Comparison of optical and electron microscopy for evaluating airborne asbestos

In order to assess the very low asbestos exposures found outside the workplace it is necessary to make fibre counts by electron microscopy. This causes difficulties in relating such exposure estimates to industrial experience where the optical microscope has been used in epidemiological studies and in compliance monitoring. We have compared results from scanning electron microscope (SEM) and transmission electron microscope (TEM) and phase contrast optical microscope (PCOM) methods. We have used, firstly, different types of laboratory prepared asbestos sample and, secondly, actual industrial and non-occupational samples.The SEM analysis at X2000 magnification proved unreliable in assessing fibre density and evaluation of the SEM results in relation to PCOM and TEM was restricted to data obtained at X10000 magnification. This may reflect a general difficulty in using SEM to evaluate fibre concentration at low magnifications.The fibre size data for all types of sample conformed with a bivariate log normal distribution. A comparison of estimated mass using fibre density and the bivariate size distribution summary statistics showed good agreement with the nominal mass of asbestos in the laboratory samples prepared from liquid suspensions. Estimates of mean fibre mass derived in this way showed substantial differences between samples prepared in the laboratory and the airborne environmental samples. This demonstrates the difficulty of using laboratory calibrations for estimating mass concentrations of airborne fibres. We have not made such estimates for our environmental samples because the small number of fibre size measurements would have produced imprecise estimates.The studies showed that TEM produced total fibre number assessments which were greater than the SEM (X10000) which in turn produced fibre counts greater than the PCOM. However, when only fibres longer than 5 vim were counted the two electron microscope methods provided equivalent results to each other. This indicates that TEM is only advantageous in comparison with SEM (X10000) when counting or sizing of short fibres is required, no advantage being gained for fibres longer than 5 pm in length.For the pure asbestos samples, considering fibres longer than 5 ��m, the geometric mean ratio EM to PCOM ranged from 0.9 to 3.4 depending on fibre type and microscope method.The corresponding mean ratio for chrysotile asbestos samples collected in the textile factory was 1.2 for SEM (X10000) and 6.1 for TEM. The ratio for asbestos fibres longer than 5 ��m to PCOM count was 1.0.The samples collected at the four non-occupational environments confirmed that PCOM is a poor predictor of actual asbestos fibre number concentration. The ratio of SEM asbestos concentration to PCOM for fibres longer than 5 vim ranged from 0.05 to 13.The report concludes that to convert asbestos fibre concentrations determined by EM to PCOM equivalent concentrations the EM fibre concentration should be divided by 4.0 for chrysotile and 1.7 for amphibole asbestos. It is recognised that different factors may be required for the various amphibole types, however, the data from the present study are insufficient to make these distinctions. “”