Issues relating to Asbestos
- The hazards associated with asbestos
- How do fibres enter the lung?
- Lung cancer from asbestos exposure
- Pleural plaques and other conditions of the pleura
- Chrysotile asbestos
- Amphibole asbestos varieties
Uses of asbestos
- Synthetic mineral fibres
Asbestos is the term used for the fibrous form of a number of naturally occurring silicate minerals that have been exploited commercially for their useful properties of:
- high tensile strength
- low thermal conductivity and
- resistance to chemical attack
There are six minerals included in this definition; one, chrysotile, is in the serpentine group of minerals, while the others, including amosite, crocidolite, anthophyllite, actinolite and tremolite belong to the amphibole group of minerals.
The three types of asbestos that have found significant industrial uses are amosite (brown asbestos), chrysotile (white asbestos) and crocidolite (blue asbestos). None of these minerals is found in commercial quantities in the UK, the bulk of the material that were used by industry were imported from Canada or South Africa.
In total, over 5 million tonnes of asbestos were imported into the UK.
The maximum importation of asbestos into the UK occurred between 1970 and 1975; thereafter importation has declined and for the amphiboles it has ceased completely. The figure shows the amount of the three common types of asbestos imported into the UK during the last century.
The physical and chemical properties of asbestos determined its uses and commercial value. The very fine fibres of chrysotile and crocidolite were ideal for textile products. Their thermal stability made the asbestos minerals useful in friction products and, together with their low thermal conductivity, in insulation materials. Asbestos cements made with chrysotile asbestos were durable materials because of the chemical bonding of the lime with the surface of the fibres.
The use of asbestos was banned in Britain in 1999, but there are still a great deal of asbestos-containing materials in buildings and industrial plant
2. The hazards associated with asbestos
The very properties that made asbestos a valuable raw material also create problems when it is inhaled; namely the ability of the fibres to split along their length into fine fibres that can reach the furthest part of the lung, and the resistance of the fibres to the chemical attack of the lung's defences.Fine fibres are more likely to be inhaled than coarse fibres because they remain suspended in the air for longer. The thin fibres that are generated when asbestos is handled can penetrate deep into the lung where they can cause disease.
Asbestos has been used since prehistoric times. One of the earliest recorded uses was as lamp wicks and funeral shrouds, and the women who wove wicks for the vestal virgins' lamps apparently wore masks to keep the dust out! However, the real hazards were first recognised by the medical profession about 1900 when the first case of lung scarring, or asbestosis, was described. This was about thirty years before the first legislation was enacted to protect asbestos workers.
In the years following the recognition of asbestosis, several other diseases, including lung cancer and a cancer of the lining of the lung or gut known as mesothelioma, have been described in association with asbestos. To combat these newly recognised risks, the law in the UK has progressively been tightened with Regulations in 1931 and 1969 and 1983 and these have been further strengthened with the introduction more stringent legislation and guidance.
The International Agency for Research on Cancer (IARC) recently reaffirmed that there is sufficient evidence for asbestos to be classified as a known human carcinogen (i.e. Group 1). All commercial forms of asbestos, including chrysotile (white asbestos), cause lung cancer and mesothelioma. The IARC also concluded that there is "sufficient" evidence that asbestos causes laryngeal cancer and, perhaps surprisingly, ovarian cancer. They also found that there was "limited" evidence that asbestos exposure causes cancers of the pharynx, stomach and gastrointestinal (GI) cancers, with the evidence for an association between asbestos exposure and GI cancer being the strongest.
3. How do fibres enter the lung?
The speed that fibres settle in air is mainly dependent on their diameter. Table 1 shows the falling speed of cylindrical fibres of different diameters.
|Diameter (μm)||Falling speed (mm/s)|
Fibres with diameter less than 3 μm will remain suspended in the air for long enough to reach deep into the lung. Long thin fibres are just as likely to penetrate into the lungs as short thin ones.
When someone is working with asbestos some of the airborne fibres are inhaled into the lungs, which comprise a series of branching tubes. The first of these tubes is called the trachea, which is attached to the larynx. The trachea is about 2 cm in diameter and approximately 12 cm long. The trachea divides into the left and right bronchi. After a few centimetres these in turn divide into segmental bronchi… and so on with about 25 further sets of branches.
Broader fibres continue to impact in the upper sections of the tracheobronchial tree falling out of the air stream onto the walls of the bronchial tubes. However, most fibres with diameter less than 3 μm regardless of length, penetrate to the alveolus. This is the part of the lung where gas exchange between the air and the blood occurs. There are over 200 million alveoli with a total surface area of more than 100 m2.
Once a fibre has deposited in the lung there is a possibility that it may cause damage to the lung tissue. This is a condition in which the lung becomes scarred as a result of prolonged inhalation of asbestos fibres. It only occurs in people exposed to relatively large amounts of asbestos, normally over many years, such as in milling, weaving, lagging or asbestos removal operations. The scarring is more properly known as fibrosis. The part of the lung that is damaged is at the far end of the smallest bronchial tubes, the place where the lung transfers oxygen to the blood stream.
As we have seen, only the finest fibres, less than 3 μm in diameter, are able to reach this part of the lung. This is why when we come to evaluate the fibre concentration in the air we count only fibres of this diameter or less.
Fine asbestos fibres, once down in the lung, are not readily removed. In fact, in sufficient numbers they are able to damage the scavenging cells that arrive to remove them and this leads to a process of 'healing' by scar formation or fibrosis.
Unfortunately, scar formation in the lung destroys useful lung tissue and ultimately may result in sufficient damage to impair the lung's ability to take up oxygen. This leads to the person becoming short of breath and, as the disease progresses, may be responsible for his or her death.
It takes prolonged high exposure to cause asbestosis and so it is a disease that is really only found amongst people who worked with asbestos for several years. It is dose-related; that is very high exposures, such as occurred before the 1950s, could produce the disease in 3 or 4 years, but lower exposures may take more than a working lifetime to cause asbestosis. Hygiene standards developed in Britain in the 1960's aimed to keep the fibre levels so low that very few exposed workers developed the disease even after 40 years of working with asbestos.
In 1985, Sir Richard Doll and Julian Peto reviewed the evidence for health hazards associated with asbestos and as far as asbestosis was concerned they concluded that there was a threshold of cumulative exposure below which clinical disease did not occur. They judged that this threshold was about 25 fibres/ml.years (that is an exposure equivalent to 25 years at 1 fibre/ml, 10 years at 2.5 fibres/ml, and so on). It is still believed that the threshold for asbestosis for most people is around this level.
The first evidence of disease is shortness of breath. At that time, the chest X-ray film or chest radiograph will show irregular shadows at the bottom of the lungs and doctors will be able to detect crackling noises through the stethoscope at the lung bases. Later, the ends of the fingers and toes become abnormally rounded, like drumsticks (called 'clubbing'). Today, physicians may use CT scans to help get a more precise diagnosis of asbestosis.
Once the disease has started, it usually progresses slowly, though there is some evidence that it may stop progressing if it is detected early and the person is removed from further exposure. It sometimes appeared for the first time after the person had left the asbestos industry. As there is no known treatment and the disease is definitely progressive if found late, early detection is essential and all asbestos workers should have regular medical surveillance.
5. Lung cancer from asbestos exposure
Lung cancer is a common and usually fatal type of cancer that occurs in almost 40,000 people in the UK each year. Far and away the main cause is cigarette smoking. However, it was known from the 1940's and 1950's that excessive numbers of asbestos workers die of this disease. The risk is such that a man with asbestosis who smokes 20 cigarettes per day has a 50% chance of dying of lung cancer. A non-asbestos exposed 20 per day smoker in contrast has a 13% chance, which is bad enough! The evidence again suggests that the more asbestos a worker has been exposed to, the greater his risk of lung cancer.
According to Andrew Darnton and his colleagues at the Health and Safety Executive , asbestos-related lung cancer probably causes 2-3% of all lung cancer deaths in males in Britain, approximately one asbestos-related lung cancer death for each mesothelioma that is diagnosed. There is some evidence that the risk of lung cancer is highest with exposure to crocidolite or amosite and to a lesser extent for chrysotile, although there is a great deal of variation in the lung cancer risks between different workplaces studied.
Lung cancer usually progresses rapidly by spreading to new sites round the body and so far methods for early detection have proved unsuccessful in improving the outlook for patients. Occasionally patients may be cured by major surgery, but essentially the hope for control of this disease lies in persuading people not to smoke. This applies with special force to asbestos workers.
This is a malignant, incurable cancer of the outside lining of the lungs (called 'the pleura') or the lining of the bowels ('the peritoneum'). When it affects the pleura it causes pain in the chest and breathlessness, and the chest X-ray shows signs of fluid and tumour inside the chest wall. It progresses slowly over 1 to 2 years, making the patient suffer more and more pain and malaise, lose weight and eventually die. A similar course is followed with peritoneal disease, though here there is pain and swelling in the abdomen.The disease is clearly associated with exposure to crocidolite and amosite, but the association with exposure to chrysotile (white asbestos) is much less obvious. Although there is evidence that risk of development of mesothelioma is related to intensity of exposure, many cases have occurred after relatively short (around 6 months) exposure to amphiboles. This is historically why different hygiene standards were applied to these minerals.
John Hodgson and Andrew Darnton published an important paper investigating the risk of mesothelioma (and lung cancer) in 2000 . The concluded that for historical exposure levels the risk of mesothelioma from the three main commercial asbestos types was roughly in the ratio 1:100:500 for chrysotile, amosite and crocidolite respectively. So from this analysis for the same fibre exposure crocidolite is 500 times more likely to cause mesothelioma than chrysotile.
Mesothelioma usually develops between 20 and 50 years after exposure to asbestos first took place. This explains why the occurrence of the disease has been steadily rising in Britain over the last 20 years (the current yearly total has now reached more than 2,000 deaths), since the peak use of amosite and crocidolite in Britain occurred during and after the last war. Restrictions on the use of asbestos in Britain over the last 40 years should eventually result in a reduction in the numbers of new victims of this disease.Latest estimates from the Professor Julian Peto and colleagues from Cancer Research UK indicate that men born in the 1940's who worked as carpenters for more than 10 years before they reached age 30 have a lifetime risk for dying from mesothelioma of about one in 17. For plumbers, electricians and decorators born in the same decade who worked in their trade for more than 10 years before they were 30, the risk is one in 50 and for other construction workers one in 125. The risk is particularly marked for carpenters because they were more likely to have worked cutting asbestos insulation boards (AIB).
These researchers also showed that for every case of mesothelioma, asbestos also causes a case of lung cancer and so the overall risk of asbestos-related cancer for carpenters is about one in 10.
The risk was also increased in other industries and the study showed that two-thirds of all British men and one quarter of women had worked in jobs involving potential asbestos exposure at some time in their lives. There was also a small increased risk in those who had lived with someone who had been exposed to asbestos.
7. Pleural plaques and other conditions of the pleura
Several other conditions may affect the pleural lining of the lung as a result of asbestos exposure. They are all relatively benign conditions, though they may be frightening for the patient if he thinks he has or may develop cancer or mesothelioma. Pleural plaques are one of these conditions. These are harmless scars in the pleura that can be found in almost anyone who has been exposed to asbestos at work. They often show up on X-ray. More rarely, the pleura scars more extensively (so-called 'pleural fibrosis') and this may cause some difficulty with breathing, If, as is only very rarely the case, it needs treatment, it can be removed by an operation. Very occasionally asbestos exposure causes fluid to accumulate around the lung (pleural effusion). This usually goes away by itself, or it can be removed easily with a needle.The Scottish Parliament have passed the Damages (Asbestos-related Conditions)(Scotland) Bill, which allows people in Scotland with certain asbestos related conditions including pleural plaques who were negligently exposed to asbestos to sue for compensation. The purpose of this bill is 'to ensure that the House of Lords Judgment, which ruled that 'asymptomatic pleural plaques...do not give rise to a cause of action under the law of damages' does not have effect in Scotland. People with pleural plaques, asymptomatic pleural thickening and asymptomatic asbestosis 'caused by negligent exposure' to asbestos will now be able to raise an action for damages.
The provisions of the Bill take effect from the date of the House of Lords Judgment in Johnston v NEI International Combustion Ltd (i.e. 17th October 2007). People whose cases have not been settled or determined by a court before the date when the Bill comes into force will be covered by its provisions.
A scientific paper has recently been published in the 'Environmental Health' journal describing the relationship between plaques and asbestos exposure. This research suggests that average exposure level and time since first exposure (but not exposure duration) are the key variables that predict the prevalence of plaques. This study also shows that fifty years after first exposure to asbestos the predicted prevalence of plaques for a population with moderate exposure (1 fibre/ml) is about 60%.
8. Chrysotile asbestos
Chrysotile is the most widely used asbestos variety, and is casually referred to as 'white' asbestos. It is a member of the group of minerals known as serpentine, all of which have the same composition - magnesium silicate.The serpentine group of minerals is widespread in nature: areas of northern Britain contain large quantities. Fortunately only the chrysotile variety occurs as a genuinely fibrous form and is much less common. Chrysotile is formed by hydrothermal alteration of rocks rich in magnesium; it may be dispersed in random orientation throughout the rock or occur in discrete veins as parallel or transverse fibres. In its raw state it is pale green, cream or white and it forms a fluffy mass of curly soft white fibres when processed. It is highly flexible and deforms inelastically which makes it ideal for spinning and weaving.
Chrysotile is a sheet silicate with alternate layers of SiO4 tetrahedra (linked in a hexagonal pattern), and Mg(OH) groups. Because of the mismatch in the dimensions of the two layers, stronger crystal structure is achieved by curvature of the composite sheets forming a scrolled structure with the Mg(OH) layer on the outside. This tubular form can be seen with high magnification transmission electron microscopy.
These tubes or 'fibrils' are the finest fibres of chrysotile that can be found. Their diameter is approximately 0.025 μm.
9. Amphibole asbestos varieties
amphibole family of silicate minerals. The amphibole minerals all have essentially the same crystal structure, that of a double chain of linked SiO4 tetrahedra with cations and anions between the chains. The general formula of the amphibole group is sometimes given as AX2Y5Z8022(OH)2
We will now outline the rules for amphibole composition. They are quite complicated and you need to understand a little chemistry to make sense of the information.
- The A site may be occupied with a large monovalent ion such as Na or K or it may be empty.
- The X site is generally occupied by two divalent ions such as Ca, Mg or Fe but it may also be occupied by a monovalent ion such as Na. In this latter case the electric charge balance of the mineral will normally be maintained by further substitution of a trivalent ion for a divalent ion elsewhere in the structure.
- The Y group may be occupied by five divalent ions such as Mg, Fe or Mn, trivalent ions such as Fe3+ or Al, or quadrivalent ions such as Ti4+ with charge balance being maintained as before by other substitutions. The maximum level of trivalent ion substitution within the Y group is 2.
- The Z group contains eight Si+ ions with a maximum limit of 2 substitutions of Al3+ being permitted, with parallel substitutions elsewhere in the A group or Y group.
is an amphibole called glaucophane
is called pargasite
is called grunerite
As we warned, this all sounds complicated and there is an enormous range of possible amphibole compositions. Fortunately only a few specific types have been found as asbestiform minerals.
These are amosite (proper name grunerite), crocidolite, anthophyllite, tremolite and actinolite.
The most common asbestos amphibole is amosite, with the formula Fe7Si8O22(OH)2. Amosite is the commercial name derived from the acronym of Asbestos Mines of South Africa, the producers of the mineral. The mineral should more properly be called grunerite. It is also known casually as brown asbestos. It occurs in veins in metamorphoric iron-rich rocks only in South Africa. It is coarser and stronger than chrysotile and forms more needle-like fibres when processed. In the raw state it is dark brown or black but when processed it is grey-brown, or white if heavily milled.
Crocidolite is the correct name for the amphibole asbestos commonly known as blue asbestos. Its chemical composition is Na2Fe32+Fe23+Si8O22(OH)2 and the presence of sodium instead of iron in the X group and the smaller proportion of iron distinguish it chemically from amosite. The primary source of crocidolite was South Africa but it was also produced in Wittenoom, Australia, or in a slightly different variety in Bolivia. It is blue-black in the raw state but when processed it forms fine fluffy fibres with a distinctive smoky blue colour. When found in its non-asbestiform form it is correctly called riebeckite.Anthophyllite is a coarse white asbestos variety that was produced in Finland until the 1960s. Its chemical formula is Mg7Si8O22(OH)2- it was not in widespread use and can only be found very occasionally in commercial products or laggings.
Tremolite and Actinolite are the magnesium and iron analogues of calcic amphiboles. Their formulae are Ca2Mg5Si8O22(OH)2 for tremolite and Ca2Fe5Si8O22(OH)2 for actinolite. Both are quite rare as commercial asbestos minerals although quite common as normal crystalline minerals. Actinolite is green in colour. It is found in association with some of the other South African amphibole asbestos (where it is known as prieskaite) and it may be present as a contaminant in products manufactured with asbestos from that area.
Tremolite asbestos was also produced in moderate quantities in Taiwan and in Korea, Pakistan, India and Italy. It consists of white silky fibres similar at first sight to chrysotile, although on closer examination it has a needle-like form. Tremolite has not had a widespread use but it can be found in commercial products.
Tremolite and actinolite may occur as a trace contaminant in other mineral products like talc or vermiculite, and also in chrysotile asbestos.
While chrysotile can only occur in one crystal form (fine fibrils or bundles of fibrils), all of the amphibole minerals may occur in a variety of different forms ranging from coarse crystals of prismatic or columnar shape to needle-like crystals and alternatively in some cases to fine fibrous aggregates as in conventional asbestos. So an amphibole such as tremolite from one geological source may look like asbestos while from other sources it is clearly not asbestos nor would it break down to produce long thin fibres like asbestos. This distinction is fairly easy to make when examining bulk samples of materials of the extreme types (fibres or coarse crystals) but it is much less obvious when dealing with intermediate forms (such as needle-like crystals) or when finely powdered materials require identification. In fact, other than morphology (size and shape) there are no easily measurable properties that distinguish between powders of asbestos and non-asbestos varieties of the same mineral.
There is currently some concern in the USA about non-asbestos elongated mineral particles (EMP's) that have a needle-like (acicular) or prismatic crystal structure, and whether these materials may also present a health risk. The available epidemiological evidence shows no indication that these materials cause mesothelioma, although the evidence for the absence of a lung cancer risk is less clear. In the past IOM scientists carried out a number of experiments to try to understand whether there was a risk associated with the non-asbestiform forms of tremolite. The figures show an asbestoform and a non-asbestiform tremolite.
These studies showed that the three "asbestiform" tremolites produced mesotheliomas in almost all animals. A brittle type of fibrous tremolite, which produced a sample with relatively few asbestiform fibers produced tumors in 70% the experimental animals. Two samples of nonfibrous tremolite produced dust samples containing numerous EMP's. Both these samples produced relatively few tumors, although one had more long "fibres'' than did the brittle tremolite that produced 70% of tumors. Clearly, this study provided some reassurance that non-asbestos EMP's do not create a serious risk to health.
The US National Institute for Occupational Safety and Health (NIOSH) has published a "Roadmap" for research that it considers necessary to deal with the potential risks from EMPs 7.
10. Uses of asbestos
Chrysotile was by far the most abundant asbestos form in terms of production and usage (about 93 per cent). It can be found in a wide variety of products from yarn, rope and textiles to cement, insulation boards, friction materials, gaskets and thermoplastics.
Crocidolite had a similar widespread use although the tendency was to use it in mixtures with other asbestos varieties. Amosite, because of its coarser nature, tended to find greater use in asbestos board and other rigid products. All three varieties may be found in all proportions in old laggings of pipes and boilers. Crocidolite was little used after about 1970, amosite after about 1980 and chrysotile after about 2000
The uses of asbestos in building construction are divided into ten broad categories:
- spray coatings and lagging
- insulating board
- ropes, yarns and cloth
- millboard, paper and paper products
- asbestos cement products
- bitumen felts and coated metals
- flooring materials
- textured coatings and paints
- mastic, sealants, putties and adhesives
- reinforced plastics
Spray coatings were used for anti-condensation and acoustic control or as fire protection on structural steel. They comprised a thin layer of cement and fibre mixture applied by high-pressure spray. The main fibre type used was amosite although the other two main varieties may sometimes be found.
Laggings are found on boilers, pipes and other items of plant. These laggings may have been produced from pre-formed sections, for example on pipes, using boards or quilts or trowelled on from a thick cement mixture. These materials are sometimes known as 'monkey dung'.
Insulating boards were manufactured from cement or calcium silicate mixed with asbestos. They were produced to provide a low density, low cost fire resistant insulation and can be found in a wide variety of buildings, both commercial and domestic.
Asbestos yarns were used in the manufacture of asbestos cloth for fire protective clothing, gloves and in fire blankets. They may also have been used in gaskets or packing materials.
Asbestos millboard and papers were generally used in fairly specialist applications such as insulation of electrical equipment. They contain a high proportion of asbestos and these products are easily damaged or abraded.
The fibre cement products produced with asbestos have had widespread use. They contain about 10% asbestos, mostly chrysotile but some crocidolite or amosite were used prior to 1976. They differ from the insulating boards in their density, which is about two to three times higher.
The remaining applications, floor tiles, bitumen felts etc, have a much lower potential to release fibres and have generally not presented a great problem in use.
1 Roof/external construction
2 Internal construction
3 Heating, ventilation & electrical equipment
4 Other items
Asbestos usage in equipment and appliances is divided into:
- domestic appliances, such as cookers, washing machines etc
- house goods, kitchen mats and ironing board pads
- fire blankets
- in heating systems
- DIY products
In the past there has been concern about asbestos in heating systems where damaged asbestos blocks or panels released fibres into the room. The warm air coming out of these heaters could effectively distribute the asbestos through the room.
There are helpful images of asbestos containing materials at http://www.hse.gov.uk/asbestos/gallery.htm
11. Synthetic mineral fibres
Synthetic fibres made from minerals, are sometimes referred to as man-made mineral fibres (MMMF) or machine-made mineral fibres (also MMMF). These fibres may be either crystalline or non-crystalline. Man-made vitreous fibres (MMVF) is a term sometimes used for non-crystalline materials with a vitreous or glass-like structure. The largest amounts of synthetic mineral fibres produced are made as mineral wool, either as slag wool, glass wool or rock wool. These materials are widely used for moderate to low temperature insulation, for example in the lofts of domestic premises. Glass continuous filaments are produced for use in making glass fibre cloth and other similar applications; they tend to have an average diameter about 6 to 10 μm. Special-purpose glass fibres are used for filtration, specialist acoustic insulation or for other purposes. These fibres typically have an average diameter of 1 μm or less. Refractory ceramic fibres (RCF) refers to fibres made from aluminium silicate.
Unlike asbestos, the synthetic vitreous fibers do not break into thinner fibers when handled although they may break into shorter fibres. This means that in most situations where synthetic mineral fibres are handled the concentrations are lower than would be found with asbestos. However, just as with asbestos it is the thinner fibres that remain in the air long enough to be inhaled and it is generally considered that fibres with a diameter less than 3 μm are respirable, i.e. if inhaled will penetrate to the alveolar region of the lung.
In the 1970's experiments carried out with synthetic mineral fibres showed that if they were artificially implanted into the lung they could cause tumours. However, extensive epidemiological studies in Europe and North America have shown that glass, rock and slag wool do not cause mesothelioma and do not increase the risk for lung cancer amongst workers manufacturing these materials.
In 2002, The International Agency for Research on Cancer (IARC) published a summary of their evaluation of the cancer risks from some synthetic mineral fibres8. They concluded that:
- Continuous glass filament, glass wool, rock wool and slag wool are not classifiable as to their carcinogenicity to humans (Group 3)
- Special-purpose glass fibres such as E-glass and '475' glass fibres are possibly carcinogenic to humans (Group 2B)
- Refractory ceramic fibres are possibly carcinogenic to humans (Group 2B)
Darnton AJ, McElvenny DM, Hodgson JT. (2006) Estimating the number of asbestos-related lung cancer deaths in Great Britain from 1980 to 2000. Ann Occup Hyg;50(1): 29-38. Available at: http://annhyg.oxfordjournals.org/cgi/content/full/50/1/29
Hodgson JT, Darnton A. (2000) The quantitative risks of mesothelioma and lung cancer in relation to asbestos exposure. Ann Occup Hyg; 44(8): 565-601. http://annhyg.oxfordjournals.org/cgi/content/abstract/44/8/565
Paris C, Martin A, Letourneux M, Wild P. (2008) Modelling prevalence and incidence of fibrosis and pleural plaques in asbestos-exposed populations for screening and follow-up: a cross-sectional study. Environmental Health; 7: 30. http://www.ehjournal.net/content/pdf/1476-069X-7-30.pdf
Davis JMG, Addison J, McIntosh C, Miller BG, Niven K. (1991) Variations in the Carcinogenicity of Tremolite Dust Samples of Differing Morphology. Annals of the New York Academy of Sciences; 643: 473 - 490. http://www3.interscience.wiley.com/journal/119349749/abstract
State of the Science and Roadmap for Research. Revised Draft NIOSH Current Intelligence Bulletin. Cincinnati: NIOSH. http://www.cdc.gov/niosh/docket/pdfs/NIOSH-099B/099B-010109-Asbestos-NA-Review%20doc.pdf
Available at: http://www.inchem.org/documents/iarc/vol81/81.html
There is a great deal of helpful information on: