Fibrous dust release from asbestos substitutes in friction products

Our plain-English summary of Fibrous dust release from asbestos substitutes in friction products, by S.A. Jaffrey, A.P. Rood, R.M. Scott (The Annals of Occupational Hygiene, 1992).

The study

• Authors: S.A. Jaffrey, A.P. Rood, R.M. Scott
• Journal: The Annals of Occupational Hygiene
• Published: 1992;36(2):173-81
• Read it: PubMed · Publisher

Abstract

A small-scale study was carried out on friction and reinforcing materials containing Kevlar and glass fibres. Kevlar, an aramid fibre, and synthetic silicate fibres are being increasingly used in brake pads and shoes for cars and trucks. The fibre levels given out under abrasion in the laboratory were low. As a comparison, a brake pad containing asbestos was also tested and gave high levels of airborne fibres.

In plain English

Researchers at the UK Health and Safety Executive carried out a small-scale laboratory study of fibre release from brake and clutch products that used Kevlar (an aramid fibre), glass and other man-made fibres instead of asbestos. They rubbed one disc pad, four car brake shoes, six lorry brake shoes, two clutch plates and two retail brake pads (one with asbestos, one without) with fine emery paper for 10 minutes in a dust box. They then examined air samples by electron microscopy. They also made pure Kevlar pulp airborne, and practically all of the 302 fibres measured were in the respirable size range. Most substitute products gave low airborne fibre counts. Almost all lorry brake shoe samples were below 0.01 fibres per millilitre (f/ml), and the retail non-asbestos pad averaged 0.007 f/ml. Two lorry shoe exceptions gave up to 0.34 f/ml. A few chrysotile fibres were found in some substitute products, which the authors suggested may have been impurities. Air samples from the asbestos brake pad were overloaded with asbestos dust. The authors concluded that fibre levels from the substitute products were extremely low. They cautioned that the counts were approximate because heavy non-fibrous dust made counting difficult.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Small-scale laboratory study: friction products were abraded in a dust box and air samples were analysed by transmission electron microscopy (TEM) and energy dispersive X-ray analysis
• Who was studied: Friction products, not people. From industry: one disc pad, four car brake shoes, six lorry/van brake shoes, two clutch plates and a sample of pure Kevlar pulp. From a retailer: two brake pads, one containing asbestos and one without. Work done by the Health and Safety Executive, London, U.K.
• Exposure: Dust from brake and clutch linings containing Kevlar, man-made mineral fibres (glass and other silicate fibres), carbon fibres and metals, plus one chrysotile-containing brake pad for comparison. Each product was rubbed by hand with fine emery paper for 10 min, estimated to equal the dust released over 1000-2000 km of car use.
• Main results:
  • Kevlar pulp: 302 fibres were measured and practically all fell in the respirable range. 96% had diameters below 2.5 micrometres (printed 'nm' in the scanned text) and over 87% were below 1 micrometre. All were under 80 micrometres long.
  • Clutch plates: the plate with 6.5% Kevlar gave a total fibre concentration of 0.29 f/ml. The plate with 4% Kevlar and 3% PAN gave less than 0.1 f/ml.
  • Clutch plates: all fibres detected in the second sample were inorganic, and a single chrysotile bundle was detected in each sample.
  • Van and lorry brake shoes: almost all samples gave fibre counts below 0.01 f/ml. Two exceptions gave up to 0.34 f/ml, along with large non-fibrous deposits.
  • Van and lorry brake shoes: one sample showed a slightly raised chrysotile concentration, averaging 0.003 f/ml. The authors thought this shoe may have been a trial sample.
  • Car brake shoes: all four gave low fibre counts and low dust levels. Most fibres were judged to be glass fibres, and one or two chrysotile fibres were possibly impurities.
  • Car brake pad supplied by industry: fibre counts were below 0.1 f/ml, mostly probably glass fibres.
  • Retail non-asbestos brake pad: average airborne concentration of 0.007 f/ml.
  • Retail asbestos brake pad: all three air samples inside the box were overloaded with asbestos dust, containing large amounts of respirable chrysotile fibrils.
  • Retail asbestos brake pad: near the box ceiling, less than 14% of chrysotile fibres were longer than 5 micrometres. Near the activity, nearly 40% were.
• The authors' conclusion: The authors concluded that airborne fibre levels in dust from these substitute friction materials were extremely low. They noted that the metallic content of the newer products should be studied further.
• Limitations the authors state:
  • Fibre concentrations should be regarded as approximate, because large amounts of non-fibrous dust made counting difficult.
  • The reported counts are only an indication of high or low levels.
  • Kevlar fibres are hard to identify by EDXA and could only be deduced by elimination.

Topics

Medical terms in this study, explained in our Lexicon: Friction Products, Silicate, Brake Lining.

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