Document Gz61wJEoyQwe7p3Dm57RXD2Zv

REVIEW Mesothelioma: cases associated with non-occupational and low dose exposures Gunnar HiUerdal Abstract men 5--10 times higher (see table 3). Even in Objectives--To estimate the importance cohorts with a very heavy exposure to asbestos of low dose exposure to asbestos on the most people will die from other causes. In peo risk of mesothelioma. ple with certified asbestosis'--that is, with a Methods--A review of the literature. heavy exposure--up to 10% will develop mes Results and conclusions--There is no evi othelioma; among insulacors in the United dence of a threshold level below which States and Canada, also a heavily exposed there is no risk ofmesothelioma. Low level group, 9.3% of che dcachs have been due to this exposure more often than not contains disease; and in amphibote miners in South peak concentrations which can be very Africa or Australia, this figure is 2--4% (table high for short periods. There might exist a 1). Clearly, with exposure concentrations background level of mesothelioma occur several magnitudes lower, as occurs in the gen ring in the abscence of exposure ot asbes eral population, the risk is very small, often tos, but there is no proof of this and this impossible to measure. "natural level" is probably much laww A discussion of the risks from low exposure than the 1-2/million/year which has been must include the dose-response curve; the often cited. existence or non-existence of a threshold, and COccup Bieainn Mid 1999;56:505-513) j thus a background concentration; and should %-cry to define low exposure and estimate to what Keywords: low exposure; asbestos; mesothelioma degree chat really means a low concentration- From conflicting findings and opinions at tempts must be made to make a meaningful Mesothelioma is an incurable disease which is almost exclusively due to inhalation of asbestos fibres. Asbestos has been extensively used in industry and construction in die 20th century, especially during and after the second world conclusion. The different types of asbestos seem to differ considerably in their ability to cause mesothe liomas. Chrysotilc is considered by many authors to be a weak carcinogen in humans," war, and even if the mineral is no longer used in whereas the two amphiboles crocidolite and most rich western countries the total world tremolite are much more dangerous according production remains high. There is a worldwide to many studies.12 The third of the more pollution with asbestos, as indicated by the important amphiboles, anthophyllite, was long Department of Lung Medicine, Harolimlu HotpiaJ, Stockholm, finding of the mineral in samples of Greenland ice! and on the Yorkshire Moors/ and every citizen in the world has been exposed to some considered nor to cause mesothelioma, but such rumours have now been reported al though che risk seems to be small." There is, Sweden G Ktilerdai extent. Consequently, asbestos fibres can be none the less, a minority opinion that chrysodle found in most lungs at necropsy.' It is thus is in fact responsible for most of the pleural Cofrejpondenct to: Dr Gubov HiUerdil, Dqnronent ofLuag Median, Karolinslca Hoipini Stockholm, Swedes. understandable that there is concern about the risk of mesothelioma for the general popula tion. However, it should be remembered that mesothelioma is a rare disease with incidence in industrialised countries ranging from 1 to mesotheliomas in society'4 or should at least be considered to carry the same risk.1' This discussion, however, falls outside che present review and is not important for the conclusions drawn here. Accepted S March : 999 5/million/year among women and values for Definition and diagnosis of mesothelioma TahU l Pivporuonau martahry from moseththoma in various cohorts Mesocheiiomas are, by definition, tumours that arise from mesothelial cells and can thus arise Cohort Amphibote miners CroodoHte otfaca Xnsutebon hauiaMkatatua Deaths (n) isrs ns sldSl 253 40 39 Dvxtu dm co nsesoCAcfwmdt (16) 2 i 6 9 9 10 10 Rdftrmc* 4 3 6 7 a 9 10 from any body cavity: the pleura, che perito neum, the pericardial sac, and even the tunica vaginalis testis. Pleural mesotheliomas are the most common and pathologically the best defined ones. Pleural mesotheliomas have a male to female rate of about five to one, whereas for peritoneal tumours this ratio is 1.5 to 1.14 Thus, either che aetiology is different, the Table 2 Possible risk factors and mediators of risk of mesotheSoma (other than asbestos) Poston Stfenkc Chronic iafianuaation Radiation Beryllium Vegetable fibres Hereditary factors Immunological factor* Dietary factors Virtue! Comments Very high mbdenee ofmeuKhdionu environmental exposure in Turkey Pleural scan (tuberculom, pieurtsy, therapeutic pneumothorax) Single cast! after Thorotmr injection or radiotherapy; causality not proved 0n cate in atomic itomb survivor Tun doubtful area described No proofin humans Familial cases (explained by common asbestos exposure?) Correiaooo with parental cancer Rapidly progressive cases in pabeno with HIV inftcaon Provitamin A, ^-carotene may decrease the risk Mesotheliomas in animals Simian virus 40 DNA sequences reported in mesotheliomas Rsftrmsss 19 20 21 22 23, 18 . 24-26 27 2*29 10 31 32 diagnosis is more difficult in women, or women Incidence ofmesothelioma exposed to asbestos are more likely to develop There is a large variation in the incidence of a peritoneal chan a pleural mesothelioma. In mesothelioma in different countries and in fact, one type of mesothelioma occurring most places a steadily rising number of cases almost exclusively in women is die so called multicysric mesothelioma. This tumour has a better prognosis, is more sensitive to cytostatic drugs, and seems to have no connection with with time. In table 3, the incidence or mortality from mesotheliomas in different countries at various rimes can be seen. As mortality for practical puprposes is the same as die inci exposure to asbestos.17 dence for this disease, both figures have been The pathological diagnosis of mesothelio used in the table. Some of the differences mas requires experience, and confusion can between the countries are probably due to occur with both benign pleural lesions and with diagnostic difficulties, but most of the varia metastatic pleural diseases.11 Many countries tions can be explained by the use of asbestos in now have "mesochelioma panels", which has the particular society some decades earlier. meant a great improvement in the diagnosis. In most national cancer registries, most likely both overdiagnosis and underdiagnosis occur. Dose-response and latency time Most researchers agree that there is a positive dosc-rcsponse curve for mesochelioma--the Suggested causes of mesothelioma other ^heavier the exposure to asbestos, the greater than mineral fibres risk. This is found in cohort studies as well Apart from mineral fibres, radiarion (for exam as in analyses of amphibole asbestos fibres in ple, through the contrast medium Thorotrast) the lungs.1*-14 It was realised early that rime has been suggested to cause human mesothe since first exposure was of great importance, lioma. However, in a large retrospective study and therefore the "Cubic residence-time of more than 250 000 women treated for model" was suggested by Doll and Peto in their mammary carcinoma, a quarter of them report in 1985": initially with radiotherapy, no association I CT) = C x F x CT4-(T-D)4 between radiation and mesochelioma could be Where, I (T)=incidence at the rime T after found.a As can be seen from tabic 2, other exposure; c=a constant depending on the causes or contributing factors have also been process, F=intensrty of exposure, and suggested. D=duration of exposure. This equation has Viruses can cause mesotheliomas in animals, been used in many studies with an acceptable but this has not been described in humans. fit for normal occupational exposure concen DNA sequences associated with Simian virus trations . F in the equation is the total 40 (SV 40) transforming factors have been exposure--a combination of fibre concentra reported in a high proportion of mesothelio tions and exposure time--usually measured in mas from some countries.'1 This suggests that fibre-years. (1 fibre-year = a mean of 1 fibre/ml there is a connection between SV40, which was air for 1 working year). Thus, the dose- a contaminant of live polio vaccines in response curve is supposed to be linear, but the 1959-61, and later development of mesothe result is heavily influenced by the time factor. lioma. Thus, SV 40 might be a cofactor to Unfortunately, the exact value of F is often asbestos in some patients with mesothelioma , uncertain, even in well defined highly exposed but the results have not been confirmed and are cohorts. The equation is thus rarely useful still disputed. especially with low doses, in which F is usually In summary, then, as far as is known today, a crude guess only. As can be seen from cable 1, factors other chan mineral fibres can only even with heavy exposure only up to 10% of a explain a very small proportion of mesothelio cohort will die from mesothelioma--so the for mas, and can for practical purposes be mula is not applicable in these cases either. disregarded. Thus, a malignant mesothelioma With very heavy exposure, most patients will can be regarded either as caused by asbestos or die from pulmonary insufficiency due to asbes- belonging to a normal background level--that tosis before there has been sufficient rime to is a spontaneously occurring tumour. The rela develop a mesothelioma. tive ixnnporance of these two factors has been Even more troublesome is the time factor T, debated and win be further explored in this which quickly becomes very important in this review. equation. If the equation is correct, the risk Table 3 Incidence or mortality ofmesothelioma in various countries and areas over time (I million inhabitants /year) of mesothelioma before 1946," although such negative evidence is of questionable value. Contwyorawa ' Uriiud States Nonta America NauaSaiiiE.NBiin, France Test Mmrf dries. United States VritciSraa Barcelona, Span Great Britain Ftalaod Gnac Britain Wantcs-Same-Navairc, France Uaictd Kingdom Denmark N*nccamf*Nazarr*, France Great Britain Au*trails Great Britain Great Bricain Aitfnaiu Year S96S-$t 1972 1956-74 1970-50 1970s 1986 1983-90 1966-71 1990-94 \972-76 1975-64 1983 1976-60 2985-92 1968-71 J982-83 1982-66 1987-91 199* Male 2I 2.8 3.2 5.3 4.4.11.1 7-13 8.3 S.4 10 12.6 17.2 17.5 14.7 29.4 20.7 23.3 30. J 44.0 49.9 frynal* 0.8 0.7 0.2 2.1 1.2*3-8 1-2 4.7 2-3 2.9 2-8 0.3 3.2 7.0 4.0 4.3 3.3 4.9 6.4 4.3 mfamc* 33 34 35 36 37 38 39 40 41 40 35 16 42 35 40 43 40 40 44 McDonald and McDonald, in a recent review, estimated the background level to be 1-21 miHion/year; they came to this figure .by extrapolating backwards from epidemiological studies from various countries." * Malignant mesothelioma can occur in children," and such cases can be considered as proof of non-asbestos (spontaneous) aetiology, as the latency time with necessity must be very short in these cases. Asbestos fibres have, how ever, been reported in the lungs of children^ even in stillborn ones," " showing that asbestos fibres spread in the human body and even pen etrate through the placenta. Even if a latency time of only a few years is extremely rare in mesothelioma related to asbestos,4" it can occur. There are some published examples of would increase steeply with time, making early childhood exposure of great importance. However, there are clear indications that mineral fibres clear from the lung, albeit with different half lifes for the different types of asbestos. Chrysocile has the shortest half life, and crocidolice is generally accepted to have the longest. The halflife of crocidolite has been estimated to 7-8 years.11 This clearing of fibres would, at least theoretically, tend to actually latency times of only 5 years." Mesotheliomas also occur in animals, from baboons'7 and domestic dogs"1"1' to fish.71 Dogs are exposed environmentally to asbestos just like their human masters, which might explain some of the tumours," *" ' but in fish it would be difficult to blame asbestos for the tumour. Thus, as in other animals, there is probably a background level of spontaneous mesothelio mas in humans. decrease the risk of mesothelioma and other diseases with time. Levels of exposure In conclusion, the value of the cubic Although many authors write about low level residence-time formula is in practice low and it exposure to asbestos, there is rarely a definition should not be used for extrapolations, at least. of this term. In fact, in many articles low level not at the extreme ends of exposure. " _exposure seems to be synonymous to non- The latency time varies in different cohorts, occupatiooal exposure, which, as described and is dependent on how long a cohort is later, is certainly not true in many cases. Occu followed up. In 370 necropsy cases from Italy, pational as well as non-occupational exposure latency time could be calculated in 312." can be anything from very heavy to very low. Latency time was also dependent on exposure, varying from 29.6 years for insulators (with the Occupational exposure to asbestos highest exposure) to 31.7 in women with It must be realised that occupational exposure domestic exposure. to asbestos occurs or has occurred not only in the "classic" industries, such as asbestos mines - and factories, shipyards, insulating business. The threshold value and the background asbestos cement industry, building and con- concentration strucrion etc, but also in very many other occu- There have been strong arguments for the pations and trades. Examples are pulp and existence of a threshold value (a minimal expo- paper industry,75 oil refineries,74 electrical sure required for development of a industry,75 jewellery workers,1* sugar mesothelioma).In most studies, several refineries,77 and cigarette filter workers.7" Sea- patients with mesothelioma do not report any men and fishermen can have been exposed to occupational or other exposure to asbestos used as insulation in their boats. In the asbestos," 5*"' and thus there seems to be a reprocessed textile industry, bags heavily con- small spontaneous basal or background inci- laminated with asbestos could be reused for denes of the tumour. In a large study from various other purposes, for instance covering England, consisting of 185 cases and 159 con- heaps of rags; in an Italian investigation of such ttols who were very carefully interviewed, 3% an industry, mesotheliomas and lung cancer of the cases (and 27% of the controls) seemed were found to be fairly common among rag not to have any kind of exposure to asbestos." sorters.7' This included domestic and even residential Given the extensive use of the mineral, many exposure. However, it is of course possible that people have been occupationally exposed to some of these background cases might in fact asbestos. This exposure can have been only be due to occupational, domestic, or even envi- brief but perhaps intense during that short roomental exposure, unknown to (or forgotten period. In many or most instances the workers by) the patients themselves. have no idea of The exposure and it can be There arc authors who claim that the impossible or almost impossible to elucidate it. presumed background level must be very low. Also, die level of exposure is often very difficult and retrospective searches for the tumour in to estimate, should (he information be avail- the medical literature yield no convincing cases able. Non-occupational exposure to asbestos URBAN AIR POLLUTION DOMESTIC EXPOSURE The air in cities contains a very low concentra The family of an asbestos worker could be tion of asbestos. Near to construction of exposed to considerable amounts of asbestos demolition work the concentrations are higher. brought home on his working clothes, which all It has been suggested that the air of London too often it was the duty of die wife or daugh during the blitz was heavy with asbestos. ter to clean. Pleural thickening, calcifications, Asbestos is also released on braking, and dose and pulmonary fibrosis have been described in to a motorway the fibre concentrations in die such cases, as well as mesotheliomas."'** air will be higher than the background, but still well below the industrial threshold values. A air pollution prom asbestos mines, risk not often appreciated is in dumps. In the FACTORIES, DOCK YARDS, ETC vicinity of a waste disposal site the concentra Asbestos mines used to be great environmental tion of asbestos fibres can be 10-1000 times nuisances. They could be seen from a distance above the background concentration.41 1 because of the dust cloud. Asbestos fibres can Drinking water can contain asbestos or spread over a distance many kilometres from a asbesnform fibres from natural sources or pol mine, and the tailings from the mines were lution. The use of asbestos cement pipes can used for paving roads, parking areas, play cause a considerable number of fibres in the grounds, etc. In the crocidoiite mining areas in drinking water. Small amounts of asbestos can South Africa, the incidence of mesotheliomas be found in some wines, beers, liquors, and was increased to at least 10 times that expected other beverages, probably deriving from the fil even among women, proving a non- tration process. Fibres chat are ingested will to occupational exposure. However, whether some extent also enter the blood stream as seen some of this exposure was domestic (see later) from animal experiments.*1 *` In humans, there is not clear.*' arc few reports. However, when amphibole Another example is the formerly active fibres were found in drinking water from Lake crocidoiite mine at Wittenoom, Western Aus Superior, a very small portion could be traced tralia. At least 5000 people lived in the in urine," and when chrysotile occurred in the township ofWiccenoom without working in the drinking water, very small amounts were again mines, and in 1993 27 cases of mesothelioma found in urine." However, most ingested min had occurred among these people.' **M It has eral fibres will never be absorbed but will be been estimated that 1.1 % ofchild residents and cleared in- the normal way, and the harmful 1.9 % of the female residents of Wittenoom effects of asbestos in drinking wacer or drinks is have died or will die from mesothelioma, probably minuscule (and much smaller than whereas among the workforce this figure would the risk of drinking the alcohol) - be 6%.' > Even if ingested asbestos is not dangerous, it Around the chrysotile mines in Quebec, has to be realised that it is only a very small Canada, there have been at least 53 occupa portion of the tap water that we actually drink. tional mesotheliomas from the mines, nine Most of it is used for other purposes. Once the domestic, and two with general environmental wacer has dried after washing, cleaning, taking exposure only." In the nearby towns, she lungs showers etc the asbestos fibres will spread in of residents who have never worked in the die air. Such amounts are small but not mines have a fibre concentration which is 10 insignificant and will add to the normal rimes higher chan that of the average background exposure.** Canadian." . Mesotheliomas have also been reported from the surroundings of asbestos factories*' and in people living near dockyards. However, the risk from residential exposure is probably low. In a ASBESTOS IN PLACE = BUILDINGS CONTAINING ASBESTOS A mixture containing asbestos was popular after the second world war for spraying on ceil large study from England, this factor ac ings and walls for insulation and decoration. counted for only 3% ofall the identified cases.'* This was used well into the 1960s. There is now a public danger because of the plaster fall consumer goods ing down from natural wear and tear, vandal Asbestos has been introduced into various ism, or "artistic" carving in schools, etc. In goods used by the public. Examples are wail many Industrial buildings, asbestos was also paints and spackiiag and jointing compounds. used for spraying on the underside of the roof, Another is a hand held hair drier, of which 13 and with natural wear and tear (and for million were sold in the United States and instance birds building nests!)** there is now which has been described as a "small asbestos release of fibres to the surroundings. Asbestos spray gun".5* In commercially produced Kent was also extensively used in walls or around filter cigarettes, crocidoiite was used in the fil plumbing for insulation purposes, in cement to ters from 1952-6. The sale of these cigarerres was 11.7 billion, and in the commercials the health effects of this filter was emphasised.7" " strengthen it, or just as a cheap filler. In a modern city, asbestos can often be found in many places: in the cellar, where steam pipes Once asbestos is introduced into a home, it are insulated; in storage or laundry rooms; in will spread to all rooms and is almost impossi air conditioning sets; in theatres, museums, ble to remove even with a vaccum cleaner. It restaurants, etc.4" Whenever there is damage to will easily be disturbed into die air from the any construction or machine that contains Slightest movement, and sedimentation is very asbestos, the possibilities of exposure, some Slow.** times even fairly high, is there. Table 4 Reported mesothelioma cases after exposure to "asbestos m place" Endemic plaques are ofinterest also in other Occupation or txposnrs Comments Reference countries, as many people bom in these places and living there in their childhood and youth School teachers Attended school Office clerk Female oAce worker Asbotos imuUtion it tame 9/437 patunG with mesothelioma 1 case each 4 cases 6/262 patients with mesothelioma 99 100-103 104 102 102 1C 100 now have moved to other places, taking with them not only the plaques but also the risk of mesothelioma.01 , Concentrations of exposure OCCUPATIONAL CONCENTRATIONS It has been claimed that up to 1000 prema ture deaths from lung cancer or mesothelioma will occur >n the future among school children from schools where asbestos was used in the walls'0--calculations which, however, had to be built on extrapolations and assumptions. Sev eral case reports have been published on patients with mesothelioma, in which the only exposure to asbestos that was reported was "in place" (table 4). From various cohorts with such exposure, significant increases in radio logical findings from the lungs--such as pleural plaques--have also been reported, indicating exposure ot asbestos, but these results arc not undisputed and there is a probable overdiagr.osis, as control groups are missing.l<rT The concentration of exposure which the first workers exposed to asbestos have experienced can only be guessed. Estimated or recreated values from the past suggest fibre concentra tions from 25 up to occasional values of 1000 2000 fibres/ml. With his own recalculations, Harries in 1970 estimated the fibre concentra tions in the dockyard in 1951 as follows: sprayed asbestos insulation 171--322 fibres/cc; stripping asbestos 334; sweeping 353; adjacent passage 83; and in die passageway to die shower 25- Bagging debris 564; pipe lagging 194-200; removal of pipe lagging 171. Snap samples showed values to up to 1000--2000 fibres/'cc.I): These values are similar to the ones published by McMillan in 1983, who recreated values from the past: engine room 88; delag ging in boilers room 171; bagging debris 353 ENVIRONMENTAL MESOTHELIOMAS FROM LOCAL fi'ml. DErosrrs of fibrous minerals Measurements from working places in the "Endemic pleural plaques" were first described 1960s often showed peak doses of 20 fibres/ml from Finland and since then many such and much less in more recent years. Where findings have been reported. In these areas, asbestos is still used, many countries have there are small local pockets of asbestos which adopted a concentration of 1-2 fibres/ml as the sometimes have been quarried, often for,- upper legal concentration of exposure. These generations, for some local use. The most"-:? figures should be compared with the few avail common use is whitewashing of houses with able non-occupational measurements (table 6). tremolite, which has resulted in an extremely A problem with the legal concentration is chat high incidence of mesothelioma in some most asbestos use today occurs in developing villages (table 5). When the exposure is due to countries, many of which nave adopted stand whitewashing of the houses the risk will disap ards which they cannot enforce. As a result, pear when this procedure is stopped, but due to actual exposures may be much higher than the the long latency time this will take many standard in these countries. decades.l= A non-asbestos fibre, the zeolite erionite, has NON-OCCLTATTONAL CONCENTRATIONS been found in some Turkish villages- Roads, The fibre concentrations in domestic exposure buildings, etc, can contain this fibre in small might in fact be as high as in occupational amounts. Erionite is even more dangerous than exposure. Brushing clothes might give peaks of crocidolite and die incidence ofmesotheliomas S100 fibres/ml.'3 Ordinary vacuum cleaning is in these unfortunate villages is extremely high. not effective in removing asbestos fibres, which Table S Local deposits of mineralfibres (asbestos oreriordtej, occurrence ofplaques, and ofmati^nosu mesothelioma Country Or ana Afghanistan Austria Bulgaria Corsica Cyprus Czechoslovakia Finland Greece: Maxsov Type offibre Tremoiic* Tremolite AsthophylUte Tremolite Trcmolixc Tremolite Unknown AnthophyUite Tremoihc SV Andes, Macedonia New Caledonia x2 STUoutftchryAftica USSR . Amostce oocidotite ETrmionnoirbete Unknown '12 detail43 000 iatabaaatallO y. fOeeuatenee m tool m. Plaques (% of btvcsngassd mAoiuantt) _ 3.3 2.3 women 5.6 men 41 >50 j) 2.7-6.6 4.5-9.0 46.9 24.2 >40 y) 2.J-4.S 1.2-23 63 Locally high Metothetioma rislt (xtttTty) _ Mot increased Not increased High High Net increased 2S0 140 (1985-94) High yjot ffigh High Comment Cue lgpptc only Viceyard md add workers Tobacco j^uywuy General pottudon General poihmon Farmers washing houses White washing house* Population srooad mine White washing houses Farmers Reference 108 109 110 111 112 113 114 115 1H 117113 119-121 122 123 124 125 4 129-128 19, 128,129 130 6 Fibre eonecnmuions in air and lungs with nan-acmpadonal cxposurt to asbestos ifexfO,un Dommdt estpottae (petaocaspadoaol) NUleWaritMeebfcloaooms:aiaci. booties, etc. When route opeminf Aftodnun 0tsxbne fwflwi near mine in Canada Local aobexoi fittCiQfs: Greece: In the yard ofan abandoned house la a needy whitewashed room Corsica Naw Caledonia: While sweeping floor Road dust eiouds Asbestos in place: Teacher's aid Female oifte* worker Fibres its air (find) 0.5 0.01-0.21 0.01 0,02-17.9 78 0.06-0.67 Fibers a lungs Qkltftf dry tame) t.y-3t*.s 0.0049 Refemta 11 102 48 1.2-26.8 134 122 2i*n 114 124, 125 0.0043 31 (TEM) 102 103 39 ngim 100 times higher than control*; TEM = transmission electron aiicroseupy. Fibres in the lungs ofpatients with nan-occupational mesothelioma As has been mentioned, asbestos fibres can be found in the hings of the general population without any known exposure. In Germany; the upper normal limit was estimated, to be 300 000 fibres/g dry lung. In a large study of 324 malignant mesotheliomas* from which 40 lung samples were available, it was found that even at a fibre concentration of 100 000-200 000 fibres/g, there was a fivefold increased risk of mesothelioma, which was significant.0* In this study (which has unfortunately only been pub lished in German) as in many others, the mean number of fibres in the lungs of patients with malignant mesothelioma is much higher than the normal values, but there are usually patients with values that lie within die normal level. Can remain for years in the house and be Discussion airborne again whenever disturbed. Thus, Any asbestos fibre found in a lung must have domestic exposure is not low exposure- been inhaled. As far as is known, no truly Environmental concentrations in the villages unexposed group can be found in the world. where whitewashing occurs, low values are There is no proof of a threshold value--that is, reported when there is no disturbance, but in a a minimal lower limit below which asbestos newly whitewashed room and while sweeping fibres cannot cause che tumour--and thus it is floors, the concentrations can be quite consid plausible chat even such low exposure can erable (able 6). cause mesothelioma (even if the risk is With asbestos in place, as long as the asbes extremely tow). Patients with mesothelioma tos is undisturbed the concentrations in the air whose lungs show- fibre concentrations within are very low (zero or hardly measurable: the normal range cannot be dismissed as back ^0.001 fibres/ml), but once deterioration takes ground cases,--that is, not due to asbestos. place values can go up to 15 fibres.-ml, ar.di when being removed there can be ever, higher ; values. In schools in the United Stares, the mean concentration was estimated to be 0.003 - The only way to prove such a hypothesis would be to compare the incidence of mesothelioma in a group with Such background exposure with the incidence in a truly non-exposed group. This is not possible, as no such group can be fibres/ml, and in federal buildings 0.006.'" found. Thus, with so called aon-occuparional expo It is nevertheless possible that there is a sure, the typical exposure is a low or very low, background level of mesochelioma,--that is, almost unmeasurable, background concentra that die tumour can occur even in the complete tion, but occasional high exposure when there absence of asbestos (or erionice) fibres. How is a disturbance of some kind. It follows, firstly, ever, che data reviewed here indicate chat if so, that retrospective estimation of cumulative this background level must be very low-- exposure from history alone is an impossibility probably much <1 case/million people/year. in most cases; but secondly, and perhaps more This figure comes from studies of industrial importantly, that any person living or working ised countries, where background exposure to in (or even temporarily visiting) buildings asbestos is unavoidable. What the true figure is where asbestos has been used in construction can only be guessed. or otherwise might well have been exposed to What, then, are the consequences for the high concentrations of airborne asbestos fibres public health? From che studies of non- once or many times in their lives, and in most occupational exposures it seems probable that instances unknowingly. This includes most of tire occasional high level exposure situations usl are the ones that are most important. Although A better way of estimating lifelong exposure the background, hardly measurable, concentra might be analysis of fibres in the lungs, bur as tions of fibres in che air cannot be completely already mentioned fibres do clear from the dismissed, the cumulative risk of these expo lungs. How big the differences in clearance are sures is probably minor---and what is more, between people is unknown. Thus, the correla tion between lifetime cumulative exposure and fibre concentrations in the lungs is not excellent, buz the findings from the lungs prob ably give a better estimation of exposure than even a careful retrospective analysis of the patient's history, at least in low grade exposure. there is no way to reduce these concentrations. It is the high concentration situations which should be avoided. By knowing where asbestos occurs, such risks could be identified. Any source of pollution by asbestos which releases significant amounts of fibres should be elimi nated as soon as it is discovered, using correct equipment and techniques. Correct techniques In most studies, there is a dear dose-response are also necessary whenever rebuilding or tear relation between exposure and the number of fibres in the lungs." * '* " ing down of structures containing asbestos to avoid asbestos pollution of the environment. If iwooccai KKR i emriroomenrai evidexaoe of ctiv health effects ofexposure to crionitt fibers: a four-year study in the Cappadocian tegwn ofTjrtey. IxtJ Cancer 1987;39:10-17. . 20 HtUerdsi C, Bctgfa J. Malignant mcaodtelioiaa secondary to chrome mftaimwoaoo and old scars. To new cases sad review of the literature. Conor 198545:1968-72. 21 Cevim A, Tfcxvis LB, Travis VD, a of Kjnemdunoa aaiiynt mesothelioma. Career I9W;77:13?wj. 22 Neugm Al Abu Ki Aooun 5GL Incidence of t&aficianc pleural mesothelioma after thoracic radiotherapy, (mko 199740:448-50. 23 Minb M, Vrfcoiuge K. Abe Y, a al A ease of malignant ptcural mesotheiinma following exposure to atonic radia tion ip MitHafci. Ptsptrolofy 19974:201-5. 24 Mtorniton G, fanaon S, Zinipa (. Malignant maothdiemn m twc pahs of sibUngr is dm a hetajiory predisposingfactor? EnrJRaprDit 1954;65:179-54. 25 Dawson A, Gibbs A, Browne K, a of Fa&iEal moorbtftotni: toads of 17 cases widl hiwopathologie findings ind miaarai Malyhs Gmcm 1992;70-1187-7. 26 HirvoiKnAvFcfia.K.'IhminilchroL.rrof Inherited G5M1 and NAT2 defects as concurrent risk modifies in asbestos, elated human malignant mcinthi-lirxnn. Cancer Ra 1995,55-^981-3. 27 Kunchafek M, Kciscy K, Muscat J, a aL Parental cancer tad genetic prot&spesisett in malignant pleural mesothelioma: a caae-eonorel trudy. 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