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Fibre Design Introduction Fibre Types They also frequently occur in composite materials and many aspects of our society would find it difficult if not impossible to function without these materials. Applications include brake and clutch linings, floor coverings, building boards, media for filtration of liquids, and advanced composite materials such as reinforced metals. Mainly because of their experience with asbestos, which is a heterogeneous sub-group of naturally occurring mineral fibres, many people suspect that all fibres are potentially hazardous. One of the most frequently asked questions is whether manmade mineral fibres (MMMFs) pose a comparable health hazard to asbestos. This is an indication of a lack of knowledge of the important differences in size and composition between the asbestos minerals and most MMMFs. By examining the current fibre knowledge base it is possible to build up a balanced view of the factors which determine the real health risks and thereby to assess any risk from MMMFs. Dust Generation To generate dust from these fibrous materials, it is necessary to fragment them into smaller lengths before they can become airborne. This is an important characteristic. When fibre is left relatively undisturbed as, for example, glass fibre insulation in an attic, then the potential to generate dust is very low. When the fibre has been disturbed in some way and fragments are released, it is the fibre's diameter which primarily determines the length of time the fibre will remain in the air. Both mathematical and practical considerations show that the rate of deposition, or falling speed, is proportional to the square of the diameter. For example, if you take 1 um fibres and 3 um fibres of identical length, the 1 um fibre will remain suspended in air approximately nine times as long as the 3 um fibre. This means that when dust is generated from a fibre with a range of diameters, then the coarser the fibre the lower the equilibrium dust level. Finer fibres produce more dust under most working conditions. Health
Effects and Disease Lung fibrosis a) Asbestosis (honeycomb lung) The two are distinguished by the different patterns they produce on X-ray images, but both have similar medical effects in that they reduce the ability of the lung to oxygenate blood efficiently. In extreme cases the disease can result in breathlessness and fatigue on even mild exertion and enlargement of the heart as it attempts to pump blood more rapidly to compensate for the low oxygen levels. Although fibrosis is rarely fatal in itself, the severe form is associated with heart failure consequent on the increased load placed on this organ. Less severely affected individuals may suffer varying degrees of breathlessness and those with the mildest form may be unaware of its existence unless diagnosed by a doctor, following a chest X-ray. Lung cancer a) Bronchial carcinoma (the tumour commonly associated
with smoking) Both have been associated with asbestos exposure but bronchial carcinoma shows a very strong association with smoking. Asbestos workers who smoke may be at significantly higher risk than either non-smoking asbestos workers or smokers who do not work with asbestos. Malignant mesothelioma is characterised by a very long latent period (time between first exposure and appearance of the tumour) but shows no association to smoking. Skin effects The most familiar skin effect is the transient skin irritation experienced when handling many types of synthetic mineral fibres. This irritation is caused by physical penetration of the outer layers of skin by fibres. Medically speaking, the irritation is of limited significance because it does not persist after exposure ceases. Nevertheless, it may be an important problems for workers in the industry because they suffer discomfort, may become irritable and unable to continue to work. Skin irritation is generally associated with handling the bulk fibre rather than dust although exposure to dust has occasionally been associated with irritation to the upper respiratory tract (coughing) and there are isolated reports of eye irritation. Association between Fibre Dust and Disease Irritation Long term effect To express any long term toxic potential, it is necessary for a dust particle to penetrate into the deepest spaces of the lung where it may be retained for a period of time. A 'respirable' particle is defined as one capable of this penetration and deposition. An 'inhalable' particle is defined as one with the ability to enter the respiratory tract but not necessarily be capable of penetrating the series of defence mechanisms designed to exclude dust particles, which are always present in the air, from penetrating to the deepest lung spaces. The lung can be considered to consist of a cascade of branching airways, lined with a thick mucus layer which is constantly moving upwards from the deep lung to the large bronchi and larynx. The air flow physics are such that most particles are trapped in this surface layer, and within a few hours are generally cleared to the throat where they are usually subsequently swallowed. Beyond these airways is an area of very small sacs with thin walls (the alveolar region). The region is well supplied with blood contained in very fine capillaries and this is where inhaled air gives up its oxygen and collects carbon dioxide. The ability of a particle to penetrate to the alveoli is dependent on size. However, once a particle has penetrated to the alveolar spaces it may be either deposited or remain suspended and eventually exhaled. Thus, the fate of any particle of inhaled dust can be to be deposited within the airways as it is inhaled, or if small enough there is a chance that it may be retained in the deep lung. Particle size Because particles have a range of different sizes and shapes and may be composed of materials of differing densities a unifying system of description of particle size takes these into account. Thus particles are usually expressed in terms of their aerodynamic equivalent diameter (AED) which is the diameter of sphere of unit density with the same aerodynamic characteristics as the particle in question. Particles greater than 10 um AED will not penetrate to the alveolar region in man. Particles less than 10 um AED have a probability of progressing to the deep lung, but the probability is low for particles of 5 to 10 um AED, which are mainly deposited in the ciliated airways. The exact relationship between percentage penetration and the AEDs is complex, varying for nose or mouth breathing and exercise rate. Nevertheless, it is generally true that the maximum probability for deposition in the deep lung is for particles in the region of 1 to 2 um AED, where up to one fifth of the relevant particles may be deposited. For most minerals, this corresponds to an actual diameter of about 1 um. Fibre diameter As for particles, not all 'respirable' fibres will be deposited in the lungs if inhaled. There is a substantial difference between fibres close to the 'respirable' limit and those of a finer diameter which have a much higher probability of alveolar deposition. To continue with the example of a comparison of 3 um and 1 um fibres, then from a dusty atmosphere containing equal number of these fibres, at least ten times more of the 1 um fibre will penetrate to the alveolar spaces than would be the case for the 3 um fibre. Once deposited, the potential for disease induction is also dependent on the physical as well as the chemical composition of the fibres. Long fibres are much more active than short fibres in producing fibrosis. There is also a contribution of surface chemistry. Although this latter is not fully understood, materials which are strongly fibrogenic in fibrous form also produce fibrosis as particulates. For the carcinogenic effects of fibres it is possible that different mechanisms may operate for the two tumour types seen. Very strong association between the occurrence of lung cancer, asbestos exposure and cigarette smoking has been mentioned. The association is frequently used as an example of synergy. That is, the risks of cigarette smoking and asbestos exposure are multiplied, not added. WHO definition of a respirable fibre
The mechanism by which fibres may induce tumours of this type is not known, but there is substantial epidemiological evidence that at doses where no asbestosis is seen there is a much reduced risk. As most man-made mineral fibres do not include progressive fibrosis they are, at worst, much less carcinogenic than asbestos (and probably not carcinogenic at all). Thus, while there is still scientific debate about the properties of fibres which are important in relation to induction of these tumours, there is substantial human evidence that controlling asbestos exposure to levels where fibrosis is not seen reduces the incidence of the tumour to levels seen in the general population. Most scientists also accept that the tumour is associated with exposure to asbestos fibres less than 1 um in diameter and (probably) more than 10 um long. There is little evidence that surface chemistry is implicated except insofar as it affects fibrosis and durability. Mesothelioma has proved somewhat easier to model in animals and the properties associated with its induction are therefore better understood. Systematic investigation has revealed that fibre size is more important than chemistry in determining the capacity to induce this type of tumour. A length of more than 10 um (and probably more than 20 um) is essential for carcinogenic potential to be expressed and the material must have a 'fine' diameter. All investigators agree that this diameter is less than 2 um and most would place the diameter much lower, with figures less than 0.5 um commonly quoted. Durability Fibre durability reflects its ability to resist dissolution in the surprisingly aggressive environment of the lung. The parameters governing this are poorly understood. Animal experiments have demonstrated that fibres which dissolve over a period of several months may not be toxic in the long term. These observations are mainly derived from a single dose of fibre and it is unclear whether repeated doses of the soluble fibre might induce some of the long term effects described, although there is limited evidence that they might. Nevertheless, a non-durable fibre must give an additional margin of safety as, by definition, the amount of fibre present at any one time will be less than with a similar exposure to a durable material. RCF Experiments The experiments were conducted to a very high standard, using the latest techniques and protocols. They involve the administration of size selected fibre which was highly 'respirable' by a nose-only exposure route to maximise deposition. The test material was prepared by a complex process of chopping, milling and water elutriation to separate fibres of mean diameter approximately 1 um to maximise the biological activity. The fibrosis lung tumours and small incidence of mesothelioma seen in these studies confirm the theoretical predictions relating to fibre size and establish a dose response and a clear, 'no-effect' level. Recent work suggests that this response may have been enhanced by the presence in the test sample of an excess of non-fibrous particles. Comparative
data Inevitably, comparisons have also been made with asbestos. These comparisons are difficult to justify, given the complexity of factors involved. Consideration of aerosol physics (and experience of attempts to generate intense clouds of MMMF for experimental purposes) indicate there is an intrinsic difference in dustiness between materials and in particular between the bulk of synthetic fibres and asbestos. Further, the asbestos fracture behaviour results in splitting lengthwise to produce finer fibres, with the result that the hazard may increase. All glassy MMMFs break transversely to give shorter fibres (and reduced risk). The result is that for most MMMFs exposures will be lower that those encountered with asbestos. An ideal fibre with respect to biological activity would balance the following criteria with functional performance requirements:
Design of Saffil Alumina Fibre Saffil inorganic fibres were designed to minimise the potential for biological activity. They are produced using controlled extrusion and drawing of a viscous aqueous solution to give, after heat treatment, a median diameter of around 3 microns with a narrow spread of individual diameters. Individual fibres are parellel-sided unlike many melt-spun vitreous fibres which taper down their length to finer diameters. Typically, sub-micron fibres are controlled to less than 1% by number (this implies much less than 0.1% by mass). As produced, fibre length is measured in centimetres. For Saffil to perform its technical function fibre length needs to be longer than 10 um. This property is essentially not controllable at manufacture although the properties of Saffil are such that long fibres are unlikely to be respirable since the small effect of length on AED becomes important when diameters are within such a narrow range. The expectation therefore is that Saffil fibres have little or no biological activity. This has been confirmed with a series of toxicological tests both in vivo and in vitro. Saffil has shown no potential for fibrosis in an in-vitro macrophage test or in a short-term intra-peritoneal injection in-vivo test, both sensitive models. For mesothelioma, in vitro tests have shown the fibres to be inactive and this has been confirmed by intra-pleural injection tests in animals, a test which is frequently claimed to be over sensitive. A lifetime inhalation study on Saffil fibres confirmed that they have a low respirable potential and are inert when deposited in the lung. Finally, a feeding study in which the fibre was administered in up to 2.5% of the diet of rats for the lifetime confirmed that there were no adverse effects of this material when eaten. The RCF studies provide further confirmation of the validity of the 'Saffil' fibre design specification with respect to the importance of minimising the level of fine diameter fires, especially those fibres with a diameter of less than 1 um. It should be stressed, however, that it would not
be possible to produce from bulk Saffil fibres a dust fraction similar
to that produced from RCF. Clearly therefore, extrapolation of results
from other materials to cover Saffil fibres is inappropriate. Experience with asbestos has led to a mistaken belief that all mineral fibres are hazardous. Man made mineral fibres (MMMF) are a diverse group of materials of widely varying composition and properties. Evaluation of their potential hazard must take this into account. Fibre size and fragility (the ability to generate fibrous dust) are important criteria in determining the biological activity of fibres. Fine diameter fibres produce more fibre dust than coarse fibres in most circumstances. Coarse mineral fibres (more than 5 um in diameter) are irritant to the skin, eyes and throat. An ideal 'safe' fibre is one which is non-respirable. The fibre should also be non-fibrogenic in particulate form, and soluble in lung fluid. To avoid irritation, the fibre should also be less than 5 um in diameter. Diseases associated with exposure to respirable mineral fibres (mainly asbestos) include lung fibrosis (asbestosis), lung cancer and mesothelioma. For most minerals, respirable fibres are less than 3 um in diameter, but only a small proportion of fibres close to this limit is retained in the lungs. For mineral fibres, maximum deposition (about 20% of those inhaled) in the deep lung occurs for fibres about 1 um in diameter. Fibrosis is associated with long respirable fibres rather than short ones and also depends on fibre chemistry. Materials which give strongly fibrogenic fibres also cause fibrosis as particulates. Lung cancer is associated with fibrosis. All evidence from asbestos indicates that controlling fibrosis eliminates lung cancer. Mesothemolia is strongly dependant on fibre size. Most scientists accept that fibres must be less than 1 um in diameter and more than 10 um long, to give a significant risk of mesothelioma. Solubility of fibres in the lung is also important.
Fibres which persist for only a short time (months) in the lung
have a much reduced or non-existent cancer risk. Saffil inorganic fibres were designed to fulfil as many of these criteria as possible. The median diameter of Saffil is about 3 um and typically, there are less than 1% by number sub-micron fibres. They are made from non-fibrogenic material. Although only poorly soluble in lung fluid, dust from Saffil fibres has a low probability of deposition in the lung. Only a small proportion of the fibre is more than 5 um in diameter. 'Saffil' should not be compared with other refractory mineral fibres or natural fibres with very different physical properties. A series of biological tests with Saffil fibres all gave negative results. All the predictions, both theoretical and practical, point to Saffil being a fibre with minimal biological activity and free of those problem normally associated with asbestos. The contents of this note are given in good faith but without warranty and the user must satisfy himself that the product is entirely suitable for his purpose. Freedom from patent rights must not be assumed. For further information and assistance regarding the safe handling of Saffil fibre products, please contact Saffil Ltd at the address below or alternatively your local office. Saffil Ltd |
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