Aerosol Science and Technology 35: 617–624 (2001)c° 2001 American Association for Aerosol ResearchPublished by Taylor and Francis0278-6826=01=$12.00 C .00

Collection of Bioaerosol Particles by Impaction:Effect of Fungal Spore Agglomeration and Bounce

Mikhaylo Trunov,1 Saulius Trakumas,2 Klaus Willeke, Sergey A. Grinshpun,and Tiina ReponenAerosol Research and Exposure Assessment Laboratory, Department of Environmental Health,University of Cincinnati, Cincinnati, Ohio

Calibration and performance evaluations of bioaerosol impac-tors are usually conducted with non-biological test aerosols, suchas polydisperse liquid oleic acid particles or monodisperse solidpolystyrene latex (PSL) particles. This study was undertaken toinvestigate to what degree surface properties and agglomerationof bioaerosol particles may result in different performance char-acteristics of impactors. The single-stage impaction of biologicaland non-biological particles on a sticky surface was studied uti-lizing Air-O-Cell sampling cassettes that are widely used to col-lect airborne fungal spores. The aerosol concentrations upstreamand downstream of the sampler were measured with an aerody-namic particle size spectrometer. The collection ef� ciency was de-termined for the sampler operating at different � ow rates rangingfrom 10 to 30 L/min. The tests were performed with aerosol par-ticles of about 1 to 4 m in diameter, including two fungal speciesof different surface properties (Penicillium brevicompactum andPenicillium melinii), and two types of non-biological aerosols (oleicacid and PSL). The 50% cut-off sizes determined experimentallywith non-biological particles differed from the theoretical predic-tions by 11% or less. The data obtained with biological test par-ticles, however, were found to show higher (at low sampling � owrates) or lower (at high � ow rates) collection ef� ciencies than deter-mined through the use of conventional non-biological test particles.E.g., at 30 L/min, the difference is about 50%. The differences wereattributed to the presence of spore aggregates and their possibledeaggregation during impaction. Inertial impaction, deaggrega-tion, and bounce of fungal spores from the collection surface werestudied experimentally and estimated theoretically utilizing exper-imental data on the percentages of singlets, doublets and triplets inspeci� c bioaerosols. It is concluded that the calibration and perfor-mance of bioaerosol impactors may strongly depend on the surfacecharacteristics, initial percentage of aggregates, and deaggregationrate of the speci� c bioaerosol particles being sampled.

Received 18 May 1999; accepted 5 April 2000.1On leave from Odessa University, Odessa, Ukraine.2On leave from Institute of Physics, Vilnius, Lithuania.3Address correspondence to Sergey A. Grinshpun, Aerosol Re-

search and Exposure Assessment Laboratory, Department of Environ-mental Health, University of Cincinnati, P.O. Box 670056, Cincinnati,OH, 45267-0056 .

INTRODUCTIONInertial impaction is one of the most common methods used

for the collection of airborne dust and microorganisms. To de-termine their collection ef� ciency, aerosol impactors are usu-ally calibrated with test aerosols that have known aerodynamiccharacteristics. Most of the existing theoretical models for in-ertial impaction assume that the sampled particles are sphericalwith a smooth surface, and that all the impacted particles re-main attached to the collection plate (Hinds 1982; Rader andMarple 1985). However, biological particles (e.g., fungal andbacterial spores) are generally non-spherical (Reponen 1995).Some species are known to produce spores with spiny sur-faces (Gregory 1973). Spores are often aerosolized in agglom-erates, such as chains and compacts (Eduard and Aalen 1988;Karlsson and Malmberg 1989; Lacey 1991; Reponen et al.1996). Aggregated particles have higher inertia than singlespores and are, therefore, more likely to impact on the collec-tion media. At the same time, they may deaggregate during im-paction. Some spores having very high elasticity may bounceafter impaction even if the collection plate is covered with asticky medium (Gregory 1973). These factors are expected toaffect the collection ef� ciency of bioaerosol impactors and thuslimit the accuracy of their performance evaluation or calibrationmade by traditional methods with non-biological test particles.This study shows that the collection ef� ciency of a bioaerosolimpactor depends not only on the aerodynamic particle size andsampling � ow rate but also on the surface characteristics of theparticles, the percentage of aggregates, and the deaggregationrate during impaction.

MATERIALS AND METHODSThe experimental setup used in this study is schematically

shown in Figure 1. The test particles were aerosolized by one ofthree different techniques, depending on the type of particle to bedispersed. The experiments were conducted with aerosols of fourtypes: two spherical non-biological particles, either polydisperseliquid oleic acid or monodisperse solid polystyrene latex (PSL,

617

618 M. TRUNOV ET AL.

Figure 1. Experimental setup for collection ef� ciency mea-surements. Sampler D Air-O-Cell impactor.

Bangs Laboratories Inc., Fishers, IN); and close-to-sphericalspores of two fungal species with different surface properties,either Penicillium brevicompactum (smooth surface) or Penicil-lium melinii (spiny surface). A size-fractionating aerosol gen-erator (Pilacinski et al. 1990) was used to generate oleic acidparticles of a desired size range. The PSL particles were gen-erated by a Collison nebulizer (BGI Inc., Waltham, MA). Anagar tube disperser (Reponen et al. 1997) was used to aerosolizefungal spores from the growth surface simulating their naturaldissemination. The test aerosols were dried and diluted with � l-tered compressed air, QDIL. The dilution � ow rate ranged from35 to 45 L/min depending on the aerosolization system used.The dried and diluted aerosol passed through a 10-mCi 85Krelectrostatic charge equilibrator (model 3012, TSI Inc., St. Paul,MN), and then entered the aerosol chamber housing the test sam-pler. When performing experiments with oleic acid droplets, anaerosol chamber with a volume 2.6 m3 was used. The exper-iments with the other types of aerosols were conducted in asmaller chamber of 550 cm3. The temperature, T, and humid-ity, RH, in the chamber were measured by a thermohygrometer(Fisher Scienti� c, Pittsburgh, PA) and kept at T D 20–21±C andRH D 15–20% during all experiments.

Air-O-Cell air sampling cassettes (Zefon International—An-alytical Accessories Inc., St. Petersburg, FL) were used as testimpactors (Zefon 1996). They utilize the principle of single-stage inertial impaction and have been designed for primarilycollecting bioaerosol particles. The entering airborne particlesaccelerate through a tapered two-dimensional slit with externaldimensions of 15 mm £ 11 mm (sampling ori� ce) and internaldimensions at the throat of 14.4 mm £ 1.055 mm (internal slit).The contraction angle is about 30±. Upon exiting the taper slit,the air stream is de� ected sideways by a cover slide used for

collecting the particles and subsequent analysis under an opticalmicroscope. The slide is covered by the manufacturer with asticky proprietary medium for increased particle retention.

The particles were alternatively sampled upstream and down-stream of the test impactor. The upstream aerosol concentra-tion, CUP, and the downstream aerosol concentration, CDOWN,were measured by an aerodynamic particle size spectrometer(Aerosizer, TSI—Amherst Process Instruments, Hadley, MA)operated at a � ow rate, QAER, of 5.3 L/min. In this study, theimpactor was tested at � ve sampling � ow rates: QS D 10,15, 20, 25, and 30 L/min. The impactor manufacturer recom-mends operating at QS D 15 L/min (Zefon 1996); however, theAir-O-Cell sampler is used in the � eld at � ow rates ranging from10 to 30 L/min. As QS was greater than QAER in all tests, the extraair, QBYPASS D QS¡ QAER, was bypassed at 4.7 to 24.7 L/min,monitored by a mass � ow meter. The sampling lines used forparticle concentration measurements downstream and upstreamof the sampler were of the same length of 40 cm and orientedvertically. Thus, the particle losses in these lines, if present, werethe same.

At the highest sampling � ow rate of 30 L/min, the pressuredrop in the impactor was highest at about 700 Pa. A pressuredrop of this small magnitude has no signi� cant effect on theperformance characteristics of the Aerosizer (Grinshpun et al.1997). The aerosol concentration measured in our tests rangedfrom 1 £ 106 to 3 £ 106 m¡3 for oleic acid particles and from2 £ 105 to 8 £ 105 m¡3 for PSL particles and fungal spores.The concentration of test aerosols in the chamber was kept lowenough to avoid particle coincidence that may occur in a singleparticle counter at high aerosol concentrations. The concentra-tion measurements were performed during a test period of 160swith oleic acid particles and during 60s with PSL particles andfungal spores. These sampling conditions assured that the collec-tion surface was not overloaded with the impacted particles dur-ing sampling (the Air-O-Cell has been designed for short-termsampling and subsequent microscopic counting of the collectedparticles).

Using the particle size distribution data measured upstreamand downstream of the sampler, the collection ef� ciency, EC,was determined for each particle size as follows:

EC D (1 ¡CDOWN

CUP) £ 100 (%) [1]

Although all of the Aerosizer’s 165 particle size channels weremonitored in each test, the collection ef� ciency of the samplerwas determined only for the particle size range of interest, 1 to4 ¹m (the test aerosol particles were generated within the sizerange typical for fungal spores).

The experiments were performed in randomized order. Abrand-new impactor was used for each new test (the housingis molded out of plastic material for one-time use). Data wereobtained with at least three/trials for each set of conditions.The average value of the collection ef� ciency and the standard

BIOAEROSOL COLLECTION BY IMPACTION 619

deviation were calculated. When a polydisperse oleic acidaerosol was used, the fractional collection ef� ciency was de-termined for each particle size channel. The experimental col-lection ef� ciencies for PSL particles (six different sizes rang-ing from da D 1:64 to 3.59 ¹m were tested) and fungal sporebioaerosols were determined for the particle size range extendingfrom

(da)mean=¾g to (da)mean £ ¾g;

where (da)mean is the geometric mean diameter and ¾g is thegeometric standard deviation.

The measured collection ef� ciency was plotted against theaerodynamic size of the oleic acid calibration particles, and the50% cut-off size, d50 , was determined for each experimentalcurve, depending on the sampling � ow rate. The d50 -values werealso calculated from the theory of inertial impaction (Marpleand Willeke 1976; Willeke 1978; Rader and Marple 1985). Thecollection ef� ciency of a single-stage rectangular impactor wasexpressed as a function of the Stokes number following Raderand Marple’s numerical solution of the Navier-Stokes and theparticle motion equations. The Stokes number at which 50%of particles are impacted on the collection surface, Stk50, wasdetermined from this theory. The d50-values were calculatedfrom Stk50 as

d50 D 9Stk50´W

½PVC[2]

where W is the width of the impactor nozzle, ´ is the air vis-cosity, ½P is the particle density, V is the average air velocity inthe nozzle, and C is the size-dependent Cunningham slip cor-rection factor. The available theory and experimental data havebeen found to be in reasonable agreement for standard impactorgeometries. At the same time, Jurcik and Wang (1995) haveshown that different impactor inlet geometries may affect theshape of the collection ef� ciency curve. The experimental datafor the Air-O-Cell impactor are expected to differ somewhatfrom the theoretical predictions, since the inlet geometry of theAir-O-Cell is somewhat different from one used in the theoret-ical calculations. For instance, the theory has assumed that thenozzle-throat length of the inlet is not negligibly small (as it is inthe Air-O-Cell impactor), that the inlet contraction angle is 60±

(not 30±), and that the sticky surface provides 100% attachmentof the impacted particles.

RESULTS AND DISCUSSIONFigure 2 shows the collection ef� ciency data obtained in our

tests with the oleic acid aerosol when the Air-O-Cell was op-erated at � ve different sampling � ow rates. The points in this� gure represent the average experimental values, and the curvesrepresent their regression analyses. Each vertical error bar rep-resents the standard deviation of at least three measurements.

Figure 2. Effect of sampling � ow rate on the particle-size de-pendent collection ef� ciency of oleic acid test particles. Theerror bars represent the standard deviations of at least three mea-surements.

The collection ef� ciency sharply increases with particle aerody-namic diameter reaching 100% at da ¼ 2:3 ¹m for the highestsampling � ow rate of QS D 30 L/min. At the minimum sam-pling � ow rate of 10 L/min, the 100% ef� ciency is reached atabout 3.7 ¹m; at the recommended � ow rate of 15 L/min, the100% ef� ciency is reached at about 3.2 ¹m. The collection ef-� ciencies at these � ow rates may not always be suf� cient foref� cient collecting of bioaerosol particles, as most of the bacte-rial and fungal spores are smaller (Reponen 1995; Reponen et al.1996).

The experimental values of the impactor’s 50% cut-off sizeare plotted against the sampling � ow rate in Figure 3. Althoughthe theoretical d50-values are about 11% lower than those ob-tained experimentally, the agreement between the two is reason-able. The 11% difference seems to re� ect the above-indicateddifferences between the impactor’s geometry and the geometryused in the numerical calculations (primarily the nozzlethroatlength and the contraction angle). It is concluded that the per-formance characteristics of an impactor collecting a standardliquid test aerosol, such as oleic acid, are predictable by numer-ical calculations.

The next set of experiments, Figure 4, was performed withsolid PSL particles of 3.03 and 3.59 ¹m in aerodynamic diam-eter at the minimum sampling � ow rate of 10 L/min. These twoparticle sizes were chosen to bracket the sharp increase of theS-shaped collection ef� ciency curve. The collection ef� cienciesmeasured with liquid and solid non-biological particles are seento be in a good agreement. However, the data obtained withbiological particles at QS D 10 L/min do not coincide with

620 M. TRUNOV ET AL.

Figure 3. Effect of sampling � ow rate on the particle cut-offsize. Data points D experiments with oleic acid particles; solidline D regression analysis; dotted line D impaction theory forslightly different impactor geometry.

Figure 4. Effect of particle type (oleic acid, PSL, P. brevicom-pactum, and P. melinii spores) on the collection ef� ciency atQS D 10 L/min. Vertical and horizontal error bars representstandard and geometric standard deviations, respectively.

those obtained with non-biological particles. The vertical errorbars represent the standard deviations of at least three measure-ments. The horizontal error bars represent the variation of sizedistributions, (da)mean £ (¾g¡1)=¾g for the error bar stretchingto the left and (da)mean £ (¾g¡1) for the right bar, as measuredby the Aerosizer.

The aerodynamic diameters of fungal spores measured bythe Aerosizer were (da)mean D 2:53 ¹m (P. brevicompactum) and2.82 ¹m (P. melinii)with ¾g D 1:14 (P. brevicompactum) and 1.1(P. melinii). It is believed that the Aerosizer’s sensor detects pri-marily single spores because aggregates are likely to be brokenup when being accelerated inside the instrument to near sonicvelocity. The measured spore sizes seem to be in a good agree-ment with the aerodynamic sizes previously reported for bothfungal species (Reponen et al. 1996, 1997). In general, the meanaerodynamic diameters of P. brevicompactum and P. melinii maydiffer from one study to another due to several factors, such as theage of the microbial culture, variations in the � ow rate and the airturbulence intensity inside the agar tube disperser, and the rel-ative humidity. For instance, the (da)mean;spore -values referred toby Jankowska et al. (2000) and Aizenberg et al. (2000) for thesespecies differ from those measured in this study by about 15%.

Using microscopic analyses, we found that some bioaerosolparticles entering the Air-O-Cell impactor were aggregated. Thisdemonstrates that not all spore aggregates had been broken upin the deaggregation ori� ce of the agar tube disperser. In a pre-vious study, quantitative microscopic analysis has shown thatamong the dispersed P. brevicompactum particles there are still11–13% of doublets and 3–6% of triplets and larger aggregates(of chain-like shape) after exiting the deaggregating ori� ce ofthe agar tube disperser (Reponen et al. 1996). As reported in thesame study, the P. melinii particles contain 14–23% of doubletsand 2–5% of triplets and larger aggregates (also chain-like). Innature, the percentage of aggregated bioaerosol particles maybe signi� cantly higher, depending on the fungal species and therelease mechanism, because, in contrast to our tests, no specialeffort was made to deaggregate spores in the process of theirnatural dissemination (Eduard and Aalen 1988; Karlsson andMalmberg 1989; Lacey 1991).

The data points representing our measurements withfungal spores at 10 L/min lie signi� cantly higher than those ob-tained from the theory as well as those found in the experimentsconducted with oleic acid and PSL particles: we measured EC ¼50% for P. melinii spores and ¼ 35% for P. brevicompactumspores, while both the theory and the experiments with non-biological test particles predict very low collection ef� ciency(EC ¼ 0 for da D (da)mean;spore ). This � nding is attributed to thepresence of airborne spore chains consisting primarily of twoor three single spores. These chains have higher inertia than thesingle spores and hence a higher chance to be impacted. A math-ematical estimate of the aerodynamic sizes of chain aggregateswas made using the particle dynamic shape coef� cients deter-mined by Kasper et al. (1985) for rod-shaped chains orientedparallel to the air � ow. These coef� cients are: 1.02 for doublets

BIOAEROSOL COLLECTION BY IMPACTION 621

and 1.06 for triplets. Calculation of the aerodynamic diameterfor a chain using the above shape coef� cients results in val-ues exceeding the aerodynamic diameter of single particles bya factor of 1.25 for doublets and 1.40 for triplets. These factorsincrease with increasing number of single particles in a chain,thus resulting in higher collection ef� ciency compared with thatof single spores.

The effect of spore aggregates on the collection ef� ciencyof the Air-O-Cell impactor was estimated theoretically by as-suming that 100 airborne particles (including singlets, doublets,and triplets) entered the chamber during the measurement time.When the impactor is operated at 10 L/min, the collection ef-� ciency remains close to about 0 as long as the particle aero-dynamic size does not exceed ¼ 2:8 ¹m (see Figures 2 and4). Therefore, all single spores of P. brevicompactum and mostof the single spores of P. melinii are expected topenetrate throughthe sampler. Using the data reported by Reponen et al. (1996),P. brevicompactum was assumed to aerosolize in singlets at 81%,in doublets at 13%, and in triplets at 6%. The number of sin-gle P. brevicompactum spores in the bioaerosol upstream of theimpactor is thus ((1 £ 81) C (2 £ 13) C (3 £ 6)) D 125 par-ticles. It is expected that the singlets cannot be collected dueto insuf� cient inertia. Therefore, all 81 single particles are tobe detected downstream of the impactor. To estimate the col-lection ef� ciency of agglomerates, we calculated their aerody-namic sizes, using correction factors of 1.25 for doublets and1.4 for triplets, and utilized the collection ef� ciency curves ob-tained with oleic acid particles Figures 2 and 4). For example,a triplet of P. brevicompactum has the aerodynamic diameter of2.53 £ 1.4 D 3.54 ¹m. At 10 L/min � owrate the triplet is col-lected of 93% ef� ciency (see Figure 4). We also assume thatthe agglomerates are not broken while accelerating inside theimpactor’s slit and that none of them are bounced off fromthe collection surface after impaction. Thus, the overall col-lection ef� ciency of a P. brevicompactum mixture, representing81% singlets, 13% of doublets, and 6% of triplets, is

EC D [(1 £ 81 £ 0:0 C 2 £ 13 £ 0:50

C 3 £ 6 £ 0:93)=125] £ 100% D 23:8%

that is close to the EC D 35 § 6% measured for P. brevicom-pactum at QS D 10 L/min. Using the same approach, the col-lection ef� ciency was calculated for 100 particles of P. meliniiconsisting of a mix of 75 singlets, 20 doublets, and 5 triplets(Reponen et al. 1996). The calculated collection ef� ciency isabout 40%, which is also close to our experimental data,52 § 7%. Therefore, even a relatively small percentage of aggre-gated biological particles in the air may considerably improvetheir collection by an impactor, if the aggregates are not bro-ken up inside the sampler, which we expect at relatively lowsampling � ow rates.

Figure 5 shows the experimental data obtained with the fourtest aerosols at higher sampling � ow rates ranging from 15 to30 L/min. Small differences between the collection ef� ciencies

Figure 5. Collection ef� ciency data for test aerosols of differ-ent type (oleic acid, PSL, P. brevicompactum, and P. meliniispores) at different sampling � ow rates, QS D 15, 20, 25,30 L/min. Vertical and horizontal error bars represent standardand geometric standard deviations, respectively.

of oleic acid and PSL particles of the same aerodynamic diam-eter are seen primarily for larger particles and higher � ow rates,e.g., for da >» 2:5 ¹m at QS D 25 and 30 L/min. When liquidoleic acid droplets were collected 100% ef� ciently, elastic PSLparticles of the same size demonstrated about 5 to 10% penetra-tion. This was attributed to the bounce of PSL particles from thecollection surface (the bounce was more pronounced at highersampling � ow rates).

At QS D 15 L/min, the collection ef� ciencies of fungal sporeswere closer to those obtained with PSL and oleic acid particlesthan observed at 10 L/min. However, at higher � ow rates of 20to 30 L/min, the collection ef� ciency data for fungal spores arelower than those obtained from our experiments with oleic acidand PSL particles of the same sizes. This tendency is opposite tothe one observed at QS D 10 L/min. The difference became con-siderable at 30 L/min when, e.g., P. melinii spores were collectedabout half as ef� ciently compared to non-biological particles ofthe same aerodynamic size. The collection ef� ciency of P. bre-vicompactum did not increase substantially when the sampling� ow rate increased by a factor of two from 15 to 30 L/min. Thecollection ef� ciency of P. melinii even decreased from 89 § 3%to 56 § 6% in spite of the increase in particle inertia due to the� ow rate increase.

To interpret these data, the inertial impaction of spore ag-gregates was analyzed. At relatively low sampling � ow rates,e.g., 10 L/min, the doublets or triplets (that are more likely

622 M. TRUNOV ET AL.

to be collected than single spores) do not deagglomerate aftertheir impaction and remain on the collection surface. Thus, theoverall collection ef� ciency of bioaerosol particles containingnon-breaking aggregates is generally higher than that predictedfor single spores (see Figure 4). The increase in the sampling� ow rate is supposed to improve the collection of single sporesand aggregates by inertial impaction. However, the increasingimpaction velocity may become suf� cient to break up aggre-gates during impaction. If the adhesive force between individualspores of an aggregate is smaller than that between an aggre-gate and the sticky collection medium, a chain aggregate maybreak off. The spore that contacts the surface � rst remains at-tached to the surface while other(s) break away from the chainand leave the impactor with the ef� uent air � ow. If the deag-gregation rate increases with increasing sampling � ow rate, theactual number of spores collected by an impactor may be signif-icantly reduced. This effect explains the data shown in Figure 5(A through D).

The spore surface characteristics may affect their adhesion tothe collection medium. For instance, P. brevicompactum sporeswith a smooth surface are expected to develop a stronger con-tact with the collection media than P. melinii spores with a spinysurface. Thus, spiny spores are more likely to bounce off re-sulting in a decrease of their collection ef� ciency at higher� ow rates. Indeed, the collection ef� ciency values measuredat 25 and 30 L/min (Figure 5C and D) are lower forP. melinii than for P. brevicompactum although (da)P. melinii >(da)P. brevicompactum.

The above interpretation was con� rmed through theoreticalquanti� cation of the effect of spore aggregates on the collectionef� ciency of P. brevicompactum and P. melinii. As a � rst step, EC

was estimated assuming that neither single spores not their ag-

Table 1Collection ef� ciency of Penicillium brevicompactum and Penicillium melinii spores at different sampling � ow rates

Collection ef� ciency, EC, %

Measured with oleic acid Estimatedparticles simulating single spores accounting for Measured

Fungal species

Sampling� owrate,L/min (da)oleic acid D (da)mean; spores spore aggregates directly

P. brevicompactum, 10 0 23.8 35 § 6(da)mean D 2:53 ¹m 15 33 56.2 64 § 5

20 91 74.2 69 § 625 100 80.0 78 § 630 100 80.0a 74 § 5

P. melinii, 10 0 39.8 52 § 7(da)mean D 2:82 ¹m 15 82 89.6 89 § 3

20 98 75.8 74 § 725 100 76.9b 68 § 430 100 76.9c 56 § 6

aTo � t the directly measured ef� ciency, the estimated one must be corrected by a single spore bounce rate of 8%.bTo � t the directly measured ef� ciency, the estimated one must be corrected by a single spore bounce rate of 15%.cTo � t the directly measured ef� ciency, the estimated one must be corrected by a single spore bounce rate of 35%.

gregates bounce after impaction. The calculation of EC for a mixof single spores and aggregates at different sampling � ow ratesrequires data on the collection ef� ciency of single spores. Forthis purpose, we used the collection ef� ciency values measuredwith oleic acid particles of the same aerodynamic size as the sin-gle spores. Table 1 presents the experimental data obtained witholeic acid particles that had the same mean aerodynamic diam-eter as single spores of each species, (da)oleic acid D (da)mean;spore.The particles of (da)oleic acid D 2:53 ¹m simulate single P. brevi-compactum spores that cannot bounce from the surface, and theparticles of (da)oleic acid D 2:82 ¹m simulate single non-bouncyspores of P. melinii. Table 1 also presents the collection ef� -ciency data for aerosols of P. brevicompactum and P. melinii thatinclude single spores and spore aggregates. The second to lastcolumn shows the EC-values that were estimated when account-ing for aggregates, and the last column shows EC-values mea-sured directly with the Aerosizer. The measured collection ef� -ciencies are presented with their standard deviations. As seen,our estimates accounting for spore agglomerates predict the ac-tual spore collection ef� ciency (measured directly) more accu-rately than the simple utilization of data obtained with oleic acidparticles having the same size as the mean size of single spores.At the highest � ow rates (30 L/min for P. brevicompactum and25 and 30 L/min for P. melinii) a correction of the theoreti-cally calculated collection ef� ciency for particle bounce seemsto be appropriate. The best � t was achieved by assuming thatthe bounce rate for P. brevicompactum single spores was 8% atQS D 30 L/min, and that for P. melinii it was 15% at 25 L/minand 35% at 30 L/min.

Figure 6 schematically summarizes our data interpretation forthe impaction of fungal spores and similarly sized non-biologicalparticles at different sampling � ow rates. At the low � ow rate

BIOAEROSOL COLLECTION BY IMPACTION 623

Figure 6. Impaction of PSL particles and fungal spores at different sampling � ow rates (schematic representation).

of 10 L/min, the particle inertia is not suf� cient to collect mostof the particles. Those spores that are collected are most likelyaggregated. The impaction velocity is not high enough to expectdeaggregation of spore chains during impaction, nor is particlebounce expected. At 15 L/min, the collection ef� ciencies of allthree particle types increase due to increasing particle inertia.This effect is more pronounced for P. melinii, as these sporeshave slightly larger sizes than P. brevicompactum. At 20 L/min,the inertia is high enough to assure that all the PSL and P. meliniiparticles are impacted, but not yet suf� cient to impact 100% ofthe smaller P. brevicompactum particles. At the same time, theimpaction velocity is high enough to result in deaggregation ofsome spore chains with subsequent escape of the particles thathave been separated in a break off. At 25 L/min, the collec-tion ef� ciency of PSL particles decreases due to their bounce(in spite of proprietary adhesive material on the collection sur-face). Bounce may also occur for single spores, particularly forP. melinii particles, as they have a spiny surface that does not al-low good contact with the collection medium. The single sporesof P. brevicompactum are less bouncy, as they are smooth andtherefore the area of their contact with the collection media islarger. Some spore chains deaggreagate, similar to what was ob-served for lower � ow rates. Thus, the collection ef� ciency doesnot increase and may even decrease with increasing � ow rate.At 30 L/min, the bounce of single P. melinii spores increases;the impaction velocity is suf� ciently high so that even singleP. brevicompactum spores may bounce off the collection med-ium. Deaggregation rate is still considerable, reducing the col-lection ef� ciency of fungal spores.

These � ndings should be applicable to single-stage impactorsof other geometries. In a multistage impactor, the effect of ag-gregation, deagregation, and bounce on the collection ef� ciencymay be even more complex (e.g., the spores broken up during

their impaction on the � rst stage and thus not collected by thisstage may still be collected by the subsequent stages).

CONCLUSIONThe collection of fungal spores by impaction was studied ex-

perimentally and theoretically using a single-stage Air-O-Cellimpactor. This study was focused on the physical aspects ofthe collection ef� ciency and did not consider the bioef� ciencythat deals with microbiological characteristics, such as the sporeviability and the stress caused by their collection. Our � nd-ings show that the collection ef� ciency of fungal spores in im-pactors is a complex function of several parameters, such asthe individual spore surface characteristics, the initial percent-age of aggregates, the deaggregation rate during impaction, andthe particle bounce rate. The standard procedure for the evalua-tion and calibration of bioaerosol impactors through testing withnon-biological test aerosols (e.g., PSL) may thus not adequatelyre� ect actual collection of bioaerosol particles. Conventionalnumerical calculations and the data generated experimentallywith non-biological tests aerosols may under- or overestimatethe collection ef� ciency of fungal spores (depending on the im-paction velocity). Effects such as initial aggregation of sporesand their subsequent deaggregation and bounce due to impactionshould be considered in the determination of the performancecharacteristics of bioaerosol impactors.

ACKNOWLEDGEMENTMost of the experiments with the Air-O-Cell sampling cas-

sette were conducted due to support from Zefon InternationalInc. The preparation of the fungal spores was done in collabo-ration with visiting scholar Dr. E. Jankowska from the CentralInstitute of Labor in Warsaw, Poland. The authors appreciatethese contributions.

624 M. TRUNOV ET AL.

The authors are thankful for important and useful commentsmade by the reviewers and the Associate Editor.

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