A modified Marple-type cascade impactor for assessing aerosol particle size distributions in workplaces

Knowledge of the particle size distributions for workplace aerosols is invaluable in the assessment of aerosol-related health effects. Cascade impactors have been widely used for obtaining such information, including a small number that have been developed as personal samplers of the type that can be used for the assessment of the exposures of individual workers. Common limitations for most samplers of this type have been that (a) the aspiration efficiency has not been well defined (leading to biases in particle size distribution measurement), and (b) the range of particle size has been constrained by particle bounce in impactors for particle sizes beyond about 20 micro m. This article describes a modification of the Marple personal cascade impactor that addresses these limitations. The sampler has a new entry whose aspiration efficiency is known and a new top stage that employs porous plastic foam filtration media and significantly extends the particle size range of the instrument. The new instrument is referred to as the modified-Marple sampler. A numerical simulation was performed to investigate the ability of the new instrument to accurately retrieve particle size distributions over the range typical of aerosols found in workplaces. The retrieval process was carried out using a simple inversion algorithm of the "zeroth-order" type. The results are presented in terms of the ability of the new sampler to retrieve the masses contained in the inhalable, thoracic, and respirable fractions. They suggest that the more narrowly distributed the particle size distribution, the more restricted the ability to accurately retrieve the particle size distribution. However, for most aerosols of the type encountered in the real world of industrial hygiene, the modified-Marple sampler provides particle size information of sufficient quality for most practical purposes.