a higher proportion of fibres which are not visible on the optical microscope.

In our search for what to measure, we are looking for an index of
exposure for airborne asbestos dust. The membrane filter technique at the
present time offers us the closest to what is generally agreed is needed.

It is sensitive (except in areas wheré there is high background of other dusts);
is relatively specific (at least for fibres); can be made reproducible; and

can be purchased for moderate cost. The validity is still not clear and for
this reason it is important for industry and researchers to collect data

concerning the quality of all airborne dust and fibre present in the plant.