RESUMEN

BACKGROUND AND AIMS: The Soufriere Hills volcano, Montserrat, has been erupting since July 1995 and volcanic ash has fallen on the island throughout most of the eruption. The ash contains substantial quantities of respirable particles and unusually large amounts (15-20%) of the crystalline silica mineral, cristobalite. The purpose of the surveys described here, undertaken between December 1996 and April 2000, was to determine levels of personal exposure of islanders to volcanic ash and cristobalite in order to inform advice on the associated risks to health and the measures required to reduce exposure. METHODS: Surveys of personal exposure to respirable dust and cristobalite were undertaken using cyclone samplers. In addition, direct reading instruments (DUSTTRAK) were used to monitor ambient air concentrations of PM(10) at fixed sites and also to provide information about exposures to airborne particles associated with selected activities. RESULTS: Environmental concentrations of airborne ash have been greatest in the areas where the most ash has been deposited and during dry weather. Individual exposure to airborne ash was related to occupation, with the highest exposures among gardeners, cleaners, roadworkers, and police at roadside checkpoints. During 1997 many of these individuals were exposed to concentrations of cristobalite that exceeded the ACGIH recommended occupational exposure limit. Since the population became confined to the north of the island in October 1997, even those in relatively dusty occupations have received exposures to cristobalite well below this limit. CONCLUSIONS: Most of the 4500 people who have remained on island since the eruption began have not been exposed to sufficiently high concentrations of airborne dust for long enough to be at risk of developing silicosis. However, more than a dozen individuals continued to experience frequent high occupational exposures to volcanic ash, some of whom may have had sufficient exposure to crystalline silica to be at risk of developing mild silicosis. If volcanic activity were to deposit further ash over the occupied areas of the island during the coming years, the risks of silicosis will become more substantial.

Asunto(s)

Exposición a Riesgos Ambientales/efectos adversos , Erupciones Volcánicas/efectos adversos , Adulto , Niño , Encuestas Epidemiológicas , Humanos , Exposición Profesional/efectos adversos , Factores de Riesgo , Dióxido de Silicio/efectos adversos , Factores de Tiempo , Indias Occidentales

RESUMEN

Results from animal studies have indicated some uncertainties over the validity of a single general occupational control limit for all types of "particulates (insoluble) not otherwise classified" (PNOC) (ACGIH, 2000). Therefore, to examine the extent to which a given control limit may be valid for nontoxic dusts with different physical characteristics, this study compared the pulmonary effects in rats of inhalation exposure to two poorly soluble dusts of similar density and with relatively low toxicity: titanium dioxide and barium sulfate. The objectives were to compare the dusts in (a) their buildup and clearance in the lungs during inhalation; (b) their transfer to lymph nodes; (c) the changes, with time, in the lavageable cell population; and (d) the pathological change from histology. The exposure aerosol concentrations were selected to achieve similar mass and volume lung burdens for both dusts and to attain "overload" over the common exposure periods of about 4 mo and 7 mo. Despite obtaining similar lung burdens for both dusts, there was significantly more translocation of TiO(2) to the hilar lymph nodes than with BaSO(4). It was also found that clearance of TiO(2) was retarded whereas clearance of BaSO(4) was not. Trends in these data were clarified by the use of a simple model of particle clearance. Retardation of particle clearance and translocation to the lymph nodes are markers of the condition known as "overload" in which the alveolar macrophage-based clearance of particles from the deep lung is impaired. In addition, bronchoalveolar lavage showed that TiO(2) caused significantly more recruitment of inflammatory neutrophils to lungs than BaSO(4). These differences between the dusts were not due to differences in toxicity, solubility, or lung deposition. The explanation that the different responses are due to the different particle size distributions of the two dust types is examined in a companion paper (Tran et al., this issue).

Asunto(s)

Sulfato de Bario/toxicidad , Polvo/efectos adversos , Inflamación/etiología , Pulmón/efectos de los fármacos , Titanio/toxicidad , Administración por Inhalación , Animales , Sulfato de Bario/farmacocinética , Carga Corporal (Radioterapia) , Pulmón/metabolismo , Pulmón/patología , Ganglios Linfáticos/metabolismo , Masculino , Tasa de Depuración Metabólica , Neutrófilos/efectos de los fármacos , Ratas , Ratas Wistar , Solubilidad , Titanio/farmacocinética

RESUMEN

In this article the volumetric overload hypothesis, which predicts the impairment of clearance of particles deposited in the lung in terms of particle volume, is reevaluated. The degree to which simple expressions of retained lung burden explain pulmonary responses to overload was investigated using data from a series of chronic inhalation experiments on rats with two poorly soluble dusts, titanium dioxide and barium sulfate. The results indicated that the difference between the dusts in the level of inflammation and translocation to the lymph nodes could be explained most simply when the lung burden was expressed as total particle surface area. The shape of the statistical relationship for both lung responses indicated the presence of a threshold at approximately 200-300 cm(2) of lung burden. On the basis of this and other similar results, a hypothesis regarding a generic mechanism for the impairment of clearance and associated lung responses is proposed for such "low-toxicity" dusts.

Asunto(s)

Sulfato de Bario/toxicidad , Polvo/efectos adversos , Inflamación/etiología , Pulmón/efectos de los fármacos , Titanio/toxicidad , Administración por Inhalación , Animales , Carga Corporal (Radioterapia) , Pulmón/metabolismo , Ganglios Linfáticos/metabolismo , Masculino , Tasa de Depuración Metabólica , Neutrófilos/efectos de los fármacos , Ratas , Ratas Wistar

RESUMEN

OBJECTIVES: In July 1995 the Soufriere Hills volcano on the island of Montserrat began to erupt. Preliminary reports showed that the ash contained a substantial respirable component and a large percentage of the toxic silica polymorph, cristobalite. In this study the cytotoxicity of three respirable Montserrat volcanic ash (MVA) samples was investigated: M1 from a single explosive event, M2 accumulated ash predominantly derived from pyroclastic flows, and M3 from a single pyroclastic flow. These were compared with the relatively inert dust TiO(2) and the known toxic quartz dust, DQ12. METHODS: Surface area of the particles was measured with the Brunauer, Emmet, and Teller (BET) adsorption method and cristobalite content of MVA was determined by x ray diffraction (XRD). After exposure to particles, the metabolic competence of the epithelial cell line A549 was assessed to determine cytotoxic effects. The ability of the particles to induce sheep blood erythrocyte haemolysis was used to assess surface reactivity. RESULTS: Treatment with either MVA, quartz, or titanium dioxide decreased A549 epithelial cell metabolic competence as measured by ability to reduce 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). On addition of mannitol, the cytotoxic effect was significantly less with M1, quartz, and TiO(2). All MVA samples induced a dose dependent increase in haemolysis, which, although less than the haemolysis induced by quartz, was significantly greater than that induced by TiO(2). Addition of mannitol and superoxide dismutase (SOD) significantly reduced the haemolytic activity only of M1, but not M2 or M3, the samples derived from predominantly pyroclastic flow events. CONCLUSIONS: Neither the cristobalite content nor the surface area of the MVA samples correlated with observed in vitro reactivity. A role for reactive oxygen species could only be shown in the cytotoxicity of M1, which was the only sample derived from a purely explosive event. These results suggest that in general the bioreactivity of MVA samples in vitro is low compared with pure quartz, but that the bioreactivity and mechanisms of biological interaction may vary according to the ash source.

Asunto(s)

Contaminantes Atmosféricos/toxicidad , Dióxido de Silicio/toxicidad , Erupciones Volcánicas/análisis , Adsorción , Animales , Células Cultivadas , Humanos , Técnicas In Vitro , Manitol/farmacología , Cuarzo/toxicidad , Ovinos , Titanio/toxicidad , Erupciones Volcánicas/efectos adversos , Indias Occidentales , Difracción de Rayos X

RESUMEN

This article describes the activity of an E-glass microfiber (104E) during chronic inhalation and intraperitoneal injection studies in rats. Results are compared with another microfiber of similar dissolution rate (k(dis)), code 100/475, and the more durable amosite asbestos, both of which we had previously used in similar experiments (Davis et al., 1996). Rats were exposed to aerosol concentrations of 1000 fibers (longer than 5 microm)/ml, as measured by optical microscopy, for 7 h/day, 5 days/wk. Subgroups of rats were followed for mean lung burden, early and late signs of fibrosis, and tumor incidence. At the end of 12 mo of exposure, the mean number of 104E fibers of all lengths in the lungs was approximately double that for amosite but two-thirds of that for 100/475. For fibers longer than 15 microm, the mean 104E burden was similar to that for the amosite and more than twice that of the 100/475. After a 12-mo recovery period, the retained lung burdens (of fibers of all lengths) were approximately 30% of those at 12 mo for both microfibers, and somewhat higher (approximately 44%) for amosite. Amosite and 100/475 fibers longer than 15 microm were more persistent in the lungs than 104E fibers. The chemical composition of 104E fibers did not appear to have been significantly altered by up to 24 mo of residence in lung tissue, whereas the composition of 100/475 was substantially altered over the same time period. From the inhalation study, out of the pathology subgroup of 43 animals exposed to 104E microfibers, 10 had lung tumors (7 carcinoma, 3 adenoma) and 2 had mesotheliomas, whereas in 42 rats exposed to amosite asbestos, there were 16 lung tumors (7 carcinoma, 9 adenoma) and 2 mesotheliomas. The 104E- and amosite-treated animals had similar levels of fibrosis. In contrast, 38 animals treated with 100/475 had little fibrosis, 4 lung tumors (adenomas), and no mesotheliomas. The greater pathogenicity of the 104E fibers, compared to 100/475 fibers, might be partly explained by the greater numbers of long fibers retained in the lung after 12 mo of inhalation. However, we speculate that modification of surface properties by extensive selective leaching of some glass components reduces the toxic potential of 100/475. In a parallel intraperitoneal injection study, 104E caused considerably more mesotheliomas (21 rats out of 24) than 100/475 (8 rats out of 24). In addition, 104E appeared to be more active than amosite asbestos, since mesotheliomas appeared much more quickly in the 104E-treated animals. In conclusion, we have shown that two microfiber types, 100/475 and 104E, of similar dissolution rates, had markedly different pathogenicity in rats. We believe that this contrast is only partly due to differences in numbers of long fibers and that differences in surface properties of the fibers, possibly due to proportionately greater leaching of 100/475 fibers, play an important role.

Asunto(s)

Asbesto Amosita/toxicidad , Vidrio , Exposición por Inhalación , Administración por Inhalación , Aerosoles , Animales , Asbesto Amosita/administración & dosificación , Asbesto Amosita/farmacocinética , Carga Corporal (Radioterapia) , Carcinoma/etiología , Carcinoma/mortalidad , Carcinoma/patología , Inyecciones Intraperitoneales , Pulmón/efectos de los fármacos , Pulmón/metabolismo , Pulmón/patología , Neoplasias Pulmonares/etiología , Neoplasias Pulmonares/mortalidad , Neoplasias Pulmonares/patología , Masculino , Mesotelioma/etiología , Mesotelioma/mortalidad , Mesotelioma/patología , Fibras Minerales , Neoplasias Experimentales/etiología , Neoplasias Experimentales/mortalidad , Neoplasias Experimentales/patología , Tamaño de la Partícula , Fibrosis Pulmonar/etiología , Fibrosis Pulmonar/mortalidad , Fibrosis Pulmonar/patología , Ratas , Ratas Wistar , Análisis de Supervivencia , Tasa de Supervivencia , Pruebas de Toxicidad

RESUMEN

In view of the extended debate and differing opinions on whether crystalline silica is a human carcinogen, we have reviewed a selection of epidemiological reports, to identify the areas of uncertainty and disagreement. We have chosen to examine the papers which in a recent review were considered to provide the least confounded examinations of an association between silica exposure and cancer risk. We also refer to a study of the mortality of coalminers very recently reported by ourselves and colleagues. We find that parts of the evidence are coherent but there are contradictions. On examination this resolves mostly into differences between types of studies. The three types of epidemiological study included are: (i) exposure-response studies, the most powerful for the confirmation of a relationship between a specific exposure and a health effect; (ii) descriptive studies in which incidence of disease in an exposed population is compared with that in a reference population; and (iii) studies of incidence of disease in subjects on silicosis case-registers. Descriptive studies frequently though not invariably suggest an excess lung cancer risk in silica-exposed workers compared with the general population, but exposure-response studies consistently fail to confirm that the cause is exposure to quartz. A single exposure-response study of cristobalite suggests a positive relation. Both sets of evidence have weaknesses. There are uncertainties on whether the excess risks in the descriptive studies are related to silica exposure or to lifestyle, including smoking habits. There are doubts on whether the exposure estimates in some of the exposure-response studies were sufficiently reliable to detect a small risk or weak association, though they are unlikely to have missed a strong effect. Studies of subjects on silicosis case registers consistently show an excess of lung cancer, but it is not clear to what extent these increased risks represent a direct effect of silica exposure, a secondary effect of the silicosis, preferential inclusion of subjects suffering from the effects of smoking, or bias in diagnostic accuracy. This not unnaturally leads to differences in opinion, exacerbated by variations in the strength of proof required by different experts. The main scientific uncertainties in the evidence are: 1. Whether, in the descriptive studies, the excess lung cancer rates in silica-exposed workers are explicable in terms of smoking habits, socio-economic class differences and inappropriate comparison populations. Better smoking information and more carefully chosen comparison populations are needed; 2. Whether the exposure-response studies could have missed a real relationship between silica exposure and lung cancer, if one exists. Many of the exposure-response studies were conducted with great care, but weaknesses, in the available data on which the exposure estimations were based, could have caused a real relationship of lung cancer and silica exposure to be missed. These studies were sufficiently powerful to demonstrate relationships of silica exposure with silicosis and silico-tuberculosis, so it is unlikely that they would have missed any but a small risk, or weak relationship, for lung cancer. Our own recent study of coalminers used uniquely detailed and reliable exposure data, and failed to demonstrate convincingly an increased risk. This negative finding, though, applies only to a dust in which the proportion of quartz in the dust is usually less than 10%. Exposure-response studies are needed, with high quality exposure estimates, in populations exposed to respirable dust of which crystalline silica comprises more than 10%; 3. Whether the excess cancer risks in subjects on silicosis registers are the result of selection and diagnostic bias. Given these difficulties, case-register studies may not be capable of giving a reliable answer to the central question, though they have been useful in pointing to the possibility of a can

Asunto(s)

Carcinógenos/efectos adversos , Neoplasias Pulmonares/etiología , Dióxido de Silicio/efectos adversos , Relación Dosis-Respuesta a Droga , Estudios Epidemiológicos , Humanos , Exposición Profesional , Reproducibilidad de los Resultados , Medición de Riesgo

RESUMEN

The inflammatory effects of respirable cellulose fibres were studied in two short-term animal models: intraperitoneal injection in mice, and inhalation in rats. The mouse peritoneal cavity is particularly sensitive to fibrous compared to non-fibrous particles. Both cellulose fibres and the positive control fibre, crocidolite asbestos, were administered in doses ranging from 10(4) to 10(8) fibres and caused marked, dose-dependent recruitment of inflammatory cells to the mouse peritoneal cavity, which was highest 1 day following injection. Crocidolite was much more active than cellulose, despite the mass dose of cellulose being 66 times greater for an equivalent number of fibres. Crocidolite at the higher doses caused inflammation to persist through 7 days. For the inhalation study, rats were exposed daily, 5 days per week, to aerosols of cellulose dust for ca. 3 weeks at a concentration of 1000 fibres ml(-1). Inhalation exposure induced an early inflammatory response in rat lungs, as determined by bronchoalveolar lavage, which peaked at 1 day following the start of inhalation and thereafter declined, despite a further 13 days of exposure over a period of 18 calendar days. In vitro production of the pro-inflammatory cytokine tumour necrosis factor alpha (TNF-alpha) by lavaged alveolar macrophages was markedly depressed by the end of the exposure period in cellulose-exposed animals, compared to sham-exposed controls, and this effect was still present in rats that had been allowed to recover for 28 days beyond the end of exposure. We conclude that the cellulose material studied is less inflammogenic than crocidolite and that the extent of the inflammatory response within the lung appears to reduce with continued exposure over a 14-day period.

Asunto(s)

Celulosa/toxicidad , Inflamación/etiología , Pulmón/patología , Cavidad Peritoneal/patología , Peritonitis/etiología , Administración por Inhalación , Animales , Asbesto Crocidolita/análisis , Asbesto Crocidolita/toxicidad , Líquido del Lavado Bronquioalveolar/citología , Líquido del Lavado Bronquioalveolar/inmunología , Celulosa/análisis , Inflamación/patología , Pulmón/química , Pulmón/inmunología , Macrófagos Peritoneales/inmunología , Macrófagos Peritoneales/metabolismo , Masculino , Ratones , Ratones Endogámicos C57BL , Peritonitis/patología , Ratas , Ratas Wistar , Factor de Necrosis Tumoral alfa/metabolismo

RESUMEN

The study objectives were to assess the ability of intratracheal injection methods to discriminate between nine fibre types in respect of pulmonary biopersistence, and to provide approximate estimates of relative biopersistence and durability for a study of general relationships with biological and toxicological responses. The test fibres included six samples of size-selected fibre types specially prepared for research purposes, two commercially available fibres, and amosite. A 1 mg dose of each fibre type was administered to rats by intratracheal injection. The relative biopersistence of fibres in different size categories was assessed from the changes in mean lung burden, as determined by electron microscopy, at 3 days and 1, 6 and 12 months after injection. The ability of the test materials to resist dissolution was measured in a parallel series of simple in vitro acellular experiments at two pHs and in a continuous flow dissolution test. The observed differences in the persistence of fibres of differing length recovered from rat lungs were consistent with the current hypothesis that short fibres are cleared by cellular processes and long fibres by dissolution and disintegration. Differences in persistence of long (> 20 microns) fibres were correlated with measured rates of dissolution in vitro. Differences in persistence among those fibre types also studied by others workers were consistent with their findings after inhalation and intratracheal injection. Overall, the differences in the biopersistences of the test fibres following intratracheal injection were sufficient to enable an examination of the relationship of biopersistence with other biological and toxicological responses. Biopersistence was influenced by both fibre dimensions and solubility.

Asunto(s)

Contaminantes Ocupacionales del Aire/análisis , Asbesto Amosita/análisis , Modelos Animales de Enfermedad , Monitoreo del Ambiente/métodos , Inyecciones/métodos , Pulmón/patología , Fibras Minerales/análisis , Tráquea , Contaminantes Ocupacionales del Aire/química , Animales , Asbesto Amosita/química , Biodegradación Ambiental , Concentración de Iones de Hidrógeno , Inhalación , Masculino , Ratas , Ratas Wistar , Reproducibilidad de los Resultados , Factores de Tiempo

RESUMEN

A range of respirable man-made mineral fibres were tested for evidence of carcinogenicity by injection into the peritoneal cavity of male SPF Wistar rats; and differences in carcinogenicity were related to the dimensions and biopersistence of the injected fibres. The fibres tested included an amosite asbestos, a silicon carbide whisker, a special purpose glass microfibre, and a range of other man-made vitreous fibres (MMVFs) and refractory ceramic fibres (RCFs) from the TIMA fibre repository. The injected dose of each was designed as the estimated mass required to contain 10(9) fibres > 5 microns in length, as determined by optical microscopy. The numbers of long fibres (> 15 microns) contained in these doses ranged across fibres from 0.1 x 10(9) to 0.8 x 10(9) fibres; the number of long fibres thinner than 0.95 micron ranged from 0.015 x 10(9) to 0.4 x 10(9). The treatment groups contained between 18 and 24 animals. Animals were killed when they showed signs of debilitation. At autopsy, the diagnosis of mesothelioma was usually obvious macroscopically. Otherwise, histological examination of peritoneal organs was used to search for early tumour development. Judged by median survival time, four of the fibre types, in the doses administered, presented higher mesothelioma activity than amosite asbestos. The other fibres tested were less carcinogenic than the amosite. Only a ceramic material derived by extreme heating to simulate the effect of furnace or oven conditions, produced no mesotheliomas. Attempts were made, using regression models, to relate these differences to fibre dimensions and to measures of durability from separate experiments. The results pointed principally to a link with the injected numbers of fibres > 20 microns in length and with biopersistence in the rat lung of fibres longer than 5 microns. Improved quantification of the relative importance of fibre dimensions and biopersistence indices requires experimentation with a range of doses.

Asunto(s)

Asbesto Amosita/efectos adversos , Compuestos Inorgánicos de Carbono/efectos adversos , Modelos Animales de Enfermedad , Vidrio , Mesotelioma/etiología , Fibras Minerales/efectos adversos , Neoplasias Experimentales/etiología , Neoplasias Peritoneales/etiología , Compuestos de Silicona/efectos adversos , Animales , Asbesto Amosita/análisis , Biodegradación Ambiental , Compuestos Inorgánicos de Carbono/análisis , Pruebas de Carcinogenicidad , Relación Dosis-Respuesta a Droga , Vidrio/análisis , Masculino , Mesotelioma/mortalidad , Mesotelioma/patología , Fibras Minerales/análisis , Neoplasias Experimentales/mortalidad , Neoplasias Experimentales/patología , Neoplasias Peritoneales/mortalidad , Neoplasias Peritoneales/patología , Ratas , Ratas Wistar , Compuestos de Silicona/análisis , Análisis de Supervivencia , Factores de Tiempo

RESUMEN

The objective was to examine and quantify the influence of fibre dimensions, persistence in the lung, and dissolution and cell toxicity in vitro, on the risks of developing lung tumours in rats. Data were brought together from the studies carried out at the IOM under the Colt Fibre Research Programme, and from studies carried out in Switzerland and the USA under the programme of the Thermal Insulation Manufacturers Association. In both studies, groups of rats were exposed by inhalation to a range of airborne fibres. At the end of their lives they were examined for the presence of benign and malignant lung tumours and mesothelioma. The studies differed in a number of details, but were combined on the basis of approximate equivalence of cumulative exposure to airborne fibres. Logistic regression models were used to relate differences in carcinogenicity to fibre characteristics; dimensions, persistence in the lung after intratracheal injection, dissolution rates from bench-top flow-through experiments, measures of inflammation, and other cell responses to fibres in vitro. Despite the small number of data points, the results suggested a primary influence of the airborne concentrations of the numbers of fibres thinner than 1 micron diameter and longer than 20 microns, and of the measured dissolution rate of the fibres. While these results are based on only a small number of fibre types, the statistical model fits the data reasonably well, and enables some cautious insights into the quantitative influences of dimensions and biopersistence. Results were broadly consistent with those from intraperitoneal injection studies of the same fibres, in that the responses were dependent on both the durability of the fibres and the numbers of long thin fibres. In vitro and in vivo cell responses did not predict significantly the risk of cancer following inhalation.

Asunto(s)

Contaminantes Ocupacionales del Aire/efectos adversos , Contaminantes Ocupacionales del Aire/análisis , Modelos Animales de Enfermedad , Neoplasias Pulmonares/etiología , Fibras Minerales/efectos adversos , Fibras Minerales/análisis , Neoplasias Experimentales/etiología , Contaminantes Ocupacionales del Aire/química , Animales , Inhalación , Modelos Logísticos , Ratas , Factores de Riesgo

RESUMEN

Crystalline silica (mostly cristobalite) was produced by vapor-phase crystallization and devitrification in the andesite lava dome of the Soufriere Hills volcano, Montserrat. The sub-10-micrometer fraction of ash generated by pyroclastic flows formed by lava dome collapse contains 10 to 24 weight percent crystalline silica, an enrichment of 2 to 5 relative to the magma caused by selective crushing of the groundmass. The sub-10-micrometer fraction of ash generated by explosive eruptions has much lower contents (3 to 6 percent) of crystalline silica. High levels of cristobalite in respirable ash raise concerns about adverse health effects of long-term human exposure to ash from lava dome eruptions.

Asunto(s)

Pulmón/metabolismo , Microscopía Electrónica de Rastreo/métodos , Fibras Minerales/análisis , Polímeros/análisis , Animales , Técnicas In Vitro , Masculino , Tasa de Depuración Metabólica , Polímeros/farmacocinética , Ratas , Ratas Wistar , Reproducibilidad de los Resultados

RESUMEN

A number of studies have indicated that the relative toxicity of different fibre types after inhalation is related to their durability within lung tissue. The aim of this study was to assess the relative biopersistence of respirable para-aramid fibrils, chrysotile asbestos and code 100/475 glass fibres in rat lungs. The biopersistence of all three test fibres was measured by quantifying the changes in retained lung burden through time following 10 days inhalation exposure to the same target concentration (700 fibres ml-1) for each fibre type. The lung burden analyses for all three fibre types show large reductions in number and volume of retained fibres during the 16 months following exposure. Most of this reduction in lung fibre burden occurred during the first 3 months following exposure but the pattern of clearance of different size classes varies with fibre type. The para-aramid data show rapid clearance of the longest fibrils during the first months following exposure, combined with an initial increase in the numbers of shorter fibrils. This is consistent with the disintegration of para-aramid fibrils into shorter fragments that subsequently are more readily cleared by macrophages. The code 100/475 glass fibre data also show rapid clearance of the longer fibres combined with an increase in the numbers of very short fibres, consistent with the removal of long fibres through disintegration. In contrast, the chrysotile data show a more rapid reduction in the numbers of retained short fibres than of long fibres, which is consistent with preferential clearance of short fibres by macrophages and minimal transverse breakage of fibres. The biopersistence of all three fibre types, in terms of overall percentage of fibres retained after 16 months is similar, but the durability of long (> 15 microns) chrysotile fibres is very much greater than that of long fibres of code 100/475 glass or para-aramid. The clearance of the three fibre types cannot be adequately described by the first order kinetic model that is often applied in studies of lung clearance. The apparent half time of clearance of each of the fibre types depends on the size fraction and time interval examined.

Asunto(s)

Asbestos Serpentinas/farmacocinética , Vidrio , Pulmón/metabolismo , Polímeros/farmacocinética , Contaminantes Atmosféricos/farmacocinética , Animales , Carga Corporal (Radioterapia) , Masculino , Fibras Minerales , Tamaño de la Partícula , Ratas , Ratas Endogámicas

RESUMEN

The scanning electron microscope (SEM) method of fibre sizing assumes that all fibres lie flat on filters. This appears to be true for samples deposited from aqueous suspension, but in some airborne samples only about 60% of fibres lie flat on the filter surface and many of the rest are balanced at high angles to the filter surface. These inclined fibres will appear foreshortened when the filter surface is viewed in conventional orientation during SEM analysis. This could lead to an underestimation of fibre length and some fibres may be so foreshortened that they do not appear to fulfil the criteria of a fibre. The results of a preliminary investigation of the effects of variable fibre inclination in fibre counts and size data for two fibre types suggest that the SEM data do give an underestimate of the number, length and therefore volume of longer fibres in some samples. This suggests that it is important to consider fibre inclination when SEM analysis of airborne fibre samples is performed. Our preliminary observations suggest that is unlikely that it will be possible to derive widely applicable correction factors to allow for orientation effects.

Asunto(s)

Contaminantes Ocupacionales del Aire/análisis , Filtración/instrumentación , Microscopía Electrónica de Rastreo , Fibras Minerales/análisis , Humanos , Concentración Máxima Admisible , Tamaño de la Partícula