Near real time monitoring of diesel particulate matter in 

underground mines 

S. Janisko 

NIOSH, Pittsburgh, PA, USA 

J. Noll, 

NIOSH, Pittsburgh, PA, USA 

ABSTRACT: Studies have linked long-term inhalation of diesel exhaust with adverse health effects such as 

cardiovascular disease, cardiopulmonary disease, and lung cancer. Diesel exhaust levels in underground mines 

can approach 100 times the environmental level and are therefore monitored closely in the mining industry. 

Elemental carbon (EC) (a product of diesel combustion commonly referred to as “soot”) has proven to be a good 

indicator of diesel exhaust levels. The current procedure for measuring EC levels involves taking personal 

samples from a mine worker’s breathing zone and/or stationary samples in various parts of the mine and 

subsequently sending these samples to a lab for analysis by NIOSH 5040 method. Unfortunately, the whole 

process can take up to several weeks to complete. To address the problem of waiting weeks for results, NIOSH’s 

Pittsburgh Research Laboratory is developing a portable device for measuring levels of elemental carbon. This 

optical device takes a continuous sample of air in the surrounding environment and measures the absorbance of 

light passing through a filter that collects diesel exhaust particulates within the instrument. This technology can 

provide a near real time measurement of EC with accuracy within 10% error. The ability to take near real time 

measurements, combined with the instrument’s portability (currently small enough to be worn on a miner’s belt), 

makes this device extremely useful in the mining industry. 

1 Introduction 

Diesel exhaust is a complex mixture that contains 

thousands of compounds. Diesel particulate matter (DPM) 

constitutes a large fraction of this mixture and has been 

recognized as a potential occupational carcinogen by the 

National Institute for Occupational Safety and Health 

(NIOSH). Therefore, NIOSH has recommended that 

worker exposure to DPM be limited. Occupations where 

this exposure is of high concern include mine worker, 

bridge and tunnel worker, railroad worker, loading dock 

worker, truck driver, forklift driver, farm worker, and auto 

mechanic (1). In particular, underground mine workers are 

exposed to some of the highest levels of DPM on a day to 

day basis. 

DPM itself is made up largely of particulate and 

particle bound organic carbon (OC) and particulate 

elemental carbon (EC) (commonly referred to as “soot”) 

(2). Total carbon (TC) is the summation of EC and OC. 

The Mine Safety and Health Administration (MSHA) 

initially considered TC as the best surrogate for measuring 

DPM in underground mines because it accounts for over 

80% of diesel particulate matter and because the EC to 

DPM ratio can change with engine load (3). However, 

cigarette smoke and oil mist in the sampling environment 

can add error to OC measurements which, in turn, can lead 

to flawed TC measurements (4, 5). As a result, MSHA has 

imposed an interim technique for monitoring DPM in 

mines which involves the measurement of both EC and 

TC. 

The interim 2007 DPM limit is 350 μg/m3

 TC, which 

can also be determined using EC as an analyte. The 

standard sampling method (see section 2.5) is performed 

over an entire shift and the filter is analyzed for TC and EC 

using the NIOSH 5040 method. The concentration of TC is 

calculated in two ways. In first way, the value of TC from 

the NIOSH 5040 is converted to an 8-hour shift weighted 

average (SWA), and in the second way, the EC value from 

the NIOSH 5040 is converted to an 8-hour SWA and then 

multiplied by 1.3 (a conversion factor determined by 

MSHA using the 31-mine study). If either the measured 

total carbon SWA or calculated total carbon SWA is above 

350 μg/m3

 then the mine is out of compliance (4). 

This standard method requires samples to be taken in 

the mine and then be sent to a laboratory for NIOSH 5040 

analysis. The process is time-consuming and it can take 

several weeks before a mine receives the results. If a 

portion of the mine is out of compliance, ventilation 

adjustments have to be made and once again, several 

weeks will pass before the mine obtains the new results. In 

addition, the standard method can only provide a value for 

the average EC concentration throughout the sampling 

period. It is incapable of showing concentration changes 

due to such factors as vehicle traffic or mineworker travel 

throughout the day. 

NIOSH has developed an instrument capable of

measuring EC at typical underground mining 

concentrations in near real time. This instrument, referred 
 

 

 

 

 

 

 

 

 

 

 

 

2 

to as the “EC monitor” uses a laser diode absorption 

technique similar to that used in conventional NIOSH 5040 

analysis instruments (Magee Scientific, Berkeley, CA and 

Sunset Laboratory, Tigard, OR)i

. However, the EC monitor 

is portable, lightweight and designed for near real time 

analysis. This instrument can be attached to a miner’s belt 

or to a piece of equipment to provide EC concentration 

analysis on the spot or to plot elemental carbon 

concentration with time. 

Methodology 

2.1 Design 

Please note: Due to pending manufacturing agreements, 

some design specifications cannot be discussed in depth. 

Full disclosure of design components will be released in 

future publications. 

The design of the EC monitor consists of four key 

components: an impactor, a filter, a pump, and an optical 

measuring circuit. The first component in the sampling 

train is an impactor through which air is drawn at a set 

flow rate in order to separate larger dust particles from 

DPM. This impactor can be placed in a miner’s breathing 

zone via a lapel holder. The air/DPM mix then passes 

through a filter within the instrument and EC from the air 

sample is collected onto the filter. An optical sensing 

circuit is used to measure the intensity of light transmitted 

through the filter from a laser diode source. As EC 

continues to collect on the filter, more light is absorbed and 

the output voltage of the light sensor decreases. This drop 

in voltage is compared with a previously established 

calibration curve to relate laser absorption to EC 

concentration (see section 2.2). A data logger is used to 

record these changes in voltage and a microcontroller is 

used to calculate and output EC values to an LCD screen 

on the instrument in real time. 

2.2 Calibration 

In order to establish a relationship between light 

absorbance and EC concentration, the EC monitors were 

calibrated in a controlled diesel exhaust laboratory 

(NIOSH, Pittsburgh Research Laboratory). During 

calibration, three EC monitors simultaneously sampled 

from a chamber filled with DPM. The light sensor voltages 

of these instruments were monitored throughout the test. 

At the same time, standard NIOSH 5040 sampling was 

performed to collect EC from the chamber on sets of quartz 

filters at a regulated flow rate of 1.7 liters per minute. 

Each set consisted of three DPM cassettes, and each 

cassette contained two 37mm quartz filters in series (SKC, 

Inc., Eighty Four, PA). The three EC monitors and the 

filter sets all began sampling at a common time. The DPM 

concentration inside the chamber was held steady and the 

filter sets sampled for different lengths of time so as to 

collect different amounts of EC on each set of filters. 

These times were recorded, as were the correlating sensor 

voltage drops in the EC monitors. Post analysis using the 

NIOSH 5040 Method was then performed on the filter sets 

to determine the specific amounts of EC on the filters (see 

section 2.3). The laser light transmittance was obtained 

from sensor voltage drop and the laser light absorbance 

was calculated from transmittance. The EC values were 

plotted against absorbance to obtain the calibration curve. 

2.3 NIOSH 5040 Method 

The NIOSH 5040 method is an established technique for 

measuring both organic and elemental carbon. The process 

involves drawing sampling air through a DPM cassette 

(SKC, Inc., Eighty Four, PA) at a flow rate of 1.7 liters per 

minute to collect soot onto quartz filters inside the cassette. 

After sampling, punches of these filters are placed inside a 

5040 oven for analysis. The oven first measures organic 

carbon (OC) by progressively ramping its temperature up 

to 870 °C in four steps in a pure helium environment. The 

lack of oxygen prevents EC from reacting. OC is oxidized 

to carbon dioxide and is reduced to methane. The methane 

is measured using a flame ionization detector (FID). EC is 

then measured by reducing the oven temperature and then 

raising it back up to 900 °C in a helium/oxygen 

atmosphere. EC reacts with oxygen to form carbon 

dioxide. Once again, the carbon dioxide is reduced to 

methane and the methane is measured with an FID (6). 

2.4 Drop Testing and Sensitivity 

To ensure that the monitors are not susceptible to 

interferences from sudden jerking or shock, a drop test was 

established to verify the rigidity of the instruments. This 

testing would also yield data suitable for calculating the 

limits of detection and quantification of the instruments 

while in rugged use. During these experiments, one EC 

monitor was repeatedly dropped from four inches while the 

other was shaken intermittently at the same rate throughout 

an eight-hour time period. These motions were achieved 

via mechanical means. The monitors sampled clean air and 

the sensor voltages were logged. This test was repeated 

five times, with monitors rotating positions after each test. 

The resulting ten blanks (five drops, five shakes) were used 

to calculate the limits of detection and quantification with a 

multiplying factor of 3 for LOD and a multiplying factor of 

10 for LOQ (7). 

2.5 Personal Sampling 

The current MSHA method for measuring a mineworker’s 

exposure to EC involves outfitting the miner with a beltworn pump that draws air from his or her breathing zone. 

The air sample is first drawn through a Dorr-Oliver 

cyclone pre-selector and then through a sampling cassette 

containing quartz filters (SKC, Inc., Eighty Four, PA). At 

the end of the mineworker’s shift, the quartz samples are 

sent to a lab for 5040 analysis. The data obtained represent 

an aggregate amount of EC exposure throughout the 

sampling period. 
 

 

 

 

The EC monitors are outfitted with belt loops and are 

worn in a fashion similar to the pump. The monitors 

continuously measure EC exposure throughout the day, 

which is the major benefit over the current method. At the 

end of a worker’s shift, data from the EC monitor can be 

downloaded and his or her exposure to EC throughout the 

day can be plotted with time. In addition, real time and 

shift weighted average exposures to EC are calculated onboard and displayed through an LCD screen on the 

instrument. This allows the miner to instantly know his or 

her exposure at any given time. 

Personal Sampling Case Study:
 

Note: An active stone mine was used as the location for all
 

of the case studies in this proceeding.
 

To obtain personal sampling data, two NIOSH researchers 

were outfitted with EC monitors while taking ventilation 

measurements within a stone mine. The researchers were 

also adorned with standard sampling method equipment for 

comparison with the EC monitor results. The researchers 

traveled first to the return (the area where air exits the 

mine), then to two different working areas (the face), then 

to the intake (where fresh air is brought into the mine). 

The researchers exited the mine and the data sets were 

downloaded from the EC monitors. The standard samples 

were taken to a laboratory for 5040 analysis. 

2.6 Cab Sampling 

Environmental cabs are sometimes used in the mining 

industry to protect vehicle operators from high levels of 

DPM. These cabs have filtered air conditioning systems to 

provide a clean working environment for the vehicle 

operator. The efficiency of cab filtration can easily be 

tested by mounting one EC monitor on the exterior of the 

vehicle and placing another EC monitor inside the cab. 

After the desired amount of sampling time (typically eight 

hours), the data can be downloaded from the monitors and 

efficiency can be plotted with time. Standard practice for 

NIOSH researchers is to include three standard DPM 

sampling cassettes and pumps with each monitor so that 

the EC monitor results can be verified with a 5040 

analysis. However, this exercise should depreciate with the 

acceptance of EC monitor readings. 

Cab Sampling Case Study: 

At the same stone mine used for personal sampling, a 

Catepillar 980F loader was outfitted with two sampling 

baskets, each containing an EC monitor and three 

traditional SKC DPM cassettes with pumps (per the above 

procedure). The loader was run on its normal working 

schedule, and after six hours the sampling baskets were 

removed for analysis. This procedure was followed for five 

days. 

2.7 Area Sampling 

The EC monitors can be a useful tool when measuring the 

effectiveness of a mine’s ventilation system. Since the 

monitors provide EC concentration readings over time, a 

better observation of mine ventilation changes throughout 

the day can be obtained. To measure EC levels at a 

particular location in a mine, the EC monitors are mounted 

in baskets placed on tripods. Once again, three standard 

DPM sampling cassettes are typically included to verify 

the EC monitor recordings. However, as in the case of 

personal sampling, traditional methods can only supply an 

aggregate EC number throughout the sampling period, 

whereas the EC monitors can show how EC concentrations 

raise and lower with the movement of vehicles throughout 

the day. In addition, if an area of a mine has suspect 

ventilation, an EC monitor can be taken to that location to 

provide a quick measurement without the need for 

laboratory analysis. 

Area Sampling Case Study: 

Four areas of the stone mine were monitored to test the 

mine’s ventilation system. Baskets containing an EC 

monitor and three SKC sampling cassettes were placed at 

the mine intake, return and two working areas where 

loading, drilling, roof bolting and scaling were occurring. 

After six hours of sampling, the baskets were removed for 

analysis. This procedure was followed for three days. 

3 Results 

3.1 Calibration Results 

Figure 1 Relationship between EC monitor laser 

absorption and concentration measured by standard 

method. 

The EC monitor calibration yielded a linear relationship 

between laser absorption and filter concentration for the 

concentration range tested (Figure 1). Laser transmittance 

Vi is first calculated as , where Vi is instantaneous 

Vo 

photodiode voltage and Vo is the starting, blank-filter 

photodiode voltage. Laser absorption is the negative 
 

 

⎛ V e ⎜ i ⎞ inv rse log of transmittance, or − log ⎟ . The slope ⎜ ⎟ ⎝Vo ⎠

(188.88) is programmed into the EC monitor firmware to 

convert absorption to concentration in real time. 

3.2 Drop Testing and Sensitivity Results 

The results of the drop tests demonstrated that the EC 

monitors are not prone to interferences from sudden shakes 

or drops. While sampling clean air, the sensor voltages 

remained stable for both monitors throughout the testing 

cycle. In addition, the limits of detection and quantification 

(7) for the following EC measurements are charted in 

Table 1: time weighted average (TWA), 5-minute average, 

10-minute average and 15-minute average. 

Blanks using EC Monitors 

8 hr TWA blank 

(ug/m3) 

5 min 

average 

(ug/m3) 

10 min 

average 

(ug/m3) 

15 min 

average 

(ug/m3) 

Test 1 -1.60 -19.29 -9.65 -12.85 

Test 2 -3.98 -1.93 -9.65 -6.43 

Test 3 -2.00 0.00 -9.65 -5.14 

Test 4 2.02 0.00 -6.75 -6.43 

Test 5 0.00 -9.65 -7.72 -6.43 

Test 6 -1.00 11.58 5.79 5.79 

Test 7 -0.40 -1.93 -9.65 -3.86 

Test 8 -2.00 -19.29 -9.65 -12.85 

Test 9 -2.00 -19.29 -19.28 -10.29 

Test 10 -3.98 -19.29 -4.82 -4.50 

stdev 1.80 11.04 6.17 5.36 

LOD 5.40 33.13 18.51 16.08 

LOQ 18.00 110.45 61.70 53.61 

Table 1. LOD and LOQ results 

3.3 Personal Sampling Case Study 

The real time readings measured by one of the researcher’s 

EC monitors is shown in Figure 2. The data show that 

researcher was exposed to approximately 200 μg/m3

 EC in 

the return, 250 μg/m3

 EC at the first working area (Face 1), 

150 μg/m3

 EC at the second working area (Face 2) and 30 

μg/m3

 EC at the intake. The EC time weighted average 

(TWA) from the EC monitor data during this six-hour time 

period was 110 μg/m3

. Later, NIOSH 5040 laboratory 

analysis on the standard samples yielded a TWA EC 

measurement of 112 μg/m3

 for the same time period. 

However, the results from the EC monitor are much more 

valuable, because they show the increased or decreased 

levels of exposure that the researcher encountered as he 

traveled to different areas of the mine. 

3.4 Cab Sampling Case Study 

A portion of the cab sampling case study results shows 

how real time measurement can better account for 

unforeseen sources of error. The graphs below (Figures 3 

and 4) show the interior and exterior EC monitor results 

from the same cab on the same vehicle but on two days 

with two different drivers. During the first test, the 

Figure 2 A researcher's EC exposure using a 15-minute 

running average as recorded by the EC monitor. 

measured cab efficiency was about 73% (based on an 

average EC concentration of 104 ug/m3

 inside the cab and 

an average EC concentration of 378 ug/m3

 outside the cab) 

(Figure 3). The measured efficiency for the same cab 

during the second test was 93% (Figure 4). 

Even if there were a leak in the filtration system, we 

would not expect cab efficiency to change drastically from 

day to day. However, the real time EC monitor results 

show large, unexpected peaks in EC concentration levels 

inside the cab at various times throughout the day (Figures 

3 and 4). From this, we can determine that the cab operator 

in test 1 must have either opened the door of the vehicle or 

had the window of the vehicle open three times throughout 

the shift, marked by the three spikes in interior EC 

concentration. The operator in test 2 only opened the door 

or window once, as indicated by the spike at the beginning 

of the test. This simple, but highly pertinent test flaw 

caused the discrepancy in cab efficiency results. With the 

EC Monitor in use, these time periods when a cab door or 

window is open can be recognized and discarded, and thus 

the actual efficiency can be more accurately determined. 

3.5 Area Sampling Case Study 

A sample of the EC monitor results from one of the 

working areas in the area sampling case study is shown in 

Figure 5. During this particular test, the standard laboratory 

analysis concluded that the time weighted average (TWA) 

EC concentration for six hours was 653 μg/m3

. The EC 

monitor measured a similar TWA of 704 μg/m3

 EC for the 

same test. However, the EC monitor’s real time data show 

that the EC concentration slowly built up over time (Figure 

5). 

This might indicate one of two things: either the 

ventilation was ineffective, slowly causing EC 

concentration to build up throughout the day or the number 

of vehicles operating at the working area had increased 

throughout the day. Since the number of vehicles was 

known not to change, it was determined that poor 
ventilation was the cause and that more air needed to be 

redirected to the working area to further dilute the EC 

concentration. This analysis would not have been possible 

with the laboratory results alone. 

600 

700 

800 inside cab 

outside cab 

E C (u g /m 3) 500 

400 

300 

200 

100 

0 

8:24 9:36 10:48 12:00 13:12 14:24 15:36 

Time 

Figure 3 Interior and exterior cab EC concentration using a 

5-minute running average during test 1. 

800 

outside cab 

700 

inside cab 600 

500 

E C (ug/m 3) 400 

300 

200 

100 

0 

8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48 

Time 

Figure 4 Interior and exterior cab EC concentration using a 

5-minute running average during test 2. 

4 Conclusion 

NIOSH has recently developed a portable device that 

measures EC concentration in real time by an optical 

absorbance method. In certain cases, this device may 

eliminate the need for post laboratory analysis. As shown 

in the cab sampling case study, errors can occur in the 

standard method when averaging over an eight-hour time 

period. These errors may even go undetected. Since the EC 

monitor provides near real time analysis, it has the ability 

detect these errors and improve the accuracy of the current 

system. 

This EC monitor can be worn by a mine worker to 

provide a real time measurement of individual EC 

exposure as well as shift weighted average. It can also be 

used to monitor ventilation and measure cab filtration 

efficiency. 

The EC monitor provides important information that is 

not attainable with the standard method. It is highly 

portable, lightweight and accurate. Because of these 

factors, it can prove to be a vital tool when monitoring
 

DPM in underground mines. 

1400.00 

1200.00 

1000.00 

800.00 

600.00 

EC (ug/m3) 5 min avg

400.00 

200.00 

0.00 

7:12 8:24 9:36 10:48 12:00 13:12 14:24 

Time 

Figure 5 The EC concentration using a 5-minute running 

average over six hours of a shift at a working area in a 

limestone mine. 

References 

Birch, M. E., 2004. NIOSH Manual of Analytical Methods 

(NMAM), Third Supplement to NMAM, 4th Edition. 

DHHS (NIOSH) Publication No. 2003-154; 

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Particulate Matter Exposure of Underground Metal 

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Noll, J. D., Bugarski, A. D., Patts, L. D., Mischler S. E. 

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Elemental Carbon, Total Carbon, and Diesel 

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constitute endorsement by the National Institute for 

Occupational Safety and Health (NIOSH).