Material of the month - Asbestos

Materials World magazine
,
1 Sep 2014

This month Anna Ploszajski unveils the history of asbestos.

Many years ago, a newly rediscovered wonder-material promised to enhance fire protection for the occupants of millions of buildings worldwide. But our amicable relationship with asbestos was short-lived, when it claimed the innocent individuals who were ignorant of its toxicity. Governments were slow to react, and the awful repercussions of past mistakes are still apparent today.

Asbestos refers to the fibrous form of six naturally occurring silicate minerals. These fibres are long and thin (around 0.1μm in diameter), and large deposits lie in Quebec, South Africa, China and Russia. The most commonly used is chrysotile, or white asbestos, which comes from the serpentine group of minerals and is formed by the hydrothermal alteration of rocks rich in magnesium. Its high flexibility and ability to deform elastically means it can be spun and woven into fabric. At the atomic scale, chrysotile is made from alternate layers of SiO4 tetrahedra and Mg(OH) groups. The lattice mismatch between these layers causes curvature of the sheets, resulting in a scrolled structure with the Mg(OH)2 layer on the outside. Chrysotile makes up 95% of asbestos used in buildings in the USA.

The other five forms of asbestos all come from the amphibole family of silicate materials, of which the most commonly used are amosite and crocidolite. The remaining three – tremolite, anthophyllite and actinolite – are not generally exploited. They all have needle-like fibres, which are formed from a double chain of linked SiO4 tetrahedra with cations and anions between the chains. Amosite, or brown asbestos, occurs in veins in metamorphic iron-rich rocks only found in South Africa. Crocidolite, or blue asbestos, has sodium ions and can occur in a range of forms, including coarse prismatic or columnar crystals, needle-like crystals and thin fibres.

The name asbestos comes from the Greek meaning unquenchable or inextinguishable, and it was the realisation of this fire-resistant property as well as sound absorption, low cost, chemical stability and electrical insulation that precipitated the dawn of large-scale asbestos mining in the 1870s. Used alone or made into ceramic composites, asbestos found application in construction materials including fire-retardant coatings, concrete, bricks, pipes, textured walls and ceilings, roofing, mastic and vinyl flooring. Flexible chrysotile fibres spun and woven into fabric were incorporated into corrugated asbestos cement roof sheets, plaster, and panels for ceilings, walls and floors as well as brake-pad linings and pipe insulation. Asbestos was even used in the manufacture of ammonium sulphate in Japanese rice production and briefly in cigarette filters in the 1950s.

A notorious material

Undisturbed natural sources of asbestos do not actually pose much health risk. Indeed, trace amounts are found naturally in the air and drinking water. Most people live perfectly healthy lives with millions of fibres in each lung. However, its misguided use in the built environment has given asbestos a notorious reputation as a highly dangerous material.

Individual asbestos fibres can be as narrow as 10nm across, making them invisible to the naked eye. The crystals have three cleavage planes, of which two are much weaker than the third. This causes fibres to preferentially break in the weakest directions along the fibre axis, and with repetitive fracture, one thick strand becomes hundreds of narrower fibres. It is this crystallographic weakness that gives asbestos fibres their deadlines.

The serious health hazard posed by asbestos lies not with the materials in-situ, but with its effect in the lungs once airborne, if disturbed. Fibres with diameters less than 3μm will remain suspended in the air for long enough to reach the deepest parts of the lung. Prolonged inhalation of asbestos fibres damages the sensitive tissue enough to cause lung cancer, mesothelioma (cancer of the lining of the lung or gut) and asbestosis (scar formation, which eventually inhibits the lung’s ability to take up oxygen).

Other conditions attributable to overexposure to asbestos are asbestos warts (when sharp fibres become lodged in the skin and are overgrown, causing benign callus-like growths), pleural plaques (benign fibrous thickening of lung tissue) and pneumothorax (trapped air next to the lung). The most hazardous products are the spray coatings, fire-retardant materials, woven cloths and cement composites, from which asbestos is most easily released.

Sadly, the danger posed by asbestos in the built environment was realised all too late. This is largely attributable to the long latency period of asbestosis, meaning that disease can develop up to 20 years after asbestos exposure. The most harmful asbestos-related illness is when individuals endure low-level exposure over long periods of time, making factory workers particularly vulnerable. Not only were individuals who were exposed occupationally at risk, but mesotheliomas have also been observed in their families and the nearby residents of asbestos factories and mines.

In the early 1900s, doctors noticed unusually high rates of lung complaints and deaths in asbestos mining towns. In 1924, British textile worker Nellie Kershaw’s premature death from pulmonary asbestosis was the first account of disease attributed to occupational asbestos exposure to be recorded in the medical literature. This precipitated an inquiry by Parliament into the effects of asbestos dust, which led to the publication of the first UK Asbestos Industry Regulations in 1931. These introduced regulated ventilation and ruled that asbestosis was an excusable work-related disease.

Further regulation followed in 1969 and 1983. In 1985, the safe threshold for ambient asbestos exposure was defined as 25 years at 1 fibre/ml. In the same year, the use of blue and brown asbestos was completely forbidden in the UK. Banning of the import, sale and reuse of white asbestos materials was introduced in 1999. Recognising the widespread presence of potentially lethal asbestos still in-situ, regulations introduced in 2012 ensure that owners of non-domestic buildings have a duty to manage asbestos, to mitigate its release and to remove completely if necessary. Furthermore, workers likely to come into contact with asbestos must be provided with annual training. Despite all of this litigation, asbestos is still the single greatest cause of work-related deaths in the UK.

The situation is even bleaker across the pond. Still to this day, the USA is one of a handful of developed countries yet to fully outlaw asbestos, despite the fact that court documents from the late 1970s show that asbestos industry officials had known of the dangers posed by asbestos since the 1930s. More controversial still, these documents also claimed that officials had consciously concealed the dangers from the public in their own best interest. The continued use of asbestos in clothing, vinyl flooring, disc brakes, roof coatings and many other applications is staggering, considering that asbestos litigation is the longest and most expensive mass tort in history, costing many billions of dollars. Some states have made limited progress by banning asbestos in certain applications and introducing exposure limits for drinking water and workspace air, which have been set by the US Environmental Protection Agency and the Occupational Safety and Health Administration. Nevertheless, the dormancy of asbestos-induced disease means that these minimal efforts are likely to be too-little-too-late to reverse the tragic damage already done.

There can be no greater tragedy from this lack of sufficient legislation than its impact on the terror-driven events of 9/11. Asbestos was used as a fire retardant in the construction of the Twin Towers, and upon its collapse, more than 1,000t of asbestos was released into the air. This, combined with other potential toxic fumes, has contributed to the high death-rate of emergency service workers present at the scene since the disaster, and the latent effect means that thousands more could follow.

Managing the threat

So how to manage this slow and silent killer? Buildings contaminated with large amounts of asbestos must be deconstructed by hand or removed by mechanical means. Once isolated, asbestos can be recycled by thermal decomposition at 1,000–1,250°C to produce a mixture of safe silicate phases. Alternatively, asbestos can be transformed into porcelain tiles or ceramic bricks by microwave thermal treatments.

There is no need to use asbestos in our modern age of man-made materials. Suitable substitutes include fibreglass, mineral wool, glass wool, aluminium silicate fibres and organic fibres. Such materials are safe because they do not break into thinner strands upon handling.

We are still suffering from the mistakes made over the past 150 years, when people were unaware of the destructiveness of the asbestos industry. Equipped with full knowledge of the danger at hand, let’s hope that the next generation will be able to breathe freely.