4. SAMPLING CONSIDERATIONS
Current methods for sampling airborne silica use a cyclone with a cassette to collect the respirable fraction of the aerosol. To minimize measurement bias and variability, these samplers should conform to the criteria of the International Organization for Standardization (ISO), the European Standardization Committee (CEN), and the American Conference of Governmental Industrial Hygienists (ACGIH) for collecting particles of the appropriate size [25, 26, 27]. Also, the cyclone should exhibit sufficient electrical conductivity to minimize the electrostatic effects on particle collection.
Cyclones typically used for crystalline silica measurements include the non-conductive Dorr-Oliver 10-mm nylon cyclone and the conductive Higgins-Dewell cyclone. These cyclones have been evaluated for their compliance with the ISO/CEN/ACGIH respirable aerosol sampling convention. Flow rates of 1.7 L/min for the Dorr-Oliver cyclone and 2.2 L/min for the Higgins-Dewell cyclone provide minimum bias for a wide range of particle size distributions that is likely to occur in the workplace [28]. The Dorr-Oliver 10-mm cyclone is the approved sampler for MSHA (CFR 30 Parts 70.100-10.206 and 71.100-71.206) and is commonly used in the United States, while the Higgins-Dewell cyclone is used in the United Kingdom and is sold in the United States and Canada. (Though the sampling recommendations to adopt the ISO/CEN/ACGIH respirable sampling convention have been formally accepted by MSHA for coal mine dust sampling, MSHA currently requires the use of the Dorr-Oliver cyclone at 2.0 L/min with 1.38 conversion factor to match an earlier sampling convention (British Medical Research Council [BMRC], 1961) [29].
Recently, the BGI GK2.69 cyclone [30] has become commerically available with a sampling rate equal to 4.2 L/min. The GK2.69 cyclone is expected to be at least as adequate as the nylon cyclone for conforming to the ISO/CEN/ACGIH respirable aerosol sampling convention; and it may be preferable for silica sampling since it is conductive, has well-defined dimensional characteristics and can be used at higher flow rates to achieve a lower detection limit. Given that current analysis methods do not have sufficient accuracy to monitor below current exposure standards, this sampler has promise for potentially lowering the levels of silica that can be measured and still meet the required measurement accuracy. The availability of a pre-selector such as the GK2.69 cyclone sampler, which is designed to conform to the respirable dust sampling criteria at a flow rate nearly two and a half times that of the nylon cyclone, offers the field industrial hygienist the ability to collect a quantifiable sample mass during tasks of short duration or of low dustiness, conditions which may not have yielded a quantifiable mass using the nylon cyclone. The hygienist must be mindful of such things as ambient conditions, analytical detection limit, working analytical range and required air volume when selecting the appropriate sampler.
Each type of cyclone exhibits its own unique particle collection characteristics and the three major methods of analysis (XRD, IR, and colorimetric) are subject to different particle size effects. Therefore, the use of a single cyclone type for each application is advised until evidence becomes available indicating that bias among cyclone types will not increase interlaboratory variability. In practice, the Dorr-Oliver 10-mm nylon cyclone has been used since the 1960's and has been a basis for some epidemiological studies. While augmenting the list of recommended samplers with the Higgins-Dewell and the BGI GK2.69 cyclones would
