Real-time diesel particulate monitor for underground

mines

James Noll,*a

a

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, PA

15236, USA. E-mail: jnoll@cdc.gov; Fax: +1 412-386-4917; Tel: +1 412-386-6828

Samuel Janiskob

b

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, PA

15236, USA. E-mail: sjanisko@cdc.gov; Fax: +1 412-386-4917; Tel: +1 412-386-4509

and Steven E. Mischlerc

e

c

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, PA

15236, USA. E-mail: smischler@cdc.gov; Fax: +1 412-386-4917; Tel: +1 412-386-6588

The standard m thod for determining diesel particulate matter (DPM) exposures in underground metal/

nonmetal mines provides the average exposure concentration for an entire working shift, and several

weeks might pass before results are obtained. The main problem with this approach is that it only

indicates that an overexposure has occurred rather than providing the ability to prevent an

overexposure or detect its cause. Conversely, real-time measurement would provide miners with timely

information to allow engineering controls to be deployed immediately and to identify the major factors

contributing to any overexposures. Toward this purpose, the National Institute for Occupational Safety

and Health (NIOSH) developed a laser extinction method to measure real-time elemental carbon (EC)

concentrations (EC is a DPM surrogate). To employ this method, NIOSH developed a person-wearable

instrument that was commercialized in 2011. This paper evaluates this commercial instrument, including

the calibration curve, limit of detection, accuracy, and potential interferences. The instrument was found

to meet the NIOSH accuracy criteria and to be capable of measuring DPM concentrations at levels

observed in underground mines. In addition, it was found that a submicron size selector was necessary

to avoid interference from mine dust and that cigarette smoke can be an interference when sampling in

enclosed cabs.

Introduction

Exposure to diesel particulate matter (DPM) is a health concern

for miners since it is classied as a potential occupational

carcinogen by both the National Institute for Occupational

Safety and Health (NIOSH) and the Environmental Protection

Agency (EPA).1–4 In addition, diesel exhaust has been potentially

linked to other chronic and acute adverse health conditions

such as asthma and eye irritation.5–8 Since underground miners

work alongside diesel equipment in a conned environment,

they are correspondingly exposed to some of the highest levels

of diesel exhaust in the country. Therefore, the Mine Safety

and Health Administration (MSHA) promulgated a rule to limit

exposures of metal/nonmetal underground miners to DPM to

an eight-hour time-weighted average (TWA) of 160 mg m-3 total

carbon (TC).10,11

The standard method for determining DPM exposures is to

collect the particulate onto a quartz ber lter for an entire shi

and then analyze for elemental carbon (EC) and TC using

NIOSH method 5040. TC is used as a surrogate to determine

DPM exposures because direct DPM measurement is not sufficiently accurate and because TC represents over 80 percent of

DPM.11–13 However, TC can be inuenced by non-DPM organic

aerosols such as cigarette smoke and oil mist. Therefore, in

some cases, MSHA uses submicron EC as an alternative surrogate because it is selective to DPM in underground mines and is

a major component of DPM. Consequently, the nal eight-hour

TWA permissible exposure limit (PEL) is a TC value that, in

order to avoid the inuence of interferences, is determined in

two ways.15 In the rst test, a full-shi personal sample is

collected and sent to a laboratory for analysis of EC and TC

using NIOSH method 5040. In the second analysis, the EC

concentration of the personal sample is multiplied by a

conversion factor to calculate an equivalent TC value. If both the

calculated TC value and the measured personal TC are above

160 mg m-3

, the mine is out of compliance.

Although NIOSH method 5040 is an accurate method for

determining DPM exposures, it only provides the average

concentration over an entire working shi, and several weeks

might pass before results are obtained. The main problem with

this approach is that it only indicates that an overexposure has

occurred rather than providing the ability to prevent an overexposure or even detect its source. Conversely, real-time
measurement with readout capability would provide workers

with almost instantaneous information to identify the major

factors contributing to overexposures and to allow timely

deployment of engineering controls. As an example, using NIOSH

method 5040, it would take weeks to determine the effect of

ventilation changes used to reduce DPM. By the time the data are

analyzed, the miners could beworking in different sections of the

mine, and the ventilation could be different due to changes in

atmospheric conditions. In contrast, real-time analysis would

allowventilation to be re-directed to provide timely DPM dilution.

One approach that has been taken for real-time DPM analysis is to utilize the Thermo Scientic Personal Dust Monitor

(PDM).16 The PDM is a portable instrument which uses a

tapered element oscillating microbalance (TEOM) technology to

measure particulate mass. The PDM is utilized to provide

continuous mass measurements of respirable dust in coal

mines. It has been shown by extensive eld measurements to

meet the NIOSH accuracy and precision criteria for eld

instruments for dust measurements.17,18 In a study performed

in coal mines in Australia, an impactor with a 0.8 mm cutpoint at

1.7 lpm was used to separate the DPM from the coal dust and

thus enable the PDM to measure ambient DPM mass concentrations. The samples were mostly collected during longwall

moves when the dust concentrations were below 500 g m-3 m .

Under these conditions, the PDM seemed to measure DPM well.

At certain concentrations, however, dust may still be an interference. For example, in studies performed at a NIOSH Pittsburgh laboratory, two different coal dusts were evaluated and

about two to four percent of the respirable dust penetrated

through the impactor.19 This would result in a bias of 20–40

percent when sampling DPM in dust concentrations at the

compliance levels (200 g m 3 m - DPM 2000 mg m-3 and dust). In

metal/nonmetal mines, the dust concentrations commonly can

be over 2000 mg m-3

, resulting in a greater interference to DPM

measurements.19 In addition, in metal/nonmetal mines, cigarette smoke and oil mist can also cause interferences to the

PDM measurements.

Light-scattering instruments have also been used for realtime DPM analysis but with highly variable results.20–27 DPM

measurements via light scattering can be inuenced by

humidity, cigarette smoke, oil mist, composition and particle

size of aerosol, and dust at certain concentrations, even when

an impactor is used.23–27 Analysts have corrected for the effects

of humidity, composition, and particle size of the aerosol when

using light scattering for dust measurements by performing a

eld calibration.22–24 This usually entails collecting a standard

sample (e.g., gravimetric for dust) along with the light-scattering

instrument and calibrating the light-scattering data with the

standard results. The problem with this correction method is

that it extends the analysis time since the real-time data from

the light scattering cannot be evaluated until the standard

sample measurement is completed. Contrary to humidity and

aerosol composition, analysts using light scattering instruments have not developed a method for addressing cigarette

smoke, dust, or oil mist in underground mines. However,

researchers have reported lter-based methods using multiwavelength light extinction to determine cigarette smoke and

some dusts concentrations in the presence of black carbon in

atmospheric samples.28–30 In order to use these methods to

correct for interferences of DPM in occupational settings,

further research and optimization would be necessary.29

Borak et al. evaluated the Aethalometer for determining realtime DPM concentrations at three occupational settings

including one underground mine.31 The Aethalometer is a lterbased method which measures real-time black carbon in the

atmosphere via absorption. Borak et al.31 found a correlation

between the Aethalometer and NIOSH method 5040 but not

agreement. Therefore, the Aethalometer would need to be

calibrated before being used to measure DPM concentrations in

occupational settings. The authors also found problems with

the pump and that the instrument was affected by vibration. In

addition, the power requirements and bulkiness are limitations

of this instrument in relation to personal sampling. In the past

few years, Magee Scientic introduced a miniature version, the

micro-Aethalometer, which can be used for measuring personal

exposures to black carbon.32 However, besides the need for

calibration, this instrument lacks the dynamic range to

measure DPM concentrations present in underground mines.

Other limitations are that it does not have the readout capability

or size selection capability necessary to limit the inuence of

dust present in underground mines.

In order to measure DPM concentrations in underground

mines and possibly other occupational settings, the Office of

Mine Safety and Health Research (OMSHR) developed a personwearable real-time EC monitoring instrument. EC was chosen

as the analyte since this fraction is a major portion of DPM, is

not prone to interferences, and, as previously stated, is one of

the surrogates used by MSHA for compliance sampling. Filterbased laser extinction was determined to be a feasible method

because EC concentration is proportional to optical density and

also because this technique is simple and can be adapted for a

small instrument. With a limited and preliminary dataset, the

rst-generation instrument demonstrated good agreement

(within ten percent) with the standard method for measuring

EC from DPM (NIOSH method 5040 (ref. 33)).34,35 This initial

success launched the manufacturing of a pre-commercial

version, which was produced by FLIR Inc. Laboratory and eld

testing was subsequently performed on a beta prototype developed by FLIR. Based on the ndings of these studies, a

commercial version of the monitor was developed, tested, and is

now available. This paper provides detailed laboratory analysis

of the commercial version (Airtec) including calibration curve,

NIOSH accuracy criteria, limit of detection (LOD), dynamic

range, and responses to potential interferences. A future paper

will discuss the results of eld evaluations.

Methods

Description of Airtec

The Airtec measures light extinction, which incorporates the

effects of light absorption and scattering. With DPM particles

only, the absorption will be the dominant effect on light

extinction. Light scattering may have more of an inuence if

other scattering aerosols are collected with the DPM.36
Fig. 1 Picture of the Airtec, the commercial version of the real-time EC monitor.

The Airtec device (see Fig. 1) weighs approximately 1.5 lbs

and can be worn by a worker or positioned for area sampling. A

diaphragm pump draws ambient air at a set ow rate that

enables a pre-selector to make around a 1 mm size cut.

Conductive tubing allows EC to reach the Teon membrane

lter without sticking to the tubing walls. The Teon membrane

lter is housed in a specially designed cassette that has a

dened volume chamber as well as a carefully constructed ow

path to achieve uniform distribution of EC on the Teon lter.

Teon lters were chosen to minimize the inuence of

absorption and light scattering from the lter material. A laser

with a wavelength of 650 nm penetrates through the sample

while collecting DPM. Optical density is converted to mg of EC

collected on the lter using a calibration curve.

The Airtec displays the ve-minute rolling average EC

concentrations. Every minute during sampling, the voltage

from a point is divided by the initial voltage (set at the beginning of the sampling period) to calculate the transmission. The

optical density (-log(transmission)) is then multiplied by the

calibration factor to determine the mg EC collected. The EC

collected ve minutes previously is then subtracted from the mg

EC at this time. This number is then inserted in the following

equation to determine the ve-minute average concentration:

( 3

) EC ðmgÞ EC mg m- 5 minute average 1⁄4 x 100 flow rate ðlpmÞ x 5 min

(1)

A data point is collected every minute, which represents the

average concentrations over the previous ve minutes.

Depending on the limit of quantication (LOQ) needed, a tenminute and 15 minute rolling average may be desirable because

the longer times provide more sensitivity. Eqn (1) is also used

when calculating these longer times; instead of ve minutes,

ten or 15 minutes is used as the sampling time, and the EC from

ten or 15 minutes earlier is subtracted from the current EC

collected to determine the mg of EC.

The Airtec also records and displays the eight-hour TWA EC

concentration, which is calculated by inserting the mg of E

collected into the following equation:

( 3

) EC ðmgÞ EC mg m- 8 h TWA 1⁄4 x 100 flow rate ðlpmÞ x 480 min

(2)

C

The Airtec has the capability to sample at 1.7 lpm (high ow)

and 0.85 lpm (low ow) in order to expand the dynamic range.

The 1.7 lpm ow rate allows for a more sensitive measurement,

and the 0.85 lpm allows a larger 8 h TWA concentration to be

measured. The 1.7 lpm ow rate results in a 0.8 mm cutpoint

with the impactor currently used, and the 0.85 lpm ow rate

results in about a 1 mm cutpoint.

Calibration curve

A calibration curve was developed for the Airtec (commercial

version), which was accomplished by collecting DPM onto

quartz ber lter samples used for determining EC concentrations via NIOSH method 5040 and Airtecs simultaneously in a

Marple chamber, as described in a previous publication.37

At least three Airtecs were set up and checked for ow rate.

Tubing was attached to the instruments and inserted into the

chamber. In addition, three-piece cassettes (Sureseal plastic

cassettes containing quartz ber lters with cellulose backing

pads) to measure EC via NIOSH method 5040 were placed inside

the chamber and attached to tubing, critical orices (ow rate

set at 1.7 lpm), and a vacuum pump. The instruments and

vacuum pump were turned on. A 4-cylinder Kubota engine with

a Genset applying a load at 50 percent was then operated for 15

minutes, aer which part of the diesel exhaust was inserted into

the chamber to be collected by the Airtecs and NIOSH method

5040 samples. When the Airtecs showed a reading of about an - eight-hour TWA concentration of 10 mg m 3

, two of the NIOSH

method 5040 samplers were turned off. This was repeated for

eight-hour TWA concentrations around 20, 50, 100, 200, 300,

and 400 mg m-3 EC. The quartz ber lter samples were

analyzed for EC via NIOSH method 5040 at NIOSH OMSHR

Pittsburgh. The procedure was repeated seven times for EC

concentrations ranging from about 20–400 mg m-3

.

The data from each Airtec was downloaded to a laptop. The

voltage reading from the Airtec when each NIOSH method 5040

sample was stopped was divided by the initial voltage to

determine the transmission of the lter during the time the

NIOSH method 5040 sample was collected. The optical density

was determined by taking the -log of the transmission. The

NIOSH method 5040 EC results as well as the Airtec results were

averaged, and both were plotted to develop a calibration curve.

NIOSH accuracy criteria

Kennedy et al. developed a document containing the NIOSH

accuracy criteria, which requires an average bias within ten

percent and an accuracy within 25 percent with 95 percent

condence when comparing a new analytical method to a

standard method.38 The document also provides recommended

procedures for determining the bias and accuracy. Therefore,

the next part of the Airtec evaluation was to determine if the

instruments would meet this criteria when compared to NIOSH

method 5040.

At least eight Airtecs were attached to conductive tubing that

was inserted into the Marple chamber. In conjunction, the same

number of cassettes (Sureseal three-piece cassette) containing

quartz ber lters was placed in the chamber to collect EC

samples for NIOSH method 5040 analysis. These cassettes were

attached to critical orices and a vacuum pump via tubing to
collect a sample at 1.7 lpm. The Airtecs and the quartz ber

lters were turned on and run at 1.7 lpm. Diesel from a Kubota

engine (50 percent load) was inserted into the chamber aer

warming the engine for ten minutes. When the Airtecs indicated that a 10 mg m-3 eight-hour TWA was collected, both the

Airtecs and NIOSH method 5040 samples were turned off. This

was repeated for EC concentrations of about 100, 120, and

240 mg m-3

. These concentrations were recommended in the

NIOSH publication for determining accuracy. The bias, precision, and total accuracy were determined using the equations

and nomogram in the NIOSH publication.38

LOD/LOQ

The LOD and LOQ were determined by running eight Airtecs for

eight hours in the laboratory where no EC was present. The

eight-hour TWA of each Airtec was calculated aer which the

standard deviation was determined and multiplied by three to

establish the LOD and by ten to determine the LOQ. The veminute, ten-minute, and 15 minute rolling averages were

calculated for the entire eight hours. The standard deviation

was then determined for each concentration and multiplied by

three for LOD and ten for LOQ.

Interferences

Absorbing particles, non-absorbing particles, and humidity

have been shown to interfere with lter-based absorption

techniques.36,39–42 Non-absorbing particles can affect the light

extinction by light scattering or can enhance the absorption by

multiple scattering which causes more opportunity for light

absorption.36 The inuence of these effects depends upon lter

type, light wavelength, and aerosol.36,39–42 Therefore, the inuence of these potential interferences specically for the Airtec

needs to be evaluated.

Aerosols present in underground mines which could be

interferences would be dust, humidity, cigarette smoke, and oil

mist.14,20 In this paper, the effects of these parameters on the

Airtec were evaluated by introducing these interferences into a

Marple Chamber and recording the response of the Airtecs to

these aerosols. In addition, DPM was inserted with these

interferences, and the Airtecs were compared to NIOSH method

5040 samples in this type of atmosphere.

Dust

In order to evaluate the effects of some dusts on the Airtec, a

dust collected from a gold mine was sent to Penn State

University to be sized to a particle size distribution representing

what was observed at the gold mine. The particles were then

inserted into a TSI 3400 Aerosol generator. A previous publication provides more details, such as particle size distribution, on

the dust used.34 A Thermo Scientic Tapered Element Oscillating Microbalance (TEOM) 1400 was set up to collect the total

dust mass concentration inside the chamber. Three SKC Inc.

DPM cassettes containing quartz ber lters were placed inside

the chamber and connected to critical orices for 1.7 lpm ow

rates to collect NIOSH method 5040 EC samples. SKC DPM

cassettes were chosen because they segregate the dust and

diesel to obtain an accurate DPM measurement via NIOSH

method 5040 in the presence of mine dust.37

Conductive tubing was extended from nine Airtecs into the

chamber. In order to evaluate the response of the Airtec to

respirable dust, three Airtecs were attached to a Dorr-Oliver

cyclone and run at 1.7 lpm. Six Airtecs were attached to an

impactor (SKC DPM cassette without the quartz lters and

backing pad) to indicate if these size selectors could prevent the

interference of dust on the monitors, because these impactors

are used to eliminate dust interference on NIOSH method 5040

samples.11,14,37 Three Airtecs with impactors were operated at

1.7 lpm and three at 0.85 lpm in order to determine the effects

of dust at the two operating ow rates. The ow rates for each

sample were checked and recorded.

The TEOM and Airtecs were turned on but the NIOSH

method 5040 samples remained off. Dust was inserted into the

chamber to the highest level achievable. Aer about two hours,

in addition to the dust, DPM via the Kubota Genset at 50 percent

load was inserted into the chamber, and the NIOSH method

5040 samples were turned on. DPM was collected until the EC

would represent a sample at about 100 mg m-3 eight-hour TWA.

The dust and diesel samplers were then shut down and the

chamber was cleared. The SKC DPM cassettes with quartz lters

were analyzed for EC and TC via NIOSH method 5040 at NIOSH

OMSHR Pittsburgh. The data from the TEOM and Airtecs were

downloaded. The experiment was repeated using limestone

dust, which also was sent to Penn State University to be sized

according to what was observed in the eld.

Humidity

Another possible interference to real-time measurements is

humidity. In order to test the effect of humidity on the Airtec,

the humidity in the Marple chamber was increased using a

Miller-Nelson Research Control System model HCS-401.

Because this alone could not bring the humidity up to the

desired 90 percent, water vapor was also inserted into the

chamber using a Vicks Humidier until the humidity was about

90 percent, determined using a Vasaila GM70 gas monitor. DPM

was then inserted into the chamber with the Kubota engine at

50 percent load, and NIOSH method 5040 samples (at least 9)

using a three-piece cassette with a quartz lter and cellulose

backing pad were collected while the Airtecs (at least 7) were

operating. The NIOSH method 5040 samples and Airtecs were

turned off when enough DPM was collected to represent about

12 mg m-3 EC 8 hour TWA. This process was repeated to collect

enough EC mass representing concentrations of 120 mg m-3 EC

and then 240 mg m-3 EC. The results from the Airtec and NIOSH

method 5040 samples were then compared.

Cigarette smoke

The response to cigarette smoke was determined by rst

introducing DPM (Kubota Genset at 50 percent load) into the

chamber while six Airtecs with impactors and twelve NIOSH

method 5040 samplers (SKC DPM cassettes) were running at

1.7 lpm. In addition, a TEOM 1400 was used to measure the

total particulate mass concentration. An Airtec and two SKC
Fig. 2 Apparatus used to introduce cigarette smoke into chamber.

DPM cassettes stopped sampling aer collecting DPM that

represented 25 mg m-3 eight-hour TWA EC. This process was

repeated until an Airtec collected enough EC representing 50,

70, 100, 140, and 200 g m-3 m eight-hour TWA. The DPM was

subsequently ushed out of the chamber.

Two Airtecs with clean lters and impactors were turned on,

and tubing was extended from the instruments into the

chamber. Again, the six Airtecs containing different DPM

concentrations started sampling inside the chamber as well as

six new SKC DPM cassettes for TC analysis of the cigarette

smoke. Cigarette smoke was puffed into a metal container

(Fig. 2) and then drawn into the Marple chamber. The cigarette

smoke was sampled until about 250 3 mg m- eight-hour TWA

cigarette smoke mass was collected. NIOSH method 5040

samples were stopped periodically throughout the test. At the

end of the test, the Airtecs were turned off and the cigarette

smoke was ushed out of the chamber.

The quartz lters were analyzed for EC and TC via NIOSH

method 5040 at NIOSH OMSHR Pittsburgh, and the Airtec and

TEOM data were downloaded.

Oil mist

Three Airtecs with no size selector, three Airtecs with submicron

impactors, and the TEOM (mass) were turned on and collected

the air in the Marple chamber. An oil mist was introduced into

the chamber using a TSI 3076 Constant Output Atomizer containing Chevron Aries ISO220, a type of oil used in drills in some

metal mines. Aer about six hours, the samplers were turned off.

In order to determine the inuence of DPM combined with

the oil mist, enough EC to represent about 100 mg m-3 eighthour TWA was collected with three Airtecs with impactors, three

with no size selector, and three SKC DPM cassettes using the

same chamber and diesel source described in the Cigarette

smoke section, including using the TEOM for determining total

particulate mass concentrations. The pumps for the SKC DPM

cassettes were stopped. The DPM was ushed out of the

chamber, and oil mist was inserted into the chamber (as

described above). Aer about three hours the samplers were

turned off.

There is little information on what concentration of oil mist

in the chamber would relate to real-world applications. Therefore, a sample was collected in an area of a metal mine where

drilling was prevalent but where no diesel vehicles were operating. An SKC DPM cassette attached to an MSA elf pump was

Fig. 3 Optical density vs. EC in (mg per filter) including points when the mg of EC

on the filter exceeds 276 mg.

worn by a miner who performs pneumatic drilling at a metal

mine. On the day of sampling, the miner worked in an area

where he was drilling but where no diesel vehicles were operating. The sample was analyzed for EC and TC using NIOSH

method 5040 at NIOSH OMSHR Pittsburgh.

Results and discussion

Calibration curve

Fig. 3 presents the calibration data plotting the Airtec results vs.

EC mass per lter determined using NIOSH method 5040. This

graph shows a strong correlation between these two data sets

with an R2 of 0.98. The equation for this line can be used to

calculate EC mass from the Airtec measurements. The calculated EC mass can then be inserted into eqn (1) and (2) to

calculate the EC concentration in the area being measured.

Note that this calibration curve is only pertinent for samples

collected with 37 mm Pall Teon lters, and a revised curve

would be necessary if different lter types or sizes are used. The

slope and intercept could also possibly be slightly different

between instruments.

As also seen in Fig. 3, the correlation between optical density

ow

rate of 0.85 lpm.

and EC seems to start to diverge from the line when the EC

collected onto the lter exceeds 276 mg. At this mass quantity,

the aerosol deposit reaches optical saturation to the point that

the optical density measurement is inaccurate. Converting 276

mg into concentration, this calibration curve becomes questionable when determining 8 hour TWA EC concentrations over

338 g m-3 m at a ow rate of 1.7 lpm or over 676 -3 mg m at a

NIOSH accuracy criteria

The method used to determine if an instrument meets the

NIOSH accuracy criteria was outlined in the Methods section.

Although this study was limited by the number of Airtec

instruments available, using this data in the equations and

nomogram developed by Kennedy et al.,

38 the method met the

NIOSH accuracy criteria (accuracy of approximately 12 percent

at 95 percent condence). Table 1 presents the biases and

coefficient of variations (CV) calculated for the Airtecs using the

Kennedy method.
Table 1 The bias and coefficient of variation for the Airtecs vs. NIOSH method 5040

NIOSH 5040 Airtec NIOSH 5040 Airtec

Number of

samples

Average 8 hour TWA EC

concentration (mg m-3

)

Average 8 hour TWA EC

concentration (mg m-3) Bias (%) CV (%) CV (%)

10

9

9

8

113

311

12 13

130

107

282

8.33

132

5.31

9.32

-

-1.54

2.5

1.8

6.1

7.8

7.8

3.7

6.1

4

Table 2 Airtec limit of detection and quantification

5 minute

average

concentration

10 minute

average

concentration

15 minute

average

8 h TWA concentration

LOD ( -3 mg m ) 1.42 21.63 13.1 10.04

LOQ ( -3 mg m ) 4.75 72.11 43.67 33.47

LOD/LOQ

Table 2 presents the LOD for the eight-hour TWA and the 5, 10,

and 15 minute real-time measurements. Using the LOQ for the

minimum quantiable concentration and the calibration curve

to determine the highest measurable concentration, the dynamic

range forthe Airtec is 4.75 to 338 mg m-3 eight-hour TWA ECat 1.7

lpm and 9.5 to 676 mg m-3 eight-hour TWA EC at 0.85 lpm.

Though the real-time data has no upper limit, the Airtec can only

accurately detect ECconcentrations until 276 mg of ECis collected

on the lter. This dynamic range is good for most concentrations

observed in underground mines.2,20 However, this instrument

could be optimized for other environments; e.g., multiple ow

rates and different Teon lter sizes would enable users to

balance sensitivity with lter longevity in order to use the

instrument in different types of scenarios.

Interferences

Dust. As seen in Fig. 4, at concentrations greater than 10 mg

m-3 (MSHA's nuisance dust standard for non-coal mines) metal

Fig. 4 The concentration of total dust from a metal mine in the test chamber,

with and without DPM, and the measurement of the Airtecs at the same time.

mine dust resulted in Airtec readings around 100 mg m-3 in the

absence of DPM and if no size selector was used. However, if a

submicron impactor was employed, these dust concentrations

did not result in a false positive measurement using the Airtec.

When DPM and dust were both present in the test chamber and

no size selector was used, the dust resulted in a bias of -109%

(see Table 3); however, this bias was reduced to less than ten

percent when an impactor was utilized. This approximate 275

mg m-3 interference is due to several effects. About 100 mg m-3

is probably due to light extinction from the dust particles, as

shown by the dust absorbing some of the light when only dust

was being collected. The other 175 mg m-3 is probably due to

several other factors as described below.

DPM can coagulate with dust particles resulting in some of

the DPM being collected by the impactor.43,44Thiswould result in

more DPM being present on the lter with no size selector

compared to when an impactor is used. The light extinction

would then be greater when no impactor is utilized. The quantitative effect of this phenomenon is notwell known but is part of

a study being performed currently at NIOSH. Besides coagulation, light scattering could also have contributed to the bias.

Bond et al. reported that even non-absorbing particles can

increase light extinction when deposited on the lter via light

scattering.36 This effect resulted in a 2–9% increase in optical

density at atmospheric concentrations. Bond et al. also reported

that scattering of non-absorbing aerosols mixed with the

absorbing aerosols can increase light extinction by 20–30%.36

The authors attributed this to multi-scattering by the particles

allowing more opportunities forlight absorption. Given the high

concentration of dust utilized for evaluating the Airtec, these

effects could easily be greater than those reported by Bond et al.

Using limestone dust, at the concentrations shown in Fig. 5,

the Airtec did not detect the dust whether an impactor was used

Table 3 Comparison of EC readings from NIOSH method 5040 and Airtec in the

presence of dust and DPM

Size selector

on Airtec

Flow

rate

NIOSH 5040

EC ( -3 mg m )

Airtec

EC ( -3 Dust mg m ) Bias%

Metal None 1.7 251.20 525.76 -109

Metal Impactor 0.85 251.20 230.03 8

Metal Impactor 1.7 251.20 229.87 8

Limestone None 1.7 621.52 808.72 -30

Limestone Impactor 0.85 621.52 591.64 5

Limestone Impactor 1.7 621.52 603.81 3
Fig. 5 The concentration of limestone dust and DPM in the test chamber along

with the Airtec measurements.

or not. This dust was not expected to absorb the light but some

effect on the light extinction might have been observed due to

scattering. This effect was not observed when using the Teon

lter. However, when the mixture of limestone dust and diesel

was present in the test chamber, as seen in Fig. 5, the

measurement of the Airtec with no pre-selector resulted in

higher concentrations of DPM than the Airtec with an impactor.

The results from this test (Table 3) convey a -30 percent bias

when no pre-selector was employed but less than 10 percent

bias when an impactor was used.

One possibility for this bias is that the DPM coagulated onto

the dust particles, causing less DPM to be collected by the

impactor. Again, this could also be the result of light scattering

or enhanced absorption when the dust is mixed with the DPM

particles, especially since studies have reported a 30% increase

in light extinction due to non-absorbing aerosols even when not

embedded in the lter.36

Some mine dusts can absorb at 650 nm causing a bias in the

Airtec readings. Non-absorbing mine dusts can also cause a bias

when comparing Airtec without a pre-selector to the standard

method for measuring DPM in mines, which entails using a

submicron impactor. This bias could be caused by coagulation,

light scattering, or enhanced absorption due to multi-scattering. The submicron impactor eliminated any interference

issues with mine dust. Therefore, when using the Airtec in

underground metal/nonmetal mines, an impactor (or equivalent size selector) should be used to avoid possible interference

from dust.

Humidity. Table 4 presents the bias results comparing the

Airtec with NIOSH method 5040 in an environment of DPM and

90 percent humidity, potentially present in some mines during

the summer months. The bias was less than 10 percent (within

bias range normally observed (see NIOSH accuracy criteria

section)) for two of the concentrations but was 15.8 percent for

the lowest concentration. However, the condence intervals for

the two lowest measurements overlap, indicating no statistical

difference in the two values. Therefore, it was determined that

the Airtecs were not affected by high humidity.

Table 4 Comparison of Airtec and NIOSH method 5040 results measuring EC in

90% humiditya

Average 8 h TWA EC

concentration ( -3 mg m )

via NIOSH method 5040

Average 8 h TWA EC

concentration ( -3 mgm)

via Airtec Bias%

14.39 � 1.37 16.66 � 0.91 -15.77

135.87 � 2.54 122.97 � 7.73 9.49

229.89 � 7.32 208.02 � 8.61 9.51

a 95% condence limits: student t x standard deviation/(number of 45 samples)0.5.

Fig. 6 The response of the Airtec to cigarette smoke.

Cigarette smoke. As seen in Fig. 6, when only cigarette smoke

was present in the chamber at concentrations well above what is

observed in underground metal/nonmetal mines, it was not

detected by the Airtec. There is limited data on cigarette smoke

concentrations in underground mines, but one study showed

that the eight-hour TWA TC concentrations were typically 50–

100 mg m-3 and at times 250 mg m-3 higher in enclosed cabs

with smokers compared to enclosed cabs with non-smokers.14

Another study, in an experimental mine, quantied that each

cigarette may contribute up to 5 mg m-3 eight-hour TWA TC

when smoking occurs in an area with dimensions of approximately 18.5 feet wide and 6.2 feet high (115 2 cross-sectional

area) and when the ventilation was 6000 cfm.46 To put this into

perspective, if 14.1 cigarettes (average number of cigarettes per

day per smoker in California in 2004)47 is used to represent the

number of cigarettes smoked in a shi, the cigarette smoke

concentration within a few feet of the miner would be about 70

mg m-3 eight-hour TWA TC for a ventilation rate of 6000 cfm

and a 115 2 cross-sectional area. Since most mines have a

larger cross-sectional area and higher ventilation rates,46,48–50

the concentration of cigarette smoke would be expected to be

lower.46

Cigarette smoke on the Teon lter did not affect the light

extinction in the Airtec. However, when mixed with DPM

particles, cigarette smoke was a potential interference. As seen

in Fig. 6, when DPM is on the lter, the Airtec does detect some
Fig. 7 The concentration measured by the Airtec when sampling in just cigarette smoke after the collection of EC from DPM.

of the cigarette smoke. As mentioned earlier, studies have

shown that non-absorbing particles can affect light extinction

due to light scattering and multi-scattering of the particles,

increasing the opportunity for light absorption.36,41,42 Therefore,

one plausible explanation for the interference of cigarette

smoke is that the multi-scattering of the cigarette smoke

particles could increase the amount of light exposed to the DPM

particles, resulting in more light absorption.

As seen in Fig. 7, adding cigarette smoke to the environment

aer collecting DPM on the Airtec lter resulted in two trends.

One trend was identied when enough DPM mass representing

concentrations between 25 and 50 mg m-3 eight-hour TWA EC

was present on the lter and another when DPM mass representing greater than 70 mg m-3 eight-hour TWA EC was

collected. The graphs show that when sampling at concentrations of DPM around the nal permissible exposure limit

(120 mg m-3 EC), concentrations of cigarette smoke between 50

and 250 mg m-3 TC (observed when smoking in enclosed cabs)

can result in readings between 8 and 98 mg m-3 of interference

on the Airtec, which would result in a bias between 7 and 81

percent. When exposed to concentrations of cigarette smoke

below 50 mg m-3 TC, the bias would then be below 7 percent

when sampling at the DPM PEL.

At this time, in underground mines, the concentrations of

cigarette smoke (above 50 mg m-3 TC) where the resulting bias is

of concern have only been observed when sampling smokers in

enclosed cabs. One way of avoiding the problem would be to

simply have the miners not smoke in the enclosed cab during

sampling. A topic of a future study would be to determine the

actual effects of the cigarette smoke on the instrument in the

eld. Then, some correction methods may be investigated. For

example, other studies have had success with using multiwavelengths to determine the contribution of cigarette smoke

in the presence of black carbon.28–30

Oil mist. As can be seen in Fig. 8a and b, the Airtec did not

detect oil mist even if DPM was present on the lter. For these

tests, the concentration of oil mist in the chamber was the

highest achievable concentration with the experimental setup

Fig. 8 Airtec measurement in the presence of oil mist in the test chamber with

(b) and without (a) DPM collected on the filter.

and was close to what was observed in a metal mine. In an area

in a metal mine where oil mist was present but where there was

little DPM, 164 g m-3 m OC was measured. DPM contributed

little to this OC value as seen by the EC value only being 16 mg

m-3

, which converts to about 21 mg m-3 TC or 5 mg m-3 OC of

the sample from DPM using the TC/EC ratio of 1.3 (typical ratio

found in mines14). The largest submicron OC-containing aerosol in the area was oil mist.

Conclusions

NIOSH has developed a method for measuring real-time DPM

concentrations in underground mines by calculating EC

concentrations from a laser extinction measurement. The Airtec

is a small, lightweight, person-wearable instrument which

incorporates this laser extinction method. This instrument has

been shown in the laboratory to meet the NIOSH accuracy

criteria. In addition, this instrument is not prone to interferences from humidity and oil mist. Laboratory studies have

shown that dust can interfere with the instrument measurements if a submicron pre-selector is not used; therefore, a

submicron impactor or some comparable pre-selector should

be used with the instrument when measuring DPM in underground mines. Cigarette smoke was also shown to cause a

measurement bias greater than ten percent under specic

circumstances with this instrument. However, the
concentration of cigarette smoke where this is a concern has

only been observed when collecting samples inside enclosed

cabs where miners are smoking. This bias can be avoided by

preventing miners from smoking inside the enclosed cab

during sampling. Other precautions and potential correction

methods to be used with this instrument are currently being

investigated. The next step in evaluating the Airtec is to test its

reliability during eld operation in underground mines. This

type of study may also provide a better idea of the role of

potential interferences such as cigarette smoke on the data and

will be a topic of a future study.

Disclaimer

Mention of a company name or product does not constitute an

endorsement by the National Institute for Occupational Safety

and Health. The ndings and conclusions in this report are

those of the authors and do not necessarily represent the views

of the National Institute for Occupational Safety and Health.

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