Man-made Mineral Fibers Research Articles

The WHO/EURO man-made mineral fiber reference scheme. By the WHO/EURO Technical Committee for Monitoring and Evaluating MMMF.

At a consultation arranged by the World Health Organization's Regional Office for Europe in 1980 a reference method was specified for sampling and evaluating airborne dust samples collected in man-made mineral fiber (MMMF) workplaces. This method involves sampling onto a membrane filter and counting the fibers with the use of a phase contrast optical microscope (PCOM). The evaluation procedure has been tested by members of the WHO/EURO MMMF technical committee through a series of interlaboratory sample exchanges. The results from these trials have shown a consistent improvement in the reproducibility of MMMF counts, maximum systematic interlaboratory differences being reduced from 2.5 times to 1.4 times. This reduction has been achieved through harmonization of the subjective judgements made by microscopists. As this improved agreement was achieved, each laboratory's counting level changed; preliminary comparisons suggest that this occurrence would produce an approximately two- to threefold increase in the fiber concentrations reported during the epidemiology study of the Joint European Medical Research Board. Further work is currently being undertaken to quantify this change better. A reference method for sizing and counting airborne MMMF with a scanning electron microscope (SEM) has also been developed, and on the basis of preliminary experimental tests there has been some harmonization between the participating laboratories.

Man-made mineral fiber size fractions and their interrelation.

The fiber fractions RFOM (respirable fibers, determined by optical microscopy), SF (Stanton fibers), PF (Pott fibers), and FLOM (total length of optically visible, respirable fibers) and their ratios have been (i) determined experimentally in fiber sizes obtained by scanning electron microscopy of air samples of man-made mineral fibers and (ii) calculated theoretically for a range of fiber size distributions, assumed to be bivariate log-normal. For realistic values of fiber size parameters, RFOM and even FLOM are good predictors of Stanton and Pott fibers, the better the more-detailed the size information available is. The results point towards a possibility of estimating past exposure to Stanton or Pott fiber fractions of airborne man-made mineral fibers, even though only RFOM were determined.

The solubility of man-made mineral fibers in the lung in contrast to natural fibers

The solubility of man-made mineral fibers in the lung in contrast to natural fibers

Sidak-type simultaneous prediction intervals for the mortality measures RSRR i about the corresponding S ePMR i for several competing risks of death in an epidemiologic study

Simultaneous prediction intervals are obtained involving the externally standardized proportional mortality measures S ePMR i which include the corresponding RSRR i with probability 0.95, or better, for several competing risks of death, under a reasonable assumption, namely, that the total number of deaths d.. due to all risks in the mortality study be large. The results are obtained under a very general situation when several risks compete for an individual's life in the study, irrespective of any covariance structure among the risks of death, known or unknown, positive or negative, or zero. As a particular case, if the risks of death may be assumed as independent, then the simultaneous prediction intervals reduce to those of the present authors (1985a). It is demonstrated that asymptotically (as d.. → ∞) the joint c.d.f. of the statistics Z 1, Z 2, ·, Z M , where Z i = RSRR i /S ePMR i , may be approximated by means of the joint c.d.f. of the multivariate ( M-variate) normal distribution. This is the basic result from which the prediction intervals are derived, using Sidak's (1967, 1968) multivariate normal probability inequalities. Our results are empirically evaluated using real data from a recent retrospective cohort mortality study of Enterline and Marsh (1983), concerning respiratory cancer among man-made mineral fiber workers. These prediction intervals do indeed quantify the observation that for each cause i of death, the measure S ePMR i is close in value to the corresponding RSRR i .

Mortality and incidence of cancer of workers in the man made vitreous fibres producing industry: an international investigation at 13 European plants.

A total of 25 146 workers at 13 plants producing man made mineral fibres (MMMF) in seven European countries (Denmark, Finland, Federal Republic of Germany, Italy, Norway, Sweden, and United Kingdom) were studied in a historical cohort investigation. At 12 of the 13 plants an environmental survey was carried out to determine present day concentrations of airborne man made mineral fibres that showed levels of respirable fibres usually below 1 f/ml and most commonly in the range 0.01 to 0.1 f/ml. Workers were entered into the cohort at the moment of their first employment at one of the 13 factories (which started to operate between 1900 and 1955), and were followed up to at least 31 December 1977. Three per cent of the workers were lost to follow up. National death rates and national cancer incidence rates, where applicable, were used for each of the seven countries for comparison with the workers' cohort. A single death from mesothelioma was reported out of a total of 309 353 person-years of observation. No consistent differences (within and between plants) were noted between observed and expected numbers concerning individual causes of death or individual cancer sites, apart from lung cancer. For this cause a tendency was observed for the standardised mortality ratio (SMR) to increase with time from first employment. When the data from all the plants were pooled a statistically significant raised SMR of 192 (17 observed, 8.9 expected; 95% confidence interval 117-307) appeared in the group with 30 years or more since first employment. The relevance of this finding, to which different factors including uncontrolled confounders such as smoking habits may have contributed, cannot be established at present. The result is suggestive, however, of an increased risk associated with the man made mineral fibres working environment of 30 or more years ago.

Mortality Among Employees of an Ontario Asbestos-Cement Factory1,2

Mortality among 535 asbestos-exposed and 205 nonexposed employees of an asbestos-cement factory was investigated. In the period beyond 20 yr from first exposure, the exposed workers had standardized mortality ratios of 175 for all causes of death, 370 for all malignancies, 480 for lung cancer, 240 for gastrointestinal cancers, and 17 deaths from mesothelioma; the factory control subjects had mortality rates similar to the general population. The cell-type distribution of the lung cancers was similar to that occurring in middle-aged smokers. Cumulative fiber exposures were calculated for the production workers, and mortality rates for the asbestos-associated malignancies were found to have significant trends with exposure. Exposure-related lung cancer risks were noted, with a large margin of uncertainty, to be similar to those observed in an American study of manmade mineral fiber workers.

On the Chemical Changes of Asbestos Fibers and MMMFs in Biologic Residence and in the Environment: Part 1

Chemical and physical microanalysis of asbestos fibers and man made mineral fibers recovered from samples of animal and human tissues showed chemical and crystalline changes in the fibers. Scanning electron microscopy, electron microprobe analysis, and mass spectroscopic analysis were used in these investigations. The degree of partial or total leaching could be assessed. The amount of leaching varied greatly among fibers which had undergone the same treatment. Certain fibers proved to be chemically resistant and were unchanged whereas for others all minerals but silica were partially or completely leached. The biological significance of the results of these measurements is discussed. In addition the physical and chemical changes of fibers resulting from milling and weathering in air and water are discussed.

Measurement and Analysis of Chemically Changed Mineral Fibers after Experiments in Vitro and in Vivo

Asbestos, as well as other natural and man-made mineral fibers used for in vitro and in vivo experiments, must be described and defined physically and chemically as exactly as possible before any application. The interactions of fibers with the physical, chemical (air, water, etc.) and biological (cells, tissues, etc.) environments cause important changes in fiber chemistry and crystalline structure. Also, these should be detected as precisely as possible after each experiment. Our recent investigations dealt with the development of a complex analytical system for such measurements and with some applications of these analytical procedures for fibrous material sampled in the environment and from biological materials. Chemical and physical microanalyses of asbestos and glass fibers obtained by environmental sampling (air, water) and from human and animal tissue have shown chemical and crystalline changes in these particles. Scanning electron microscopy, electron microprobe analysis and mass spectrometry analysis were used in these investigations. A partial or total leakage of elements could be observed. The leakage of elements in fibers is of a statistical nature. Some fibers remained chemically unchanged; in some fibers some elements were partially leached; and in some fibers the majority of metallic elements were leached. The potential meaning of this effect is also discussed.

Influence of Well-Defined Mineral Fibers on Proliferating Cells

The effects of well-defined asbestos and man-made mineral fibers, as well as glass and synthetic fluoroamphibole, on phagocytizing permanent rat tumor cells were tested. The following parameters were compared: cell proliferation as determined by cell count and 3H-thymidine incorporation, RNA synthesis by 3H-uridine uptake, protein synthesis by incorporation of 3H-labeled amino acids, protein content and plasma membrane permeability by release of lactic dehydrogenase. The dosage of most of the dusts was estimated gravimetrically, but for some dusts also numerically. Because of the wide range of different fibers lengths, diameters and specific weights, it was sometimes difficult to compare chemically and physically differing fiber fractions with the same fiber counts. In some cases, resulting weights are so different that a direct comparison of the conclusions is impossible. The results with fibers of diverse sources showed the same trends: the toxicity of fibers increases with increasing length and dose. In this test system we found an inhibition of DNA and RNA synthesis. Protein synthesis as measured by amino acid uptake per total cell culture decreased, but the protein content of the single cell increased as determined by the Lowry method. The increase of plasma membrane permeability as determined by lactic dehydrogenase was also dependent on fiber length and concentration. Generally the thinner the fiber, the greater the toxicity when gravimetrical dosage and the same length distributions are employed. Beyond that we can state that the toxicity of fibers from different sources with similar fiber dimensions is similar. One of the glass fiber fractions has a comparable geometry (length, diameter) to the UICC fraction of chrysotile and exhibits the same high toxicity.

Macrophage functions after exposure to mineral fibers.

UICC, other well-defined asbestos samples and different man-made mineral fibers (MMM) such as glass fiber and synthetic amphibole asbestos were studied in vitro by using rat and guinea pig lung macrophages. These samples had relatively narrow length and diameter spectra. Most of the fiber samples were added to the cultures on a gravimetric basis, although some were added on a numerical basis. Electrocorundum and DQ12 (Dorentruper Quartz) were used as controls at comparable gravimetrical concentrations. The assays used were the release of lactate dehydrogenase (to demonstrate plasma membrane permeability) and the release of beta-glucuronidase (to indicate lysosomal permeability). Carbohydrate metabolism was monitored by the measurement of lactic acid production and, as one of the tests for macrophage function, the production of lysozyme was determined. The phagocytic ability of the cells was measured, after the addition of opsonized zymosan, by bioluminescence following luminol enhancement. Only some results could be evaluated, however, due to technical difficulties. A length- and dose-dependent cytotoxicity of the fibers was found in this system which was similar to that previously described with permanent cell lines. No great differences were found between fibers having different physicochemical compositions if their geometric dimensions were similar. Long, very thin fibers of glass, chrysotile, crocidolite and synthetic fluoroamphiboles were all toxic in the test system.

Measurement and analysis of chemically changed mineral fibers after experiments in vitro and in vivo.

Asbestos, as well as other natural and man-made mineral fibers used for in vitro and in vivo experiments, must be described and defined physically and chemically as exactly as possible before any application. The interactions of fibers with the physical, chemical (air, water, etc.) and biological (cells, tissues, etc.) environments cause important changes in fiber chemistry and crystalline structure. Also, these should be detected as precisely as possible after each experiment. Our recent investigations dealt with the development of a complex analytical system for such measurements and with some applications of these analytical procedures for fibrous material sampled in the environment and from biological materials. Chemical and physical microanalyses of asbestos and glass fibers obtained by environmental sampling (air, water) and from human and animal tissue have shown chemical and crystalline changes in these particles. Scanning electron microscopy, electron microprobe analysis and mass spectrometry analysis were used in these investigations. A partial or total leakage of elements could be observed. The leakage of elements in fibers is of a statistical nature. Some fibers remained chemically unchanged; in some fibers some elements were partially leached; and in some fibers the majority of metallic elements were leached. The potential meaning of this effect is also discussed.

Influence of well-defined mineral fibers on proliferating cells.

The effects of well-defined asbestos and man-made mineral fibers, as well as glass and synthetic fluoroamphibole, on phagocytizing permanent rat tumor cells were tested. The following parameters were compared: cell proliferation as determined by cell count and 3H-thymidine incorporation, RNA synthesis by 3H-uridine uptake, protein synthesis by incorporation of 3H-labeled amino acids, protein content and plasma membrane permeability by release of lactic dehydrogenase. The dosage of most of the dusts was estimated gravimetrically, but for some dusts also numerically. Because of the wide range of different fibers lengths, diameters and specific weights, it was sometimes difficult to compare chemically and physically differing fiber fractions with the same fiber counts. In some cases, resulting weights are so different that a direct comparison of the conclusions is impossible. The results with fibers of diverse sources showed the same trends: the toxicity of fibers increases with increasing length and dose. In this test system we found an inhibition of DNA and RNA synthesis. Protein synthesis as measured by amino acid uptake per total cell culture decreased, but the protein content of the single cell increased as determined by the Lowry method. The increase of plasma membrane permeability as determined by lactic dehydrogenase was also dependent on fiber length and concentration. Generally the thinner the fiber, the greater the toxicity when gravimetrical dosage and the same length distributions are employed. Beyond that we can state that the toxicity of fibers from different sources with similar fiber dimensions is similar. One of the glass fiber fractions has a comparable geometry (length, diameter) to the UICC fraction of chrysotile and exhibits the same high toxicity.

In Vitro and In Vivo Response After Exposure to Man-Made Mineral and Asbestos Insulation Fibers

In Vitro and In Vivo Response After Exposure to Man-Made Mineral and Asbestos Insulation Fibers

Respiratory disease among workers exposed to man-made mineral fibers.

Fibrous glass and mineral wool workers exposed to airborne man-made mineral fibers prior to 1964 were identified and followed for deaths caused by respiratory disease. Fiber exposure levels were estimated for each worker. Average fiber concentrations were very low in relation to a National Institute of Occupational Safety and Health recommendation that exposure to man-made mineral fibers less than 3.5 microns in diameter be limited to 3 fibers per cubic centimeter of air. Fibrous glass workers were exposed to about one hundredth and mineral wool workers to about one tenth of that amount. Respiratory cancer death rates were not excessive for the fibrous glass workers, but they were significantly elevated for the mineral wool workers. When respiratory cancer deaths were compared with fiber exposure levels, however, there was in no instance a strong positive correlation. There was a significant excess in nonmalignant respiratory disease deaths (excluding influenza and pneumonia) for fibrous glass workers and for the total cohort. When these nonmalignant respiratory disease deaths were compared with fiber exposure levels, however, there was in no instance a strong positive association. A special study was made of workers exposed to small diameter fibers. Respiratory cancer deaths were not in excess; however, there was an excess in nonmalignant respiratory disease deaths. This study provided no consistent evidence of a respiratory disease hazard related to exposure to man-made mineral fibers among the workers who produce these fibers. Results must be interpreted in relation to the relatively low exposure levels for these workers.

Man-made mineral fibre size distributions utilizing unbiased and fibre length biased counting methods and the bivariate log-normal distribution

The bivariate log-normal distribution and the marginal and conditional distributions are introduced for the description of diameter ( D) and length ( L) distributions of airborne man-made mineral fibres (MMMF). Measured fibre sizes fit this distribution well. The accumulated length-diameter distribution has several unique properties regarding characterization of bulk MMMF and dispersed dust as well as of aerodynamic and biological properties of airborne MMMF. This distribution is shown to be the distribution function of a stochastic variable D L defined as the diameter of a fibre drawn from a fibre population with a probability proportional to the fibre length. The distribution of D L is easy to measure and some underlying formulae can be taken directly from stereology. If ( D,L) is bivariate log-normally distributed, then D L is log-normal. The variances of In D L and In D are identical and a simple formula relates the parameters of the distribution of In D L to the parameters of the bivariate log-normal distribution.

Determination of asbestos fibres in air transmission electron microscopy as a reference method

Asbestos fibres are present everywhere in our environment. A series of questions concerning, for example, their toxicity or their acceptable levels still remain unanswered. The elaboration of an as accurate as possible reference method for the determination of mineral fibres in air which would be sensitive enough for use in environments with a very low level of contamination is thus called for. From a very short survey of the available methods it can be concluded that a transmission electron microscopy (TEM) method fulfils these requirements. This method is very long and expensive and should be used only in those environments where the level of fibres is low, or in complicated situations where a reference method is required. In other types of environments, such as occupational or paraoccupational situations, other less accurate but more rapid and convenient methods may be used. It is stressed that for any of these methods, and especially for the TEM method, a detailed standardization of the procedure is essential. As a scanning electron microscopy (SEM) method has also been considered for monitoring the ambient environment, the characteristics of both methods are compared, illustrated by photomicrographs and discussed. A TEM method is described in detail as follows: sampling (included recommended air volumes for different contaminated areas), sample treatment, mounting of collected fibres on electron microscopy grids, identification and counting, expression of results and detection limit. Finally, this method is applied to two different paraoccupational situations: two buildings insulated with asbestos. It is then compared with other, more simple methods. For the case of the air contaminated by long fibres (mostly crocidolite) the agreement between the different methods is fairly good. However, for the case where the fibres are short (mixture of man-made mineral fibres and chrysotile) this is not true. These differences are discussed and it is concluded that the choice of the method is dependent on the considered situation and may require preliminary investigations.

Ophthamologic effects of man-made mineral fibers.

The effect of man-made mineral fibers on the human eye was investigated in a cross-sectional study of 15 workers exposed to Rockwool and a matched reference group of 15 people. Eye symptoms, changes in the cellular and mucous content of the conjunctival fluid, break-up time of the precorneal film, the number of microepithelial defects, and the number of dead and degenerated cells on the cornea and bulbar conjunctiva were used as measures of effect. The number of fibers accumulated in the eye and conventional dust sampling methods were used as measures of dose. A significantly higher frequency of eye symptoms related to work conditions (p less than 0.001) was found among exposed workers. Similarly, the number of microepithelial defects on the medial bulbar conjunctiva increased significantly (p = 0.009) after 4 d of exposure. Six exposed workers had a pathological increase in the neutrophil count of the conjunctival fluid after 4 d of exposure, and an increase was seen in only one worker after a weekend free from exposure. Significant correlation was found between microepithelial defects on the medial bulbar conjunctiva and measures of dose (p less than 0.01). The symptoms and cellular changes can be explained by the assumption that man-made mineral fibers have the same mechanical and reversible effect on the eye as on the skin. The described dose-effect relationship suggests that a much lower hygienic standard is needed for man-made mineral fibers than what has been recommended by the American Conference of Governmental Industrial Hygienists.

DEPOSITION OF INHALED MAN-MADE MINERAL FIBRES IN THE LUNGS OF RATS

The lung retention of man-made mineral fibres by rats, sacrificed 5 days after the end of a 6 day period of exposure to the airborne fibre, has been examined as a function of fibre length, diameter and aerodynamic equivalent diameter. The retention of fibres < 0.5 μm dia. showed a peak of about 7.6% at 21 μm fibre length and was significantly higher than the retention (< 1%) of fibres in the diameter categories 0.5-0.7 μm and 0.7-1.0 %mUm. In terms of aerodynamic equivalent diameter, retention was a maximum, ca. 7%, at de = 1 μm, but decreased rapidly to about 0.5 % at de between 1.5 and 2.5 μm amd 0.01% at ca. 4 μm. When fibre length only was considered, irrespective of diameter, retention fell sharply as length increased from 2 to 10 μm, then more slowly to < 0.05% at ca. 40 μm.

Accumulation of fibers in the eyes of workers handling man-made mineral fiber products.

The external eye can be considered a passive dust sampler. Foreign material deposited in the conjunctival cul-de-sac becomes enclosed by mucous fibrils. The fibrils adhere to the conjunctival mucous thread. Hence, by removing the mucus, one can study, eg, man-made mineral fibers deposited in the eye. In this study a simple method for this purpose has been developed, and the sampling characteristics of the eye for nonrespirable fibers is described. The numbers of nonrespirable fibers accumulated in the eyes correlated with the total dust exposure dose (correlation coefficient r = 0.92) and with the nonrespirable fiber exposure dose (r = 0.82). There was no correlation between the airborne fiber concentration and the number of fibers in the mucous thread alone.

A fine fibrous silica contaminant of flour in the high oesophageal cancer area of north-east Iran.

We report here the discovery and characterization of a fibrous mineral contaminant of the diet in that area of north-east Iran where oesophageal cancer has a very high incidence. This contaminant has a smoothly tapering shape and is between 50 and 150 micrometers long. The greatest diameter is between 1 and 10 micrometers and this decreases to a sharply pointed tip with a radius of curvature of between 0.25 and 0.60 micrometers. Electron microscope X-ray analysis shows that this fibre consists almost entirely of silica. It is free from alkali metals, aluminium and iron, and therefore differs from other known natural or manmade mineral fibres. Examination of the seeds of more than sixty different species of weed know to contaminate the wheat in this area of the Middle East shows that the fibre originates from the seeds of the common Mediterranean grass Phalaris minor. This seed bears fibres of the same dimensions, composition and birefringence, borne upon the inflorescence bracts which envelop the pericarp of the seeds of this and other members of the phalaris genus. They are broken off from the seed when the wheat is milled but persist in the flour, where up to 3,000 are found in each gram. Similar fibres can be isolated in quantity from the seeds of related species which are grown commercially, and they have a similar size and composition. When cells of the 3T3 mouse fibroblast line are exposed to these fibres in semi-solid suspension culture, their proliferation is stimulated more than 100-fold. We present an hypothesis for the involvement of these plant mineral fibres in the aetiology of oesophageal cancer in Iran and in other areas of high incidence.