Scanning electron microscope sizing of airborne fibre samples: a preliminary investigation into the effects of the variable inclination of fibres to filter surfaces

The scanning electron microscope (SEM) method of fibre sizing assumes that all fibres lie flat on filters. This appears to be true for samples deposited from aqueous suspension, but in some airborne samples only about 60% of fibres lie flat on the filter surface and many of the rest are balanced at high angles to the filter surface. These inclined fibres will appear foreshortened when the filter surface is viewed in conventional orientation during SEM analysis. This could lead to an underestimation of fibre length and some fibres may be so foreshortened that they do not appear to fulfil the criteria of a fibre. The results of a preliminary investigation of the effects of variable fibre inclination in fibre counts and size data for two fibre types suggest that the SEM data do give an underestimate of the number, length and therefore volume of longer fibres in some samples. This suggests that it is important to consider fibre inclination when SEM analysis of airborne fibre samples is performed. Our preliminary observations suggest that is unlikely that it will be possible to derive widely applicable correction factors to allow for orientation effects.