Selecting fiber materials to improve mist filters

Aerosol Science 34 (2003) 1481–1492www.elsevier.com/locate/jaerosci

Selecting �ber materials to improve mist �ltersGina M. Letts, Peter C. Raynor∗, Rebecca L. Schumann

Division of Environmental and Occupational Health, School of Public Health, University of Minnesota, Mayo MailCode 807, 420 Delaware St. SE, Minneapolis, MN 55455, USA

Received 18 March 2003; received in revised form 3 May 2003; accepted 6 May 2003

Abstract

As a �lter captures droplets and retains liquid, the e0ciency of the �lter declines while the pressuredrop rises. Making �lters that drain more e2ectively and retain less liquid may minimize e0ciency lossesand pressure drop increases. Glass, polyester, and polyaramid �bers were observed microscopically as theycollected droplets. Liquid spread much more readily on the polyaramid �bers than on the other kinds. Complete�lters were then formed from glass and polyaramid �bers and tested for e0ciency, pressure drop, and liquidretention as they collected droplets. Although the �lters made from polyaramid �bers exhibited less liquidretention and pressure drop increase, the reduction in e0ciency between �lters made from the two �ber typeswas not statistically di2erent. These �ndings suggest that using higher surface energy �bers in mist �ltersmay allow lower levels of liquid retention that result in wet �lters with a lower pressure drop.? 2003 Elsevier Ltd. All rights reserved.

Keywords: Filtration; Mist; Droplets; Fiber material; E0ciency; Pressure drop

1. Background

Fibrous �lters collect airborne droplets during chemical production, compressed gas production,industrial gas cleaning, and other processes. One gas cleaning process of interest is �ltration ofmetalworking :uid (MWF) mist droplets. MWFs are used during machining to cool and lubricatethe chip-tool-workpiece interface (Leith, Raynor, Boundy, & Cooper, 1996). Due to heat generatedduring machining processes, MWFs can evaporate and then condense into small droplets (Thornburg& Leith, 2000). Fluid droplets can also be ejected into the air during application and from movingmachine or part surfaces. Woskie et al. (1994) measured the mass median droplet diameters ofsamples taken in metalworking facilities to be between 5.5 and 8:0 �m, with geometric standard

∗ Corresponding author. Tel.: +1-612-625-7135; fax: +1-612-626-0650.E-mail address: [email protected] (P.C. Raynor).

0021-8502/03/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0021-8502(03)00102-2

mailto:[email protected]

1482 G.M. Letts et al. / Aerosol Science 34 (2003) 1481–1492

deviations between 2.4 and 3.3. Piacitelli et al. (2001) reported that particle size distributions ofMWF aerosols had an average mass median aerodynamic diameter of 5:3 �m.

An estimated 1.2 million workers are exposed to MWFs in the United States (NIOSH, 1977).In the occupational environment, people can be exposed to MWFs through inhalation of aerosolmists and by absorption of the :uid through contact with the skin. These exposures are associatedwith a variety of adverse health e2ects, most commonly skin disorders such as irritation, rashes, oilacne, dermatitis, folliculitis, and keratosis. Eye, nose, and throat irritation and respiratory disorderssuch as breathing problems, changes in respiratory function, coughing, asthma, lipid pneumonia,hypersensitivity pneumonitis, chronic bronchitis, and tightness of the chest are also health e2ectsassociated with exposure to MWFs. Some MWFs are associated with an increased risk of larynx,rectum, pancreas, skin, scrotum, and bladder cancers (NIOSH, 1998).

In workplaces, areas where MWFs are used are usually surrounded by exhaust hoods or enclosuresthat capture the mist to reduce worker exposure. The contaminated air is pulled through a ventilationsystem to mist collectors that are designed to remove the droplets e0ciently. Many times, this airis recirculated back into the work area. Mist collectors often employ �brous �lters that are left inplace for extended periods of time. Most collectors are comprised of multiple stages with the �rststages removing the largest droplets using baIes, metal-mesh �lters, roughing �lters, open cell foamdesigned to coalesce droplets, centrifugal collectors, or electrostatic precipitators. Intermediate stageseliminate succeedingly smaller droplets using �brous �lters such as hanging pocket �lters, large �berbeds, or thin pleated �lters. High E0ciency Particulate Air (HEPA) �lters or 95% dioctyl phthalate(DOP) �lters have proven e0cient as a �nal stage (Leith et al., 1996).

Because of the recirculation of the air, and because �lters are used for long periods, decreasesin �lter e0ciency can lead to increased worker exposure to potentially harmful MWF mists. Thus,improving the recirculating air �ltration systems can reduce worker exposures to MWFs and othercompounds. Many studies have demonstrated that e0ciency of �brous �lters increases with timewhen collecting solid particles. However, other studies have shown that the opposite is true when�brous �lters collect mist droplets (Leith et al., 1996; Raynor & Leith, 2000). E0ciency mea-surements of mist �lters show that accumulation of liquids on �brous �lters leads to decreases inthe e0ciency of the �lters, particularly for droplets smaller than 1 �m in diameter. Raynor andLeith (2000) postulated that the e0ciency reductions are caused by increased air velocity throughthe �lters as liquid accumulates, which leads to a decrease in small droplet capture by Browniandi2usion. In addition, the pressure drop through the �lters increases as liquid accumulates. Thispressure drop increase can cause the air:ow through ventilation systems to decrease to the point atwhich hoods and enclosures cannot adequately capture the mists (Boundy, Leith, Hands, Gressel, &Burroughs, 2000; Raynor & Leith, 2000).Fibrous �lters are usually made using either glass or polymer �bers. Among the di2erent types of

polymers used are polyamide, polyaramid, polyester, polypropylene, polyacrylonitrile, polyimide, andpolytetra:uoroethylene. The �lters are comprised of numerous �bers, with diameters ranging from0.5 to 30 �m, depending on the material and the �lter design. These �bers are oriented essentiallyrandomly in the plane of the �lter and are mixed and intertwined to create :ow passages whereparticles are trapped (Dickenson, 1997).

When mist droplets encounter a �lter, many of them collect and accumulate on the �bers. Raynorand Leith (2000) found that the liquid retained on a mist �lter coalesced as larger drops attachedalong the lengths of �bers or at �ber intersections. Much of this accumulated liquid drains down

G.M. Letts et al. / Aerosol Science 34 (2003) 1481–1492 1483

the �lter and out the bottom if the air:ow is perpendicular to gravity. Some of the accumulated:uid will evaporate back into the passing air stream. Raynor and Leith (2000) developed empiricalmodels using �tted parameters to describe the performance of a mist �lter. The goal of a �brousmist �lter is to maximize e0ciency, and at the same time minimize pressure drop and evaporation.The most important factor in:uencing changes in e0ciency and pressure drop with use for a wetted�lter is how much liquid the �lter retains. The goal of this research was to see if changing �ltermaterials could decrease the liquid retention of the �lter, which could improve �lter performance byminimizing e0ciency reductions and pressure drop increases.

2. Methods

Two sets of tests were conducted to investigate the in:uence of �ber material on mist �lterperformance. First, thin layers of �bers were imaged and photographed through a microscope asthey collected mist droplets. The purpose of these tests was to assess if di2erent kinds of �bersallowed liquid to spread more easily across their surfaces. Second, test �lters were made from thesedi2erent kinds of �bers. The �lters were exposed to high concentrations of mist for several hoursto determine how e0ciency, pressure drop, and liquid holdup changed with loading as a function of�ber material and :uid.

2.1. Fiber photography tests

To study the in:uence of �ber materials upon drop formation, dilute aqueous suspensions of�bers were made and drawn through a polymer support mesh. The �bers collected on the meshwere oriented randomly in the plane of the mesh. The suspension was dilute enough so that �berswere deposited individually on the mesh; the �bers were not layered.

The mesh was mounted in the test apparatus shown in Fig. 1. Air was drawn into the apparatusunder vacuum through an e0cient �lter. After the :ow passed through a calibrated ori�ce, mist wasintroduced using a Collison nebulizer (BGI Inc., Waltham, MA). Eventually, the mist droplets :owedinto the test chamber in which the mesh was mounted. Air velocity past the �bers was 25 cm=s.

Fig. 1. Diagram of �ber photography test apparatus.


Fig. 2. Diagram of complete �lter test apparatus.

Mist concentrations were approximately 2000 mg=m3. The �bers on the mesh were observed throughwindows in the chamber using a stereomicroscope. As mist collected on the �bers, the growth ofdrops upon the �bers was observed. Using a camera mounted on the microscope and backlightingprovided by a string of lights in the apparatus downstream from the test chamber, the �bers anddrops were photographed repeatedly over a 4-h test period.

Three kinds of �bers and three kinds of :uids were used during the tests. Included in the testswere glass �bers, polyester �bers, and polyaramid �bers. All of these �bers were between 10 and20 �m in diameter on average. The :uids tested included bis(2-ethylhexyl) sebacate (BEHS), astraight mineral oil MWF (Ilocut 5468, Castrol, Downers Grove, IL), and an aqueous syntheticMWF (Syntilo 9930, Castrol, Downers Grove, IL). Each combination of �ber and :uid was testedtwice for a total of 18 runs.

The negatives of the photomicrographs taken at the conclusion of the 4-h tests were scanned andprinted on a color printer. Of the 18 images, only 15 were focused well enough for further analysis.The total length of �bers in the images was measured and the drops were counted. The ratio ofnumber of drops divided by �ber length was calculated to give a “drop density” in units of drops/cm.These drop density results were analyzed statistically to determine if numbers of drops varied as afunction of �ber material or :uid.

2.2. Complete 6lter tests

Fig. 2 illustrates the apparatus used to expose the �lters to mist during experiments to evaluatethe performance of complete �lters made from di2erent kinds of �bers. Room air was drawn bya vacuum pump through a high e0ciency �lter, into Tygon tubing, and then into PVC pipe. Mistfrom a triple-jet Collison nebulizer (BGI, Inc., Waltham, MA) was introduced into the :ow via thePVC pipe before the air entered two 90◦ turns and was returned to Tygon tubing. The :ow thenentered a 30:5 cm long stainless-steel expansion which ended in a 5:1 cm× 10:2 cm opening, wherethe test �lters were placed. The test �lters were inserted into the apparatus through removable plasticwindows on either side of a 10:2 cm wide × 15:2 cm high × 7:6 cm deep stainless-steel chamber.The bottom of the chamber consisted of a 7:6 cm long contraction leading to a drain with a closure


through which the liquid draining from the test �lter could be vacuumed into a :ask when theclosure was opened. After leaving the chamber, the :ow passed through a 10:2 cm long contractioninto a 3:6 cm diameter pipe. Identical 0:64 cm diameter, sharp-edged sampling probes entered theapparatus pre- and post-�lter to obtain measurements of droplet counts upstream and downstreamfrom the test �lter.

The test �lters were made in the laboratory from two of the �ber types imaged in the �berphotography tests: polyaramid �bers with an average diameter of 8:0 �m and glass �bers with anaverage diameter of 8:5 �m. Test �lters were custom made in the laboratory by suspending �bers ina mixture of water and acetic acid, with a pH of approximately 3.0. Bringing the pH to this levelcaused the �bers to disperse evenly in the water. The �bers were then poured into a 5:08 cm ×10:16 cm × 0:88 cm �lter holder, which was pre-weighed, and drawn through a �ne mesh to forma �ber bed in which �bers were oriented randomly. The �lter was then rinsed with deionized waterand methanol to remove the acid and to prevent �ber bunching during the drying period. Wet �berswere then compressed into the holder so each �lter had a �nal thickness of 0:88 cm. The �lterholders had a coarse metal mesh on their downstream sides to prevent the �lters from collapsingduring the tests. A braided 0:95 cm diameter glass �ber wick was pre-weighed and incorporated intothe �lters to draw :uid reaching the bottom of the �lter into the drain.

After the �lter was dry, it was weighed and the solidity of the �lter was calculated using themasses of the dry �lter, wick, and holder and the density of the �ber. The solidity of the �lter, �,was calculated using the equation

�=mf

V�f; (1)

in which mf is the mass of the �bers in the �lter, V is the volume of the �lter, and �f is the densityof the �bers. The density of the polyaramid �bers was 1:4 g=cm3 and the density of the glass �berswas 2:5 g=cm3. The solidity of the polyaramid �lters ranged from 0.049 to 0.050 with an averagesolidity of 0.050. The solidity of the glass �ber �lters ranged from 0.044 to 0.050 with an averagesolidity of 0.047.

The nebulizer was used to generate mist from BEHS and the same straight mineral oil MWFused during the �ber photography tests. Filters were exposed to mists until they reached a steadyoperating state, which was considered to be achieved when pressure drop was stable for at least30 min and the drainage rate stopped increasing. For these tests, �lters were exposed to the oil mistfrom 5 to 9 h. Liquid that drained from the �lter was weighed throughout the tests to determine thedrainage rate.

The amount of liquid retained within a �lter is represented by S, the saturation ratio. S is thefraction of the void volume of a �lter �lled with liquid. The saturation ratio was calculated usingthe equation

S =ml

�lV (1− �) ; (2)

in which ml is the mass of the liquid trapped in the �lter at steady state and �l is the liquid density.The density of the straight mineral oil was 0.897. The density of BEHS was 0.914.

Pressure drop across the �lter was monitored continuously during the tests using a Dwyer mano-meter (Dwyer Industrial, Michigan City, IN) attached to pressure taps upstream and downstreamfrom the �lter.


Because �ltration e0ciency depends on the diameter of a droplet being collected, measurementsof e0ciency were made for a range of droplet sizes. Raynor and Leith (2000) collected data fore0ciency measurements for test �lters using a real-time particle counting and sizing instrument.However, they had to reduce the incoming mist concentration during measurements dramaticallyto avoid overloading the instrument. This lowering of the concentration may have altered �lterperformance.

To avoid decreasing the mist concentration, data for size-speci�c e0ciency measurements weretaken in this study through the sampling probes using Model 298 Eight Stage Marple PersonalCascade Impactors (Graseby Andersen, Smyrna, GA). The cascade impactor inlets were adaptedto �t the sampling ports. The impactors were attached via Tygon tubing to GilAir 5 Tri-modeConstant Flow air sampling pumps (Sensidyne Inc., Clearwater, FL). The pumps were calibratedwith a Gilibrator Soap Bubble Flow Meter (Sensidyne Inc., Clearwater, FL) to 2:0 l=min. At thelow air speeds used in these experiments, approximately 100% aspiration e0ciency was expected fordroplets entering the sampling probes. The two sampling lines were matched to make transmissione0ciency as similar as possible for the droplets. The Marple Impactors were loaded with 34 mmdiameter mylar collection substrates. These substrates were weighed pre- and post-sample to obtainparticle size distributions and particle mass concentrations. E0ciency () of the �lter was calculatedfor each particle size range using the following equation:

= 1− cDcU; (3)

in which cD is the particle mass concentration downstream from the �lter and cU is the particle massconcentration upstream of the �lter. E0ciency measurements were taken at the beginning of the testsand after steady state had been reached. Visual observations of mist :ow indicated su0cient mixingin the test apparatus to assume uniform droplet concentrations upstream and downstream from thetest �lter.

The mist droplets that were generated for the experiments ranged in size from 0 to 21 �m indiameter, with most droplets having a diameter of 0.52–6:0 �m. The :ow of air entering the �l-ter was 32:5 l=min, which converted to a face velocity through the �lter of 10:5 cm=s. The aver-age mist concentration generated by the nebulizer was 2560 mg=m3. Most industrial mist collectionsystems would have a mist concentration of less than 10 mg=m3 (Boundy et al., 2000). Such a highconcentration was chosen to accelerate the saturation of the �lters.

A total of 12 tests were conducted. Three tests were conducted with glass �ber �lters and straightoil, three with glass �ber �lters and BEHS, three with polyaramid �ber �lters and straight oil, andthree with polyaramid �ber �lters and BEHS. The �rst four tests conducted used one of each �berand :uid combination. The order of the �nal eight tests was random.

3. Results and discussion

3.1. Fiber photography tests

Fig. 3 shows photomicrographs of BEHS drops retained by each of the �ber types after 4 h ofmist collection. The black grids were the polymer mesh used to support the �bers. The grid spacingwas approximately 0:163 cm in the horizontal direction by 0:145 cm in the vertical direction. The


Fig. 3. Photomicrographs of BEHS drops collected on (a) glass �bers, (b) polyester �bers, and (c) polyaramid �bers.

density of drops collected on the polyaramid �bers was much lower than the drop density for theglass and polyester �bers. Moreover, the drops that were present on the polyaramid �bers werelarger and more likely to be located at intersections of �bers.

Table 1 presents the measurements for drop density for all combinations of :uid and �ber material.As indicated in the table, far fewer drops were observed on the polyaramid �bers than on the other�bers even though the rate of mist fed to the �bers was similar. The data analysis indicates that thisdi2erence is signi�cant statistically. In addition, the table and statistical analysis showed that moredrops formed when the synthetic :uid was nebulized than when the other :uids were used. Otherdi2erences were not signi�cant statistically.

The results indicate that :uids are more likely to spread on polyaramid �bers than on glassor polyester �bers. This easier spreading suggests that the polyaramid �bers have higher surfaceenergy than the glass or polyester �bers. Furthermore, for the purposes of low velocity �ltration,this spreading may be advantageous. If the liquid accumulating within a mist �lter spreads moreeasily, the liquid is likely to move to �ber intersections readily and then drain from the �lter morerapidly. Thus, �lters made from polyaramid �bers are likely to retain less liquid at a steady operatingstate than �lters made from glass or polyester �bers.


Table 1Drop density, in drops/cm of �ber, for each combination of �ber material and :uid

Fluid Glass �bers Polyester �bers Polyaramid �bers

BEHS 43.8 47.7 3.2(n= 2) (n= 2) (n= 2)

Straight oil 38.5 51.3 1.8(n= 1) (n= 2) (n= 1)

Synthetic :uid 56.8 78.1 18.4(n= 2) (n= 1) (n= 2)

The number of tests for each condition is shown in parentheses.

Fig. 4. Average steady-state saturation ratios for mist �lters made from glass and polyaramid �bers and tested with BEHSand straight mineral oil. Error bars represent one standard deviation.

3.2. Compete 6lter tests

Fig. 4 shows the saturation ratios of the di2erent �ber types and test :uids after the complete�lters reached steady state. This graph indicates that �lters made from polyaramid �bers drain morereadily and retain less liquid at a steady operating state than �lters made from glass �bers. Astatistical analysis of the saturation ratio data shows that the di2erence between the �ber types is


Fig. 5. Pressure drop increase, in percent, versus time for mist �lters made from glass and polyaramid �bers and testedwith BEHS and straight mineral oil. Error bars represent one standard deviation.

likely signi�cant (p=0:0003), whereas the di2erence between the :uid types is likely not signi�cant(p= 0:66).These results make sense when compared to the �ber photography tests. The photographs showed

that liquids spread more readily on individual polyaramid �bers than on glass �bers and that retaineddrops grew only at �ber intersections on polyaramid �bers rather than along the individual �bers.Fig. 4 suggests that this spreading may occur in entire �lters made of polyaramid �bers, causingthese �lters to drain faster and retain less liquid at a steady operating state than �lters made fromglass �bers.

Filters made from glass �bers had an average initial pressure drop of about 60 Pa; �lters madefrom polyaramid �bers had an initial pressure drop of 37 Pa. Fig. 5 shows the percentage increasein pressure drop across the �lter during the tests for the various combinations of �ber type and test:uid. This graph shows that �lters made from glass �bers exhibit substantially higher percentageincreases in pressure drop as liquid accumulates than �lters made from polyaramid �bers. Theabsolute di2erences were even more pronounced because the glass �ber �lters had a larger pressuredrop initially. The di2erence in pressure drop increase between the polyaramid �ber �lters and theglass �ber �lters is signi�cant statistically, with a p-value of 0.0020. The use of the di2erent :uidswas not signi�cant (p= 0:82).These results are important from a �lter performance standpoint. Boundy et al. (2000) and Raynor

and Leith (2000) pointed out that an increase in pressure drop through the �lters can cause theair:ow through the entire ventilation system to decrease to the point where hoods and enclosurescannot adequately capture the mists. Therefore, making �lters out of polyaramid �bers might leadto smaller pressure drop through mist �ltering equipment and help to ensure that these ventilationsystems capture mist droplets adequately when the �lters are wet, as well as when they are new.


Fig. 6. Initial and steady state �lter e0ciency as a function of droplet diameter for glass �ber and polyaramid �ber �lterstested using BEHS.

Fig. 6 shows the e0ciency for the glass and polyaramid �ber �lters when clean and after collectingBEHS droplets until the �lters reached a steady operating state. The e0ciency declined for �ltersmade from both kinds of �bers. Although the decrease in e0ciency was slightly larger for theglass �ber �lters for most particle sizes, the magnitude of the e0ciency decreases did not exhibit astatistically signi�cant dependence on �ber type using a p¡ 0:05 criterion. Fig. 7 displays e0ciencyresults for the �lters that collected straight oil droplets. Again, the change in e0ciency between whenthe �lters were clean and when they were at a steady operating state did not depend signi�cantly on�ber material. The statistical analysis also indicated that the magnitude of the e0ciency reductionsdid not depend signi�cantly on the type of :uid used.

These results do not show the hoped for minimizations in �lter e0ciency reduction. Speci�cally,the tests do not indicate that the polyaramid �ber �lters, which retained less liquid than the glass �ber�lters, had less reduction in e0ciency than the glass �ber �lters. As mentioned earlier, e0ciencyfor small droplets may decrease with liquid loading because air velocity within the �lter increases,causing less collection by Brownian di2usion. At any time, the liquid within a �lter will be comprisedof “stationary” drops growing on �bers and rivulets of liquid draining by gravity through the �lter.The �ber material will probably have a strong e2ect on the stationary drops, but less in:uenceon the draining rivulets. At the high incoming droplet concentrations used in this study, much ofthe liquid present within the �lter was :uid draining through the �lter. Therefore, the e2ects of�ber material on liquid retention and, in turn, e0ciency were likely to be relatively small. At morerealistic concentrations, the �ber material might have more in:uence on e0ciency. Despite the lackof an improvement in e0ciency, the wet polyaramid �ber �lters have an advantage over the glass�ber �lters because they exhibit less of a pressure drop increase.


Fig. 7. Initial and steady state �lter e0ciency as a function of droplet diameter for glass �ber and polyaramid �ber �lterstested using a straight mineral oil.

The test results may be in:uenced some by di2erences between the glass and polyaramid �ber�lters. Although the �lters looked similar visually and had similar solidities, the polyaramid �ber�lters had somewhat lower initial pressure drop and e0ciency than the glass �ber �lters. The in:u-ence of this di2erence on the tests was considered as the data were analyzed by conducting analyseson percentage change in pressure drop and percent change in calculated single �ber e0ciency.However, the di2erent initial behavior may still a2ect the conclusions somewhat.

4. Conclusions

Tests were conducted to determine if utilizing alternative �ber materials might o2er the opportunityto build �lters that perform better when wet. The accumulation of liquid on several kinds of �berswas observed microscopically and photographed. Results showed that polyaramid �bers allowedcollected :uids to spread more easily than glass or polyester �bers. Then, experiments were run ontest �lters made of glass and polyaramid �bers to investigate the e2ect of �ber material on wetted�lter performance. Filter e0ciency, pressure drop, saturation ratio, and drainage rates were measured.Filters made of glass �bers retained more liquid and showed larger increases in pressure drop than�lters made of polyaramid �bers. However, �lters made from glass �bers exhibited only a slightlylarger e0ciency decrease than �lters made from polyaramid �bers.

These experiments suggest that using �bers that allow liquid to spread and drain out of a �ltermore e2ectively might make better �lters for use in mist collectors. Using �bers with higher surfaceenergy might decrease pressure drop through wet �lters and help to ensure that mist collectionsystems capture mists adequately when the �lters are wet.


However, these experiments did not show clear-cut minimizations in wet �lter e0ciency reduction.At lower droplet concentrations like those found in real mist collection systems, the polyaramid�bers and other �bers with higher surface energy might have a better chance to minimize e0ciencyreductions.

Acknowledgements

This research was made possible by Grant No. 1 R03 OH04164 from the National Institute forOccupational Safety and Health (NIOSH) and by University of Minnesota Grant-in-Aid for Research,Artistry, and Scholarship #17860. The contents of this paper are solely the responsibility of its authorsand do not necessarily represent the o0cial views of NIOSH. The authors also wish to acknowledgeMs. Harpreet Bhatia for her help in test method development.

References

Boundy, M., Leith, D., Hands, D., Gressel, M., & Burroughs, G. E. (2000). Performance of industrial mist collectors overtime. Applied Occupational and Environmental Hygiene, 15, 928–935.

Dickenson, C. (1997). Filters and 6ltration handbook (p. 623). New York: Elsevier Science, Inc.Leith, D., Raynor, P. C., Boundy, M. G., & Cooper, S. J. (1996). Performance of industrial equipment to collect coolant

mist. AIHA Journal, 57, 1142–1148.NIOSH (1977). National occupational health survey, Vol. III, DHEW(NIOSH) Publication No. 78-114. Cincinnati, OH:

National Institute for Occupational Safety and Health.NIOSH (1998). Criteria for a recommended standard: Occupational exposure to metalworking ;uids, DHHS (NIOSH)

Publication No. 98-102. Cincinnati, OH: National Institute for Occupational Safety and Health.Piacitelli, G. M., Sieber, W. K., O’Brien, D. M., Hughes, R. T., Glaser, R. A., & Catalano, J. D. (2001). Metalworking

:uid exposures in small machine shops: An overview. AIHA Journal, 62, 356–370.Raynor, P. C., & Leith, D. (2000). The in:uence of accumulated liquid on �brous �lter performance. Journal of Aerosol

Science, 31, 19–34.Thornburg, J., & Leith, D. (2000). Size distribution of mist generated during metal machining. Applied Occupational and

Environmental Hygiene, 15(8), 618–628.Woskie, S. R., Smith, T. J., Hallock, M. F., Hammond, S. K., Rosenthal, F., Eisen, E. A., Kriebel, D., & Greaves, I. A.

(1994). Size-selective pulmonary dose indices for metal-working :uid aerosols in machining and grinding operationsin the automobile manufacturing industry. AIHA Journal, 55, 20–29.

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