Evaluation of a Wearable Monitor for Measuring Real-Time

Diesel Particulate Matter Concentrations in Several

Underground Mines

J. D. Noll and S. Janisko

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,

Pittsburgh, Pennsylvania

Keywords diesel, measurement, real-time

Address correspondence to: J. D. Noll, 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; e-mail: JIN1@cdc.gov.

The standard method for determining diesel particulate

matter(DPM) exposures in underground metal/nonmetal mines

provides the average exposure concentration for an entire

working shift, and it can take weeks to obtain results. This

approach is problematic because, although it reports that

an overexposure has occurred, it fails to provide critical information about cause or prevention. Conversely, real-time

measurement would provide miners with timely information

to identify the major factors contributing to overexposures

and would allow engineering controls to be deployed immediately. Due to these potential benefits, the National Institute

for Occupational Safety and Health (NIOSH) developed a

wearable instrument that measures real-time elemental carbon (EC) concentrations (EC is a DPM surrogate) via laser

extinction. This instrument was later constructed into a commercial version (Airtec). This article evaluates the Airtec’s

performance in several underground metal/nonmetal mines by

comparing it to the standard method for determining DPM

exposures (NIOSH method 5040). The instrument was found

to meet the NIOSH accuracy criteria and to show no statistical

difference from NIOSH method 5040 results. In addition, the

instrument’s measurements were found to be unaffected by dust

and humidity.

INTRODUCTION

Exposure to diesel particulate matter (DPM) is a health

concern for miners because it has been classified as a

potential occupational carcinogen by the National Institute

for Occupational Safety and Health (NIOSH) and as likely to

be carcinogenic to humans by the Environmental Protection

Agency (EPA).(1–5) In addition, diesel exhaust has been potentially linked to other chronic and acute adverse health effects

such as asthma, eye irritation, nausea, and headaches.(6–8) Since

underground miners work alongside diesel equipment in a confined environment, they can be exposed to some of the highest

levels of diesel exhaust in the country.(2,9) Therefore, the Mine

Safety and Health Administration (MSHA) promulgated a rule

to limit exposures of metal/nonmetal underground miners to

DPM to an 8-hr time-weighted average (TWA) of 160 μg/m3

total carbon (TC).(10,11)

The standard method for determining DPM exposures is to

collect the particulate onto a quartz fiber filter for an entire

shift using a submicron impactor to segregate dust from DPM.

The sample is then sent to a laboratory to be analyzed for

elemental and total carbon using NIOSH method 5040. TC

is used as a surrogate to determine DPM exposures since

direct DPM measurement is not accurate enough and since

TC represents over 80% of DPM.(11–13) However, non-DPM

organic aerosols such as cigarette smoke and oil mist can

influence TC concentrations.(14) Therefore, in some cases,

MSHA uses submicron elemental carbon (EC) as an alternative

surrogate for DPM because it is a major component of DPM,

and it eliminates the interference from aerosols containing

organic carbon. A worker’s 8-hr TWA exposure level is measured in terms of TC concentration and, in order to avoid the

influence of interferences, is determined in two ways: (1) a

TC value is directly measured using a personal sample, and

(2) a TC value is calculated by multiplying the EC value

from the personal sample by a conversion factor (determined

from an area sample).(15) Both TC values have to be over the

permissible exposure limit (PEL) for the mine to be 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 shift, and the results can

take weeks to obtain. This approach is problematic because,

although it reports that an overexposure has occurred, it fails to
provide critical information about cause or prevention. Conversely, real-time measurement would provide miners with

timely information to identify the major factors contributing

to overexposures and would allow engineering controls to be

deployed immediately. As an example, NIOSH method 5040

would take weeks to assess the effect a ventilation strategy

has had on actual DPM concentrations. By the time the data is

analyzed, the miners could be working in different sections of

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

in atmospheric conditions. Real-time analysis would allow

ventilation to be redirected to provide timely DPM dilution.

To address the above issues, NIOSH developed an instrument to measure real-time EC exposure via laser extinction.(16–19) EC was chosen as the analyte because it makes

up a major portion of DPM, is not prone to interferences,

and is one of the surrogates used by MSHA for compliance

sampling. Filter-based laser extinction was determined to be

a feasible method because EC concentrations are proportional

to laser absorption and because this simple technique can be

adapted into a small instrument. 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 influence if other scattering aerosols are

collected with the DPM (see the interference section under

Methods for more detail).(19,20) For example, light scattering

from some non-absorbing aerosols can enhance the absorption

of the DPM particles. With a preliminary data set, the firstgeneration instrument demonstrated good agreement (within

10%) with the standard method for measuring EC from DPM

(NIOSH method 5040(21)) in laboratory and limited field studies.(16–18)

Taking into account what was learned from the laboratory

and field testing on a prototype beta model, a commercial

version was developed, tested, and is now available from FLIR,

Inc. Laboratory tests have shown that the commercial version

(known as Airtec) meets the NIOSH accuracy criteria and has

the capability to measure DPM at concentrations commonly

observed in underground metal/nonmetal mines.(19) The study

also showed that a submicron impactor should be used to avoid

the interference of mineral dust and that oil mist and humidity

did not affect the Airtec readings. Cigarette smoke was also

revealed as a potential interference when sampling a smoker

inside an enclosed cab.

The purpose of this study was to evaluate the performance

of the Airtec in the field. The accuracy of the instrument when

operating in actual working mines was assessed, as was the

variability of measurements relative to that of the standard

method. In addition, the effects of dust and humidity were also

evaluated.

METHODS

Comparison to NIOSH Method 5040

Before commercial Airtec units were available, the firstgeneration or beta units constructed by NIOSH and FLIR

(Figure 1) were compared to the standard method for measuring DPM exposures by running them along with two to

three NIOSH method 5040 samples in various areas in four

metal and seven stone mines. In the beta models, a diaphragm

pump draws ambient air at 1.7 l pm, which enables a preselector to make a 0.8-μm size cut. Conductive tubing allows

EC to reach the Teflon filter without attaching to the tubing

walls. A Teflon filter is housed in a specially designed cassette

that includes a defined volume chamber as well as a carefully

constructed flow path to achieve uniform distribution of EC on

the Teflon filter. A laser penetrates through the sample while

collecting DPM, and the optical density (-log(transmittance))

of the laser’s energy is measured and converted to μg of EC

collected on the filter using a calibration curve established in

previous laboratory studies.(16–18) The beta models provided

5-min, 10-min, and 15-min rolling averages as well as the 8-hr

time-weighted average as described by Noll et al.(19) The 8-hr

time-weighted average was used to compare the beta model

samples with the NIOSH method 5040 samples.

The sampling train for NIOSH method 5040 samples consists of a Dorr-Oliver cyclone followed by SKC DPM cassettes

(consisting of two quartz fiber filters in tandem inside an

impactor with a cutpoint of 0.8 μm at 1.7 lpm) with flow

powered by an MSA Elf pump via tubing. The DPM samples

are then analyzed for EC concentration via thermal-optical

analysis (following procedures outlined in NIOSH method

5040). This same sampling setup is used for regulatory compliance measurements in underground metal and nonmetal

mines. The NIOSH method 5040 results were converted to

an 8-hr time-weighted average.

Based on lessons learned after evaluating the beta model, a

commercial unit was manufactured with improvements. These

improvements included a quieter pump, a different casing to

incorporate an internal battery and decrease size, and a metal

clip instead of a plastic one. The revised commercial version

(Airtec) (Figure 2) was then tested, using the same procedures

as with the beta models, at three stone mines under different

FIGURE 1. The prototype (beta) models of the wearable instrument for measuring real-time elemental carbon concentrations: (a)

constructed by NIOSH (b) constructed by FLIR.
FIGURE 2. The commecial version (Airtec) of the real-time DPM

monitor.

conditions (Table I). Area samples consisting of three SKC

DPM cassettes for NIOSH method 5040 analysis (using the

same setup as described for the beta testing) and at least one

Airtec were inserted in a basket, hung along the rib, or placed

on a tripod in an area of a mine. In other cases, the same

types of samplers were inserted in a basket or connected to a

metal plate and then attached to a vehicle or placed inside an

enclosed cab.

The bias between NIOSH method 5040 and the Airtecs

was calculated using the equations found in Kennedy et al.(22)

The 95% confidence interval for the bias was determined by

multiplying the t-value by the standard deviation and dividing

by the square root of the number of samples.(23,24)

Analytical method errors, pump errors, and matrix effects

can all affect the accuracy of the instrument. In underground

mines, another source of error (spatial variability) can cause

a difference between side-by-side samplers even if both are

reading accurately. In fact, spatial variability can lead to measurement biases of over 20%.(25,26) Therefore, in this study,

the Airtec and NIOSH method 5040 results were compared

using a method that takes spatial variability into account. The

bias between the Airtec and NIOSH method 5040 and the

percent difference between duplicate NIOSH method 5040

samples (randomly chosen from the triplicate samples) were

compared. If the Airtec is equivalent to NIOSH method 5040,

the variability between the Airtec and NIOSH method 5040

results should be similar to the variability between two NIOSH

method 5040 samples since both experience similar spatial

variability. This evaluation was performed by comparing the

TABLE I. Samples Collected With Airtec

Type of Area On Vehicle In Enclosed

Mine Mine Sample Sample Cab

Limestone B 1

Limestone J 15 4

Granite K 27 21

Total 28 36 4

median and spread between the biases of the Airtec and NIOSH

method 5040 results and the percent differences of duplicate

NIOSH method 5040 samples, which were recorded at identical underground locations on the same dates. Since the two

data sets were non-normally distributed, the Mann-Whitney

rank sum test was performed using Sigma Plot 12.0 instead

of the t-test to determine if there are statistical differences

between medians. In addition, the Levene’s test using Sigma

Plot 12.0 instead of an F-test was performed to assess for

statistical differences between variances.

Interferences

Absorbing particles, non-absorbing particles, and humidity

have been shown to interfere with filter-based light extinction

techniques.(19,20,27) 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 to occur.(20) The influence of these effects depends

on filter type, light wavelength, and aerosol.(19,20) Of the known

aerosols present in underground mines, potential interferences

are dust, humidity, cigarette smoke, and oil mist.(14)

The effects of these interferences on the Airtec were evaluated in a laboratory analysis.(19) These results showed that

mineral dust can interfere with the Airtec when no size selector

was used, but using a submicron size selector removed the

influence of the dust on the Airtec. Even though it is submicron

in particle size, oil mist as well as humidity were found not to

interfere with the Airtec. The Airtec did not detect the presence

of cigarette smoke, another submicron particle, unless DPM

was on the filter. This was attributed to multi-scattering of

the cigarette smoke particles increasing the opportunities for

light absorption from the DPM on the filter. So far, the concentrations of cigarette smoke resulting in a bias to the Airtec

were reached only when sampling in enclosed cabs where

the equipment operator was smoking. Due to the potential

interference of cigarette smoke in some situations, it would

have been beneficial to have some samples in the presence of

cigarette smoke in the field, but none of the operators in the

field studies described in this article were known to smoke

within proximity of the sampling locations.

There were locations where the dust concentrations and

humidity were measured so that the effects of these on the

Airtec measurements could be evaluated in the field. In order

to determine if mineral dust affected the Airtec readout, dust

measurements as described by Cecala et al.(28) were collected

along with the Airtec and NIOSH method 5040 samples in

several locations at a limestone mine. Dust concentrations

were then compared to biases between the Airtec readings

and NIOSH method 5040 results to identify any correlation

between the two parameters. The effect of humidity was tested

in mines where the humidity was greater than 75%. A Vasaila

(Vantaa, Finland) GM70 portable gas monitor, located near the

Airtec and NIOSH method 5040 samples, was used to measure

relative humidity.
RESULTS AND DISCUSSION

Comparison of Airtec with NIOSH Method 5040

in the Field

The comparison of NIOSH method 5040 samples and the

Airtec readings resulted in an average % bias of 3 ± 3 with

96% of samples having a % bias within ±25%, and 90% of

the samples having a % bias within ±16%. This data meets

NIOSH accuracy criteria—an average overall bias equal to or

less than 10% and 95% confidence that the bias will be within

25%(23)—but may overestimate the bias of the instrument since

it does not account for spatial variability, which can result in

20% error in some cases.

All the Airtec samples at 40 μg/m3 EC and below resulted

in a positive bias, with two of the six samples providing biases

(>40%) above the values of the other samples (Figure 3).

Whether this phenomenon involves just a few outliers or is

caused by environmental issues, such as interferences with

greater effects at lower concentrations or higher influence of

spatial variability at lower concentrations, cannot be determined at this time, for there are only six points (6 out of 68)

at and below 40 μg/m3 EC.

Given the potential uncertainty for samples at lower concentrations, the biases for all samples greater than 40 μg/m3 EC

were also evaluated. These concentrations represent the majority of mining samples and contain the PEL (about 120 μg/m3

EC). For samples greater than 40 μg/m3 EC, the average% bias

was 0.9 ± 2 with 95 % of % biases being within ±16% and

90% being within ±13%, even considering potential spatial

variability issues.

As described in the Methods section, since the direct comparison of NIOSH method 5040 results and the Airtec readings

do not account for spatial variability (potentially resulting in

overestimation of the Airtec bias), the biases between the

Airtec and NIOSH method 5040 were compared to the differences between side-by-side NIOSH method 5040 samples

to determine the equivalency of the Airtec to NIOSH method

5040. The analysis found that the accuracy of the Airtec

is not statistically different from the accuracy of side-byside NIOSH method 5040 samples. As seen in Figure 4,

the percent biases between the Airtec and NIOSH method

5040 results (range −18% to 52%) were within the percent

differences between duplicate NIOSH method 5040 samples

(range −22% to 65%). The Mann-Whitney rank sum test

showed no statistical difference between the medians of the

biases of the Airtecs and the percent differences of the NIOSH

method 5040 samples. In addition, the Levene’s test (p =

0.678) showed no statistical difference between the spreads or

variances of the two data sets.

FIGURE 3. The bias between the Airtec and NIOSH method 5040.
TABLE II. Comparison of Airtec and NIOSH Method 5040 Samples in the Presence of Respirable Dust

EC (μg/m3) via Respirable Dust

EC (μg/m3

) via Airtec NIOSH 5040 Concentration (μg/m3) Bias (μg/m3) %

199 242 190 −43 −18

227 242 190 −15 −6

307 298 610 9 3

302 298 610 4 1

237 250 770 −13 −5

247 250 770 −3 −1

208 202 780 6 3

77 71 780 6 9

61 40 1160 21 52

46 40 1160 6 14

103 106 1400 −3 −3

163 163 2820 0 0

182 163 2820 19 12

208 198 4430 10 5

Bias

Interferences

As mentioned earlier, interference of dust (when an impactor was used) and humidity was not observed when performing the laboratory evaluation on the Airtec (19), and this

was validated for some samples in the field. As seen in Table II,

bias in the Airtec did not depend on dust concentration when

FIGURE 4. The bias between the Airtec and NIOSH method 5040 and the percent difference between duplicate NIOSH method 5040 samples

when compared to the average concentration of triplicate NIOSH method 5040 samples.
FIGURE 5. The bias between the Airtec and NIOSH method 5040 results vs. the average concentration of triplicate NIOSH method 5040

samples showing the partitioning of the different types of samples.

an impactor was used. This was most evident when the EC

concentrations were similar (between 200–250 μg/m3

) and the

dust concentrations were variable (within 190–4,400 μg/m3

).

At these parameters, the bias did not increase with concentrations of dust. In addition, a consistent positive or negative bias

was not observed due to the presence of dust. Table III shows

that high humidity did not cause biases greater than 13% for

EC concentrations between 100–300 μg/m3

.

During testing, the Airtec was attached to a vehicle that

traveled over rough terrain and performed mining tasks, thus

TABLE III. Comparison of Airtec and NIOSH Method

5040 Samples in High Humidity

EC (μg/m3) EC (μg/m3) Relative

via via NIOSH Humidity %

Airtec 5040 (%) Bias

103 91 94 13

206 192 78 7

289 266 95 9

265 275 86 −4

introducing it to a potential interference from vibration. As

seen in Figure 5, the results from this field study provided no

evidence that vibration affected the Airtec readings; i.e., there

was no obvious increase in bias between the Airtec and NIOSH

method 5040 results when attached to a vehicle compared to

area samples.

CONCLUSION

I

n this field study, the Airtec readings were shown to be

equivalent to NIOSH method 5040 samples. The difference

between the Airtec readings and NIOSH method 5040 results

were similar to the differences observed when comparing duplicate NIOSH method 5040 samples. When an impactor was

used, dust and high humidity did not seem to affect the Airtec

readings in the field. In these field studies, another known

potential interference (cigarette smoke) was not tested because

smoking was not known to occur at the sampling locations.

Besides the known potential interferences, if other submicron

aerosols are present, they could potentially cause a bias in

the Airtec results at high enough concentrations.(19,20) Future

studies will address their effects if any additional aerosols that

may potentially interfere are identified.
DISCLAIMER

Mention of a company name or product does not constitute

an endorsement by the National Institute for Occupational Safety and Health. The findings 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. This article is not subject to US copyright law.

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