Size and shape of airborne asbestos fibres in mines and mills.

There is increasing evidence that fibre size and shape play an important part in the production of health effects related to asbestos. The dimensions of airborne fibres collected at various stages of fibre processing in three mines and six mills producing three types of asbestos were measured using phase contrast light microscopy and transmission electron microscopy. Airborne fibres of different asbestos types had appreciably different size and shape distributions. For a given asbestos type, fibres collected at different stages of processing differed in their size distributions but the differences were considerably less than between fibre types. Most of the airborne fibres to which miners and millers were exposed were short and thin and thus respirable. The physical properties which best differentiated crocidolite fibres from other types of asbestos and which had higher values determined for crocidolite fibres than those obtained for the other types, were median aspect ratio and the proportion of long thin fibres--that is, fibres less than or equal to 0.2 micron in diameter and greater than 5 micron in length as the percentage of total fibres. The median true diameter of amosite fibres was about four and three times higher than the median true diameters of chrysotile and crocidolite fibers respectively. The median true length of amosite fibres was more than four and two times higher than the median true lengths of chrysotile and crocidolite fibres respectively. The observed differences in size and shape of airborne fibres have important implications for the setting of work environmental standards and in explaining differences in health risks associated with different types of fibre.     

Non-malignant consequences of decreasing asbestos exposure in the Brazil chrysotile mines and mills

Aims: To investigate the consequences of improvement in the workplace environment over six decades (1940–96) in asbestos miners and millers from a developing country (Brazil).

Methods: A total of 3634 Brazilian workers with at least one year of exposure completed a respiratory symptoms questionnaire, chest radiography, and a spirometric evaluation. The study population was separated into three groups whose working conditions improved over time: group I (1940–66, n = 180), group II (1967–76, n = 1317), and group III (1977–96, n = 2137).

Results: Respiratory symptoms were significantly related to spirometric abnormalities, smoking, and latency time. Breathlessness, in particular, was also associated with age, pleural abnormality and increased cumulative exposure to asbestos fibres. The odds ratios (OR) for parenchymal and/or non-malignant pleural disease were significantly lower in groups II and III compared to group I subjects (0.29 (0.12–0.69) and 0.19 (0.08–0.45), respectively), independent of age and smoking status. Similar results were found when groups were compared at equivalent latency times (groups I v II: 30–45 years; groups II v III: 20–25 years). Ageing, dyspnoea, past and current smoking, and radiographic abnormalities were associated with ventilatory impairment. Lower spirometric values were found in groups I and II compared to group III: lung function values were also lower in higher quartiles of latency and of cumulative exposure in these subjects.

Conclusions: Progressive improvement in occupational hygiene in a developing country is likely to reduce the risk of non-malignant consequences of dust inhalation in asbestos miners and millers.

     

Size distribution of airborne dust in Labrador iron mines

The size distribution of the total airborne dust and its components approximates a straight line on a plot of log mass undersize against log aerodynamic size. The quartz component distribution has a reasonably constant slope, is close to the distributions found in other mines and mills, and follows the normal breakage laws. The results for the iron component suggest that the top size present is similar to that of quartz and that the slope is also similar. The slopes for total dust by weight are not as steep as for quartz and suggest a bimodal distribution consisting of a coarser mineral fraction mixed with a finer fraction from other sources, such as combustion products, that are often found. The slopes for manganese are sometimes much less than for iron or quartz and suggest that manganese is associated with a more crushable rock. The one finding of chromium suggests that it is associated mostly with very fine particles and, therefore, may have been dispersed as fume from an operation such as welding.     

Physical parameters of airborne asbestos fibres in various work environments-preliminary findings.

The results of a pilot investigation to describe the physical parameters, length, aspect ratio, mass and shape of airborne fibres in a variety of industries producing processing and handling chrysotile, amosite and crocidolite are described. Samples of airborne dust were collected on nucleopore membrane filters and examined by scanning electron microscopy. The diameters and lengths of airborne fibres collected during the dumping of raw amosite at an asbestos products plant were greater than those of fibres collected during the application of amosite insulation. Chrysotile fibres collected in the carding area of an asbestos textile plant also tended to have smaller diameters than fibres collected in the dryer and bagging areas of an asbestos mill. The measurements of fibre dimensions indicate that the degree of protection afforded a worker by optical counts using the membrane filter technique is likely to depend on variety of asbestos and stage of processing. Preliminary results are not in conflict with experimental data suggesting that asbestosis might be related to the mass of airborne dust and primary malignant mesothelial tumors to exposure to fibres in a specific range of fibre diameter and length.     

Potential for recognition of airborne asbestos fibres from spatial laser scattering profiles

This Short Communication describes some preliminary results of research which seeks to develop a method by which individual airborne asbestos fibres may be detected in real-time within a contaminated environment. The method proposes using an analysis of the spatial laser scattering profile (i.e. the complex manner in which individual particles scatter laser light), recorded from individual airborne particles, as a means of classifying the particles in terms of their morphological characteristics. Scattering profiles have been recorded from both crocidolite and chrysotile fibres and these display differentiating features which suggest that in certain cases it may be possible to discriminate each type of fibre from other airborne particulates. Examples of scattering profiles are presented and methods of automated pattern recognition, which are currently under development, are briefly discussed.     

Risk factors for asbestosis in workers of asbestos mills

The analysis was based on a set of fifteen factors, characterising: the value of exposure to dust hazard, social status of the subjects involved, presence of diseases in the anamnesis, habits, etc., with the aim of establishing their effect on the possibility of developing asbestosis among the female workers of asbestos mills. Mathematical methods of pattern recognition were used for this multifactorial analysis. The decisive rule, obtained on the "training" principle, helped to differentiate the observations made in a reference group with the following accuracy: "without asbestosis": 87.5% correct answers, and "with asbestosis": 88.9% correct answers, which confirms the possibility of predicting the individual risk of developing asbestosis by analysing the set of risk factors. The use of the improved dermatoglyphic technique contributed to demonstrating the significance of the genotype in producing a predisposition towards asbestosis. The experience acquired demonstrates the possibility of medico-occupational selection for work involving asbestos exposure with simultaneous improvement of dust abatement measures at the factory.     

Amphibole fibres in Chinese chrysotile asbestos

Ten chrysotile bulk samples originating from six Chinese chrysotile mines were studied for amphibole fibres. Five of the mines operate on ultramafic rocks whereas one exploits a dolomite-hosted deposit. The asbestos fibre content in lung tissue was examined from seven deceased workers of the Shenyang asbestos plant using these raw materials. The bulk samples were pretreated with acid/alkali-digestion, and thereafter, scanning and transmission electron microscopy, X-ray microanalysis, selected area electron diffraction and X-ray powder diffractometry were used to identify the minerals. Sample preparation of lung tissue involved drying and low-temperature ashing.All of the bulk samples contained amphibole fibres as an impurity. The amphibole asbestos contents were between 0.002 and 0.310 w-%. Tremolite fibres were detected in every sample but anthophyllite fibres were present only in the sample originating from the dolomite-hosted deposit. In comparison, anthophyllite (71%), tremolite (9%) and chrysotile (10%) were the main fibre types in the lung tissue samples indicating faster pulmonary clearance of chrysotile fibres. The total levels ranged from 2.4 to 148.3 million fibres (over 1 microm in length) per gram of dry tissue, and they were consistent with heavy occupational exposure to asbestos.     

Effect of preparation methods on the assessment of airborne concentrations of asbestos fibres by transmission electron microscopy

The usual indirect and direct preparation methods used to assess the airborne asbestos fibre concentrations by transmission electron microscopy (TEM) have been investigated in detail. It has been shown that ultrasonics, often used in the indirect preparation method to facilitate the recovery of dust, may significantly increase the fibre number concentration. This effect is highly variable and dependent on the characteristics of the workplace where the samples were collected. If the indirect preparation method is modified to avoid the use of the ultrasonic treatment, both the direct and indirect preparation methods give comparable results for fibres of length greater than 5 μm. By contrast, the number concentrations of fibres shorter than 5 μm are always different but the reason for this is not clear.     

Urinary fibres in occupational exposure to asbestos

Urine samples from 10 workers from an asbestos cement factory and from a control group of 10 workers from a foundry, were obtained; drastic precautions were taken to avoid contamination. Each urinary mineral fibre was sized and identified by transmission electron microscopy. Results show that contamination problems encountered by other authors have been overcome and that the workers exposed to chrysotile appear to excrete more chrysotile fibres, but that this difference is not statistically significant. Possibly only a few of the exposed workers are significantly exposed to asbestos, the overall exposure level being very low. The degradability of chrysotile fibres in biological fluids or the retention of fibres in some organ could explain the lack of apparent correlation between exposure and urinary concentration. Unexpectedly high concentrations of crocidolite fibres of unknown origin were detected in both groups of workers.     

Retention patterns of asbestos fibres in lung tissue among asbestos cement workers.

Retention patterns in lung tissue (determined by transmission electron microscopy and energy dispersive spectrometry) of chrysotile, tremolite, and crocidolite fibres were analysed in 69 dead asbestos cement workers and 96 referents. There was an accumulation of tremolite with time of employment. Among workers who died within three years of the end of exposure, the 13 with high tremolite concentrations had a significantly longer duration of exposure than seven in a low to intermediate category (medians 32 v 20 years; p = 0.018, one sided). Crocidolite showed similar patterns of accumulation. In workers who died more than three years after the end of exposure, there were no correlations between concentrations of amphibole fibres and time between the end of exposure and death. Chrysotile concentrations among workers who died shortly after the end of exposure were higher than among the referents (median difference in concentrations 13 million fibres (f)/g dry weight; p = 0.033, one sided). No quantitative differences in exposure (duration or intensity) could be shown between workers with high and low to intermediate concentrations. Interestingly, all seven workers who had had a high intensity at the end of exposure (> 2.5 f/ml), had low to intermediate chrysotile concentrations at death, whereas those with low exposure were evenly distributed (31 subjects in both concentration categories); hence, there was a dependence between last intensity of exposure and chrysotile concentration (p = 0.014). Among 14 workers with a high average intensity of exposure, both those (n = 5) with high tissue concentrations of chrysotile and those (n = 10) with high tissue concentrations of tremolite fibres had more pronounced fibrosis than those with low to intermediate concentrations (median fibrosis grades for chrysotile: 2 v 1, p = 0.021; for tremolite: 2 v 0.5, p = 0.012). Additionally, workers who died shortly after the end of exposure with high concentrations of chrysotile and crocidolite had smoked more than those with low intermediate concentrations (medians for chrysotile 35 v 15 pack-years, p = 0.030; for crocidolite 37 v 15 pack-years, p = 0.012). The present data indicate that chrysotile has a relatively rapid turnover in human lungs, whereas the amphiboles, tremolite and crocidolite, have a slower turnover. Further, chrysotile retention may be dependent on dose rate. Chrysotile and crocidolite deposition and retention may be increased by tobacco smoking; chrysotile and tremolite by fibrosis.     

Implications of dose distribution on monitoring requirements in U mines and mills

In U mines and mills, mean doses from gamma radiation and 222Rn daughters, respectively, range from 10-30% of the individual limits recommended by the International Commission on Radiological Protection (ICRP), while the mean exposure to long-lived dust can be as low as a few percent or as high as 30% of the ICRP recommended limit. In certain mines, 220Rn daughters are present and should also be measured and accounted for. When the doses (or dose equivalents) from all the components of the radiation sources are taken into account, according to the ICRP notions of effective dose equivalent and committed effective dose equivalent, the mean of the combined doses can reach 30-50% of the combined permissible limit of dose. It is generally observed that individual doses and exposure to radiation are log-normally distributed. Since individual exposures to each specific hazard are generally not correlated, there is a probability that a number of individuals belong to the upper part of each exposure distribution. Therefore, it can happen that non-negligible fractions of the populations are liable to be close to the combined dose limit or to be overexposed. Consequently, in view of the observed nature of the distributions and the need to account for all sources of radiation, it is essential that appropriate radiation monitoring techniques be used to measure and record all significant doses and exposures. The analysis of the results of appropriate monitoring practices will lead to improved engineering controls of radiation hazards and optimum use of preventive resources.