Validation of a New Concept for Measuring Respirable Dusts

Sampling of airborne dust in industry is influenced by the potential health effects associated with human exposure. Health effects depend on mass concentration and particle size which influences the site of pulmonary deposition. Occupational diseases tend to be associated with dust deposition in specific regions of the respiratory tract. The ACGIH size selective TLVs are expressed in three forms: Inhalable Particulate Matter, Thoracic Particulate Matter, and Respirable Particulate Matter. In pneuomoconioses, the amount of dust deposited in the lungs can be estimated by sampling the respirable fraction. Dose-response relationships are derived by relating the health experience of workers to the extent of their exposure to respirable dust. It can be readily seen that validity of these relationships depends primarily on the accuracy of estimation of the exposure dose. Different sampling techniques, such as impactors, horizontal elutriators and cyclones were used for decades to estimate the exposure to respirable dusts. Cyclones have been the most widely utilized. However, the performance curve of the cyclone is considerably different from observed dust deposition data in the human alveolar compartment. Current methods of measuring respirable dust overestimate that dust fraction, which results in underestimating the agent’s toxicity in dose-response relationships. In this investigation, a new concept for sampling respirable dust was proposed and validated. The goal of this study was to design a combined impactor/cyclone device that provides better estimation of the amount of respirable dust. The objectives of this study were: 1) to calibrate ten single-stage impactors previously deigned and machined by Dr. Hammad, 2) to obtain the collection efficiency curves of ten impactor-cyclone combinations by superimposing the collection efficiency curves of impactors on the well-defined cyclone efficiency curve, and 3) to compare the combined efficiency curves to actual human alveolar deposition data, and thus validate this new concept for sampling of respirable dust. The experiment was conducted in a 20’’x20’’x20’’ aerosol testing chamber constructed from aluminum with a glass window. A LoveLace nebulizer with a nominal droplet size of 7 micrometers was used to generate fluorescent monodisperse polystyrene latex aerosols 0.5, 1, 2, and 3 micrometers in diameter. A Vilnius aerosol generator was used to generate fluorescent PSL dry powders 6 micrometers in diameter. The generated aerosols were collected on 37 mm polyvinyl chloride filters positioned after the impactors. Sample fluorescence was determined using a GloMax-Multi Jr fluorometer. Impactor efficiencies at the various sizes were used to construct the collection curves of impactors. Efficiency curves were subsequently superimposed on the cyclone efficiency curve to obtain the final efficiencies of the sampling devices. The results indicated that the cut-off diameters increased with impactor jet size. The new efficiency curves of the sampling devices had similar shapes to actual alveolar deposition as determined experimentally in human subjects. Actually they fell between actual alveolar deposition curves 2 and 4 seconds for mean residence times. The findings from this work can be applied to design a novel respirable dust sampler that provides a realistic estimate of pulmonary deposition to be used in dose- response relationships for the various mineral dusts encountered in general and mining industries. The under estimation of the dust toxicity associated with the current sampling methodology may be one of the reasons for continuous lowering of the TLV and PEL for silica