Chane-Yu Lai

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.

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...

Reducing Particle Bounce and Loading Effect for a Multi-Hole Impactor

Aerosol sampling is used to evaluate the health hazards associated with particles deposited in the human breathing system. Impactors, which are extensively employed as aerosol samplers, have low collection efficiency because of particle bounce. The impaction plate is typically coated with oil or grease to prevent particle bounce. However, such coating materials cannot sustain long-term heavy particle loading. In this study, the impaction plate was recessed, forming a cavity filled with Trypticase Soy Agar (TSA) to reduce particle bounce and re-entrainment. An ultrasonic atomizing nozzle was employed to generate challenge aerosols. An Aerodynamic Particle Sizer (APS) was utilized to measure the number concentrations and the size distributions upstream and downstream of the size-selective devices. A multi-hole impactor and Personal Environmental Monitor PM 2.5 (PEM–PM 2.5 ) were used to evaluate particle bounce and heavy particle loading. Liquid type-Dioctyl phthalate (DOP), soluble solid type-potassium sodium tartrate tetrahydrate (PST) and insoluble solid type-polymethyl methacrylate (PMMA) were investigated, as were different impaction surfaces/surface combinations. The multi-hole impactor coated with silicone oil was compared with a TSA-filled plate. Laboratory results demonstrate that the solid PST particles bounced off the TSA-filled plate less than off the silicone-coated aluminum plate. This study also used a 700-μm-thick layer of silicone oil to prevent TSA dehydration. The experimental results revealed that the silicone-TSA double layer minimized PST particle bounce during the two-hour heavy sampling (mass concentration was around 7.22 mg/m 3 ). Moreover, the PEM-PM 2.5 impactor yielded consistent results when the silicone-TSA double layer method was used. These results are useful for designing bounce-free impaction substrates during heavy load sampling.

Performance characteristics of PM10 samplers under calm air conditions.

ABSTRACT The size range of airborne particles that is closely related to specific deposition regions in the human respiratory tract and excess lung burden of these deposited particles is associated with disease. Size-selective sampling, therefore, needs to be performed to assess the related health risks. Performance criteria applied to these samplers must be well characterized in order to provide accurate and reliable results. The PM10 samplers that have been used in place of the total suspended particulate samplers for the collection of ambient air particles are more relevant to potential inhalation hazards. In order to be certified, a PM10 sampler must meet reliable performance specifications, primarily the aerosol penetration test with liquid and solid particles in a wind tunnel (wind speeds of 2, 8, and 24 km/hr). This testing is intended to assure reasonable accuracy in aerosol measurements. However, the sampler performance under calm air conditions has not been well studied. In the present study, the sampling heads of three devices—the Harvard impactor, the Personal Environmental Monitor (PEM), and the Sierra Andersen model 241 dichotomous sampler PM10 inlet head—were tested for aerosol separation efficiency. With the consideration of bias and imprecision of the measurements, five specimens of each type of sampler were chosen for performance testing, repeating the tests 5 times for each specimen. An ultrasonic atomizing nozzle was used to nebulize potassium sodium tartrate tetrahydrate and dioctyl phthalate particles as the solid and liquid challenge aerosols, respectively. The aerosol number concentrations and size distributions upstream and downstream of the samplers were measured by using an aerosizer calibrated against a settling velocity chamber. The results showed that among the samplers tested, the dichotomous sampler PM10 inlet head had the best fit to the PM10 convention, while the other two samplers not only appeared to have a steeper separation-curve slope but also had significant particle bounce when challenged with solid particles. Analysis of variance also confirmed the superiority of the dichotomous samplers. Surface-coating with oil or grease greatly reduced the problem of particle bounce.

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.

Filter quality of electret masks in filtering 14.6–594 nm aerosol particles: Effects of five decontamination methods

This study investigates the effects of five decontamination methods on the filter quality (qf) of three commercially available electret masks—N95, Gauze and Spunlace nonwoven masks. Newly developed evaluation methods, the overall filter quality (qf,o) and the qf ratio were applied to evaluate the effectiveness of decontamination methods for respirators. A scanning mobility particle sizer is utilized to measure the concentration of polydispersed particles with diameter 14.6–594 nm. The penetration of particles and pressure drop (Δp) through the mask are used to determine qf and qf,o. Experimental results reveal that the most penetrating particle size (MPS) for the pre-decontaminated N95, Gauze and Spunlace masks were 118 nm, 461 nm and 279 nm, respectively, and the respective penetration rates were 2.6%, 23.2% and 70.0%. The Δp through the pretreated N95 masks was 9.2 mm H2O at the breathing flow rate of heavy-duty workers, exceeding the Δp values obtained through Gauze and Spunlace masks. Decontamination increased the sizes of the most penetrating particles, changing the qf values of all of the masks: qf fell as particle size increased because the penetration increased. Bleach increased the Δp of N95, but destroyed the Gauze mask. However, the use of an autoclave reduces the Δp values of both the N95 and the Gauze mask. Neither the rice cooker nor ethanol altered the Δp of the Gauze mask. Chemical decontamination methods reduced the qf,o values for the three electret masks. The value of qf,o for PM0.1 exceeded that for PM0.1–0.6, because particles smaller than 100 nm had lower penetration, resulting in a better qf for a given pressure drop. The values of qf,o, particularly for PM0.1, reveal that for the tested treatments and masks, physical decontamination methods are less destructive to the filter than chemical methods. Nevertheless, when purchasing new or reusing FFRs, penetration should be regarded as the priority.

Precision Evaluation of Size-Selective Aerosol Samplers

The importance of assessing exposure to atmospheric particles has recently increased, partially owing to epidemiological studies that have identified the negative health effects of particulate matter. Although size-selective sampling devices appear to be promising for measuring the degree of exposure to aerosol particles, the performance testing criteria of these devices may still be limited. Aerosol sampler performance can be measured in terms of bias and imprecision. The bias map, as a function of particle size distribution, is extensively used to evaluate sampling inaccuracy. However, procedures for constructing the imprecision map remain undetermined. The imprecision map should provide another useful indicator of sampler performance. This study develops a semi-empirical model of imprecision under statistical premises. The binomial distribution assumption, rather than the conventional normal assumption of an ANOVA test, was made to model the imprecision of the sampler. Analytical results indicated that the size distribution of challenge aerosols, total particle number count, number of specimens, and number of replicates all affected the imprecision map. The “One Standard Error Shift,” similar to the “Mean Square Error,” combining the bias and imprecision maps was a novel and effective indicator of sampler performance.