Professor V. Silano opened the meeting and welcomed the participants on behalf of the Ministry of the Environment, Italy. Professor G. Donelli greeted participants on behalf of the host institute (Instituto Superiore di Sanità), and Professor F. Valic addressed the meeting on behalf of the International Programme on Chemical Safety (IPCS).
1.1 Objectives
The objectives of the meeting were to review documentation and experience relating to the presence of asbestos in the environment resulting from:
a) the construction, maintenance, and demolition of buildings, including school buildings;
b) the use of asbestos in the construction, maintenance and use of vehicles;
c) collection, transportation, storage and disposal of asbestos-containing waste.
Occupational exposure was not considered, except in relation to objective b).
1.2 Approach
1) To discuss methods and approaches for the reduction of asbestos in the environment.
2) To consider data relevant to the identified approaches and to outline associated priorities.
3) To propose the identified approaches in the form of general guidelines.
For the application of guidelines and recommendations developed by the Working Group, it is important that each country assess their relevance in the light of other local priorities in health protection. Economic, technical and manpower constraints also need careful consideration, these factors being particularly important in developing countries.
The Working Group reviewed five background papers prepared for the meeting:
€ Asbestos Risk in Buildings and Building Maintenance, G. Burdett, United Kingdom
€ Reducing Exposure to Asbestos in the United States, S. Vogt, USA
€ Asbestos Risk in Vehicle Manufacture, Maintenance and Repair, F. Valic, Yugoslavia
€ Background Information on Management of Asbestos Waste Collection, Transportation,
Storage and Disposal, S. Roy, USA
€ Industry's Views on the Needs and Feasibility of Environmental Asbestos Reduction,
N. Stack, Asbestos International Association, U.K.
Although several countries have well-defined policies on the control of environmental exposure to asbestos, only that of the USA was described in the background documents provided to the Working Group.
1.3 Evaluation of health risks - environmental exposure to asbestos
While it was considered inappropriate to accept unconditionally the full report of the IPCS Environmental Health Criteria (EHC) monograph 53: Asbestos and Other Natural Mineral Fibres, since it reflected the consensus of a different panel of experts, the summary of this monograph was felt to be a relevant basis for the development of guidelines and recommendations concerning environmental exposure.
The Working Group recognized differences in interpretation of the available data as a basis for evaluation of the risks to health associated with exposure to asbestos in the environment. In particular, there were different opinions on the adequacy of the data concerning:
€ variations in the potency of different fibre types (chrysotile and amphiboles) and sizes;
€ variations in the risks in different industries;
€ the validity of the use, for quantitative risk estimation, of linear dose-response models based
on cumulative exposure.
However, the Working Group decided that it was possible to develop recommendations and guidelines on the reduction of asbestos in the environment while acknowledging the lack of consensus on these aspects.
(From: Environmental Health Criteria 53 - Asbestos and Other Natural and Mineral Fibres, World Health Organization, Geneva, 1986, 194 pp.)
2.1 Identity, physical and chemical properties, methods of sampling and analysis
The commercial term asbestos refers to a group of fibrous serpentine and amphibole minerals that have high tensile strength, conduct heat poorly, and are relatively resistant to chemical attack. The principal varieties of asbestos used in commerce are chrysotile, a serpentine mineral, and crocidolite and amosite, both of which are amphiboles. Anthophyllite, tremolite and actinolite asbestos are also amphiboles, but they are rare, and the commercial exploitation of anthophyllite asbestos has been discontinued. Other natural mineral fibres that may be considered potentially hazardous because of their physical and chemical properties are erionite, wollastonite, attapulgite and sepiolite.
Chrysotile fibres consist of aggregates of long, thin, flexible fibrils that resemble scrolls or cylinders. The dimensions of individual chrysotile fibres depend on the extent to which the sample has been manipulated. Amphibole fibres generally tend to be straight and splintery. Crocidolite fibrils are shorter with a smaller diameter than other amphibole fibrils, but they are not as narrow as fibrils of chrysotile. Amosite fibrils are larger in diameter than those of both crocidolite and chrysotile. Respirable fractions of asbestos dust vary according to fibre type and manipulation.
Several methods involving optical phase contrast microscopy have been developed for determining levels of asbestos fibres in the air of workplaces. Only fibres over 5um in length with an aspect ratio > 3:1 and a diameter of less than 3 um are counted. Thus, the resulting fibre count can be regarded only as an index of actual numbers of fibres present in the sample (fibres with diameters less than the resolution of the light microscope are not included in this essay). Fibres with diameters smaller than approximately 0.25 um cannot be seen by light microscopy, and an electron microscope is necessary for counting and identifying these fibres. Electron microscopes that are equipped with auxiliary equipment can provide information on both structure and elemental composition.
The results of analysis using light microscopy can be compared with those using transmission or scanning electron microscopy, but only if the same counting criteria are used.
2.2 Sources of occupational and environmental exposure
Asbestos is widely distributed in the earth's crust. Chrysotile, which accounts for more than 95% of the world asbestos trade, occurs in virtually all serpentine rocks. The remainder consists of the amphiboles (amosite and crocidolite). Chrysotile deposits are currently exploited in more than 40 countries; most of these reserves are found in southern Africa, Canada, China and the USSR. There are, reportedly, thousands of commercial and industrial applications of asbestos.
Dissemination of asbestos and other mineral fibres from natural deposits may be a source of exposure for the general population. Unfortunately, few quantitative data are available. Most of the asbestos present in the atmosphere and ambient water probably results from the mining, milling and manufacture of asbestos or from the deterioration or breakage of asbestos-containing materials.
2.3 Environmental levels and exposures
Asbestos is ubiquitous in the environment because of its extensive industrial use and the dissemination of fibres from natural sources. Available data using currently accepted methods of sampling and analysis indicate that fibre levels (fibres <5 µm in length) at remote rural locations are generally below the detection limit (less than 1 fibre/litre), while those in urban air range from <1 to 10 fibres / litre or occasionally higher. Airborne levels in residential areas in the vicinity of industrial sources have been found to be within the range of those in urban areas or occasionally slightly higher. Non-occupational indoor levels are generally within the range found in the ambient air. Occupational exposure levels vary depending on the effectiveness of dust-control measures; they may be up to several hundred fibres/ml in industry or mines without, or with only poor, dust control, but are generally well below 2 fibres / ml in modern industry.
Reported concentrations in drinking water range up to 200 x 10 6 fibres/litre (all fibre lengths).
2.4 Toxicological effects on animals
Fibrosis in many animal species, and bronchial carcinomas and pleural mesotheliomas in the rat, have been observed following inhalation of both chrysotile and amphibole asbestos. In these studies, there were no consistent increases in tumour incidence at other sites, and there is no convincing evidence that ingested asbestos is carcinogenic in animals. Data from the injection / implementation studies have shown that shorter asbestos fibres are less fibrogenic and carcinogenic.
The length, diameter and chemical composition of fibres are important determinants of their deposition, clearance and translocation within the body. Available data also indicate that the potential of fibres to induce mesotheliomas following intrapleural or intraperitoneal injection in animal species is mainly a function of fibre length and diameter; in general, fibres with a maximum carcinogenic potency have been reported to be longer than 8 µm and less than 1.5 µm in diameter.
2.5 Effects on man
Epidemiological studies, mainly on occupational groups, have established that all types of asbestos fibres are associated with diffuse pulmonary fibrosis (asbestosis), bronchial carcinoma, and primary malignant tumours of the pleura and peritoneum (mesothelioma). That asbestos causes cancers at other sites is less well established. Gastrointestinal and laryngeal cancer are possible, but the causal relationship with asbestos exposure has not yet been firmly established; there is no substantial supporting evidence for cancer at other sites. Asbestos exposure may cause visceral and parietal pleural changes.
Cigarette smoking increases the asbestosis mortality and the risk of lung cancer in persons exposed to asbestos but not the risk of mesothelioma. Generally, cases of malignant mesothelioma are rapidly fatal. The observed incidence of these tumours, which was low until about 30 years ago, has been increasing rapidly in males in industrial countries. As asbestos-related mesothelioma became more widely accepted and known to pathologists in western countries, reports of mesothelioma increased. The incidence of mesothelioma prior to, e.g. 1960, is not known. Mesotheliomas have seldom followed exposure to chrysotile asbestos only. Most, but not all, cases of mesothelioma have a history of occupational exposure to amphibole asbestos, principally crocidolite, either alone or in amphibole-chrysotile mixtures.
There is strong evidence that one non-asbestos fibrous mineral (erionite) is carcinogenic in man. This fibrous zeolite is likely to be the cause of localized endemic mesothelioma in Turkey.
Non-malignant thickening of the visceral pleura is frequently associated with asbestosis. Thickening of the parietal pleura, sometimes with calcification, may occur in the absence of detectable asbestosis. It is seen in those occupationally exposed to asbestos and also occurs endemically in a number of countries, but the causes have not been fully established. Tremolite fibre has been implicated as an etiological agent in some regions.
2.6 Evaluation of health risks
At present, past exposure to asbestos has not been sufficiently well defined to make an accurate assessment of the risks from future levels of exposure, which are likely to be low.
A simple risk assessment is not possible for asbestos. In making an assessment, the emphasis is placed on the incidence of lung cancer and mesothelioma, the principal hazards. Two approaches are possible, one based on a comparative and qualitative evaluation of the literature (qualitative assessment), the other based on an underlying mathematical model to link fibre exposure to the incidence of cancer (quantitative assessment). Attempts to derive the mathematical model have had limited success. Data from several studies support a linear relationship with cumulative dose for lung cancer and an exponential relationship with time since first exposure for mesothelioma. However, the derived "coefficients" within these equations cover a wide range of values from zero upwards. This numerical variability reflects the uncertainty of many factors including historical concentration measurements, fibre size distributions associated with a given fibre level and variations in the potency of different fibre types. Furthermore, smoking habits are rarely well defined in relation to bronchial cancer. The variability may also reflect uncertainty in the validity of the models. These factors have complicated the quantitative extrapolation of the risk of developing these diseases to levels of exposure such as those in the general environment, which are orders of magnitude below levels of exposure in the populations from which the estimates have been derived.
The following conclusions can be drawn on the basis of qualitative assessment:
(a) Among occupational groups, exposure to asbestos poses a health hazard that may result in asbestosis, lung cancer and mesothelioma. The incidence of these diseases is related to fibre type, fibre dose, and industrial processing. Adequate control measures should significantly reduce these risks.
(b) In para-occupational groups, including persons with household contact, those living in the vicinity of asbestos-producing and using plants, and others, the risks of mesothelioma and lung cancer are generally much lower than for occupational groups. The risk of asbestosis is very low. These risks are being further reduced as a result of improved control practices.
(c) In the general population, the risks of mesothelioma and lung cancer, attributable to asbestos cannot be quantified reliably and are probably undetectably low. Cigarette smoking is the major etiological factor in the production of lung cancer in the general population. The risk of asbestosis is virtually zero.