Point-counting technique for friable asbestos-containing materials
The "1% Rule" concerning the definition of asbestos-containing materials (ACM) has created a dilemma for microscopists when analyzing samples containing low concentrations of asbestos. An experienced microscopist is able to detect asbestos in concentrations of <1%, but quantitating such low concentrations is difficult, if not impossible. Studies at RTI indicate that the point-counting method, used as a compliment to routine PLM analysis, provides a relatively accurate and precise tool (as compared to visual estimation) for quantitation of asbestos concentration. In order to achieve representative results by point counting, care must be taken in 1) homogenizing the samples; 2) preparing the slide mounts; and 3) point counting the slides. Conclusions from the RTI point counting studies are that: <br>- Visual estimates of asbestos concentrations tend to be higher than concentrations determined by point counting.<br>- Proper homogenization and slide preparation are necessary in order to achieve representative results by point counting.<br>- Point counting is time-consuming and thus expensive, but results are more accurate and precise.