Effects of Diesel Exhaust

Aftertreatment Devices on

Concentrations and Size Distribution

of Aerosols in Underground Mine Air

ALEKSANDAR D. BUGARSKI,*
 

* Corresponding author phone: +1 412 386 5912; fax: +1 412 386 

4917; e-mail: abugarski@cdc.gov. 

GEORGE H. SCHNAKENBERG, JR., †
 

† Now retired. 

JON A. HUMMER, EMANUELE CAUDA,
 

SAMUEL J. JANISKO, AND
 

LARRY D. PATTS
 

U.S. Department of Health and Human Services, Public

Health Service, Centers for Disease Control and Prevention,

National Institute for Occupational Safety and Health,

Pittsburgh Research Laboratory, 626 Cochrans Mill Road,

Pittsburgh, Pennsylvania 15236

Three types of uncatalyzed diesel particulate filter (DPF)

systems, three types of high-temperature disposable filter

elements (DFEs), and one diesel oxidation catalytic converter

(DOC) were evaluated in underground mine conditions for their

effects on the concentrations and size distributions of diesel

aerosols. Those effects were compared with the effects of a

standard muffler. The experimental work was conducted directly

in an underground environment using a unique diesel laboratory

developed in an underground experimental mine. The DPF

systems reduced total mass of aerosols in the mine air

approximately 10-fold for light-load and 20-fold or more for highload test conditions. The DFEs offered similar reductions in

aerosol mass concentrations. The efficiency of the new DFEs

significantly increased with accumulation of operating time

and buildup of diesel particulate matter in the porous structure

of the filter elements. A single laundering process did not

exhibit substantial effects on performance of the filter element.

The effectiveness of DPFs and DFEs in removing aerosols by

number was strongly influenced by engine operating mode. The

concentrations of nucleation mode aerosols in the mine air

were found to be substantially higher for both DPFs and DFEs

when the enginewas operated at high-loadmodes than atlowload modes. The effects of the DOC on mass and number

concentrations of aerosols in mine air were relatively minor

when compared to those of the DPF and DFE systems.

Introduction

In recent years, health issues associated with exposure to 

diesel particulate matter (DPM) and other, primarily combustion-generated, nano and ultrafine aerosols have received 

substantial attention from the public, government agencies, 

and in academia. Long-term exposure to combustion-related 

fine particulate pollution is perceived as an important risk 

factor for cardiopulmonary and lung cancer mortality (1). 

Extensive utilization of diesel-powered equipment by the 

mining industry makes the reduction of underground miners’ 

exposure to particulate matter and gaseous emissions from 

diesel-powered equipment a major challenge. Techniques 

for reducing worker exposure typically involve one or more 

of the following methods; improvements in mine ventilation, 

the curtailment of DPM and toxic gaseous emissions through 

improved engine maintenance, exhaust aftertreatment technologies, and the use of alternative fuels. 

Diesel particulate filter (DPF) systems are recognized as 

an effective technology for removing DPM from the exhaust 

of diesel-powered equipment (2-4). Diesel exhaust filtration 

systems with disposable filter elements (DFEs) have been 

extensively used to control DPM emissions from permissible 

heavy-duty diesel-powered coal mining equipment since the 

early 1990s (5). Various models of DPFs and DFEs are 

currently accepted by the U.S. Mine Safety and Health 

Administration (MSHA) for controlling DPM emissions from 

underground coal mining equipment (6-9). Although a 

limited number of underground mining vehicles in operation 

are currently retrofitted with DPF and DFE systems it can be 

expected that this number will increase with advancements 

in engine, DPF and DFE technologies. 

The findings of laboratory (10) and field studies (3, 4, 11) 

indicate that the introduction of various diesel exhaust 

aftertreatment technologies dramatically changes the physical and chemical properties of diesel aerosols and potentially 

changes their associated health effects. Growing evidence 

suggests that particle number, surface area, size, or perhaps 

some associated structural properties may affect nanoparticle 

toxicity, when compared with larger respirable particles of 

the same composition (12). 

This paper summarizes the results of a study conducted 

to evaluate the effects that several types of DPFs, DFEs, and 

a DOC have on the concentration and size distribution of 

diesel aerosols in an underground mine. Previous studies on 

size-resolved characterization of diesel aerosols have typically 

been performed in well-controlled and ideal laboratory 

environments (13-16), on roads (11, 16), and in tunnels 

(17, 18). Since the size and concentration of diesel aerosol 

and semivolatile materials emitted by diesel engines have 

been shown to be strongly influenced by a number of complex 

processes (19), the National Institute for Occupational Safety 

and Health (NIOSH) chose to assess the aforementioned 

effects in an underground mine setting. In order to achieve 

this goal, while still preserving relative precision, a unique 

diesel laboratory was developed in a nonoperational limestone mine; the NIOSH Lake Lynn Experimental Mine 

(LLEM), near Fairchance, PA (20). Although the measurements were taken underground, the findings should also be 

applicable to other occupational settings where workers are 

performing their duties in proximity to diesel-powered 

equipment. 

Experimental Section

A schematic of the laboratory layout is shown in Figure 1. 

The D-drift is approximately 530 m (1750 ft) long, 6 m (20 

ft) wide, and 2 m (7 ft) high. The major components of the 

laboratory are an engine/dynamometer system, three sampling and measurement stations, and a ventilation measurement and control system. 

The Isuzu C240, one of the most popular light-duty engines 

in U.S. underground coal mines, was operated at four steadystate engine operating modes (Table 1). 

The modes were selected to cover a wide range of engine 

operating parameters such as exhaust temperatures and 

emission rates. The average exhaust temperatures at the inlet 
and outlet of the tested DPFs, DFEs, DOC, and muffler 

observed for test modes are summarized in Table 2. 

FIGURE 1. NIOSH Diesel Laboratory in D-drift of LLEM (not to scale).

TABLE 1. Test Modes

engine speed torque power

mode description (rpm) (Nm) (kW)

R50 rated speed 50% load 2950 55.6 17.2

R100 rated speed 100% load 2950 111.2 34.3

I50 intermediate speed

50% load

2100 69.1 14.9

I100 intermediate speed

100% load

2100 136.9 30.6

Fresh air was supplied to the underground facility via a 

ventilation shaft located in E-drift (Figure 1). A series 1000 

model 23017-3450 Axivane fan (Joy Technologies Inc., 

Franklin, PA) and a subsonic Venturi meter (Primary Flow 

Signal Inc., Tulsa, OK) were used to maintain and measure 

constant flow of fresh air through the drift throughout the 

tests. The measurements showed the average volumetric flow 

rate of 5.687 ( 0.047 m3

/s (12050 ( 100 ft3/min). The very 

low test-to-test variability in flow rate (0.86%) eliminated 

the need for normalization of the data with respect to it. 

The average exhaust dilution ratios for R50, R100, I50, and 

I100 engine operating modes were calculated to be 148, 

149, 186, and 188, respectively. The average ambient 

temperatures upstream of the engine were between 10.5 

and 18.7 °C. The corresponding average ambient temperatures at the downstream measurement station ranged 

between 14.7 and 22.3 °C. 

ExhaustAftertreatmentTechnologies.Three types of DPF 

systems (representative of that which is currently available 

to the underground mining industry) were evaluated in this 

study: 

1. Catalytic Exhaust Products (CEP), Toronto, ON, model 

912-SXT with uncatalyzed Corning EX-80 Cordierite element 

(31 cell per cm2 and 0.3 mm wall thickness) (Cordierite DPF). 

2. DCL International Inc., Concord, ON, model Minex 

Sootfilter 5.66 × 10 with uncatalyzed Ibiden silicon carbide 

element (31 cell per cm2 and 0.36 mm wall thickness) (SiC 

DPF). 

3. Mann+Hummel GMBH, Speyer, Germany, model SMFAR with uncatalyzed sintered metal element (10 μm mean 

pore size, 45% porosity, and 0.38 mm wall thickness) used 

with Satacen 3 fuel additive (SM DPF). 

The DPF systems with Cordierite and silicon carbide wall 

flow monoliths similar to those from CEP and DCL meet 

MSHA criteria, and they are listed as 85 and 87% efficient, 

respectively, in the removal of total DPM mass (6). The SM 

DPF system was recently added to the list as 99% efficient 

when used with ultra low sulfur fuel. 

All of the DPF systems were installed at an identical 

location in the exhaust system for their respective tests. The 

maximum allowable pressure drop across the DPF systems 

for all engine modes and DPM loads was set to 14.9 kPa (60′′

H2O). Consequently, the DPF systems with a 25.4 × 30.5 cm 

(10 × 12 in) Cordierite element and with a 2.75 m2 sintered 

metal element were evaluated as single element units, while 

the SiC DPF system was evaluated with two 14.4 × 25.4 cm 

(5.66 × 10 in) elements mounted in parallel. A butterfly valve 

was installed in the exhaust pipe between the engine and 

DOC (or muffler) and was used during the DOC and muffler 

tests to generate pressure drops comparable to those 

observed for the corresponding DPF and DFE tests. Prior to 

the start of the first evaluation runs, the DPFs were “degreened” and fully regenerated using a CombiClean automatic cleaning station from Engine Control Systems, Newmarket, ON. 

Three types of high-temperature DFEs with synthetic 

filtration media were tested using a DFE system consisting 

of a custom filter housing and an air-to-air heat exchanger 

designed and built by Mac’s Mining Repair Service (Huntington, UT). The heat exchanger was installed between the 

engine and the DFE housing to cool exhaust below 343 °C 

(650 F). The following DFEs were evaluated: 

1. Donaldson Company, Minneapolis, MN, model P604516 

(DFE-A); 

2. A Laundered DFE, identical to DFE-A, but laundered 

by Mac’s Mining Repair Service, Huntington, UT, following 

standard protocols (LDFE-A); 

3. FST Systems Corporation, Price, UT, model FST-115-26 

(DFE-B). 

DFE-A and DFE-B meet MSHA criteria for permissible 

and nonpermissible applications, and they are listed as 83 

and 80% efficient, respectively, in the removal of total DPM 

(6). The laundering process is used by some coal operators 

to extend the life cycle of the high-temperature DFEs. The 

laundered version of DFE-A (LDFE-A) tested in this study 

was laundered only once. 

Two brand new elements (DFE-A and DFE-B) and the 

laundered element (LDFE-A) were conditioned prior to the 

4-mode tests through 13 and 6 h of operation at R50 mode, 

respectively. The results of measurements performed during 

this conditioning period were used to assess the effects the 

filter element loading had on the effectiveness of DFEs. 

The observed pressure drops across the DFEs varied with 

engine operating mode and DPM load, but they were well 

below the manufacturer-recommended maximum limit of 

14.95 kPa (60′′ H2O) for all test modes. 

A DOC used in this study was manufactured by Engine 

Control Systems Ltd., Newmarket, ON (model A16-0130) 

with Cordierite substrate and proprietary catalyst formulation. The baseline measurements were made with a generic 

muffler installed in place of the DPF systems and DOC. 

Fuel. Single-consignment ultralow sulfur diesel fuel (11 

ppm S by weight) supplied by Guttman Oil (Belle Vernon, 

PA) was used for all tests in this study (see Supporting 
Information (SI) Table 1 document for the properties). For 

the majority of the tests, the fuel was supplied to the engine 

from a 200 L main fuel tank (Rohmac Inc., Mt. Storm, WV). 

The exceptions were the tests involving the SM DPF system, 

which was evaluated using fuel from the same batch, but 

treated with fuel additive from Innospec Limited, Cheshire, 

United Kingdom, known under the name of Satacen 3. It is 

important to note that, at the time of testing, this additive 

was not EPA registered and therefore it was not approved for 

use in U.S. underground mines. The additive was premixed 

with the fuel at approximately 0.677 mL per liter of diesel 

fuel. The additive-treated fuel was supplied to the engine 

from a designated 45 L fuel tank. Procedures were followed 

to avoid cross-contamination of the fuels. 

TABLE 2. Average Exhaust Temperatures at the Inlet and Outlet of the Devices

DPFs DFEs DOC Muffler

mode inlet (°C) outlet (°C) inlet (°C) outlet (°C) inlet (°C) outlet (°C) inlet (°C) outlet (°C)

R50 310 ( 5 257 ( 13 203 ( 6 154 ( 4 298 ( 2 274 ( 2 302 ( 1 255 ( 1

R100 535 ( 10 427 ( 17 328 ( 0 238 ( 4 500 ( 2 455 ( 2 536 ( 4 440 ( 2

I50 256 ( 5 213 ( 14 157 ( 4 120 ( 9 249 ( 1 226 ( 1 253 ( 1 213 ( 1

I100 532 ( 9 424 ( 21 313 ( 6 230 ( 10 518 ( 6 460 ( 5 550 ( 2 451 ( 2

Aerosol Measurements. The effects of the high temperature DFEs and the muffler on aerosols were established by 

examining results of measurements conducted at two 

measurement stations established in the D-drift. A downstream station was located about 60 m (197 ft) downwind of 

the dynamometer, and an upstream ambient monitoring 

station was located approximately 60 m (197 ft) upwind of 

the dynamometer and in relatively clean background air. 

The downstream sampling location was selected to allow for 

sufficient residence time for initial formation and transformation of diesel aerosols (21) and full mixing in the drift. 

Background corrections were made by subtracting the results 

of measurements performed at the upstream station from 

the corresponding results obtained at the downstream 

station. 

FIGURE 2. Percentage of change in total mass (TEOM) and number (SMPS) concentrations for (a) DPFs and (b) DFEs (reduction is

positive number).

Measurements were initiated at the end of the initial hour 

of each test (which was dedicated to achieving thermodynamic and concentration equilibriums). The results in this 

report are based on the average data obtained over the last 

hour of each test. The purpose of this approach was to allow 
enough time for DPM to accumulate within the pore structure 

of the DPFs and DFEs so that the aftertreatment devices 

were operating at maximum efficiencies during the test. 

A tapered element oscillating microbalance (TEOM) series 

1400a ambient particulate monitor from Thermo Scientific 

(Franklin, MA) was used at the downstream station, and a 

second was used at the upstream station to measure total 

DPM mass. A 10 mm Dorr-Oliver cyclone followed by a DPM 

cassette (SKC, Eighty Four, PA) with its collection filter 

removed were used to preclassify aerosols entering the TEOM, 

allowing only particles with an average aerodynamic diameter 

(50dae) smaller than 0.82 μm to reach the TEOM. The inlet to 

the instrument was heated to 50 °C following accepted 

standard practices used to reduce the interference from 

particle bound water and to minimize thermal expansion of 

the tapered element. 

A scanning mobility particle sizer (SMPS) (22) from TSI 

(St. Paul, MN) was used at the downstream station, and 

another was used at the upstream station to measure size 

distribution and number concentrations of aerosols between 

10 and 408 nm. The SMPS at the downstream station was 

configured with an electrostatic classifier (EC) model 3080 

L and a condensation particle counter (CPC) model 3025A. 

The SMPS at the upstream station consisted of an EC model 

3080 L and a CPC model 3776. 

FIGURE 3. Size distributions for the DPFs and the DOC at: (a) R50, (b) R100, (c) I50, and (d) I100 modes.

Results and Discussion

Effects on Concentrations of Aerosols. The total aerosol 

mass and number concentrations were measured at the 

downstream and upstream sampling locations at prevailing 

ventilation conditions. The results of the measurements 

performed during the last hour of each test were used to 

calculate representative average values (see SI Table 2). The 

average efficiencies of each aftertreatment device in reducing 

DPM mass and number concentrations were calculated by 

comparing the average concentrations measured for the 

device with those observed for the muffler at the same engine 

mode (Figure 2). The results are background corrected. The 

error bars represent one standard deviation of the mean. 

All three DPFs reduced aerosol mass concentrations by 

approximately 10-fold for R50 and I50, by more than 20-fold 

for high-emissions R100, and by approximately 100-fold for 

I100 conditions (Figure 2a). For R100, I50, and I100 modes 

the DFEs reduced the aerosol mass concentrations by 20­

fold or more. At R50 the reductions were less than 10-fold 

for both DFE-A and DFE-B (Figure 2b). 

The engine operating modes were found to have a more 

pronounced effect on the number than on the mass 

concentrations of aerosols. The effects of DPFs ranged from 

20-fold reductions to slight increases in total number 

concentrations (Figure 2a). For the light load modes, R50 

and I50, the DFEs reduced total number concentration 

between 93.3 and 99.6%. Significantly lower number reductions, ranging from 65.5 to 75.0%, were observed for all DFEs 

at R100 and for DFE-A at I100 mode. The effectiveness of 

LDFE-A in the removal of aerosol mass and number was 

found to be comparable to that of a new DFE-A, indicating 

that a single laundering resulted in no substantial reduction 

in the performance of DFE-A. 

A series of tests was conducted in order to assess the 

effects of DFE loading on performance of the new and 

laundered DFE elements. Once a DFE was installed, the 

engine was operated at R50 mode for a period of 13 h (DFE-A 

and DFE-B) or 6 h (LDFE-A) and aerosol data were collected 

during those periods. All SMPS measurements were initiated 

after a one-hour equilibration time. With accumulation of 

operating time and buildup of DPM within the pore structure 

of the DFEs the total number concentration of aerosols 

gradually decreased over the first several hours of operation. The corresponding efficiencies of the DFE-A and 
DFE-B increased over the test periods from 83.0 to 99.6 and 

for 83.0 to 98.0%, respectively. It appears that tested DFEs 

asymptotically approached but never reached their terminal 

efficiency over the test periods. 

When compared with DPFs and DFEs, the DOC produced 

relatively modest effects on aerosol mass and number 

concentrations (see SI Table 2). The effects of the tested DOC 

on total mass and number concentrations and size distributions of aerosols were found to be influenced strongly by 

engine operating mode. Substantial data variability and 

relatively modest reductions resulted in higher uncertainty 

for DOC results than for the other technologies tested. The 

most substantial reductions of 42% in mass and 24% in 

number concentrations were found for the I100 mode. 

EffectsonSizeDistributionofDieselAerosols.The results 

of size distribution measurements performed with an SMPS 

at the downstream station are summarized in Figures 3 and 

4. The presented distributions are not corrected for dilution 

ratio or for background concentrations. 

For all test modes during muffler and DOC tests, the 

majority of aerosols were found to be in a single agglomeration mode. Although the corresponding aerosol size distributions for the muffler and the DOC were found to be similar, 

the geometric mean diameters were consistently found to 

be lower for the DOC cases (40.2 nm vs 40.6 nm for R50, 43.4 

nm vs 45.2 nm for R100, 46.7 nm vs 49.6 nm for I50, and 73.8 

nm vs 95.5 nm for I100). This probably resulted from a modest 

catalytic oxidation of the organic fraction of particulate 

matter, which appeared to be the most effective at I100 

conditions. Slightly elevated concentrations of nucleation 

aerosols were found only when the engine fitted with the 

DOC was operated at I100 mode. This is consistent with 

observations made by Maricq and coauthors (10). 

For the DPFs, the majority of aerosol size distributions 

were found to be bimodal with pronounced nucleation (50dem 

< 50 nm) and accumulation modes (50dem > 50 nm). The 

aerosol concentrations were found to be primarily in the 

nucleation mode when the engine was operated at the heavy 

load and high exhaust temperature conditions, R100 (Figure 

3b) and I100 (Figure 3d), and in the accumulation mode 

when the engine was operated in the light load and lower 

temperature conditions I50 (Figure 3c). All three types of 

DPFs were found to be similarly effective in reducing the 

accumulation mode aerosols from mine air for all test 

conditions. Although the concentrations of nucleation mode 

aerosols at R100 and I100 conditions were found to be 

substantially different between the DPF tests, due to experimental limitations it was not possible to differentiate 

the effects of the filtration media and a number of the other 

parameters. 

High number concentrations of nucleation mode aerosols 

in the mine air during the evaluation of SiC and SM DPFs 

at R100 (Figure 3b), I100 (Figure 3d), and even R50 (Figure 

3a) modes, resulted in low or even negative number 

efficiencies calculated for those modes (Figure 2a). However, 

for I50 mode, the DPF systems were found to be almost 

equally effective in removing number and mass of aerosols 

from mine air (Figure 2a). This can be attributed to low 

concentrations of nucleation mode aerosols in downstream 

mine air during the I50 mode tests (Figure 3c). 

The size distributions for all three types of DFEs were 

similar in shape (Figure 4) with aerosols found in both 

nucleation and accumulation modes.

FIGURE 4. Size distributions for DFEs: (a) R50, (b) R100, (c) I50, and (d) I100 mode.

 At high load modes, 

the concentrations of aerosols in the nucleation modes were 

higher than those in the accumulation modes, which explain 

the modest total aerosol number concentration reductions 

observed for all DFEs at those modes. For all test conditions 

and for all three DPFs, the peak concentrations of nucleation aerosols did not substantially exceed corresponding concentrations measured during the evaluation of the 
muffler. The size distributions for LDFE-A were found to 

be comparable to those of a new DFE-A, showing that a 

single laundering did not substantially affected performance of DFE-A. 

Figure 4a and c show that the size distributions measured 

for DFE-A and DFE-B tests at R50 and during DFE-A at I50 

had two accumulation modes. The relatively high concentration of secondary accumulation mode aerosols with 50dem 

> 100 nm was attributed to background. The presence of 

those aerosols can explain the modest total mass reductions 

found for those conditions (Figure 2a). 

The results of size distribution and concentration measurements for DPFs and DFEs, but not for the muffler and 

DOC, show a positive correlation between concentrations of 

nucleation mode aerosols in the mine air and exhaust 

temperatures (Table 2). The concentrations of nucleation 

mode aerosols were found to be substantially higher when 

the DPFs were examined at the heavy load and higher exhaust 

temperature conditions, R100 (Figures 3b and 4b) and I100 

(Figures 3d and 4d), than at the lighter load and lower exhaust 

temperature conditions, R50 (Figure 3a and b) and I50 

(Figures 3c and 4c). Substantially lower concentrations of 

nucleation mode aerosols were observed when the DFEs were 

examined at high load modes. It is important to note that, 

due to the effects of the heat exchanger, the DFEs were 

operated at much lower corresponding exhaust temperatures 

(Table 2). 

The rate of nucleation is likely related to the propensity 

for hot hydrocarbon-rich gases to pass through the DPFs 

and DFEs under high exhaust temperature operating conditions and subsequently nucleate downstream as the exhaust 

cools. Due to the high efficiency in removing the accumulation mode aerosols, the DPFs and DFEs reduce the potential 

adsorption sites and scavenging of particle precursors, thus 

promoting heterogeneous and homogeneous nucleation, 

while at the same time decreasing coagulation of nucleation 

mode aerosols with accumulation mode aerosols (23, 24). 

It is important to note that the reported number efficiencies (Figure 2) were calculated using aerosol counts for the 

SMPS configuration measurement range, 10-408 nm. In 

several cases where the size distribution measurements 

revealed substantial concentrations of nucleation mode 

aerosols (Figures 3 and 4), one could construe that the 

concentration of aerosols with diameter smaller than 10 nm 

contributed significantly to the total number of aerosols. 

Therefore, if aerosols with diameters smaller than 10 nm 

were considered, the theoretical increases in number of 

aerosols in those cases would be substantially higher than 

those reported in Figure 2. 

Variability in the quality of background air (see SI Figures 

1 and 2) introduced a higher level of uncertainty into mass 

and number measurements during the evaluation of very 

efficient DPF and DFE systems. In those cases, the downstream mass concentrations were found to be comparable 

to the upstream concentrations and even slight test-to-test 

variations in concentrations or size distributions of aerosols 

at the upstream sampling station were found to have a major 

impact on aerosol mass results. The certainty of number 

concentration measurements was affected by temporal 

fluctuations in concentrations of nucleation mode aerosols 

and background concentrations (Figures 3 and 4). Since the 

upstream number concentrations were on average between 

0.2 and 20.2% of the corresponding downstream concentrations, the effects of background were somewhat less pronounced on number than on mass results. Lastly, changes 

in the performance of the DPF and DFE systems with 

accumulation of DPM in filter media over the one-hour 

averaging period contributed to the overall uncertainty of 

both mass and number measurements. This raises the more 

general question of test length and amount of device 

conditioning needed to properly evaluate such filtration 

technologies in both laboratory and field environments. 

This study demonstrated the potential of certain diesel 

emissions control devices to reduce mass and, in the majority 

of cases, number concentrations of diesel aerosols in 

occupational settings such as underground mines. The results 

showed evidence of the formation of high concentrations of 

nucleation mode aerosols when DPFs and DFEs were used 

in the exhaust system of engines operated at high exhaust 

temperature conditions. Future work is necessary to fully 

understand how these changes in the characteristics of diesel 

emissions may impact the toxicological properties of these 

aerosols and the potential health risks associated with worker 

exposure to DPM. 

Acknowledgments

We acknowledge support from NIOSH Lake Lynn Laboratory 

personnel for their help and assistance in the experimental 

phase of this study. Disclaimer: The findings and conclusions 

of this publication have not been formally disseminated by 

the National Institute for Occupational Safety and Health 

and should not be constituted to represent any agency 

determination or policy. 

Disclaimer

Mention of any company or product does not constitute 

endorsement by NIOSH. 

Supporting Information Available

Additional information available. This material is available 

free of charge via the Internet at http://pubs.acs.org. 

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