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FILE NAME Household Contact HC DATE 1999 DOC HC020 DOCUMENT DESCRIPTION Journal Article - Mesothelioma Cases Associated with Occupational and Low Dose Exposures Mesothelioma cases associated with occupational andlow dose exposures Gunnar Hillerdal men 5 1t0imes higher see table 3 Even in importance Objectives exposure estimate the cohorts with a very heavy exposure to asbestos of low dose to asbestos on the most people will die from other causes In peo- ve mesothelioma risk of ple with certified asbestosis is with a Methods review of the literature heavy exposure to 10 will develop mes- Results and conclusions is no evi- othelioma among insulators in the United dence of a threshold level below which States and Canada also a heavily exposed thereis no risk of mesothelioma Low level exposure more often than not contains peak concentrations which can be very high for short periods There might exist a background level of mesothelioma occurring in the abscence of exposure ot asbestos but there is no proof of this and this natural level is probably much lower than the million which has been often cited Occup Environ Med 505 513 group 9.3 of the deaths have been due to this disease and in amphibole miners in South Africa or Australia this figure is 2-4 table ) Clearly with exposure concentrations several magnitudes lower as occurs in the general population the risk is very small often impossible to measure A discussion of the risks from low exposure must include the response curve the existence or existence of a threshold and thus a background concentration and should try to define low exposure and estimate to what Keywords low exposure asbestos mesothelioma degree that 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 conclusion almost exclusively due to inhalation of asbestos The different types ofasbestos seem to differ fibres Asbestos has been extensively used in considerably in their ability to cause mesothe- industry and construction in the 20th century liomas Chrysotile is considered by many especially during and after the second world 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 al most rich western countries the total world tremolite are much more dangerous according ro production remains high There is a worldwide to many studies The third of the more a Department of Lung Medicine Karolinska : Hospital Stockholm - Sweden G Hillerdal pollution with asbestos as indicated by the 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 extent Consequently asbestos fibres can be found in most lungs at necropsy. It is thus understandable that there is concern about the important amphiboles anthophyllite was long considered not to cause mesothelioma but such tumours have now been reported although the risk seems to be small There is none the less a minority opinion that chrysotile is in fact responsible for most of the pleural mesotheliomas in society or should at least be Correspondence to risk of mesothelioma for the general popula- considered to carry the same risk This . Dr Gunnar Hillerdal ry,Department of Lung = Medicine Karolinska tion discussion however falls outside the present ke However it should be remembered that review and is not important for the conclusions 5 Hospital Stockholm | mesothelioma is a rare disease with incidence drawn here XSweden in industrialised countries ranging from 1 to reste BFi)rr,Atyccepted 8 March 1999 million among women and values for Definition and diagnosis of mesothelioma tzey h. 1 Table Proportionate mortality from mesothelioma in various cohorts Cohort Amphibole miners te miners om MB Inswagon workers er Patients with asbestosis oe. Deaths n 1225 118 4951 283 40 39 Deaths due to mesothelioma % 2 + 6 9 9 10 10 Reference 4 5 6 7 8 9 10 that Mesotheliomas are by definition rumours arise from mesothelial cells and can thus re from any body cavity the pleura the peritos pee neum the pericardial sac and even the ttunuic~nic~ rene vaginalis testis Pleural mesotheliomas arethe ee ones PlePulreaulralmesotheliomas have most common and pathologically the best , - rate one defined male to female rate of about five to ratio ratio is DE, 1.5 whereas for peritoneal tumours this ratio 1.5 to 1. Thus either the aetiologyis different ne: weer 506 Hillerdal Table 2 Possible risk factors and mediators of risk of mesothelioma other than asbestos Factors Erionite ? Chronic inflammation ' Radiation ^' * Beryllium * Vegetable fibres Hereditary factors : Immunological factors Dietary factors Viruses . Comments Very high incidence of mesothelioma environmental exposure in Turkey Pleural scars tuberculosis pleurisy therapeutic pneumothorax Single cases after Thorotrast injection or radiotherapy causality not proved One case in atomic bomb survivor Two doubtful cases described No proof in humans Familial cases explained by common asbestos exposure Correlation with parental cancer Rapidly progressive cases in patients with HIV infection Provitamin A carotene may decrease the risk Mesotheliomas in animals Simian virus 40 DNA sequences reported in mesotheliomas References 19 12238 12238 22 12238 12238 24-26 27 28 29 30 31 32 diagnosis is more difficult in women or women exposed to asbestos are more likely to develop a peritoneal than a pleural mesothelioma In fact one type of mesothelioma occurring almost exclusively in women is the so called Incidence of mesothelioma There is a large variation in the incidence of mesothelioma in different countries and in most places a steadily rising number of cases with time In table 3 the incidence or mortality 4 @ @ ie # multicystic mesothelioma This tumour has a better prognosis is more sensitive to cytostatic drugs and seems to have no connection with exposure to asbestos The pathological diagnosis of mesotheliomas requires experience and confusion can occur with both benign pleural lesions and with metastatic pleural diseases Many countries now have mesothelioma panels which has various times can be seen As mortality for practical puprposes is the same as the inci- F dence for this disease both figures have been @& used in the table Some of the differences between the countries are probably due to # diagnostic difficulties but most of the varia- 3 tions can be explained by the use of asbestos in s 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 response and latency time Most researchers agree that there is a positive response curve for mesothelioma , Suggested causes of mesothelioma other than mineral fibres . heavier the exposure to asbestos the greater the risk This is found in cohort studies as well 4 Apart from mineral fibres radiation for exam- ple through the contrast medium Thorotrast has been suggested to cause human mesochelioma However in a large retrospective study of more than 250 000 women treated for mammary carcinoma a quarter of them as in analyses of amphibole asbestos fibres in J the lungs It was realised early that time qm since first exposure was of great importance and therefore the cubic residence # model was suggested by Doll and Peto in their # report in 1985 swe f initially with radiotherapy no association between radiation and mesothelioma could be T4- T ^ F 4 Where I incidence at the time T after the found As can be seen from table 2 other causes or contributing factors have also been suggested Viruses can cause mesotheliomas in animals but this has not been described in humans DNA sequences associated with Simian virus concentra 40 SV 40 transforming factors have been reported in a high proportion of mesothelio- fibre mas from some countries This suggests that there is a connection between SV40 which was a contaminant of live polio vaccines in 1959-61 and later development of mesothe- lioma Thus SV 40 might be a cofactor to asbestos in some patients with mesothelioma , but the results have not been confirmed and are still disputed as Insummary then as far is known today only factors than mineral fibres explain a very small propproportioon rtion of mmeseosthoeltioh-elmieo-sothelioexplain mesothelio- either mas and can forpractical _ purposes be disregarded Thus a malignant mesothelioma regarded can be either as caused by asbestos or belonging to a normal background level is a spontaneously occurring tumour The relative imnportance of these two factors has been debated and will be further explored in this exposure = constant depending on process intensity of exposure and J duration of exposure This equation has % been used in many studies with an acceptable fit for normal occupational exposure concen- trations F in the equation is the total exposure combination of fibre J tions and exposure usually measured in years 1 fibre = a mean of 1 air for 1 working year Thus the dose- response curve is supposed to be linear but the result is heavily influenced by the time factor Unfortunately the exact value of F is often uncertain even in well defined highly exposed cohorts The equation is thus rarely useful especially with low doses in which F is usually a crude guess only As can be seen from table 13 even with heavy exposure only up to 10 of a cohort will die from mesothelioma the for mula is not applicable in these cases With very heavy exposure most patients will die from pulmonary insufficiency due to asbes tosis before there has been sufficient time % develop a mesothelioma 3 Even more troublesome is the time factor which quickly becomes very important in COSSAct the risk 3. Incidence or mortality of mesothelioma in various countries and areas over time Imillion inhabitants / year of mesothelioma before 1946 although such negative evidence is of questionable value Year Male Female Reference McDonald and McDonald in a recent review estimated the background level to be 1-2 Nazaire France 1968-81 1972 1950-74 1976-80 2.1 2.8 52 5.8 0.8 33 0.7 34 0.2 35 2.1 36 million they came to this figure by extrapolating backwards from epidemiological studies from various countries cities United States Nazaire France Nazaire France 1970s 1986 1983-90 1968-71 1990-94 1972-76 1975-84 1983 1978-80 1985-92 1968-71 1982-88 1982-86 1987-91 1994 4.4-11.1 7-13 8.3 8.4 10 12.6 17.2 17.5 14.7 194 20.7 28.3 30.5 44.0 49.9 1.2-3.8 37 1-2 38 4.7 39 2.3 40 2.9 41 2.8 40 0.8 35 3.2 16 7.0 42 4.0 35 4.3 40 3.3 43 4.9 40 6 40 4.8 +4 Malignant mesothelioma can occur in children and such cases can be considered as proof of 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 65 even in stillborn ones showing that asbestos fibres spread in the human body and even penetrate through the placenta Even if a latency time of only a few years is extremely rare in mesothelioma related to asbestos it can occur There are some published examples of would increase steeply with time making early latency times of only 5 years childhood exposure of great importance Mesotheliomas also occur in animals from feter However there are clear indications that baboons"and domestic dogs to fish Dogs mineral fibres clear from the lung albeit with are exposed environmentally to asbestos just different half lifes for the different types of asbestos Chrysotile has the shortest half life and crocidolite is generally accepted to have the longest The half of crocidolite has been estimated to 7-8 years This clearing of fibres like their human masters which might explain 69 72 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- would at least theoretically tend to actually mas in humans decrease the risk of mesothelioma and other diseases with time cubic In conclusion the value of the cubic residence formula is in practice low and it should not be used for extrapolationast least not at the extreme ends of exposure The latency time varies in different cohorts and is dependent on how long a cohort is followed up In 370 necropsy cases from Italy latency time could be calculated in 312 Levels of exposure Although many authors write about low leve exposure to asbestos there is rarely a definition of this term In fact in many articles low level exposure seems to be synonymous to non- occupational exposure which as described later is certainly not true in many cases Occupational as well as occupational exposure 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 i i highest exposure to 51.7 in women with It must be realised that occupational exposure : i to asbestos occurs or has occurred not only in domestic exposure the classic industries such as asbestos mines and factories shipyards insulating business je The threshold value and the background concentration asbestos cement industry building and con- struction 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 oil refineries electrical sure required for development of a industry jewellery workers sugar sen mesothelioma In most studies several refineries and cigarette filter workers Sea- rrT er patients with mesothelioma do not report any occupational or other exposure to men and fishermen can have been exposed to asbestos used as insulation in their boats In the asbestos and thus there seems to be a reprocessed textile industry bags heavily con- small spontaneous basal or background inci- taminated with asbestos could be reused for dence 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 trols who were very carefully interviewed % 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 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 ronmental exposure unknown to or forgotten period In many or most instances the workers e by the patients themselves There are authors who claim that the have no idea of the exposure and it can be impossible or almost impossible to elucidate it presumed background level must be very low Also the level of exposure is often very difficult en and retrospective searches for the tumour in to estimate should the information be avail- a oe Wetane the medical literature yield no convincing cases able Pet. 508 Hillerdal ang AGS to occupational occupational exposure asbestos EXPOSURE DOMESTIC asbestos URBAN AIR POLLUTION The air in cities contains a very low concentra- The family of an asbestos worker could be exposed to considerable amounts of asbestos brought home on his working clothes all often too it was the duty of the wife or daugh- ter to clean Pleural thickening calcifications and pulmonaryfibrosis have been described in : such cases as well as mesotheliomas tion of asbestos Near to construction or demolition work the concentrations are higher It has been suggested that the air of London during the blitz was heavy with asbestos Asbestos is also released on braking and close to a motorway the fibre concentrations in the air will be higher than the background but still well below the industrial threshold values A AIR POLLUTION FROM 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 nuisances They could be seen from a distance because of the dust cloud Asbestos fibres can tion of asbestos fibres can be 10-1000 times above the background concentration Drinking water can contain asbestos or spread over a distance many kilometres from a asbestiform 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 crocidolite mining areas in South Africa the incidence of mesotheliomas drinking water Small amounts of asbestos can 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 ---- ---tration process Fibres that are ingested will to occupational exposure However whether some of this exposure was domestic see later some extent also enter the blood stream as seen " from animal experiments In humans there is not clear.33 are few reports However when amphibole Another example is the formerly active crocidolite mine at Wittenoom Western Aus- tralia At least 5000 people lived in the township of Wittenoom without working in the mines and in 1993 27 cases of mesothelioma 84 had occurred among these people It has been estimated that 1.1 of child residents and 1.9 % of the female residents of Wittenoom have died or will die from mesothelioma fibres were found in drinking water from Lake Superior a very small portion could be traced in urine and when chrysotile occurred in the drinking water very small amounts were again found in urine However most ingested mineral fibres will never be absorbed but will be cleared in the normal way and the harmful effects of asbestos in drinking water or drinks is probably minuscule and much smaller than whereas among the workforce this figure would be % Around the chrysotile mines in Quebec Canada there have been at least 53 occupational mesotheliomas from the mines nine the risk of drinking the alcohol Even if ingested asbestos is not dangerous it has to be realised that it is only a very small portion of the tap water that we actually drink Most of it is used for other purposes Once the domestic and two with general environmental exposure only In the nearby towns the lungs of residents who have never worked in the mines have a fibre concentration which is 10 water has dried after washing cleaning taking showers etc the asbestos fibres will spread in the air Such amounts are small but not insignificant and will add to the normal times higher than that of the average background exposure Canadian.86 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 large study from England this factor accounted for only % of all the identified cases CONSUMER GOODS Asbestos has been introduced into various goods used by the public Examples are wall paints and spackling and jointing compounds Another is a hand held hair drier of which 13 million were sold in the United States and which has been described as a small asbestos spray gun In commercially produced Kent filter cigarettes crocidolite was used in the filters from 1952-6 The sale of these cigarettes was 11.7 billion and in the commercials the 89 health effects of this filter was emphasised Once asbestos is introduced into a home it will spread to all rooms and is almost impossi- ble to remove even with a vaccum cleaner It will easily be disturbed into the air from the slightest movement and sedimentation is very ASBESTOS IN PLACE = BUILDINGS CONTAINING ASBESTOS A mixture containing asbestos was popular after the second world war for spraying on ceil- ings and walls for insulation and decoration This was used well into the 1960s There is now a public danger because of the plaster falling down from natural wear and tear vandalism or artistic carving in schools etc. In many industrial buildings asbestos was also used for spraying on the underside of the roof and with natural wear and tear and for instance birds building nests there is now release of fibres to the surroundings Asbestos was also extensively used in walls or around plumbing for insulation purposes in cement to : found strengthen it or just as a cheap filler In a modern city asbestos can often be in many places in the cellar where steam pipe are insulated in storage or laundry rooms in air conditioning sets in theatres museums restaurants etc. Whenever there is damage to any construction or machine that contains asbestos the possibilities of exposure some- * Table 4 Reported mesothelioma cases after exposure to asbestos in place Table Endemic plaques are of interest also in other eaen Occupation or exposure School teachers School Attended school Comments living vuntewa Reference countries as many people born in these places and there in their childhood and youth 9/487 patients with mesothelioma 99 now have moved to other places taking with 1 case each 4 cases 100-103 104 them not only the plaques but also the risk of 102 mesothelioma Off Tk Fer ffice worker Asbestos insulation at home 6/262 patients with mesothelioma 102 105 106 Concentrations of exposure OCCUPAIONALCONCENTRATIONS It has been claimed that up to 1000 prema- ture deaths from lung cancer or mesothelioma will occur in the future among school children from schools where asbestos was used in the walls calculations which however had to be built on extrapolations and assumptions Several 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 radiological findings from the lungs as pleural plaques also been reported indicating exposure ot asbestos but these results are not undisputed and there is a probable overdiagnosis as control groups are missing 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 concentrations from 25 up to occasional values of 10002000 fibres With his own recalculations Harries in 1970 estimated the fibre concentrations in the dockyard in 1951 as follows sprayed asbestos insulation 171-322 fibres stripping asbestos 334 sweeping 353 adjacent passage 83 and in the passageway to the shower 25. Bagging debris 564 pipe lagging 194-200 removal of pipe lagging 171. Snap samples showed values to up to 1000-2000 fibres These values are similar to the ones published by McMillan in 1983 who recreated values from the past engine room 88 delag- ENVIRONMENTAL MESOTHELIOMAS FROM LOCAL ging in boilers room 171 bagging debris 353 ml DEPOSITS OF FIBROUS MINERALS Measurements from working places in the Endemic pleural plaques were first described 1960s often showed peak doses of 20 fibres 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 sometimes have been quarried often for adopted a concentration of 1-2 fibres as the 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 occupational measurements table 6 tremolite which has resulted in an extremely A problem with the legal concentration is that 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 have adopted stand- whitewashing of the houses the risk will disap- ards which they cannot enforce As a result aT E pear when this procedure is stopped but due to actual exposures may be much higher than the a the long latency time this will take many standard in these countries decades A asbestos fibre the zeolite erionite has OCCUPATIONAL CONCENTRATIONS e TT been found in some Turkish villages Roads The fibre concentrations in domestic exposure Lome 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 0 crocidolite and the incidence of mesotheliomas 100 fibres Ordinary vacuum cleaning is in these unfortunate villages is extremely high not effective in removing asbestos fibres which Table 5 Local deposits of mineral fibres asbestos or erionute occurrence of plaques and of malignant mesothelioma Country or area Type of fibre Afghanistan Austria Bulgaria Corsica Cyprus Czechoslovakia Finland Greece Metsovo Tremolite Tremolice Anthophyllite Tremolite Tremolite Tremolite Unknown Anthophyllite Tremolte SW Aridea Macedonia New Caledonia x2 a S frica TL USSR Amosite crocidolite Tremolite Erionite Unknown 12 cases 000 inhabitants y Occurrence in local area fe Plaques % of investigated inhabitants ~~ 5.3 2.8 women 5.6 men 41 50 y 2.7-6.6 6.5-9.0 46.9 24 2 40 y 2.5-6.6 1.2-25 65 Locally high Mesothelioma risk x110 _ Not increased Not increased High High _ Not increased 280 140 1985-94 High 8.3 300+ High High Comment ' Case report only Vineyard and field workers Tobacco growers General pollution General pollution Farmers White washing houses White washing houses Population around mine White washing houses Farmers Reference 108 109 110 112 114 115 16 117 113 119-121 122 123 124 125 4 126-128 19 128 129 130 510 Table 6 Fibre concentrations in air and lungs with occupational exposure to asbestos Type of exposure Domestic Domestic exposure paraoccupational ) parocupational Near asbestos mines factories etc In Wittenoom When mine operating After closure of mine Farmers near mine in Canada Local asbestos findings Greece In the yard of an abandoned house In a newly whitewashed room Corsica New Caledonia While sweeping floor Road dust clouds Asbestos in place Teacher's aid Female office worker Fibres it air fbal Fibers in lungs x10g x10g dry tissure . 5.3-319.5 0.0049 Reference 41 102 0.5 0.01-0.21 1.2-26.8 48 134 001 0,02-17.9 * 21 123 114 78 0.06-0.67 124 125 0.0043 31- TEM 102 103 - 39 m100 times higher than controls TEM = transmission electron microscopy RAR: Hillerdal nP aeah Fibres in the lungs of patients with ele occupational mesothelioma sabtot As has been mentioned asbestos fibres can be ft found in the lungs of the general population without any known exposure In Germany the - upper normal limit was estimated to be 300 000 fibres dry lung In a large study of 324 malignant mesotheliomas from which 46 lung samples were available it was found that even | at a fibre concentration of 100 000-200 000 fibres there was a fivefold increased risk of mesothelioma which was significant In this study which has unfortunately only been published 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 the normal 137-141 level.25 can remain for years in the house and be airborne again whenever disturbed domestic exposure is not low exposure e Environmental concentrations in the villages where whitewashing occurs low values are reported when there is no disturbance but in a newly whitewashed room and while sweeping floors the concentrations can be quite consid- erable table 6 With asbestos in place as long as the asbes- tos is undisturbed the concentrations in the air are very low zero or hardly measurable /0.00f1 ibres but once deterioration takes place values can go up to 15 fibres and when being removed there can be even higher values In schools in the United States the mean concentration was estimated to be 0.003 fibres and in federal buildings 0.006.15 Thus with so called occupational exposure the typical exposure is a low or very low almost unmeasurable background concentration but occasional high exposure when there is a disturbance of some kind It follows firstly that retrospective estimation of cumulative exposure from history alone is an impossibility in most cases but secondly and perhaps more importantly that any person living or working in or even temporarily visiting buildings where asbestos has been used in construction or otherwise might well have been exposed to high concentrations of airborne asbestos fibres once or many times in their lives and in most instances unknowingly This includes most of us A better way of estimating lifelong exposure might be analysis of fibres in the lungs but as already mentioned fibres do clear from the lungs How big the differences in clearance are between people is unknown Thus the correla- tion between lifetime cumulative exposure and fibre concentrations in the lungs is not excellent but the findings from the lungs probably give a better estimation of exposure than even a careful retrospective analysis of the patient's history at least in low grade exposure In most studies there is a clear response relation between exposure and the number of Discussion Any asbestos fibre found in a lung must have been inhaled As far as is known no truly unexposed group can be found in the world There is no proof of a threshold value is | a minimal lower limit below which asbestos fibres cannot cause the tumour thus it is plausible that even such low exposure can cause mesothelioma even if the risk is extremely low Patients with mesothelioma whose lungs show fibre concentrations within the normal range cannot be dismissed as back- ground thaistnot due to asbestos 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 exposed group| This is not possible as no such group can be found It is nevertheless possible that there is a background level of mesothelioma is| that the tumour can occur even in the complete absence of asbestos or erionite fibres However the data reviewed here indicate that if so this background level must be very low- probably much < million people , This figure comes from studies of industrial- ; ised countries where background exposure to | asbestos is unavoidable What the true figure is ; can only be guessed i What then are the consequences for the : public health From the studies of non- occupational exposures it seems probable that ~fl the occasional high level exposure situations : are the ones that are most important Although 4: the background hardly measurable concentray: tions of fibres in the air cannot be completely dismissed the cumulative risk of these exposures is probably minor what is more 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 are also necessary whenever rebuilding or tear- ing down of structures containing asbestos to the asbestos is well contained and not dis- 33 Enterline PE Henderson VL Geographic patterns for pleu- e! e turbed it is usually better to leave it in place In TRUE e many instances encapsulation is also better ral mesothelioma deaths in the United States 1968 871 Nad Cancer Inst 31-7 34 McDonald AD McDonald JC Malignant mesothelioma in than removal 35 CNhaoirltlheuAxmeEriPciaocChaencDer C1 ho6 pra5S0et al EEpid^'miolpgie id6 ^'miologiedu me re m^'soth^'liomepleural malin dans la r^'gion de Nantes- e te a 2 J Nazaire Evolution 1956-92 Rev Mal Respir 1995 1 Langer AM Inorganic particles in human tissue and their ft association with neoplastic disease Environ Health Perspect 353-7 229 33 36 Weiss NS Martin J Suarez L et al Review of mesothelioma Gloag D. 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Asbestos fiber analysis in 27 malig- 141 Dodson RF O'Sullivan M Corn CJ Analysis of asbestos nant mesothelioma cases AJm Ind Med 193 207 138 Tuomi T Konttinen M Tammilehto L et al fiber burden in lung tissue from mesothelioma patients Ultastruct Pathol 321 36 Mineral fiber concentration in lung tissue of mesothelioma 142 Ferguson D. level asbestos the priorities are wrong patients in Finland AmJ Ind Med 247 54 Me J Ad ust 617 18 3 : ' Occupational and Environmental Medicine - http://www.occenvmed.com Visitors to the world wide web can now access Occupational and Environmental Medicine either through the BMJ Publishing Group's home page http://www.bmjpg.com or directly by using its individual URL http://www.occenvmed.com There they will find the following fi Current contents list for the journal Contents lists of previous issues @ Members of the editorial board Fe @ Subscribers information 7 Instructions for authors iB | @ Details of reprint services ee 5 .. hotlink gives access to fi BMJ Publishing Group home page @ British Medical Association website Online books catalogue @ BMJ Publishing Group books The web site is at a preliminary stage and there are plans to develop it into a more sophisticated site Suggestions from visitors about features they would like to are welcomed They can be left via the opening page of the BMJ Publishing Group site or alternatively via the journal page through about this site i: 1 Aid: F ! i: | T a E ? a ; i: ! | |