Review and demonstration of equations applied to models of filtering facepiece respirator particle capture efficiency

The use of filtering facepiece respirators (FFRs) of various types increased dramatically by both workers and the public during the ongoing COVID-19 pandemic. This increased use has, likewise, instigated a proliferation of research on the qualities of FFRs. An aspect of FFR development and optimization involves the use of mathematical models that predict filter efficiency based on various filter characteristics while also considering a number of particle capture forces. An evaluation of current literature failed to identify a publication that provides a comprehensive assessment of the models developed to predict filter efficiency. The purpose of this review was, therefore, to describe models developed to include the forces associated with diffusion, interception, impaction, and electrostatic attraction as they contribute to the efficiency of an entire filter. The literature review was augmented with figures created with the use of many of the models discussed to compare different models of the same force as well as to illustrate the influence of electrostatic forces on overall filter efficiency.

Particle Filtration Efficiency Testing of Sterilization Wrap Masks

Objectives

Non-traditional materials are used for mask construction to address personal protective equipment shortages during the coronavirus disease 2019 (COVID-19) pandemic. Reusable masks made from surgical sterilization wrap represent such an innovative approach with social media frequently referring to them as “N95 alternatives.” This material was tested for particle filtration efficiency and breathability to clarify what role they might have in infection prevention and control.

Methods

A heavyweight, double layer sterilization wrap was tested when new and after 2, 4, 6, and 10 autoclave sterilizing cycles and compared with an approved N95 respirator and a surgical mask via testing procedures using a sodium chloride aerosol for N95 efficiency testing similar to 42 CFR 84.181. Pressure testing to indicate breathability was also conducted.

Results

The particle filtration efficiency for the sterilization wrap ranged between 58% to 66%, with similar performance when new and after sterilizing cycles. The N95 respirator and surgical mask performed at 95% and 68% respectively. Pressure drops for the sterilization wrap, N95 and surgical mask were 10.4 mmH₂O, 5.9 mmH₂O, and 5.1 mmH₂O, respectively, well below the National Institute for Occupational Safety and Health limits of 35 mmH₂O during initial inhalation and 25 mmH₂O during initial exhalation.

Conclusions

The sterilization wrap’s particle filtration efficiency is much lower than a N95 respirator, but falls within the range of a surgical mask, with acceptable breathability. Performance testing of non-traditional mask materials is crucial to determine potential protection efficacy and for correcting misinterpretation propagated through popular media.

The "double eights mask brace" improves the fit and protection of a basic surgical mask amidst COVID-19 pandemic

STUDY OBJECTIVE

The COVID-19 pandemic has resulted in widespread shortages of personal protective equipment, including N95 respirators. Although basic surgical facemasks are more commonly available, their efficacy is limited due primarily to their poor face seal. This pilot study examined the impact of a rubber band mask brace on a basic surgical mask, as determined by quantitative fit testing.

METHODS

Subjects wearing a basic surgical facemask and the rubber band mask brace underwent quantitative fit testing using machinery designed to certify N95 mask fit. Subjects were tested with the brace anchored behind their ears, with a paperclip behind the head, and on the side knobs of their face shields. The primary outcome measure was whether the subject passed the quantitative fit test at or above the Occupational Safety and Health Administration (OSHA)-verified standard for N95 masks.

RESULTS

Subjects (n = 11) were 54.5% female, with a median height of 70 inches (interquartile range [IQR] = 68-74), weight of 170 pounds (IQR = 145-215), and body mass index (BMI) of 24.6 (IQR = 22.2-27.2), and encompassing 5 distinct N95 mask fit types. We found that 45%, 100%, and 100% of subjects passed the quantitative fit test when the brace was anchored behind the ears, with a paperclip and on a face shield, respectively.

CONCLUSION

Of the 11 subjects included in the analysis, across a range of body habitus and N95 mask fit types, all passed the quantitative fit test when the mask brace was anchored on either face shield or with a paperclip. This data suggests that although the brace does not create an N95 equivalent in terms of filtration, it would offer improved protection from airborne viruses when worn with a basic surgical mask.

Differences in Particle Deposition Between Members of Imaging-Based Asthma Clusters

Background: Four computed tomography (CT) imaging-based clusters have been identified in a study of the Severe Asthma Research Program (SARP) cohort and have been significantly correlated with clinical and demographic metrics (J Allergy Clin Immunol 2017; 140:690-700.e8). We used a computational fluid dynamics (CFD) model to investigate air flow and aerosol deposition within imaging archetypes representative of the four clusters. Methods: CFD simulations for air flow and 1-8 μm particle transport were performed using CT-based airway models from two healthy subjects and eight asthma subjects. The subject selection criterion was based on the discriminant imaging-based flow-related variables of J(Total) (average local volume expansion in the total lung) and Dh*(sLLL) (normalized airway hydraulic diameter in the left lower lobe), where reduced J(Total) and Dh*(sLLL) indicate reduced regional ventilation and airway constriction, respectively. The analysis focused on the comparisons between all clusters with respect to healthy subjects, between cluster 2 and cluster 4 (nonsevere and severe asthma clusters with airway constriction) and between cluster 3 and cluster 4 (two severe asthma clusters characterized by normal and constricted airways, respectively). Results: Nonsevere asthma cluster 2 and severe asthma cluster 4 subjects characterized by airway constriction had an increase in the deposition fraction (DF) in the left lower lobe. Constricted flows impinged on distal bifurcations resulting in large depositions. Although both cluster 3 (without constriction) and cluster 4 (with constriction) were severe asthma, they exhibited different particle deposition patterns with increasing particle size. The statistical analysis showed that Dh*(sLLL) plays a more important role in particle deposition than J(Total), and regional flow fraction is correlated with DF among lobes for smaller particles. Conclusions: We demonstrated particle deposition characteristics associated with cluster-specific imaging-based metrics such as airway constriction, which could pertain to the design of future drug delivery improvements.

Filter penetration and breathing resistance evaluation of respirators and dust masks

The primary objective of this study was to compare the filter performance of a representative selection of uncertified dust masks relative to the filter performance of a set of NIOSH-approved N95 filtering face-piece respirators (FFRs). Five different models of commercially available dust masks were selected for this study. Filter penetration of new dust masks was evaluated against a sodium chloride aerosol. Breathing resistance (BR) of new dust masks and FFRs was then measured for 120 min while challenging the dust masks and FFRs with Arizona road dust (ARD) at 25°C and 30% relative humidity. Results demonstrated that a wide range of maximum filter penetration was observed among the dust masks tested in this study (3-75% at the most penetrating particle size (p < 0.001). The breathing resistances of the unused FFRs and dust masks did not vary greatly (8-13 mm H₂O) but were significantly different (p < 0.001). After dust loading there was a significant difference between the BR caused by the ARD dust layer on each FFR and dust mask. Microscopic analysis of the external layer of each dust mask and FFR suggests that different collection media in the external layer influences the development of the dust layer and therefore affects the increase in BR differently between the tested models. Two of the dust masks had penetration values < 5% and quality factors (0.26 and 0.33) comparable to those obtained for the two FFRs (0.23 and 0.31). However, the remaining three dust masks, those with penetration > 15%, had quality factors ranging between 0.04-0.15 primarily because their initial BR remained relatively high. These results indicate that some dust masks analysed during this research did not have an expected very low BR to compensate for their high penetration.

Performing T-tests to Compare Autocorrelated Time Series Data Collected from Direct-Reading Instruments

Industrial hygienists now commonly use direct-reading instruments to evaluate hazards in the workplace. The stored values over time from these instruments constitute a time series of measurements that are often autocorrelated. Given the need to statistically compare two occupational scenarios using values from a direct-reading instrument, a t-test must consider measurement autocorrelation or the resulting test will have a largely inflated type-1 error probability (false rejection of the null hypothesis). A method is described for both the one-sample and two-sample cases which properly adjusts for autocorrelation. This method involves the computation of an "equivalent sample size" that effectively decreases the actual sample size when determining the standard error of the mean for the time series. An example is provided for the one-sample case, and an example is given where a two-sample t-test is conducted for two autocorrelated time series comprised of lognormally distributed measurements.

Assessment of swine worker exposures to dust and endotoxin during hog load-out and power washing

Field measurements of personal and area dust and endotoxin concentrations were obtained while agricultural workers performed two work tasks that have been previously unreported: hog load-out and swine building power washing. Hog load-out involves moving hogs from their pens in finishing buildings into a truck for transport to a meat processor. High pressure power washing is conducted for sanitation purposes after a building has been emptied of hogs to remove surface and floor debris. This debris consists of feed, feces, and hog dander as dust or an encrusted form. The hog load-out process necessarily increases pig activity which is known to increase airborne dust concentrations. An unintended consequence of power washing is that the material covering surfaces is forcibly ejected into the atmosphere, creating the potential for a highly concentrated aerosol exposure to workers. The load-out process resulted in a median personal inhalable mass concentration of 7.14 mg m(-) (3) and median endotoxin concentration of 12 150 endotoxin units (EU) m(-) (3). When converted to an 8-h time-weighted average for a 'total' sampler, one of the 19 samples exceeded a regulatory limit of 15 mg m(-) (3). An impinger was used to sample power washing endotoxin concentrations, which resulted in a median personal concentration of 40 350 EU m(-) (3). These concentrations were among the highest found in the literature for any occupation. With the lack of engineering controls present to reduce airborne contaminant concentrations in swine buildings, either respirator use or a reduction in exposure time is recommended while performing these tasks.

A strategy for assessing workplace exposures to nanomaterials

This article describes a highly tailorable exposure assessment strategy for nanomaterials that enables effective and efficient exposure management (i.e., a strategy that can identify jobs or tasks that have clearly unacceptable exposures), while simultaneously requiring only a modest level of resources to conduct. The strategy is based on strategy general framework from AIHA® that is adapted for nanomaterials and seeks to ensure that the risks to workers handling nanomaterials are being managed properly. The strategy relies on a general framework as the basic foundation while building and elaborating on elements essential to an effective and efficient strategy to arrive at decisions based on collecting and interpreting available information. This article provides useful guidance on conducting workplace characterization; understanding exposure potential to nanomaterials; accounting methods for background aerosols; constructing SEGs; and selecting appropriate instrumentation for monitoring, providing appropriate choice of exposure limits, and describing criteria by which exposure management decisions should be made. The article is intended to be a practical guide for industrial hygienists for managing engineered nanomaterial risks in their workplaces.

CFD simulation of contaminant decay for high reynolds flow in a controlled environment

This study examines the usage of computational fluid dynamics (CFDs) for estimating the time-elapsed decay of contaminants within a chamber experiencing high Reynolds flow. CFD results were compared with measurements taken at a controlled facility. In addition, parameters of the CFD simulation were examined; namely the effects of turbulence and inertial transport at high Reynolds number ventilating flows, as well as inlet duct configuration and its effect on the inlet velocity profile. The agreement between the computational and experimental clearance times was quite good, with percent errors as low as -5.32% at high flow rate and -11.8% at the lower flow rate. This study determined that for high Reynolds flow, diffusive transport effects may be ignored as the majority of mass is transported via the bulk stream, i.e. momentum transport. In addition, resolving the inlet velocity profile was of prime importance for accurate simulation of ventilating flows and prediction of contaminant washout. This was done by including the inlet duct geometry in the computational domain. In addition, it was found that despite different flow rates, the predicted contaminant washout took approximately 12-13% longer than predicted assuming instantaneous mixing. Furthermore, percent error between computational and experimental data as low as -5.32% shows that CFD is a useful tool for studying ventilation phenomena.

Airborne monitoring to distinguish engineered nanomaterials from incidental particles for environmental health and safety

Two methods were used to distinguish airborne engineered nanomaterials from other airborne particles in a facility that produces nano-structured lithium titanate metal oxide powder. The first method involved off-line analysis of filter samples collected with conventional respirable samplers at each of seven locations (six near production processes and one outdoors). Throughout most of the facility and outdoors, respirable mass concentrations were low (<0.050 mg/m(3)) and were attributed to particles other than the nanomaterial (<10% by mass titanium determined with inductively coupled plasma atomic emission spectrometry). In contrast, in a single area with extensive material handling, mass concentrations were greatest (0.118 mg m(-3)) and contained up to 39% +/- 11% lithium titanium, indicating the presence of airborne nanomaterial. Analysis of the filter samples collected in this area by transmission electron microscope and scanning electron microscope revealed that the airborne nanomaterial was associated only with spherical aggregates (clusters of fused 10-80 nm nanoparticles) that were larger than 200 nm. This analysis also showed that nanoparticles in this area were the smallest particles of a larger distribution of submicrometer chain agglomerates likely from welding in an adjacent area of the facility. The second method used two, hand-held, direct-reading, battery-operated instruments to obtain a time series of very fine particle number (<300 nm), respirable mass, and total mass concentration, which were then related to activities within the area of extensive material handling. This activity-based monitoring showed that very fine particle number concentrations (<300 nm) had no apparent correlation to worker activities, but that sharp peaks in the respirable and total mass concentration coincided with loading a hopper and replacing nanomaterial collection bags. These findings were consistent with those from the filter-based method in that they demonstrate that airborne nanoparticles in this facility are dominated by "incidental" sources (e.g., welding or grinding), and that the airborne "engineered" product is predominately composed of particles larger than several hundred nanometers. The methods presented here are applicable to any occupational or environmental setting in which one needs to distinguish incidental sources from engineered product.

House dust collection efficiency of the high volume small surface sampler on worn carpets

The High Volume Small Surface Sampler (HVS3) is a dust-sampling vacuum that allows for set airflow and back pressure during sampling, increasing precision. Total dust collection efficiency of the HVS3 has been evaluated only on new carpets-not worn carpets. We performed a factorial study to assess the impact of carpet wear, dust deposition level, carpet type, and relative humidity during sampling on HVS3 collection efficiency. House dust was aerosolized in a 1-m3 exposure chamber and allowed to settle on test carpets and reference filters. Dust was embedded into the carpets and later extracted with the HVS3 under controlled environmental conditions according to established protocols. Overall collection efficiency was high, 88.3%. Collection efficiency was significantly higher at low relative humidity levels (30%) relative to high (75%) (p = < 0.0001), though differentially between cut-pile and closed-loop carpets. Collection efficiency of carpets with high wear was significantly lower than those with midlevel wear (p = 0.01). These results demonstrate that the design of the HVS3 partially corrects for differences in dust load and carpet type. However, collection efficiency of the HVS3 is affected by high levels of carpet wear and ambient humidity during sampling.

Effect of nitrogen dioxide exposure on allergic asthma in a murine model

STUDY OBJECTIVES

The purpose of this study was to examine the effects of NO(2), a major component of air pollution, on airway eosinophilic inflammation and bronchial hyperreactivity, using a mouse model of asthma.

SETTING AND SUBJECTS

BALB/c mice (eight mice per experimental group) were studied in a basic research laboratory at the University of Iowa.

INTERVENTIONS

Using a standard murine model of asthma, BALB/c mice were sensitized to ovalbumin (OVA) by intraperitoneal (IP) injections (days 1 and 7) and were challenged with aerosolized OVA (days 13 and 14). Some mice were exposed to NO(2) (2 ppm) in an exposure chamber for 24 h before undergoing OVA aerosol challenge. A control group was exposed to OVA alone.

MEASUREMENTS AND RESULTS

The outcomes assessed included airway inflammation, bronchial hyperreactivity to inhaled methacholine, and goblet cell hyperplasia. We found that NO(2) exposure modestly increased airway neutrophilia but not airway eosinophilia in OVA-exposed mice. These mice exhibited epithelial damage and loss of epithelial mucin. Surprisingly, nonspecific bronchial hyperreactivity (ie, enhanced pause index) was not increased, although baseline smooth muscle tone was increased (p < 0.05) in the mice exposed to NO(2).

CONCLUSIONS

These data indicate that relatively short-term (24 h) exposure to NO(2) causes epithelial damage, reduced mucin expression, and increased tone of respiratory smooth muscle. Reduced mucin production may be a mechanism of injury following long-term exposure to inhaled NO(2). Despite enhancing epithelial damage in OVA-exposed mice, NO(2) exposure does not otherwise alter the expression of allergen-induced airway responses.

Modulation of murine allergic rhinosinusitis by CpG oligodeoxynucleotides

BACKGROUND

Allergic rhinosinusitis is characterized by eosinophilic inflammation of the upper airway, which is induced by TH-2 cytokines. CpG oligodeoxynucleotides (ODN) are known to induce TH-1 and to suppress TH-2 cytokines in a variety of settings, including murine models of asthma.

OBJECTIVE

To examine the effect of CpG ODN in a murine model of upper airway allergic inflammation and to correlate with reduction of its manifestations of sneezing and nasal scratching.

METHODS

BALB/c mice were sensitized using Ovalbumin (Ova) intraperitoneally and challenged with aerosolized Ova. CpG ODN were administered at the time of Ova sensitization. Outcomes measured included nasal symptoms, submucosal eosinophilia in the areas lined by respiratory or olfactory epithelium, and bone marrow eosinophilia. To delineate the mechanism of CpG ODN-induced suppression of eosinophilic inflammation, in vitro experiments were carried out to examine the effect of stimulation with Ova on splenocytes obtained from mice that were treated with CpG or control ODN (or no ODN) in vivo. Supernatant was collected after 72 hours of incubation and cytokines were measured by enzyme linked immunosorbent assay.

RESULTS

CpG ODN administered at the time of Ova sensitization effectively abrogated nasal symptoms and eosinophilic upper airway inflammation compared with mice treated with control ODN or with no ODN. Cytokine data revealed that Ova sensitization suppressed IFN-gamma and induced IL-4 and IL-5 compared with non-sensitized mice. CpG ODN treatment reversed these effects.

CONCLUSION

CpG ODN prevents the development of TH-2-mediated eosinophilic inflammation and symptoms in a murine model of allergic rhinosinusitis.

Endotoxin responsiveness and subchronic grain dust-induced airway disease

Endotoxin is one of the principal components of grain dust that causes acute reversible airflow obstruction and airway inflammation. To determine whether endotoxin responsiveness influences the development of chronic grain dust-induced airway disease, physiological and airway inflammation remodeling parameters were evaluated after an 8-wk exposure to corn dust extract (CDE) and again after a 4-wk recovery period in a strain of mice sensitive to (C3H/HeBFeJ) and one resistant to (C3H/HeJ) endotoxin. After the CDE exposure, both strains of mice had equal airway hyperreactivity to a methacholine challenge; however, airway hyperreactivity persisted only in the C3H/HeBFeJ mice after the recovery period. Only the C3H/HeBFeJ mice showed significant inflammation of the lower airway after the 8-wk exposure to CDE. After the recovery period, this inflammatory response completely resolved. Lung stereological measurements indicate that an 8-wk exposure to CDE resulted in persistent expansion of the airway submucosal cross-sectional area only in the C3H/HeBFeJ mice. Collagen type III and an influx of cells into the subepithelial area participated in the expansion of the submucosa. Our findings demonstrate that subchronic inhalation of grain dust extract results in the development of chronic airway disease only in mice sensitive to endotoxin but not in mice that are genetically hyporesponsive to endotoxin, suggesting that endotoxin is important in the development of chronic airway disease.

Synthesis and structural characterization of Sm(II) and Yb(II) complexes containing sterically demanding, chelating secondary phosphide ligands

Metathesis between [(Me3Si)2CH)(C6H4-2-OMe)P]K and SmI2(THF)2 in THF yields [([Me3Si]2CH)(C6H4-2-OMe)P)2Sm(DME)(THF)] (1), after recrystallization. A similar reaction between [(Me3Si)2CH)(C6H3-2-OMe-3-Me)P]K and SmI2(THF)2 yields [([Me3Si]2CH)(C6H3-2-OMe-3-Me)P)2Sm(DME)].Et2O (2), while reaction between [(Me3Si)2CH)(C6H4-2-CH2NMe2)P]K and either SmI2(THF)2 or YbI2 yields the five-coordinate complex [([Me3Si]2CH)(C6H4-2-CH2NMe2)P)2Sm(THF)] (3) or the solvent-free complex [([Me3Si]2CH)(C6H4-2-CH2NMe2)P)2Yb] (4), respectively. X-ray crystallography shows that complex 2 adopts a distorted cis octahedral geometry, while complex 1 adopts a distorted pentagonal bipyramidal geometry (1, triclinic, P1, a = 11.0625(9) A, b = 15.924(6) A, c = 17.2104(14) A, alpha = 72.327(2) degrees, beta = 83.934(2) degrees, gamma = 79.556(2) degrees, Z = 2; 2, monoclinic, P2(1), a = 13.176(4) A, b = 13.080(4) A, c = 14.546(4) A, beta = 95.363(6) degrees, Z = 2). Complex 3 crystallizes as monomers with a square pyramidal geometry at Sm and exhibits short contacts between Sm and the ipso-carbon atoms of the ligands (3, monoclinic, C2/c, a = 14.9880(17) A, b = 13.0528(15) A, c = 24.330(3) A, beta = 104.507(2) degrees, Z = 4). Whereas preliminary X-ray crystallographic data for 4 indicate a monomeric structure in the solid state, variable-temperature 1H, 13C(1H), 31P(1H), and 171Yb NMR spectroscopies suggest that 4 undergoes an unusual dynamic process in solution, which is ascribed to a monomer-dimer equilibrium in which exchange of the bridging and terminal phosphide groups may be frozen out at low temperature.

Mechanisms of asbestos-induced nitric oxide production by rat alveolar macrophages in inhalation and in vitro models

To evaluate the contribution of reactive nitrogen species to inflammation by asbestos, Fischer 344 rats were exposed to crocidolite or chrysotile asbestos by inhalation to determine whether increases occurred in nitric oxide (NO.) metabolites from alveolar macrophages (AMs). AMs from animals inhaling asbestos showed significant elevations (p < .05) in nitrite/nitrate levels which were ameliorated by NG-monomethyl-L-arginine (NMMA), an inhibitor of inducible nitric oxide synthase (iNOS) activity. Temporal patterns of NO. generation from AMs correlated with neutrophil influx in bronchoalveolar lavage samples after asbestos inhalation or bleomycin instillation, another model of pulmonary fibrosis. To determine the molecular mechanisms and specificity of iNOS promoter activation by asbestos, RAW 264.7 cells, a murine macrophage-like line, and AMs isolated from control rats were exposed to crocidolite asbestos in vitro. These cells showed increases in steady-state levels of iNOS mRNA in response to asbestos and more dramatic increases in both iNOS mRNA and immunoreactive protein after addition of lipopolysaccharide (LPS). After transfection of an iNOS promoter/luciferase reporter construct, RAW 264.7 cells exposed to LPS, crocidolite asbestos and its nonfibrous analog, riebeckite, revealed increases in luciferase activity whereas cristobalite silica had no effects. Studies suggest that NO. generation may be important in cell injury and inflammation by asbestos.

Comparative proliferative and histopathologic changes in rat lungs after inhalation of chrysotile or crocidolite asbestos

Patterns of cell proliferation in lung and pleura and development of histopathologic lesions were studied in lungs from Fischer 344 rats after inhalation exposure to chrysotile or crocidolite asbestos at average airborne concentrations of approximately 8 mg/m3 air for 5 and 20 days and after 20 days of exposure followed by an additional 20 days in room air (20 + 20 days). To assess cell proliferation rats were injected with 5-bromo-2'-deoxyuridine (BrdU) at various time points after initiation of exposure to asbestos. Image analysis was used to quantitate the effects of chrysotile and crocidolite on BrdU labeling indices in the following lung compartments: (1) interstitium, (2) alveolar duct region, (3) bronchial epithelium, and (4) visceral mesothelium. With the exception of mesothelium, which exhibited significant increases in BrdU incorporation in rats exposed to crocidolite at 20 + 20 days, asbestos-induced elevations in BrdU uptake in other compartments were transient with labeling comparable to sham controls at later time points. Histopathology of rat lungs revealed fibrotic lesions of a greater extent and severity at 20 days in rats exposed to crocidolite, but fibrosis occurred in both asbestos-exposed groups after an additional 20 days in clean air (20 + 20). Quantification of fiber burden in rat lung after inhalation of comparable airborne concentrations of either fiber type demonstrated that inhalation of crocidolite asbestos led to a higher fiber retention when compared to chrysotile asbestos. Our results indicate that chrysotile and crocidolite asbestos induce different patterns of cell proliferation in lung and pleural cells. The protracted increases in BrdU labeling of mesothelial cells by crocidolite may reflect increased retention of fibers and/or inherent differences between types of asbestos.

Nonuniform air flow in inlets: the effect on filter deposits in the fiber sampling cassette

Smoke stream studies were combined with a new technique for visualizing a filter deposit from samples used to monitor asbestos or other fibers. Results clearly show the effect of secondary flow vortices within the sampler under anisoaxial sampling conditions. The vortices observed at low wind velocities occur when the inlet axis is situated at angles between 45 degrees and 180 degrees to the motion of the surrounding air. It is demonstrated that the vortices can create a complex nonuniform pattern in the filter deposit, especially when combined with particle settling or electrostatic interactions between the particles and the sampler. Inertial effects also may play a role in the deposit nonuniformity, as well as causing deposition on the cowl surfaces. Changes in the sampler, such as its placement, may reduce these biases. The effects noted are not likely to occur in all sampling situations, but may explain some reports of high variability on asbestos fiber filter samples. The flow patterns observed in this study are applicable to straight, thin-walled inlets. Although only compact particles were used, the air flow patterns and forces involved will have similar effects on fibers of the same aerodynamic diameter.