Monitoring and evaluating man-made mineral fibres: work of a WHO/EURO Reference Scheme.

In the mid-1970s, a meeting held at WHO in Copenhagen launched a research programme on the health effects of man-made mineral fibres. As part of this programme, a WHO/EURO Reference Scheme was initiated in 1980 to improve comparability of data obtained from epidemiological studies and thus reduce the level of uncertainty in the risk assessment process. The scheme involved developing reference methods for evaluating airborne MMMF concentrations and size distributions by phase contrast optical microscopy (PCOM) and scanning electron microscopy (SEM). These methods were tested in a series of inter-laboratory counting trials. Interim progress in the scheme was published in 1985. This paper updates the work of the scheme and confirms that the previously published improvements in reproducibility of measurement have been maintained. I N T R O D U C T I O N THE REGIONAL Office for Europe of the World Health Organization (WHO) is concerned with the problems of industrialized society. The adverse health effects of exposure to chemicals, harmful dusts and fibres in all aspects of daily life rank high in the WHO priorities. In the mid-1970s, a meeting held at WHO in Copenhagen launched a research programme on the health effects of man-made mineral fibres (MMMF). This programme was sponsored by the European MMMF industry through its Joint European Medical Research Board (JEMRB) and comprised: animal experiments conducted by the British Medical Research Council Pneumoconiosis Unit; an international epidemiological study co-ordinated by the International Agency for Research on Cancer (IARC); and environmental surveys performed by the Institute of Occupational Medicine (IOM), Edinburgh, U.K., to determine airborne fibre levels and fibre size distributions in several European MMMF factories. For the epidemiological study, it was necessary to give special attention to the quality of measurements of airborne MMMF. Different measurement methods were used in different countries (OTTERY et al., 1982). Furthermore, different laboratories did not always produce comparable results even when using the same method; this was particularly true for the subjective procedures of fibre counting and sizing. This is similar to experience with asbestos fibre counting which has resulted in the establishment of national and international quality assurance schemes such as RICE (CRAWFORD and COWIE, 1984), PAT (SCHLECHT and SHULMAN, 1986) and AFRICA (CRAWFORD et al., 1984). Consequently, a WHO consultation (WHO, 1981) held in Copenhagen in 1980 initiated a similar scheme, known as the WHO/EURO Reference Scheme. Its aims were to produce reference methods for sampling and evaluating 557 D ow naded rom http/academ ic.p.com /nnw eh/article-at/31/4B/557/161895 by gest on 06 M arch 2019 558 N. P. CRAWFORD et al. MMMF and to minimize inter-laboratory variation in the results obtained with these methods. A technical committee of experts, the WHO/EURO Technical Committee for Monitoring and Evaluating MMMF, was established in 1981 to manage the scheme. This committee comprises representatives of the WHO Regional Office for Europe, JEMRB, IARC, the IOM and national laboratories in six countries (Czechoslovakia, West Germany, France, Poland, Sweden and the United Kingdom). The scheme has had three key operations: (a) Development of a reference method for measuring airborne MMMF concentrations by phase contrast optical microscopy (PCOM). (b) Development of a reference method for sizing airborne MMMF by scanning electron microscopy (SEM). (c) Implementation of practical quality assurance schemes to minimize interlaboratory differences in fibre evaluation by PCOM and SEM. The technical committee co-ordinated the work through an appointed Central Reference Laboratory (the IOM) which has been responsible for organizing technical meetings and discussions, preparing and distributing samples for evaluation by participating laboratories, statistical treatment of the data and reporting the results. This paper updates the progress reported in 1985 which the WHO/EURO Technical Committee has made in harmonizing fibre counts and size distributions obtained by the use of standard evaluation procedures and regular inter-laboratory sample exchanges (WHO/EURO TECHNICAL COMMITTEE, 1985a). REFERENCE METHODS The PCOM reference method is based on the membrane filter technique used to determine airborne respirable asbestos fibre concentrations (CEC, 1983). The sample is collected by drawing a measured quantity of air through a membrane filter by means of a battery-powered sampling pump. The filter is mounted on a glass slide and made optically transparent with acetone/triacetin. The number of respirable fibres in randomly selected areas of the filter is counted using PCOM at a magnification of about 500 x . Respirable fibres are those objects longer than 5 ^m, narrower than 3 fxm, and with a length/diameter ratio (aspect ratio) ^3 :1 . In contrast to the asbestos fibre counting rules, fibres in contact with particles or other fibres are counted, provided they meet the above criteria; this avoids underestimation of the number of fibres present and minimizes ambiguities in the decisions microscopists have to make in these circumstances. The PCOM method also permits determination, if required, of the concentrations of non-respirable fibres, i.e. these fibres with the same length and aspect ratio characteristics as respirable fibres but with diameters > 3 nm. In the SEM reference method, samples are collected onto a polycarbonate filter (Nuclepore) or a PVC membrane filter (Gelman DM800) using the same sampling methodology as the PCOM method. After preparation, the samples are observed on an SEM at a magnification of 5000 x. Series of photomicrographs are recorded from randomly selected fields and fibre lengths and diameters are measured from optically enlarged images of these photomicrographs. Whilst the SEM method was primarily developed to determine fibre size, it also permits fibre number concentration to be evaluated if required. D ow naded rom http/academ ic.p.com /nnw eh/article-at/31/4B/557/161895 by gest on 06 M arch 2019 Monitoring and evaluating MMMF 559 Detailed specifications of both reference methods are given in a WHO Environmental Health Report (WHO, 1985b).