245 =

required sample on a membrane filter or by other means. A satisfactory air
elutriation method for asbestos fibres has not yet been reported, but it is
hoped that one may be available in due course. In the meantime, it is
recommended that the sampling orifice be so arranged (e.g. filter facing
downwards) so that the large particles not of biological interest are sub-
stantially excluded from the sample. ‘The daily average weighted exposure
can be determined by using the following formula:

Total of (duration x dust level for each operator)
8 hours

Total sampling duration = 8 hours

(d) LIMITATIONS AND ACCURACY OF MEMBRANE FILTER METHOD

With the method at present only fibres greater than 5 ym in length
are measured and a fibre is defined as having a length:breadth ratio of 3 to
1. There have been many studies to determine inter-laboratory variation
including an international study. While the differences between counters
can in some instances be high (+ several hundred percent) there is evidence
that the adoption of a rigorous and detailed protocol for counting and
regular contact with counters in their laboratories makes this difference
tolerable (+ 30 - 45%). Major problems arise from failure to define how to
count fibres sticking out of a particle. The identification of asbestos
fibres in dust clouds containing other fibres is not readily possible ona

routine basis.

Counting fibres on an optical microscope is tedious and time con-
suming. Automated techniques are being tested and if fibre counts are to
be the accepted method of monitoring the environment in the asbestos industry,
such methods will become essential and will prove to have a considerable
advantage over manual methods. There is one major concern about the present
membrane technique and standard and this is the fact that only a very select
fraction of the airborne fibre population is observed on the optical microscope.
Fibres with diameters less than about 0.3 micron will not be observed on the
microscope and single chrysotile fibres may be of the order of 0.03 pam in

diameter. These may exceed 5 ym in length. The size of fibres can also

change with the degree of processing or utilization of the product may contain