Tung-Sheng Shih

Development of Respirable Aerosol Samplers Using Porous Foams

Workplace aerosols must be sampled to assess the degree of health hazard caused by the particulate matter. By adjusting the sampling flow rate, most of the samplers can match the 50% cutoff size, but not the slope of the respirable convention defined by the American Conference of Governmental Industrial Hygienists, the International Organization for Standardization, and the European Committee for Standardization (CEN). Combinations of foams (or other porous material) of different nominal sizes (10-100 ppi) and thicknesses (5-35 mm) were employed to overcome this bias. A foam disk 25 mm in diameter was placed in an asbestos sampling cowl. Dioctylphthalate was the liquid test agent. An aerodynamic particle sizer and an Aerosizer were calibrated against a settling chamber and were employed to measure the aerosol number concentrations and size distributions upstream and downstream of the foams. The sampling efficiency data showed that the 50% cutoff size could be met for foams in series, but that the slope remained sharper than the new definition. Foams in parallel showed great flexibility and many of the parallel combinations flattened the slope, closer to that of the new international respirable convention. For instance, when the total flow rate is set at 10.1 L/min the aerosol penetration through foams in parallel (100 ppi, 20 mm thick, diameter 25 mm + 10 ppi, 20 mm thick, diameter 13 mm) nearly matched the new international standard for respirable fraction. This sampler can be further miniaturized for smaller sampling flow rates to fit the capacity of personal sampling pumps.

Development of a Size-Selective Inlet-Simulating ICRP Lung Deposition Fraction

A size-selective inlet made of polyurethane filter foam was designed and fabricated to simulate a portion of the ICRP respiratory deposition curve. A downstream aerosol measuring device then could be used to generate aerosol concentration data that represented the fraction reaching the respiratory system. This article introduces useful knowledge about porous foam penetration for particle size ranges below those reported in the previous studies. Different porosities of polyurethane foam filters were tested for aerosol penetration. Among the parameters operated in this work were (1) foam porosity (ppi), (2) filter thickness, (3) face velocity, and (4) packing density of the filter foams. Di-octyl phthalate was used as the test agent. A constant output atomizer and an ultrasonic atomizing nozzle were used to generate polydisperse submicrometer–and micrometer-sized particles, respectively. Aerosol concentrations and size distributions upstream and downstream of the filter foams were monitored by using a scanning mobility particle sizer (for particles with diameters smaller than 0.7 µ m) and an aerodynamic particle sizer (for particles larger than 0.7 µ m). The aerosol output was neutralized by a radioactive source. A lognormal-distribution curve with a mode of 0.25 µ m and a GSD of 6.2 was set as the primary target curve simulating the light-work ICRP deposition model. The results showed that the most penetrating size (also referred to as collection minimum) of the filter foams decreased upon increasing the foam porosity, packing density, and face velocity. In this work, the highest foam porosity and packing density we could acquire were 100 ppi and 0.2, respectively. By adjusting the face velocity, the most penetrating size was moved to 0.25 µ m, which happened to be the most penetrating size for ICRP light-work criterion. The whole aerosol penetration curve could further fit to the modified ICRP curve by adjusting the filter thickness. There are numerous ways to match the ICRP definition. This size-selective inlet becomes even more versatile if the auxiliary detector and vacuum system are operated under different flow rates to simulate light-to-heavy workloads.

The virtual cyclone as a personal respirable sampler

Aerosol exposure via the inhalation route is a primary concern in occupational health. A researcher must perform aerosol size-selective sampling because respiratory deposition is dependent on aerosol size. The optimal sampling instrument is therefore the one that provides the most accurate measurement of the atmospheric dust component, thus showing the extent of a health hazard. However, the most commonly used respirable samplers today can only meet the 50% cut-off point and not the slope of the respirable convention prescribed by the American Conference of Governmental Industrial Hygienists (ACGIH), the Inter national Standard Organization (ISO), and the Comite European de Normalisa tion (CEN). These conventional cyclones are also found to be affected by the aerosol deposition and accumulation on the wall of the cyclone body, which leads to a significant decrease in aerosol penetration. In the present study, a miniature, compact, rugged virtual cyclone, which employs a nonimpact particle separation, was ...

Laboratory performance comparison of respirable samplers

Three respirable samplers (nylon cyclone, SKC cyclone, and foam sampler) were tested for aerosol penetration as a function of aerosol size, to examine the precision and the accuracy with respect to the newly defined respirable convention. An ultrasonic atomizing nozzle was used to generate micrometer-sized liquid dioctylphthalate or solid potassium sodium tartrate aerosol particles, with count median diameters of 3 µm or 8 µm, and geometric standard deviation of 1.6, depending on the properties of the solution to be nebulized. The aerosol number concentration and size distribution upstream and downstream of the samplers were measured by using an aerodynamic particle sizer, which was calibrated against a settling velocity chamber. The results showed that a newly developed foam sampler meets the requirements of the 50% cutoff size as well as the slope of the international respirable convention. For considering both the accuracy and precision of the samplers, it may be inappropriate to select the estimator w...

Determination of Uniformity of Filter Deposits

Certain measurement techniques (such as the asbestos method using phase contrast microscopy) require uniform deposits of the sample on a filter. The asbestos fiber analytical methods require such uniform deposition because the analysis only observes small, randomly chosen locations on the filter. In this study, a vibrating orifice monodisperse aerosol generator was used to generate methylene blue particles. The aerosols were dried by filtered compressed air and then neutralized by inducing a charge on the droplet stream that emerged from the vibrating orifice. An Aerodynamic Particle Sizer was used to measure the number concentration and size distribution of the generated aerosol particles. Meanwhile, the filter deposits were examined via image processing, combined with statistical methods for defining uniformity. In order to better define uniformity and make the indicator more universal, the uniformity was defined as the exponential of the negative CV (coefficient of variation) value which was a transformation for easily understanding the uniformity of the filter deposits. The experimental results demonstrated that, when aerosol counting was performed, the equal area approach was superior to the equivalent distance approach.

Overall Performance Evaluation of Aerosol Number Samplers

At present, there is neither an officially accepted size-selective fiber (aerosol number) sampler, nor are there established performance criteria. In this work, a prototype preclassifier (multihole impactor) was used to connect a conventional asbestos sampler so that the aerosol penetration test and particle counting process could be performed. The bias, as a function of particle size, was defined as the difference between the measured penetration curve and the target ISO/ACGIH/CEN respirable convention. The imprecision was the standard error with reference to the mean aerosol penetration curve. A statistical term, one standard error shift (OSES) was used in a previous study to combine the sampling bias and imprecision. The bias and imprecision could be for aerosol number, aerosol mass, or even surface area. In this work, an additional step was taken by introducing another statistical term, maximum sampling shift (MSS), to further combine the OSES with the counting imprecision. For the surrogate sampler tested, the particle counting imprecision increased with increasing particle diameter and decreased with increasing geometric standard deviation. The particle counting imprecision was comparable with the OSES, and the resultant MSS map was actually the summation of imprecision and OSES.

Computer simulation of particle overlap in fiber count samples.

Fibrous aerosols are of great importance to industrial hygienists because of the severe health risks that may be associated with inhaling such particles. Previous studies on measurement error due to overloading of fibers and nonfibrous particles on the collected sample indicate that a 100-1300 fiber/mm2 filter area is the best filter loading density to reduce bias in fiber counts. The present study investigated the upper fiber and particle concentration limits for reliable counting and identification and the possibility of a procedure for correcting observed fiber counts to account for fiber masking due to overlapping particles or fibers. A computer-generated grid was used to simulate the light microscope graticule field. The resolution of 2000 x 2000 was found to accurately represent the shape of the fibers and nonfibrous particles. Bivariate lognormal distributions were used to describe the length and width distributions of the fibers. The capability of distinguishing particle-overlapped fibers (defined as the resolution index), the coverage of the graticule field, the filter surface loading density, size distributions of fibers and particles, and the fiber-to-particle concentration ratio were the primary parameters in this study. The counting efficiency was found to consistently decrease with increasing filter surface loading density and decreasing resolution index. The recommended upper limit of filter surface fiber density depended not only on the number concentration ratio but also on the filter surface loading densities and size distributions of fibers and particles. The advantage of using a thoracic preseparator on counting efficiency was calculated and found to improve counting efficiency significantly when the count median diameter of nonfibrous particles was close to or larger than the thoracic 50% cutoff point of 10 microm.