147

Evaluation of dust exposure to truck

drivers following the lead haul truck

W.R. Reed and J.A. Organiscak

Mining engineers with NIOSH, Pittsburgh Research Laboratory, Pittsburgh, Pennsylvania

Abstract

Haul trucks have the potential to generate large amounts of respirable dust. This respirable dust has been

shown to be a health hazard to personnel, especially if it has a high silica content. Lack of dust sampling data

from haul trucks prompted the completion of a study to quantify respirable dust concentrations. These field

studies were conducted measuring instantaneous respirable dust at stationary locations from passing haul

trucks at a stone quarry and at a coal mine site. Fugitive dust levels from haul trucks were analyzed to

characterize dust dispersion. This analysis indicates safe following distances for haul trucks and other heavy

equipment to avoid overexposure of respirable dust from the lead haul truck.

Introduction

Almost all surface mining operations use mining haul trucks.

These trucks are used to move material within or from the

mining property. Past research, using the U.S. EPA’s emissions factors for unpaved haul roads, has shown that haul

trucks generate the majority of dust emissions from surface

mining sites, approximately 78% to 97% of total dust emissions (Cole and Zapert, 1995; Amponsah-Dacosta and

Annegarn, 1998; Reed et al., 2001). Observations of dust

emissions from haul trucks show that if the dust emissions are

uncontrolled, they can be a safety hazard by impairing the

operator’s visibility. Reducing the operator’s visibility increases the probability for haul truck accidents. However, the

greatest long-term health hazard of dust generated from hauling operations is due to inhalation of respirable dust, which

contains dust particles whose median diameter is 4.0 μm and

PM10, which is the size fraction whose medium diameter is 10

μm (Soderholm, 1989; ISO, 1993; Lippmann, 1995; U.S.

GPO, 2002).

Exposure to respirable dust has long been considered a

health hazard at surface mining operations, especially if silica

dust is present. Respirable dust containing silica has caused

more than 250 deaths annually (MSHA, 1997). There is also

the potential for 106,000 to 182,000 mine production, development and exploration workers to be exposed to respirable

dust from surface mining operations (U.S. Census Bureau,

2001; NIOSH, 2002). The large range in the number of

workers results from differences in the statistical databases

from the U.S. Census Bureau and the Mine Safety or Health

Administration (MSHA).

An evaluation of a recent MSHA database of respirable

dust samples containing silica from 1996 through 2000 shows

the exposure for truck drivers and road grader operators. At

surface coal mines, approximately 10% of the dust samples

taken exceeded the exposure limit for silica dust for truck

drivers. For road grader operators, that rate was approximately 5% (Hale, 2002). For stone mining operations, the

number of respirable dust samples taken that exceeded the

exposure limit for silica dust was approximately 5% for truck

drivers. The overexposure rate was approximately 29% for

road grader operators (Hale, 2002). At nonmetal mines, the

rates were approximately 5% and 14% for truck drivers and

road grader operators, respectively (Hale, 2002), and the rates

for metal mining operations were approximately 8% and 3%

for truck drivers and road grader operators, respectively

(Hale, 2002).

Although the health and safety concerns for dust overexposure focus on respirable dust, PM10 has also been shown to

represent a health hazard. Many epidemiologic studies have

been completed that show that PM10, by itself, causes harm to

humans. A 50 μg/m3 increase in the 24-hour average PM10

concentration was statistically significant in increasing mortality rates by 2.5% to 8.5% (U.S. EPA, 1996). Long-term

effects from PM10 are dependent on the exposure to PM10 over

the life of the worker.

Of particular concern is the use of haul trucks outside of the

mining industry. There are many off-road and over-the-road

haul trucks that are used in the construction industry. Many of

the over-the-road haul trucks enter quarry sites to obtain

material to deliver it to construction sites, and these trucks

generally use the mine site haul roads. Some of these quarry

sites have a high traffic volume of over-the-road trucks. These

operators can be exposed to high respirable silica dust concentrations during their time at the quarry site. Even though their

time at the quarry site may be minimal, they may return

several times during the day to the quarry for material thus,
148

increasing their potential exposure to respirable silica dust.

The delivery locations are generally construction sites that

also have the potential to expose the truck operators to high

respirable dust concentrations, particularly at highway or road

construction sites. Highway construction sites also use some

of the same heavy equipment as surface mining operations.

This equipment may be operated in close proximity to laborers, equipment operators and the general public. There may be

a high probability for potential exposure of respirable dust to

these personnel at these locations. However, data to substantiate this supposition is almost nonexistent.

There are several dust-control practices currently in use for

haul trucks. These are reducing haul truck speed, watering

haul roads, treating haul roads and maintaining equipment

cabs. Reducing the haul truck speed is a simple control

method. The reduction in dust is attributable to the lower

amount of disturbance to the haul road at lower speeds.

Watering haul roads with a water truck once an hour has

been shown, through past research, to have a control efficiency of 40% for total suspended particulates (TSP). If

watering is increased to once every half hour, the control

efficiency for TSP increases to 55% (Rosbury and Zimmer,

1983). The control efficiency was defined as a comparison of

the controlled (watered) emission rate to the uncontrolled

emission rate. The United States Environmental Protection

Agency (U.S. EPA) reported several test results of watering

haul roads. The results ranged from a control efficiency of

74% for TSP for the 3 to 4 hours following the application of

water at a rate of 2.08 L/m2 (0.46 gal per sq yd) to a control

efficiency of 95% for TSP for 0.5 hours after the application

of 0.59 L/m2 (0.13 gal per sq yd) (U.S. EPA, 1998).

Treating haul roads is generally completed through the

application of chemicals and requires a significant amount of

road maintenance. Control efficiencies were shown to be 95%

for magnesium chloride and 70% for a petroleum derivative

for controlling haul truck generated dust (Olson, 1987).

Maintaining equipment cabs in good operating conditions

also reduces operator exposure to respirable dust. A study

conducted on dozers and drills demonstrated that properly

maintained cabs can attain dust reductions of 90% for drills

and between 44% and 100% for dozers (Organiscak and Page,

1999). The variations of the dust reductions for dozers were

attributed to reentrainment of internal cab dust (Organiscak

and Page, 1999). An additional study completed on haul

trucks, which involved the retrofitting of a cab with a filtration/pressure air conditioning system to produce positive

pressure in the cab, showed that properly maintained cabs can

produce a potential 52% reduction of respirable dust (Chekan

and Colinet, 2003).

Field study

Due to a lack of data characterizing airborne dust generated

from haul trucks, a field study that measured dust from haul

trucks at surface mining operations was completed by

Organiscak and Reed in the summer of 2002. The study was

completed at two different sites: a surface stone quarry in

Virginia and a coal preparation plant waste hauling operation

in Pennsylvania. Both sites had a significant number of trucks

that passed by the sampling instruments. Therefore, analysis

for the potential exposure of haul truck drivers to respirable

dust was completed, in addition to the haul truck dust characterization. It should be noted that the stone quarry site had

more trucks following closer together than the coal preparation plant site.

This study utilized seven sampling stations located at

varying distances from the haul road. However, the analysis of

the respirable dust exposure of truck drivers required only the

review of data from the two sampling stations that were

adjacent to and on opposite sides of the road. The test section

of the haul road was a straight unwatered section of road

approximately 30 m (100 ft) in length. The test section for the

Virginia site had a slight grade and was the main entrance road

to the pit, while the Pennsylvania site was relatively flat and

was the main haul road to the waste dump.

This study used MIE personal data RAMs, MSA Escort

ELF personal sampling pumps, 10-mm Dorr-Oliver cyclones

and BGI GK2.69 cyclones at each measurement location.

Samples were collected on 37-mm filters. To measure respirable dust, Escort ELF personal sampling pumps were fitted

with 10-mm Dorr-Oliver cyclones. The pumps were set to run

at 1.7 L/min to collect the respirable dust sample (Bartley et

al., 1994). The MIE personal data RAMs were used to collect

instantaneous respirable dust concentration measurements.

They recorded dust concentrations every two seconds. The

MIE personal data RAMs were fitted with 10-mm DorrOliver cyclones and were operated at 1.7 L/min. Because the

PM10 size fraction is similar to thoracic fraction of dust

(Lippmann, 1995), BGI GK2.69 cyclones, which measure

thoracic dust, were used to represent the PM10 size fraction.

Thoracic dust was measured using Escort ELF personal sampling pumps fitted with the BGI GK2.69 cyclones. The

sampling pumps for the thoracic portion were operated at 1.6

L/min to collect the PM10 sample (Maynard, 1999). Total dust

concentrations were measured by attaching 37-mm filters

directly to the Escort ELF personal sampling pumps, set to

operate at 1.7 L/min. In addition, Cascade Impactors that

contained six stages and were connected to Escort ELF

personal sampling pumps operating at 2.0 L/min were used.

This flow rate allowed for the measurement of dust concentrations for the cut-off size ranges of 21.3, 14.8, 9.8, 6.0, 3.5 and

1.55 μm (Andersen Instruments Inc., 2002).

The sampling stations (A and B) for this evaluation contained the previously mentioned respirable dust sampler, MIE

Personal Data RAM respirable dust sampler, thoracic dust

sampler, total dust sampler and Cascade Impactor. Weather

data consisting of temperature, both dry and wet bulb, and

barometric pressure were recorded hourly. A wind speed and

direction station was placed nearby and recorded wind speeds

and directions every 30 seconds. Dust measurements were

collected for approximately 6 to 7 hours per day.

A time study was also conducted of the haul trucks using

the haul road throughout the entire time period of the study.

The haul truck time entering and exiting the test section of the

road, type of haul truck and speed and direction of travel were

recorded. The types of haul trucks at the Virginia stone quarry

were mostly various tandem-axle over-the-road trucks, with

some trailer trucks, capable of carrying 18-t (20-st) payloads.

Additionally, some 45-t (50-st) off-road haul trucks were

included in this study. Their average speed during the study

was 7.0 m/s (15.6 mph). The Pennsylvania site operated 45- to

54-t (50- to 60-st) off-road trucks (Cat, Terex and Payhauler)

that traveled at an average speed of 7.1 m/s (15.9 mph). The

road conditions were similar at both sites, with average road

surface material specific gravity, moisture content and silt

content being 2.86, 0.16%, and 22.3%, respectively, for the

Virginia stone quarry. The Pennsylvania site’s average road

surface material specific gravity, moisture content and silt

content were 2.48, 0.62%, and 21.9%, respectively. The

specific gravity, moisture content and silt content were determined according to ASTM standards.
149

Results

Average particle size distributions of dust

generated from haul trucks were calculated from cascade impactor sampler data

from the two samplers located along the

haul road from both sites. The average

particle size distribution of the dust generated by haul trucks during this study is

shown in Fig. 1. Figure 1 reveals that, on

average, 14.5% of the airborne dust generated from haul trucks consists of material <10 μm and 3.5% is material <3.5

μm. The majority (85.5%) of the airborne

dust consists of larger particles that do not

pose a respirable threat to the truck operator, but may be a visibility hazard.

Instantaneous analysis of the airborne

dust concentration data was completed

for both sites. Figures 2 and 3 shows a

typical average instantaneous respirable

dust concentration curve for the airborne

dust generated from haul trucks for Stations A and B, respectively.

Stations A and B were located adjacent to the haul road and on opposite sides. These graphs were

constructed from data from the coal mine preparation plant

study and show the average instantaneous respirable dust

concentrations with their 95% confidence intervals. The confidence intervals were calculated using:

CI x s

n

=± × ⎛

⎝

⎜ ⎞

⎠ 1 96 . ⎟ (1)

where

CI is the confidence interval,

x

– is the sample mean,

s is the sample standard deviation and

n is the number of data values.

The negative time intervals represent the time before the

haul truck arrived at the sampling station, and the positive

time intervals represent the time after the haul truck passed the

sampling station. In addition, percentile plots were created

from the data used to create Figs. 2 and 3. These plots present

the statistics for the median and the 25th, 75th and 90th

percentiles and are shown in Figs. 4 and 5.

Prior evaluations on the instantaneous dust data for the

coal preparation plant showed a linear relationship existing

between the respirable and thoracic dust (Reed, 2003;

Organiscak and Reed, 2004). This relationship was represented as a ratio that was shown to be 3.93 respirable dust to

thoracic dust (3.93 x respirable dust concentration = thoracic

dust concentration). Graphs showing the instantaneous thoracic dust calculated from the instantaneous respirable dust

graphs are presented in Figs. 6 and 7.

Instantaneous dust concentration data from the Virginia

stone quarry were used to analyze dust concentrations with the

time lag of a following haul truck. In this case, there were

numerous trucks passing the sampling stations with the majority of the trucks following a lead truck. Box and whisker plots

are presented of the stone quarry data in Figs. 8 and 9.

These results show the instantaneous airborne respirable

dust concentration data versus time lag in seconds. These

graphs present the instantaneous airborne respirable dust

concentration for the following trucks at the point of passing

the sampling stations, or time zero on Figs. 2 and 3. Figures 10

Figure 2 — Average instantaneous respirable dust

concentrations for Station A of the coal preparation plant

study.

Figure 3 — Average instantaneous respirable dust

concentrations for Station B of the coal preparation plant

study.

Figure 1 — Average size distribution for airborne dust generated by haul trucks

for the entire sampling period.
150

and 11 show a “zoomed-in” version of the box and whisker

plots shown in Figs. 8 and 9.

The box and whisker plots show the data grouped in

percentiles. The horizontal line inside the box represents the

median of the data. The lower end of the box is the 25th

percentile line. All data below this line represents <25% of the

data. Data above this line represents >25% of the data. The

upper end of the box is the 75th percentile line. Similar to the

25th percentile line, all data below this line represents <75%

of the data. Data above this line represents >75% of the data.

The lower “whisker” represents the 10th percentile and the

upper “whisker” represents the 90th percentile.

Discussion of results

The measured particle size distributions from the haul truck

study (14.5% <10 μm and 3.5% <3.5 μm) are much less than

previously determined airborne particle size distributions for

road and soil dust of 52.3% <10 μm and 10.7% <2.5 μm

(Chow and Watson, 1998). The mass fractions of 14.5% <10

μm and 3.5% <3.5 μm are more representative of airborne

dust from haul trucks, than the mass fractions from previous

research. A review of the procedure used to determine the

prior mass fractions (52.3% <10 μm and 10.7% <2.5 μm)

showed that bulk road samples were taken from the unpaved

road and resuspension of the dust was completed in the

laboratory (Ahuja et al., 1989; Houck et al., 1989). In this field

study, the sampling stations were located right on the edge of

the haul road and measured the airborne dust directly from the

haul trucks. Therefore, the prior mass fractions of 52.3% <10

μm and 10.7% <2.5 μm portray the potential mass fractions

for airborne road dust, while the mass fractions of 14.5% <10

μm and 3.5% <3.5 μm depict a more representative description of the mass fractions of road dust that emanate from haul

trucks.

Examination of the instantaneous respirable dust data

shown in Figs. 2 and 3 show that the peak dust concentration

occurs within 20 seconds after the truck has passed the

sampling stations. Figure 2 shows the dust concentrations

starting to peak before the truck arrives at the sampling

station. This effect is caused by varying wind directions.

Sometimes wind directions originate from the same directions

that the trucks originate. This can cause the dust concentrations to precede the truck. The average peaks, within the time

frame of 20 seconds, are generally within 2.0 to 2.5 mg/m3,

showing that a driver following a truck can be potentially

exposed to dust concentrations above the legal limit of 2.0

mg/m3, although these are instantaneous concentrations and

not eight-hour time-weighted-average concentrations. However, exposures can sometimes be significantly higher for the

following truck driver, as seen by the percentile graphs of

Figs. 4 and 5.

The percentile graphs show where the data lie in the

spectrum of the data collection. It can be seen that, at the

peaks, half of the respirable dust concentrations are above

1.76 mg/m3 for Station A and 1.70 mg/m3 for Station B.

Twenty-five percent of the data lie between the median and

the 75th percentile curves shown on Figs. 4 and 5. This shows

that 25% of the peaks occur within the region of 1.76 to 3.71

mg/m3 for Station A and 1.70 to 3.55 mg/m3 for station B. It

can also be seen that 25% of the peaks are greater than 3.71

mg/m3 for Station A and 3.55 mg/m3 for Station B. It can be

concluded from these graphs that the following truck driver

can be exposed to respirable dust concentrations greater than

3.55 mg/m3 for approximately 25% of the time the driver is

following the lead haul trucks.

Figure 4 — Percentile graph of airborne respirable dust

concentrations generated from haul trucks for Station A of

the coal preparation plant study.

Figure 5 — Percentile graph of airborne respirable dust

concentrations generated from haul trucks for Station B of

the coal preparation plant study.

Figure 6 — Estimated average instantaneous thoracic

dust concentrations for Station A of the coal preparation

plant study.
151

Graphs of average instantaneous thoracic dust concentrations were created, as shown in Figs. 6 and 7. These graphs

were created from the average instantaneous respirable dust

concentration graphs of Figs. 2 and 3 by using the 3.93

respirable-thoracic ratio calculated in previous research (Reed,

2003). These two graphs show that the estimated average peak

of thoracic dust is approximately 9.36 mg/m3 for Station A

and 9.16 mg/m3 for Station B and that the following truck

driver can be exposed to these high concentrations within a

time frame of zero to 20 seconds behind a haul truck. Again,

these concentrations are instantaneous concentrations rather

than the 24-hour time-weighted-average concentrations.

The evaluation of the data from the stone quarry in Virginia

was completed in a different manner. Due to the high traffic

volume, instantaneous dust concentration curves representing

dust generation from individual haul trucks could not be

reliably evaluated because of the interference from other

trucks. In this case, the haul truck emissions were examined

using dust concentrations versus time lag. Box and whisker

plots were created as shown in Figs. 8 and 9. Figures 8 and 9

show that the instantaneous respirable dust concentrations for

Station A are lower than the concentrations for Station B. This

is due to the fact that the wind directions were predominately

towards Station B. Therefore, Station B would record a higher

amount of respirable dust than Station A.

It can be seen from the plots in Figs. 8 and 9 that the

instantaneous respirable dust concentrations are lower than

the instantaneous respirable dust concentrations from the coal

preparation plant study. Part of the explanation for this phenomenon is that these plots show the instantaneous dust

concentrations as the following truck passes by the sampling

station. It is seen from Figs. 2 and 3 that the maximum average

respirable dust concentrations generally occur within a time

frame of 5 to 15 seconds behind the truck. Many of the

following truck concentrations in Figs. 8 and 9 did not occur

at the maximum instantaneous concentration. Another explanation is that almost all the trucks at the stone quarry were

over-the-road haul trucks, not off-road mine trucks as in the

coal preparation plant study. It was seen from visual observation that the off-road mine trucks generated more dust than the

over-the-road haul trucks. Although, this cannot be quantified, as separation of the truck type was not completed.

The box and whisker plots do support the premise that the

maximum dust exposure of the haul trucks occurs within the

time frame of zero to 20 seconds behind the lead haul truck

(max. occurs at the 10 to 20 second time period). In addition,

Figs. 10 and 11 show a close-up view of Figs. 8 and 9,

respectively. Figure 11 shows that the following truck driver

can be exposed to instantaneous respirable dust concentrations in the range of 0.75 to 2.75 mg/m3, excluding the outliers

for approximately 15% of the time that they follow 10 to 20

seconds behind a haul truck. Additionally, 10% of this time

the respirable dust concentrations can exceed 2.75 mg/m3.

Using this information, an additional control technique can

be implemented to help protect truck drivers from overexposure. This technique consists of monitoring the locations of

the haul trucks. By instituting traffic-control procedures to

require haul trucks to maintain more than 10 to 20 seconds of

separation, there should be a resultant reduction in dust

exposure to the haul truck operator.

In many cases, such as in large open pits with many loadout locations, implementing such a policy may not be economically feasible. It may also not be feasible for road grader

operators to avoid the trucks in their duty of removing material

from the haul roads. In these cases, the cabs and air conditionFigure 7 — Estimated average instantaneous thoracic

dust concentrations for Station B of the coal preparation

plant study.

Figure 8 — Airborne respirable dust concentrations at

Station A for various haul truck time lag intervals from the

stone quarry study.

Figure 9 — Airborne respirable dust concentrations at

Station B for various haul truck time lag intervals from the

stone quarry study.
152

ing systems should be properly maintained so that in instances where trucks are following one another, the cabs have

a proper seal. However, many cases, such as quarries, overthe-road trucks usually enter the site in groups. It would be

easy enough to install traffic procedures such that trucks enter

the loading area at 20 to 30 second time intervals, instead of

back-to-back, which frequently occurs. One possible method

could involve the use of a traffic light that monitors the traffic

nearby the weighing station. Once a truck passes the light, the

light turns red stopping traffic until 20 to 30 seconds have

passed. The light then changes to green allowing the next

truck to proceed. This could also help alleviate the backup of

trucks at the load-out point, which can also be a source of dust

exposure.

Summary and conclusions

In summary, from respirable and PM10 dust sampling conducted during the study, it was found that airborne dust

generated by haul trucks consists of, on average, 14.5% <10

μm and 3.5% <3.5 μm.

At the coal preparation plant in Pennsylvania, the instantaneous respirable concentration levels ranged anywhere from

0.001 to 21.50 mg/m3 within 5 to 15 seconds after the 45- to

54-t (50- to 60-st) off-road haul trucks traveling at 7.1 m/s

(15.9 mph) had passed. However, the average peak instantaneous respirable concentration level was approximately 2.35

mg/m3 within the same time period. The percentile graphs

showed that a truck driver might be exposed to respirable dust

concentration levels above 3.55 mg/m3 for 25% of the time

that the driver is following 5 to 15 seconds behind other trucks.

At the Virginia stone quarry, the instantaneous respirable

concentration levels ranged from 0.01 to 8.03 mg/m3 within 0

to 20 seconds after the haul trucks had passed. Most of these

were over-the-road tandem axle trucks, with some trailer

trucks capable of 18-t (20-st) payloads, traveling at 7.0 m/s

(15.6 mph). The high traffic volume at this site precluded the

calculation of an average peak respirable concentration level.

However, the box and whisker graphs showed that a truck

driver might be exposed to respirable dust concentration

levels within a range of 0.75 to 2.75 mg/m3 for 15% of the time

that the driver is following 10 to 20 seconds behind other

trucks. Additionally, the respirable concentration levels can

exceed 2.75 mg/m3 for 10% of this time.

From the results of this study, an additional control technique is proposed — one that monitors the locations of haul

trucks with respect to one another. A review of Figs. 2, 3, 8 and

9 shows that the critical time period of following a truck is

from 0 to 20 seconds, with maximum exposure occurring

between 4 and 15 seconds. Implementing a policy to ensure

that trucks do not follow within 20 seconds of another truck

can result in a 41% to 52% reduction in airborne respirable

dust exposure to the following truck.

By reviewing the effects of haul truck generated dust on

following equipment, another method of dust control by using

traffic procedures can be implemented. This knowledge will

help in the understanding of dust exposure for haul truck

operators and give mine operators additional insight as to how

to effectively reduce it.

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