The Letter from the Editor

The industrial handling procedures for bulk materials, for example, to store, to convey, to mix, and to fill, etc. often lead to dust emissions. The generated dust is closely related to health hazards and environmental pollution, and is also a cause of f ires and explosions. The particle size distribution and concentration determine the risks. The dust liberation property of disperse particle systems socalled powders depends on a multitude of variables and also on the method and intensity of stressing. The present paper describes practically all the dust measuring methods published in literature. It also presents related particle sizing and counting techniques and systematically summarises the integrated measurement methods. * Rainer-Gruenter-Str. Geb. FF D-4211g Wuppertal † Accepted: June, 2003 that nano-sized particles are emitted during most treatment processes [13]. Occupational safety as an example shows that the evaluation of the dustiness potential during the handling of disperse materials is of significant importance. If new materials are to be used, the expected dustiness will be an important judgment criterion. 2. Aims and requirements The currently increased interest in dustiness estimation is based on the involved fields of occupational safety, health, hygiene, and fire and explosion prevention. The reduction of the airborne dust load as early as the planning phase of manufacturing processes through improved estimation models is the principal goal [5, 6]. This enables the reduction of secondary measures such as extraction systems, provided the technical requirements of the products are no obstacle. Due to the integration nature of the total dust method, conclusions about particle size distribution are impossible. More appropriate methods will permit conclusions about particle size distribution. Corresponding requirements are, for example, standardized in DIN EN 481 [14], which classifies inhaled particles into 3 categories. The penetration depth depends on the particles’ aerodynamic diameter. Fig. 1 displays the classification of aerosols in the respiratory system, according to occupational health criteria. The characteristics of materials, especially the physical ones, are often of increased interest. They are related to the dwell time of the particles in the air and their correlated particle mobility. Increasing particle mobility leads to a higher contamination risk. The correlation between material stressing and the amount of emitted dust must be retraced by the measurement methods. Basically, the acceptance of the methods is increased through simple handling and high reliability. Additionally important for the economic aspects are versatility in use, easy handling, fast and reliable results, and easy cleaning. High efficiency leads to lower costs [16]. 3. Historical methods of dustiness measurement In the face of the variety of methods used for measuring the dustiness capacity of materials, the choice of appropriate apparatus is difficult. Basic factors of choice are comparability to field conditions and the kind of output data (loaded filter or displayed results). The comparability of results of different methods is often difficult because of the different time and kind of stressing, as well as the means of evaluation and sampling. By using modern particle measurement techniques, conclusions about the particle size fraction are possible. Early studies were made in 1922. Andreasen et al. [17], for example, investigated the dustiness of 24 sample substances by using a single-drop apparatus (Fig. 2). Additionally, they described the inf luence of small particles on particle collectives and their capability of generating dust. Andreasen used a device consisting of a modular-design dust chamber and two fitted and sealed falling tubes. An iris is used as the shutter. The sample is placed on the iris and covered by a f lat-turned tray to seal the sample against the inf luence of ambient air. Six metal slides are inserted successively from bottom to top into the dust chamber after the sample has been dropped. The time interval of insertion is defined by a metronome. After the test, the slides are weighed. The height of the falling tube has not been documented. ANSI/ATSM D547-41 standardized a modified method to investigate the index of dustiness of coal and coke. The initial method has been known since 1939 [18]. This method also uses the single-drop dustiness technique (Fig. 3). The dust chamber is 8 KONA No.21 (2003) Alveolar-Deposition Exhalable Part Non-Inhalable Part Total Deposited Part Nose-Pharynx-Larynx Deposition (Extratorac-Part) Tracheo-BronchialDeposition Inhalable Part Aerosol within the respiratory Air Fig. 1 Classification of airborne aerosols according to occupational health criteria [15]. sealed after the sample (22.7 kg) has been placed on the inserted upper slide. After drawing out the upper slide abruptly, the sample drops into the bottom drawer. Exactly 5 s later, both lower slides are inserted quickly. The top slide is pulled out after two minutes, the lower slide after 10 minutes. By weighing the two slides, a dust index is generated for coarse and f loat dust. 4. Characterization of airborne dust 4.1. Filter samplers Filter samplers consisting of filter housing and filter are used as a standard measurement method. The whole airborne dust is collected in the filter, whereas conclusions about the particle size distribution are only possible by means of further measurement. Fractional sampling units are used for the fields of occupational health and safety to deposit inhalable and respirable dust fractions [19]. With the aid of these techniques, non-conformity during further measurement can be avoided. 4.2. Foams Metal and polymer foams with defined pore sizes are used alternatively [20]. The results are in accordance with the demands of DIN EN 481 (Fig. 4). 4.3. Impactors Impactors are often used to fractionally measure the dustiness. They can easily replace filter samplers in the test apparatus. Impactors can measure only low particle concentrations. Therefore the dust-laden air has to be diluted. The air velocity at the inlet has to be adequate to prevent particle sedimentation. OtherKONA No.21 (2003) 9 Fig. 2 Single-drop dustiness apparatus used by Andreason. [17] a. cover, b. iris, c. slides, d. modular casing, e. falling tube part 1, f. falling tube part 2 Fig. 3 ANSI/ATSM D547-41 [18] standardized device to investigate the dustiness index of coal and coke, a. cover, b. upper slide (sample to be tested), c. stop watch, d. guides, e. slide for coarse dust, e. slide for f loat dust, f. drawer Sample a