Document 6Rd4O8oyV8mVXwr2mxE5OKXMg

Section ^ Jccupationai Medicine Table i Chrywtik ftitMSios content of brake drum dints Great Britain West Germany Franc* Unued States of America Finland Western Australia Xv. ot samples 8 tt \ X-ray diffraction {step-scam ....-................. ' : ; Xu. positive MV/yAr percent Mean kontff 6* 1.8 0.7-2.3 5* 2.4 0.5-3.2 l 2.5 10 10 53 7J J.5 2.0-15.1 1.8 U.8-2.5 J.4 0.5-5.3 39 29 r II I simple possibly positive Transmission electron niteroscupv Xn. pft.Mtirc 8 8 1 to < 7 39 properties consistent with chrysotile. been obser ved with this technique. X-ray dijjractometry: The ten brake dust samples were analysed by X-ray diffractometry, in both continuous and step-scan mode. The diagnostic reflection selected for chrysotile (3.66 A; hkl~ (004)) was step-scanned in the fixed-count mode. By comparison with external standards, the amount of chrysotile present can be determined. Chrysotile reflections were observed in all samples, with weight percentages estimated to range from 2 to 15 (average 4.5). Lead compounds, quartz, carbonate minerals, clays, halite (NaCl). graphite, micas and alpha-iron were also identified in vari ous samples. Twenty-nine additional brake drum dust samp les were collected by colleagues in four European countries and Australia. The samples were ob tained from areas representing variable circum stances. such as driving conditions, friction material composition, type of automobile and climate. The results of X-ray diffraction analyses are presented in Table 1. Of the 39 total samples analysed (including 10 United States samples) chrysotile was found in 29, or about three-fourths. The mean chrysotile content varied from 1.4*;, in the seven Australian samples to 4.5 "c in the New York City samples. The thermal breakdown product of chrysotile. i.e.. forsterite. which might be expected to form as a result of reervstalibation, was not unambigu ously detected in any sample. 7ransmission electron microscopy (TEST): In order to verify the results of X-ray diffraction analysis, the brake dust samples were prepared for electron microscopic analysis by means of a technique which disperses the dust particles in a nitrocel lulose film without altering particle sbes. Both free chrysotile fibre bundles and fibrils were observed in all 39 samples. Most of the chrysotile retained its characteristic morphology without significant alteration, and selected area electron diffraction patterns obtained on representative fibres dem onstrated the preservation of crystal structure as well (Fig 1). Fibre size distribution: In ten brake dusts sampled in New York City, free asbestos fibres were sbed by TEM at magnification 42 000. The results showed that about 80 "0 ofchrysotile is in free fibril form and is shorter than 0.4 pm in length. Over 57% have lengths of about 0.2 pm. At magnifi cation 40 000, such a fibre would be about I cm long. If lower magnification were used to scan for asbestos, significant numbers of fibres might not be detected. It is obvious that most fibres are too small to be seen by optical microscopy. The Asbestos Stan dard adopted by the Occupational Safety and Health Administration (OSHA) of the US Depart ment of Labour is limited to optica) microscopy and neglects to count or control fibres less than 5 pm long. On the other hand, accumulating evidence suggests that such small asbestos fibres may produce disease (Holt et al. 1964. Davis 1965. Pott et al. 1972. Wagner et al. 1973. Hilscher et al. 1970. Bouhuys 1975). Air sampling: Eight personal air samples taken during brake repair work were selected for electron microscopic analysis. This was done in order to positively identify chrysotile in the samples and to determine whether a systematic relationship existed between optically and subnticroscopically visible fibres for this type of exposure. The memb rane fillers were ashed in plasma oxygen to elimin ate organic materials and the residue was prepared for electron microscopy by a 'rubout' technique (Nicholson et al. 1971). which comminutes large chrysotile fibre bundles into individual fibres and fibrils. Large inorganic particles are likewise re duced in size, permitting all chrysotile to be seen and measured. Chrysotile was identified in the eight samples, in both fibre and fibril form (Fig 2). By measuring a large number of fibres at magnifi cation 42 000 and converting the volumetric data into mass, a concentration per volume of 3ir is determined. Comparison of optical microscopic 60DJ 0016 PROTYl 'rPTT PY FOPT)