldngs of ths sector have unnhabted basements preventng the penetraton of radon to lvng quarters. The thrd sector ncluded prvate, manly 1-storey houses bult at dfferent tme perods and the qualty of buldng materals was poorly controlled. Most houses were constructed of local buldng materals. Some houses of ths sector have basements openng to the nhabted rooms. The sgnfcant part of the sector has a hgh level of standng groundwater and ths can be a determnant of the process of brngng the radoactve radon gas to lvng quarters. The fourth sector conssted of prvate houses of Ostrogorka vllage joned to Lermontov n Ths sector had been manly bult of envronmentally frendly materals wth low concentratons of natural radonucldes brought from other regons of Russa under proper radaton control. Basements were well solated, thus preventng the penetraton of radon to nhabted spaces. The dstnctve feature of Ostrogorka was that not all houses were connected to the central water supply system and 15% to 20% of the populaton used water from wells and artesan wells for drnkng and cookng. So the exposure of the populaton of Lermontov to onzng radaton vared dependng on the sector and the type of buldng materals of ther resdences. Takng nto account the fact that gamma radaton can also contrbute to the total background radaton n the town, we establshed quanttatve ndces of ths type of the radaton exposure. Table 1 shows dose rates establshed by SanEpd specalsts n Lermontov n the course of the pedestran g-survey. Accordng to them, n a half of the town the rate of the ambent equvalent of the dose of g-radaton exceeded 0.30 μsv/hour and n 2% (about 4.4 hectares) t was over 0.60 μsv/hour. Table 1. The rate of ambent dose equvalent n the town of Lermontov Dose rates, μsv/hour The percentage of the town area < Average: The analyss of medcal statstcs showed a hgh cancer prevalence rate n Lermontov compared to that n the Stavropol Regon as a whole. In 2003 the rate of mortalty from cancer of all stes was 255 per 100,000, and that from lung cancer 58 per 100,000. The comparson of the standardzed rates of mortalty from lung cancer n Lermontov and n the Stavropol Regon showed that the former s 1.5 tmes hgher (for the general populaton), and 1.9 tmes hgher when recounted for the male populaton. The occupatonal pathology of Almaz workers was analyzed for the perod from 1980 to 1990, the years of the sustanable and stable operaton of the uranum mne and mll. Lung cancer ranked frst n the structure of the occupatonal pathology (58%), and the percentage of other occupatonal dseases, such as slcoss, dust bronchts, and occupatonal hearng mparment, vared from 4% to 8%. The study was based on the followng prncples: 1. Accordng to avalable publshed data the man medco-bologcal effect of the exposure to radon and ts daughters, s the development of lung cancer. At the same tme, lung cancer, just lke the majorty of other tumors, belongs to the dseases nduced by a combnaton of dfferent factors ncludng medco-bologcal, occupatonal, socal, household, envronmental, and others. To our opnon, such a multple factor condtonalty of lung cancer makes tradtonal sngle factor epdemologc technques neffectve. The reason for lmtatons s that case-control studes enable one to elmnate the effect of 2-3 confounders whereas t may be assumed a pror that effects of a much greater number of confounders must be adjusted for. To our mnd, ths very crcumstance leads to such polar rsk estmates of envronmental radon exposure obtaned n the result of sngle factor epdemologc studes. The most adequate to the task n queston s the epdemologc study based on the technques of a multvarate analyss. In ths case adjustment for confounders s unnecessary because the whole aggregate of data s analyzed at a tme. 2. The study must be conducted at the ndvdual level because such an approach to collectng epdemologc data yelds most correct results. Thus, specalsts thnk (10) that the man drawback of wellknown studes of Cohen et al. (11) s the substtuton of ndvdual values and characterstcs by values averaged for U.S. admnstratve dstrcts. 3. Drect measurements of ndoor radon concentratons shall be used to establsh the level of the radaton exposure of a person. The case group ncluded 122 people wth lung cancer dagnosed n the perod All dagnoses were thoroughly verfed by nstrumental methods of examnaton and confrmed morphologcally n over 70% of 4

3 cases. The control group ncluded 208 people randomly chosen from the populaton of the town wth account for the followng crtera: (a) sex and age structure corresponded to that of the adult populaton of the town as a whole; (b) t ncluded people resdng n all parts of the town n proporton close to the exstng terrtoral dstrbuton; (c) the occupatonal structure of the control group was consstent wth that of the town. As a result, when formng the control group for our study we dd our best to brng t n conformty wth the age, sex, and occupatonal structure of the populaton of Lermontov. Each of 330 study subjects was characterzed by a set of 23 ndces reflectng well-known rsk factors of lung cancer. In the course of the study we collected personal data on such factors as sex, age, natonalty, occupaton, the duraton of occupatonal exposure to carcnogens, a famly hstory, chronc lung dseases, bad habts (smokng and alcohol abuse), socal and household condtons (the floor of lvng, a gas stove n the ktchen, the type of buldng materals of the house, lnoleum floorng, the source of drnkng water supply), and the data on occupatonal and envronmental radon exposure of all subjects. One of the most mportant parts of our study was measurng ndoor concentratons of radon and thoron. To estmate radon concentratons we used archve data of radon surveys conducted by specalsts of the santary and epdemologc survellance center of Lermontov n the perod usng coal accumulaton chambers and results of specal measurements taken by ntegral track radometers of radon n the perod The perod of exposure of coal accumulaton chambers and ntegral track radometers of radon was one week and 1 to 3 months, respectvely. Measurements were performed n bedrooms and lvng-rooms,.e. n the rooms where people spent most of ther tme. The results were brought to annual average values of the radon concentraton by usng a temperature normalzaton, whch had been verfed based on a sgnfcant number of test results of SanEpd specalsts n Lermontov. In order to estmate the equvalent equlbrum concentraton of radon we used the equlbrum factor 0.4 usually appled for regons wth a warm clmate. At the ntal stage of the study we took random one-tme measurements of thoron concentratons usng a Markov-Terentyev aspraton method. Based on those results ndoor levels of accumulaton of thoron daughters n Lermontov were found nsgnfcant (<2 Bq/m 3 ) and so we dd not measure them n the rest of homes. Table 2 provdes data on ndoor radon concentratons measured n resdences of cases and controls. Table 2. Dstrbuton of average ndoor radon concentratons n Lermontov Cohorts Arthmetc mean, Bq/m 3 Geometrc mean, Bq/m 3 The percentage of resdences wth radon concentratons >100 Bq /m 3 The percentage of resdences wth radon concentratons >200 Bq/m 3 Cases % 17% Controls % 12% Based on archved and our own results of measurng ndoor radon concentratons, we assessed the exposure of cases and controls. We used a common unt workng level month (WLM) that s numercally equal to the exposure to 3,700 Bq/m 3 of radon n equlbrum wth daughters for 170 hours (the workng tme per month). We estmated the exposure D (WLM) accordng to the number of years T of lvng at each address: D EEC 7000 T = The assessment of occupatonal exposure was performed based on work hstores of Almaz employees. Those hstores contaned personal data on occupatonal hazards and adverse labor condtons ncludng radon concentratons at workplaces. For the multple factor analyss we appled a mathematcal tool of pattern recognton. The followng tasks were solved durng the data processng: (1) Evaluaton of suffcency of the selected set of factors for a relable descrpton of dfferences between observatons for cases and controls. (2) A quanttatve evaluaton of nformatonal content of each factor nterpreted as a power of ts effect. (3) Determnaton of the character (drecton) of the effect of each factor, whch wthn the framework of ths study can be nterpreted as an ncrease or a decrease n the probablty of the factor-related dsease. The frst task was solved usng methods of the dscrmnant analyss. Its essence s as follows. A part of observatons for both cases and controls (here, 15%) s allotted for an examnaton. Then the computer s taught based on the rest of observatons that form a so-called teachng sample and the dscrmnant functons (decson rules) dvdng analyzed classes are bult. The crteron of qualty of dvdng classes s the percent of correctly recognzed observatons of the examnaton sample. It s natural to suppose that the hgher s the percent, the more substantated s the concluson that the selected set of sgns (factors), wthn the scope of whch the decson rule was developed, does characterze the observatons belongng to dfferent classes. Solvng the task of teachng the computer s usually preceded by the procedure of evaluatng nformatonal content of sgns that allows one to rank them by ther usefulness for dvdng the gven classes and to determne the number and lst of sgns accordng to whch the dvson of the classes under study wll be performed. To evaluate the nformatonal content of sgns we used a method based on calculatng the dfference n the frequences of occurrence of sgns n samples of the classes under consderaton. In accordance wth ths algorthm the range of values of each sgn s dvded to z ntervals and then the relatve frequency of occurrence of the values of the sgns under study from vectors of dfferent classes n each of these ntervals. The nformatonal content of sgn s estmated usng the formula (2): 2 z k 1 k = + J ( ) ( ) = pl, t pm, t, k( k 1) (2) t= 1 l 1 m= l 1 ( ) ( ) Where, p, p l, t m, t are the frequences of occurrence of values of -sgn from vectors of classes l and m n the t-nterval. The number of ntervals z depends on the sze of class samples and s calculated n the KVAZAR package usng a well-known formula lnm, where m s the sze of the smallest sample. (1) 5

4 The obtaned values are normalzed as follows: mn J j H j 1, n 1, n : J = (3) max J j mn J j j 1, n j 1, n And they form relatve values of the nformatonal content. At ths the most nformatve sgn s gven the value 1 and the least nformatve 0. As a result, all sgns become normalzed by the extent of nformatonal content n the decreasng order n the range of 1 to 0. The frst n sgns are used as the desred subsystem. Snce the optmal value n s unknown n advance, teachng s usually performed n several dfferent subspaces wth dfferent n. The best soluton s chosen out of all obtaned. The drectvty of effects of factors was assessed based on the analyss of ther average values n the samples of classes under study. All tasks were solved n the package of appled programs KVAZAR (12). It was nterestng to compare results of drect epdemologc studes of the radon exposure and lung cancer wth the results of rsk assessment obtaned usng specal mathematcal models. To do ths, we made calculatons usng a multplcatve model BEIR VI, whch s consdered by many specalsts to be most approprate for assessment of the rsk of radaton-nduced lung cancer (2). Results and dscusson Durng the mathematcal data processng we frst decded on the suffcency of nformaton about 23 examned factors for a relable descrpton of dfferences between classes. The best result of recognzng the examnaton sample 92.3% was obtaned when usng 13 most nformatve sgns. On the whole, decson rules correctly recognzng 87% to 92% of vectors of the examnaton sample were obtaned usng dfferent subsets of 11 to 22 sgns. The fact that hgh results were obtaned durng recognton of the examnaton materal gves grounds to state that the selected complex of ndces ncludes the most mportant rsk factors of lung cancer for the contngent under study. Besdes, t should be noted that these results were obtaned by usng three dfferent algorthms of recognton, whch certanly makes them more trustworthy. Usng a specal algorthm (13) we estmated the confdence nterval of the results of recognton that equaled 81-96%. On ts bass we can state that the sze of our samples was qute suffcent for obtanng relable results. At the second stage of the mathematcal processng of epdemologc data we analyzed the power of effect of each factor on the development of lung cancer. Table 3 presents aggregated data on effects of the factors consdered n the epdemologc study on the development of ths pathology. In ths table the factors are ranked n the decreasng order by ther nformatonal content values calculated based on dfference n frequences of occurrence. Besdes, the table contans values of Student t crtera for each factor and ranks correspondng to the crtera. The last column shows the drecton of the effect of each factor: the plus ndcates a postve lnk between the factor and cancer, and the mnus a negatve one. The well-known rsk factors of lung cancer such as duraton and ntensty and smokng, sex, age, and chronc lung dseases ranked hghest. Workng at Almaz, the sgn ndcatng the occupatonal exposure to radon, J ranked among the hghest. At the same tme the ndcator that drectly characterzed the occupatonal radon exposure ranked 15, and accordng to Student t crteron even 18. We are of the opnon that the reasons for such nconsstency mght be due to nsuffcent relablty of the nformaton taken from archved data and appled to the assessment of the occupatonal radon exposure. The parameter characterzng the radon exposure n the household ranked 19 and 20 by Student t crteron. The parameter of the dstrct of Lermontov where the subject lved also ranked low. On the whole, f we consder the total effect of all factors under study on the development of lung cancer n the resdents of Lermontov as 100%, then the contrbuton of the envronmental radon exposure was 2%, whereas that of the occupatonal radon exposure exceeded 15% (Fgure 1). Smokng 23% Occupatonal radon exposure 15% Other factors 24% Sex 14% Age 7% Chronc lung dseases 15% Envronmental radon exposure 2% Fgure 1. Relatve contrbuton of the man groups of factors n the rsk of lung cancer for the populaton of Lermontov When assessng the character of effect of each factor we generally obtaned qute predctable results: men were at hgher rsk of lung cancer; as expected, ths rsk depended on the duraton and ntensty of smokng and on age; t was hgher n people wth chronc lung dseases and ex-workers of Almaz. The only excepton n ths case was the factor of havng relatves wth the verfed dagnoss of cancer that showed an nverse relatonshp wth dseases of the observed cases. The reason for such a contradcton mght be an nsuffcent relablty of nformaton about ths factor. Fgures 2 and 3 show that there exsts a drect correlaton between the rsk of lung cancer and the level of both occupatonal and non-occupatonal radon exposure controls cases < >300 WLM Fgure 2. Dstrbuton of lung cancer cases and controls dependng on the level of occupatonal radon exposure 6

5 Table 3. Informatonal content and drectvty of factors contrbutng to the lung cancer rsk for the populaton of Lermontov, Russa Factors Codng and unts Informatonal content by the dfference n frequences The rank by the dfference n the frequency of occurrence Student s t crteron The rank by Student s t crteron Vectors of effects of the factors Smokng hstory Years Age Years Male gender 0 no, 1 yes Employment at Almaz 0 no, 1 yes 0, Smokng ntensty 0 - none, 1 <10 cgarettes a day, 2 10 to 20, 3 - > Exposure to occupatonal hazards Years Smokng at home 0 never, 1 occasonally, 2 constantly Chronc lung dseases 0 no, 1 yes The area of the dwellng space per person In m Alcohol consumpton 0 none, 1 rare, 2 once a week, 3 more than once a week, almost daly The floor of lvng Floor No Famly hstory The number of relatves wth cancer Type of heatng 1 central, 2 gas, 3 oven Duraton of lvng n Lermontov Number of years Occupatonal radon exposure In WLM unts Local buldng materals n the resdence 0 not used, 1 sand, clay, etc., 2 uranum wastes Dwellng n the Lower town 0 no, 1 yes Dwellng n the prvate sector 0 no, 1 yes Resdental radon exposure In WLM unts Type of water supply 1 centralzed, 2 a well Ktchen equpment 1 an electrc stove, 2 a gas stove, 3 an oven Dwellng n Ostrogorka 0 no, 1 yes Dwellng n the Upper town 0 no, 1 yes Notes: 1. Values of the nformatonal content of factors are gven n relatve unts. 2. The + ndcates the postve lnk between the factor and lung cancer and - a negatve one < >200 WLM controls cases Fgure 3. Dstrbuton of lung cancer cases and controls dependng on the level of envronmental radon exposure Apart from drect epdemologc studes the data were analyzed usng the methodology of rsk assessment wth the multplcatve model of radaton rsk BEIR VI appled. Accordng to the results produced by ths model the contrbuton of envronmental exposure to the rsk of lung cancer among the populaton of Lermontov was 30%, and thus t could be referred to one of the man factors nducng lung cancer n the resdents of Lermontov. At the same tme the results of the multple factor analyss gave evdence of ts low contrbuton to the development of lung cancer. The followng conclusons can be made based on the study results. One of the man rsk factors of lung cancer n Lermontov s the occupatonal radon exposure (the major part of the populaton used to work for the uranum mne and mll). The contrbuton of the envronmental radon exposure, n ts turn, was sgnfcantly lower. Takng nto account the fact that the town of Lermontov s characterzed by one of the hghest levels of natural background radaton n Russa, t may be assumed that on 7

6 the whole the value of ndoor radon as a carcnogenc rsk factor n the country s small. The study results show that the effect of the ndoor radon exposure on the ncdence of lung cancer n the town of Lermontov s 15 to 20 tmes lower than that of the man rsk factors (Table 3), even though t s 2 to 3 tmes stronger than that found n the Ural towns (8, 14). Based on these facts we thnk t expedent to preserve the exstng system of radaton hygene control n Lermontov and to contnue mplementng the program of decreasng ndoor radon concentratons. At the same tme for the ndvduals makng a short vst to the town (.e. for holdays) the exstng background radaton poses no carcnogenc rsk at all. The followng arguments for the relablty of our results can be offered: A sgnfcant degree of ther consstency wth the fndngs of smlar studes conducted n the Ural towns of Pervouralsk and Karpnsk (14); The achevement of hgh results of recognzng the case and control groups when usng algorthms of pattern recognton based on dfferent prncples; The confdence nterval of obtaned values enablng one to consder the sze of the case and control groups suffcent to obtan statstcally sgnfcant results; The fact of establshng the expected character of the relatonshp between lung cancer and many well-known carcnogenc rsk factors unrelated to radaton such as smokng, occupatonal exposure to carcnogens, age, chronc lung dseases, etc. durng the systems analyss. In concluson t should be noted that the results of the mult-factor epdemologc study are consstent wth the opnon of those specalsts who nsst on a weak effect of the radaton related to low ndoor concentratons of radon and ts daughters on the development of lung cancer (7, 9). At the same tme the comparatve analyss of the contrbuton of the radon exposure to the development of lung cancer among the resdents of Lermontov and two towns of the Sverdlovsk Regon gves evdence of the ncrease n the rsk of lung cancer wth the ncrease n the ndoor radon concentraton even though ts value s generally small. 5 Stdley C, Samet J. A revew of ecologc studes of lung cancer and ndoor radon. Health Phys. 1993;3(5): Krewsk D, Lubn J, Zelnsk J, Alavanja M, Catalan V, Feld R, et al. Resdental radon and rsk of lung cancer: a combned analyss of 7 North Amercan casecontrol studes. Epdemology. 2005;2(16): Kerm-Markus I. New nformaton about human health effects of low doses of onzng radaton the crss of the rulng concept of regulatng radaton. Medcal Radology and Radaton Safety. 1997;2: In Russan. 8 Lezhnn V, Polzk E, Kazantsev V, Yakusheva M. A system analyss n cancer epdemology. Ekaternburg: UB RAS; In Russan. 9 Yarmonenko S. Problems of human radobology n the end of the 20 th century. Medcal Radology and Radaton Safety. 1998; 1(43):30-6. In Russan. 10 Lubn J, Boce J. Lung cancer rsk from resdental radon: Meta-analyss of eght epdemologc studes. J Natl Cancer Inst. 1997;1(89): Cohen B, Coldtz G. Test of the lnear-no threshold theory for lung cancer nduced by exposure to radon. Envron Res. 1994;1(64): Kazantsev V. Tasks of classfcaton and ther software (package KVAZAR). Moscow: Nauka; In Russan. 13 Duda R, Hart P. Pattern classfcaton and scene analyss. New York- London- Sydney-Toronto: A Wley-nterscence publcaton; John Wley and sons; Polzk E, Lezhnn V, Kazantsev V. To the ssue of evaluatng the radon effects on lung cancer. Radaton Bology Radoecology. 2004;2(44): In Russan. Conflct of nterest We declare no conflcts of nterest. References 1 IARC monographs on the evaluaton of carcnogenc rsk to humans. Man - made mneral fbres and radon. Monograph Vol. 43. Lyon: IARC; Natonal Research Councl. Commttee on the Bologcal Effects of Ionzng Radaton (BEIR VI). The health effects of exposure to ndoor radon. Washngton, USA: Natonal Academy Press; Lubn J, Wang Z, Boce J Jr, Xu Z, Blot W, De Wang L, et al. Rsk of lung cancer and resdental radon n Chna: pooled results of two studes. Int J Cancer. 2004;109: Darby S, Hll D, Deo H, Auvnen A, Barros-Dos J, Baysson H, et al. Resdental radon and lung cancer detaled results of a collaboratve analyss of ndvdual data on 7148 persons wth lung cancer and persons wthout lung cancer from 13 epdemologc studes n Europe. Scand J Work Envron Health. 2006;32(1):