Field Studies of CO Migration from Blasting 

By Marcia L. Harris, Michael J. Sapko, and Richard J. Mainiero 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory
 
ABSTRACT 

Since 1988, there have been thirteen documented incidents in the United States and Canada in which 

carbon monoxide (CO) is suspected to have migrated through near surface strata into structures or other 

enclosed spaces as a result of nearby trench blasting or surface mine blasting. From these incidents, 

there are thirty suspected or medically verified CO poisonings and one fatality associated with these 

incidents. To better understand the variables contributing to this hazard, the National Institute for 

Occupational Safety and Health (NIOSH) carried out a small-scale field study to identify key factors 

that may contribute to the migration of CO through the ground. NIOSH conducted single-hole heavily 

confined experimental test shots to measure the radial pressure pulse as it propagated through 

surrounding monitoring holes to determine if a relationship existed between the pressure pulse and 

resulting CO measurements afterwards. NIOSH also monitored blasts at an airport construction site. 

Following the detonations, the concentrations of CO, nitric oxide (NO), and nitrogen dioxide (NO2) 

were monitored for several days in the surrounding satellite holes. The peak CO measurements relative 

to distances from the shots were compared. Pressure histories and peak particle velocities (PPV) were 

measured at monitoring boreholes. It was found that the differential pressures recorded within the 

boreholes give some indication as to the likelihood of CO appearing, whereas the PPV measurements do 

not seem to be as good an indicator for potential CO migration. In this limited study, changes in 

barometric pressure had very little influence on removing the trapped detonation products, whereas 

applying a vacuum to the monitoring holes was effective in reducing the entrapped CO concentration. 

INTRODUCTION 

Toxic gases such as CO and the oxides of nitrogen (NOx) are produced by the detonation of explosives. 

The adverse health affects and methods for minimization of such products have been studied for 

decades.1,2 Early research addressed toxicity issues associated with blasting in confined spaces such as 

underground coal mines. Over the past decade there has been an increased interest in the toxic gases 

that are released during some large blasts in surface mines. More recently, the mining and construction 

industry has been concerned with gaseous toxic detonation products that may travel laterally through 

near surface strata and soil rather than be vented to the atmosphere. Since 1988, there have been 13 

documented incidents in the U.S. where explosive-generated CO moved through the earth and 

accumulated in nearby structures and other underground enclosed spaces.3,4,5,6 In each case, the 

overburden heavily confined the explosives in the blasts, restricting the venting of the associated 

byproduct explosive gases to the atmosphere. All the blasts associated with these incidents were in or 

near residential areas and none of the areas were excavated immediately after the blast. Five of the 

blasts were within 20 to 50 ft (6 to 15 m) of the structures or other enclosed underground spaces, three 

were 100 to 150 ft (30 to 46 m) away, and one was 400 to 500 ft (120 to 150 m) away. 

BACKGROUND 

Martel et al.4

 studied the migration of CO from blasting operations in civil engineering operations such 

as construction of piping systems under roads, pools, houses, and buildings. Their paper summarizes the 

CO intoxication cases that occurred in the Province of Québec following blasting activities. The 

primary objectives of their investigations were to evaluate the production of CO and identify the 

migration mechanism through fractures induced in the rock following the blasting of a house basement. 

They found that the structural geology of the rock formation plays a role in the direction and distance of 

gas propagation in fractures generated when blasting with explosives. Also, the type of blast 
confinement can affect the quantity of gas migrating through the fractured rock. They suggested that 

advection is the initial mechanism of CO migration in both blast induced fractures or naturally occurring 

joints present before the blast. The distance of migration by this mechanism is short (16 to 26 ft [5 to 8 

m]). In trenches of mucks (fills under roads), the distance of CO migration by advection is 39 to 66 ft 

(12 to 20 m). In the 2 to 3 days following the blast, further CO migration of up to 49 ft (15 m) was 

controlled by diffusion in the induced fractures in trenches. Finally, they found that the pathways were 

underground conduits broken by blasts and fill around underground conduits in a road trench, house 

trench, or fractured rock created by the blast. 

Martel also evaluated procedures that might minimize the travel of CO. These procedures were: (1) 

excavation and refilling (ventilating) of the muck and the overburden immediately after each blast, (2) 

installation of vents in the rock around the blast, (3) modification of the blasting sequence, and (4) 

modification of explosive type. None of these procedures were very effective. One mitigation 

technique that seemed to be effective in minimizing the CO migration after a blast was removing the 

detonation gases from within the muck by attaching pumps to boreholes and pulling the fumes out of the 

ground. 

The authors also recommended that CO detectors be put inside every house located within a radial 

distance of about 100 ft (30 m) from a blast to warn people if significant CO concentrations begin to 

infiltrate their homes. They suggested that the distance should be increased to about 250 ft (75 m) from 

the blast along roads with underground public utilities. This recommendation is consistent with an 

earlier NIOSH Hazard Alert publication recommending that CO monitors be placed in confined spaces 

near a blast site until better techniques to protect personnel and the public could be developed.7 

Due to an increased frequency of CO migration incidents over the past several years, NIOSH’s 

Pittsburgh Research Laboratory (PRL) has conducted additional research focused on the reduction and 

mitigation of CO production and migration from trench blasting operations. Initial laboratory research 

examined the relative amounts of toxic fumes produced from unconfined detonations of several 

explosive types that were involved in some of the reported CO poisonings. Results indicated that all 

explosives produced CO at rates ranging from 0.149 ft3

/lb (9.3 l/kg) to 6.3 ft3/lb (393 l/kg).8 

The primary focus of the current field studies is to help answer five questions posed by explosive 

manufacturers, blasters, regulators, and researchers: 1) Is there an obvious relationship between 

measured PPV at a given location surrounding a trench blast and the subsequent post-blast CO migration 

and accumulation? 2) Is there an obvious relationship between blast pressure history as measured at a 

monitoring borehole and subsequent CO migration and accumulation? 3) How effective is negative 

pressure at reducing the CO concentration trapped in voids within the blasted material and surrounding 

strata? 4) Is the CO measured at the remote monitoring hole associated with the initial arrival of the 

pressure pulse at that location, or is the CO transport away from the detonation controlled by post 

detonation diffusion? 5) Do changes in the barometric pressure significantly influence the rate of CO 

dissipation? 
 

EXPERIMENTAL APPROACH 

NIOSH PRL FIELD SITE 

Field studies were conducted at the NIOSH PRL site and at a location in Latrobe, PA. At the PRL site, a 

small-scale study was conducted using nineteen 21⁄2-in (6.35-cm) diameter monitoring holes drilled 

approximately 10 ft (3.05 m) deep and approximately 5 ft (1.52 m) apart (Figure 1). The holes were 

lined with 10-ft (3-m) long 2-in (5-cm) inside diameter (ID) PVC pipe (Figure 2). The bottom 5 ft (1.5 

m) of the pipe was perforated with multiple rows of 1⁄4-in (0.64-cm) diameter holes spaced at 3-in (7.62­

cm) apart vertically every 90° around the pipe circumference to allow for infiltration of gases from the 

ground. Each monitoring hole was equipped with a PVC cap that included a port for installing two 1⁄4-in 

(0.635-cm) OD sampling tubes. One line was extended approximately 8 ft (2.43 m) into the hole and 

the other was placed at a depth of approximately 1 ft (0.3 m). 

Prior to each shot, instrumentation for acquiring pressure and vibration data were set up. Pressure 

measurements at the time of detonation were obtained from pressure transducers placed on the short (1 ft 

[0.3 m] depth) tube. A total of 8 transducers were used to measure pressure histories at surrounding 

monitoring holes. Figure 1 shows the placement of mini-seismographs and pressure transducers. In 

Hole L, 3.74 lb (1694.5 g) of dynamite were detonated. The explosive was placed at the bottom of the 

hole and the remainder of the hole was stemmed with sand. 

After the detonation, the pressure transducers were removed from the various holes and on-line gas 

analyzers were installed. Two Testo 350sa

,

a

 Reference to specific products does not imply endorsement by NIOSH. 

 electrochemical cell-based gas analyzers, were used to 

periodically determine the CO, NO, NO2, and O2 in the holes surrounding the shot. To avoid sample 

dilution in the monitoring hole, the analyzer pump drew the sample from the tube placed near the bottom 

of the hole while the exhaust from the analyzer was returned to the top tube, thus maintaining a closed 

system. The instruments were allowed to sample for more than 2 minutes at a rate of 0.0423 cubic 

ft/min (CFM) (1.2 liters/minute [L/min]) before stabilized measurements were recorded. Three 

consecutive measurements were recorded at each location before the instrument was relocated to another 

hole. 

Gas mixtures consisting of 800 ppm CO, 300 ppm NO, and the balance N2 were also injected at a rate of 

0.46 CFM (13 L/min) into the ground at Hole M and the concentration was measured at Hole N. The 

gas was injected through the longer tube and the flow was regulated using a needle valve and flow 

meter. The short line of the injection hole was capped to prevent gases from venting. When applying 

vacuum (negative pressure) to Hole A, D, G, or H, a vacuum hose was inserted into the PVC pipe and 

taped to prevent leaks. A 0-50 in (0-127 cm) water column (W.G.) differential pressure gauge was 

installed in the vacuum hose to measure the hole vacuum. 

LATROBE FIELD SITE 

An opportunity for a large-scale field study of blasting induced CO migration was available in Latrobe, 

PA. A total of eight shots at a construction site were monitored by NIOSH. The blaster loaded the first 

six blasts with an ANFO/Emulsion blend. Figure 3 shows the pattern and placement of holes relative to 

the shots. The first shots consisted of 61⁄2-in (16.5-cm) diameter holes drilled in a 15 ft by 15 ft (4.6 m 
by 4.6 m) pattern to a depth of 40 ft (12m). The holes were double primed with 13⁄4-lb (793.8-g) cast 

boosters and loaded with 378-520 lbs (171.5-235.9 kg) of blasting agent. Drill cuttings were used to 

stem the top 15 feet (4.6 m) of the hole. The holes varied in depth from 35 ft to 40 ft (10.6 to 12 m). 

The gas sampling boreholes were drilled to the depth of the blast holes, on average 40 ft (12 m), which 

was the final grade for the construction project (see Figure 3 for sample hole placement). The top 5 ft 

(1.52 m) of each monitoring hole was lined with a length of 4-in (10.2-cm) diameter PVC pipe and drill 

cuttings were used to fill the annulus between pipe and drill hole, limiting the exchange of gases with 

the atmosphere. Figure 4 shows the construction of the borehole liners. 

The borehole liners were made from a 5 ft (1.5 m) length of 4-in (10.2-cm) ID PVC with a 4-in (10.2­

cm) toilet floor flange cemented approximately 1 ft (0.3 m) from the end of the top of the pipe. This 

provided a lip that would support the pipe above the hole by two short pieces of 2-in by 4-in (5.1-cm by 

10.2-cm) wood. A 4-in (10.2-cm) PVC cap with a screwed fitting was installed at the end of the pipe 

above the toilet flange (approximately 1 ft [0.3 m] from the toilet flange). Two holes were drilled into 

the cap for insertion of 1⁄4-in (0.635-cm) OD gas sampling tubes to depths of 25 ft and 5 ft (7.6 m and 1.5 

m). At the bottom end of the liner, a 4-in (10.2-cm) PVC sleeve with a fluted skirt was installed. The 

skirt caught the drill cuttings that were backfilled around the pipe to provide an effective seal to limit the 

exchange of borehole gases with the atmosphere. 

Prior to each shot, instrumentation for recording borehole pressures and vibration data were set up. 

Pressures at the time of detonation were measured using +/- 15 psig (pounds per square inch gauge) 

differential pressure transducers placed on the short (5 ft [1.5 m] depth) tube with the other 25 ft (7.6 m) 

tube referenced to atmospheric pressure. Mini-seismographs were also placed on the surface close to 

selected monitoring holes to measure PPV. 

Table 1 - Mini-Seismograph and Pressure Transducer Placement at Latrobe Field Site. 

Shot MiniSeismograph 

Pressure 

Transducer Shot MiniSeismograph 

Pressure 

Transducer 

1 Hole 1 Hole 1 5 Hole 8 Hole 8 

Hole 2 Hole 2 Hole 9 

Hole 3 Hole 3 

Hole 4 Hole 4 6 Hole 6 Hole 5 

Hole 7 Hole 6 

2 Hole 2 Hole 1 Hole 8 Hole 7 

Hole 3 Hole 2 Hole 9 Hole 8 

Hole 4 Hole 3 Hole 9 

 Hole 4 

7 Hole A Hole A 

4 Hole 5 Hole D Hole B 

Hole 6 Hole C 

Hole 7 Hole D

Hole 8 Hole E 

 

5 Hole 5 Hole 5 8 Hole F Hole F 

Hole 6 Hole 6 Hole G Hole G 
 

Table 1 - Mini-Seismograph and Pressure Transducer Placement at Latrobe Field Site. 

Shot MiniSeismograph 

Pressure 

Transducer Shot MiniSeismograph 

Pressure 

Transducer 

Hole 7 Hole 7 Hole H Hole H 

After each shot, the gases within each of the monitoring boreholes were measured using MSA FiveStars 

with pump attachments and two Testo 350s. One or both of the Testo 350s were set up to record gas 

concentration data (CO, NO, NO2, and O2) overnight with an average reading being logged at least 

every 30 minutes. The gas probe from the respective analyzer was attached to the longer tube to sample 

gas at 24 ft (7.3 m) below the surface. CO, NO, NO2, CH4, and O2 concentrations were measured 

periodically at each monitoring hole using MSA FiveStars. Gas samples were also collected in (0.92-in3 

[15-ml]) Vacutainer glass sampling bottles for subsequent gas chromatograph (GC) analysis. The GC 

analysis was used to verify CO concentrations and also identify any O2, H2, CO, CH4, and heavier 

hydrocarbons through C5 that might have been present in the detonation products. 

The final 2 shots (7-8) monitored at the Latrobe field site were substantially different from the first 6 

shots. Shallower hole depths ranged from only 10 to 12 ft (3 to 3.65 m). There were more rows and 

holes per row in a 12 ft by 12 ft (3.65 m by 3.65 m) pattern as shown in Figure 5. Thirty-five pounds 

(15.9 kg) of emulsion/AN packaged explosive were loaded in each hole with 7-9 ft (2.1-2.74 m) of drill 

cuttings for stemming. The same borehole liner configurations were used as for shots 1-6, except the 

gas sample tubes only extended to depths of 10 ft (3.05 m) and 1 ft (0.3 m). 

RESULTS AND DISCUSSION 

PRL FIELD SITE 

SHOT DATA 

A preliminary shot at the PRL field site used Hole O as the loaded hole (see Figure 1). The detonation 

hole for the shot discussed here was Hole L with all other holes as monitoring points. The pressures in 

selected boreholes were recorded along with peak particle velocity (PPV) at various locations (Figure 1). 

Figure 6 shows a typical borehole pressure history and PPV measured at the top of Hole A. The initial 

rise in the borehole pressure history occurs at the same time as the seismic signal arrives at the top of the 

hole. At Hole A, the PPV peaks at just over 4 in/s (10 cm/s) as the shock wave radiates away from the 

blast site. Within 0.2 seconds after the blast, there is very little seismic activity remaining. As the shock 

wave passes through the ground and intersects the hole, there is a sharp rise in pressure to about 0.13 

psig (0.896 kPa). The initial gas pressure pulse, due to the shock wave interaction with the borehole, 

then decays, as does the seismic activity, at about 0.2 seconds after the blast. Following this initial 

decay, the pressure builds more slowly to about 0.15 psig (1.03 kPa) and then decays to almost zero 

pressure in about 2 seconds as the detonation gases cool. This longer duration gas pressure pulse is 

believed to be associated with the compression of the gases within the fracture zone surrounding the 

blast hole. The maximum gas pressure decreases with increasing distance from the hole as is shown in 

Table 2, which is a composite of all measured pressure histories around shot Hole L. 
Table 2 – Peak Pressure, Curve Area, 

and Next-Day CO Measurements at PRL. 

Hole Next-Day CO 

Measurements, ppm 

Peak Pressure, 

psi (kPa) 

Pressure Curve Area, 

psi-sec (kPa-sec) 

E 1055 0.54 (3.72) 0.47 (3.24) 

N 276 1.07 (7.38) 0.55 (3.79) 

J 44 0.29 (2.00) 0.33 (2.28) 

C 13 0.44 (3.03) 0.39 (2.69) 

G 0 0.31 (2.14) 0.36 (2.48) 

H 0 0.14 (0.97) 0.27 (1.86) 

A 0 0.15 (1.03) 0.25 (1.72) 

T 0 0.09 (0.62) 0.19 (1.31) 

Immediate post-shot gas sampling did not indicate the presence of CO or other detonation gases. The 

CO did not appear for several minutes, or in some cases several hours, after the detonation. Shown in 

Figure 7 is the CO concentration in Hole M for 3 periods of sampling over 55 hours after the shot. Due 

to logistical problems, it was not possible to record continuous measurements throughout the 55 hours. 

The CO concentration had peaked sometime between 5 hours and 23 hours after the shot while the PPV 

and borehole pressure dissipated in less than 2 seconds after the shot. This suggests that the gas pressure 

pulse recorded at the borehole is not produced from net movement of detonation products to the 

monitoring hole but by the compression of pre-existing gases between the detonation hole and the 

monitoring hole. Therefore the controlling factor for CO migration appears to be the diffusion transport 

of CO. However, the higher the pressures and impulses suggest more open pathways for enhanced CO 

migration between them. 

Because there is a relationship between decreasing barometric pressure and methane out-gassing from 

coal mines, significant changes in barometric pressure have to be considered as a factor that might affect 

the retention of CO within the ground and its migration.9,10,11,12 One would expect to see similar 

influences on CO retention and migration. 

Figure 8 displays the rise of CO concentration in Hole M and the barometric pressure on the same day 

after the shot. In this test, the CO concentration within the hole more than triples from 4,000 ppm to 

about 14,000 ppm in about a 3 1⁄2-hour period while the barometric pressure remains relatively constant 

at about 29.7 in Hg (755 mm Hg). Figure 9 shows a decreasing CO concentration, from 12,000 ppm to 

8,200 ppm, over a 4-hour period in Hole M on the day after the shot at the PRL field site. During this 

same period, the barometric pressure rose slightly from 29.86 in Hg (758.5 mm Hg) to 29.90 in Hg 

(759.8 mm Hg), then returned to its initial value, while the CO concentration decay rate remained 

relatively constant at about 690 ppm/hour. From the data shown in figures 8 and 9, barometric pressure 

did not appear to significantly influence CO migration. 

Shown in Figure 10 is the concentration of CO measured within Hole M. The concentration remained at 

2,200 ppm 72 hours after the shot before starting to decline at a rate of 233 ppm/hr. In an attempt to 

enhance CO extraction, on the fourth day after the shot, a Shop-Vac was used to apply suction to Holes 
H and J. The suction, applied at Hole H 30 ft (9.1 m) away from Hole M, pulled about 96 CFM (2,718 

L/min), producing a borehole pressure of -8 inches (-20.32 cm) of water gauge (in W.G.). Shortly after 

the suction was applied, the rate of CO concentration decline more than doubled from 233 ppm/hr to 580 

ppm/hr. After about 90 minutes, the suction was stopped at Hole H and about 10 minutes later was 

applied to Hole J (20 ft [6.1 m] away from Hole M). The applied suction produced 109 CFM (3,087 

L/min) at a differential pressure of -19 in (-48.26 cm) W.G. The authors expected a higher CO decay 

rate since Hole J was closer to injection Hole M. In fact, the decay was less. This finding indicates 

there may be better or more selective communication pathways between Hole M and Hole H than 

between Hole M and Hole J, even though Hole J was 10 ft (3.05 m) closer to Hole M. When suction 

was discontinued at Hole J, the rate of CO concentration decay at Hole M continued at about 132 

ppm/hr, but was beginning to level off as shown in Figure 10. 

MITIGATION 

To further investigate the effectiveness of borehole vacuum as a mitigation technique, an 800 ppm 

CO/300 ppm NO/balance N2 gas was injected into Hole M. The CO concentration history in Hole N, 10 

ft (3 m) from injection Hole M, was continuously recorded while suction was alternately applied to 

holes G, H, D, and A. Figure 11 displays one concentration history measured at Hole N while the 

vacuum was applied to Hole G. At time zero, the gas injection started at Hole M; about 20 minutes 

later, the CO appeared in Hole N. The concentration climbed rapidly to about 225 ppm and when the 

gas injection was stopped, the CO concentration started to decay at about 7 ppm/hr. About 2 hours into 

the experiment, suction was started at Hole G. The CO concentration in Hole N decayed rapidly at 

about 188 ppm/hr and slowed to about 17 ppm/hr when the vacuum was stopped. When suction was 

applied at Hole A, H, and J, similar results were obtained. 

LATROBE FIELD SITE 

SHOT DATA 

Core samples were obtained at the Latrobe field site to get a better understanding of the soil profile and 

bedrock. The top 26 to 40 ft (7.9 to 15.85 m) of material was to be removed by blasting. The core 

samples indicated that only about 1 to 2 in (2.5 to 5.08 cm) of topsoil existed on the site. Beneath the 

topsoil was 9 to 11 ft (2.74 to 3.35 m) of clayey sand, a thin band of shale and coal, 12 ft of sandy 

siltstone, and finally limestone for the final grade at the bottom. 

Data collected during the shots at Latrobe included recorded peak pressure values, pressure impulse 

curve areas, and CO concentration measurements following the day of the shot (Table 3). Although one 

would think that more CO would be detected in holes located closer to the blast, this did not hold true, 

especially in the case of Hole 8. After shot 4, CO measurements were highest (>2,000 ppm) at Hole 8 

which was farther from the blast than Holes 5, 6, and 7. After evaluation of shots 5 and 6, it is clear that 

there is a preference for CO to migrate towards Hole 8 (the CO in Hole 8 was at least double that 

measured in any other hole), even though the distances to holes 5 through 8 are similar. Detailed 

geology of the area was not available to map the sequence of joints and fracture systems that run through 

the area. However, the contractor encountered numerous voids ranging 8-12 in wide (20-30 cm) wide 

that were empty or filled with mud. These void spaces generally ran in an East-West direction. Data 

was not gathered for shot 3 and is not included in Table 3. 
Table 3 – Peak Pressure, Curve Area, 

and Next-Day CO Measurements at Latrobe Field Site.
 

Shot 

Number Hole 

Distance from 

Shot 

 

Next-Day 

CO 

Measure 

ments 

Peak Pressure Curve Area PPV 

 ft. m ppm psi kPa psi-s kPa-s in/s cm/s 

1 1 100 30.5 42 0.404 2.785 6.662 45.933 6 15 

 2 150 45.7 11 0.159 1.096 4.442 30.627 4 11 

 3 200 61.0 3 0.039 0.270 0.604 4.164 4 10 

 4 250 76.2 2 0.038 0.259 0.290 1.999 3 7 

2 1 32 9.6 17 0.000 0.000 0 

 2 82 24.8 27 0.188 1.296 9.182 63.308 18 46 

 3 132 40.1 18 0.084 0.576 4.985 34.370 9 23 

 4 182 55.3 12 0.056 0.386 1.728 11.914 7 18 

4 5 50 15.2 436 0.000 0.000 20 51 

 6 100 30.5 0.000 0.000 10 26 

 7 150 45.7 106 0.000 0.000 7 17 

 8 200 61.0 >2000 0.000 0.000 4 10 

 9 215 65.5 27 0.000 0.000 0 

5 5 127 38.7 241 0.248 1.710 7.993 55.110 7 17 

 6 129 39.3 0.115 0.793 7.530 51.918 7 18 

 7 127 38.7 4 0.146 1.007 2.250 15.513 7 17 

 8 125 38.1 8895 0.534 3.682 10.523 72.554 7 18 

 9 68 20.7 301 -0.162 -1.117 6.242 43.037 0 

6 5 51 15.4 78 -0.052 -0.361 5.629 38.811 0 

 6 48 14.5 -1.540 -10.618 11.805 81.393 15 38 

 7 43 13.2 1050 -5.350 -36.887 11.010 75.911 19 48 

 8 45 13.7 4732 0.186 1.282 3.087 21.284 22 55 

 9 43 13.1 558 -1.040 -7.171 4.870 33.577 24 61 

7 A 60 18.3 23 -0.021 -0.145 0.240 1.655 3.85 10 

 B 30 9.1 1699 -0.072 -0.498 0.864 5.957 0 
 

 

 

 

 

 

 

Table 3 – Peak Pressure, Curve Area, 

and Next-Day CO Measurements at Latrobe Field Site.
 

Shot 

Number Hole 

Distance from 

Shot 

Next-Day 

CO 

Measure 

ments 

Peak Pressure Curve Area PPV 

ft. m ppm psi kPa psi-s kPa-s in/s cm/s 

C 30 9.1 23 -0.097 -0.672 1.081 7.453 0 

D 60 18.3 19 -0.034 -0.232 0.252 1.737 3.95 10 

E 45 13.7 12 -0.007 -0.050 0.177 1.220 0 

8 F 15 4.6 227 -0.917 -6.322 1.049 7.233 17 44 

G 29 8.8 5720 -0.889 -6.129 13.452 92.748 13 32 

H 76 23.2 11 -0.061 -0.421 1.420 9.791 5 12 

PRESSURE HISTORIES 

Shown in Figure 13 are the pressure histories that were recorded at holes 5, 6, 7, 8, and 9 from the 

detonation of about 23,900 lbs (10,841 kg) of explosives during Shot 4. Figure 3 is the plan view of 

blast holes and satellite monitoring holes for the Latrobe field site. Hole 8, which is 125 ft (38.1 m) 

from the edge of the blast, yielded the highest pressure of about 0.534 psi (3.68 kPa), while Hole 9, 

which was closer, showed about -0.162 psi (-1.117 kPa). Figure 13 shows that the pressure stays up for 

quite some time. It may be due to the entrapped detonation gases cooling and the underground pressure 

within the fractures equilibrating. 

In contrast, Figure 14 is the satellite borehole pressure histories for Latrobe shot 6, where all monitoring 

holes were closer to the last row of shot holes (about 43-50 ft [13.1-15.24 m]) than in previous shot 5. 

Notice that only Hole 8 showed a quasi-static positive pressure of about 0.18 psi (1.24 kPa) whereas for 

holes 5, 6, 7, and 9 the pressures were significantly negative or below ambient pressure. The negative 

pressure swing may be due to additional void spaces being created as the explosive detonates, fracturing 

and lifting the ground. The additional volume creates lower pressure, until air and gases can flow in and 

bring the pressure back to atmospheric. 

Because Hole 8 showed a dramatic increase in CO as compared to the other holes, holes 5-9 were 

examined with a borehole camera after shot 6. These holes had been drilled to a depth of 40 ft (12.2 m). 

The camera recorded perpendicular to the wall of the hole. Water infiltrated all of the holes at 

approximately 23 ft (7 m) below grade. Hole 6 had a water level of 26 ft (7.9 m) while Hole 7 and Hole 

9 had water levels of 38 and 41 feet (11.6 and 12.5 m), respectively. Hole 8 did not have any water 

accumulation. However, Hole 8 did have a very noticeable vertical fracture. This vertical fracture, 

which would most likely not allow for the accumulation of water, could also serve as a conduit for CO 

movement. 

Motivating this experiment, some researchers and blasters have asked the question: “Is there an obvious 

relationship between measured pressure history recorded at a satellite monitoring hole and the 
subsequent CO concentration or migration rate of CO to that location?” The initial thinking was that a 

higher post-blast quasi-static pressure might indicate the degree of open fractures and communication 

pathways between the shot rubble and the satellite monitoring holes. 

To help answer the above question, Figure 12 displays the pressure curves for all of the holes associated 

with the shot. Hole N and E have the highest pressures and greater area under the curve. They also 

show the highest amounts of CO measured the next day (presented in Table 2). Table 2 shows the CO 

concentration about 24 hours following the shot, the peak pressure value, and the area under the 

pressure-time curve for each of the holes measured. Although the pressure peaks or the impulse are not 

indicative quantitatively of the degree to which CO will appear, they do seem to indicate qualitatively 

that the higher the pressure recorded in the hole, the more CO is likely to appear. 

PRESSURE CURVE AREAS AND PPV 

Figures 15 and 16 show a close relationship between the pressure curve area and the next-day CO 

measurements versus the scaled distance (the distance divided by the cubed root of total weight of 

explosives used in the shot).13 In Figure 15, as the distance increases, the pressure curve area and the 

CO measured in the hole the next-day decreases. The plots of pressure curve area and the CO 

measurement exhibit a similar trend. Although the values in Figure 16 do not exhibit the same curve as 

in Figure 15, the pressure curve area and CO measurements do exhibit a similar trend. The high values 

of about 13 psi-sec (89.63 kPa-sec) and 6,000 ppm CO may be due to an overriding conduit similar to 

what was seen for Hole 8 in the earlier shots. The effects of an open pathway seem to overwhelm and 

obscure any other trend. There appears to be a general trend where the higher the pressure impulse, the 

more CO is measured. 

There are many factors involved that differed between the shots displayed on Figure 15 (shots 1 and 2) 

and Figure 16 (shots 7 and 8). Some of these factors include blasting in a different area, shorter hole 

depths (40 ft vs. 10 ft [12.2 m vs. 3 m]), degree of wetness in the holes, amount of explosives used, and 

time delays used in the shots. 

Figures 15 and 16 also display the PPVs corresponding to the next-day CO measurements and the 

pressure curve areas. Looking at Figure 15 alone, it would seem that the higher the PPV, the higher the 

CO concentration. However, this does not always prove to be true based on the results from shots 7 and 

8 (Figure 16). The high PPV of 17 in/sec (43 cm/sec) of shot 8 is associated with the low pressure curve 

area of 1.049 psi-sec (7.233 kPa-sec). The PPV of 13 in/sec (33 cm/sec) of shot 8 has the highest 

pressure curve area of 13.452 psi-sec (92.748 kPa). 

The data from shot 5 (Figure 17) shows an even more dramatic disparity of PPV and CO. While all the 

holes measure 7 in/sec (18 cm/sec) PPV, the pressure areas varied along with the CO measurements. 

Hole 8 had 4,732 ppm CO and the largest pressure curve area of 10.523 psi-sec (72.553 kPa-sec), while 

Hole 7 had the lowest next-day CO concentration of 4 ppm and the smallest pressure curve area of 2.25 

psi-sec (15.51 kPa-sec). The distance from the shot varied only by 2 ft (0.6 m). Thus, the PPV does not 

appear to be a good indicator of CO migration. 

In addition to the online gas analyzer used in this study to follow the CO concentration over time, gas 

samples were collected on a few of the monitoring holes for subsequent analysis by gas chromatography 
 

(GC). Shown in Table 4 is GC analysis of samples from Hole 7 and 8 after shots 6 and 5, respectively. 

In Hole 8 the sample results show 6,300 ppm CO, 4,500 ppm H2, 0.15% methane, 57 ppm ethylene, and 

30 ppm ethane. 

Table 4 – GC Analysis of Select Monitoring Borehole Gas Samples, Latrobe Field Site. 

Location H2 O2 N2 CH4 CO CO2 C2H4 C2H6 Ar 

ppm % % % ppm % ppm ppm % 

Hole 7 760 3.10 90.74 0.04 890 5.02 6 6 0.93 

Hole 8 4500 8.80 83.10 0.15 6300 5.95 57 30 0.93 

 The presence of high concentrations of CO and H2 can be indicative of either non-ideal 

detonation, the detonation of a fuel-rich formulation, or a combination of both. If the fuel oil was not 

distributed uniformly throughout the prills during the one- to two-hour period before the detonation, 

there may be zones within the prill column where the fuel oil concentration varies from fuel-lean to fuelrich condition. Such variations could produce a less than ideal or uniform detonation. Therefore, less 

efficient and lower temperature combustion would occur. Both would contribute to the formation of 

CO, H2, methane, ethane, and ethylene.14 

When studying explosive detonation fumes, some researchers follow the concentration of CO2 produced 

to measure the efficiency of combustion, but in this study the CO2 concentration measured at a 

monitoring hole is strongly influenced by CO2 lost by absorption in the damp soil, since CO2 is very 

water soluble compared to CO, H2, methane, ethane, and ethylene. Measurement of other detonation 

gases besides CO and NO such as H2, CH4, C2H6, and C2H4 may be helpful in understanding the relative 

in-hole detonation efficiency, in particular when compared with the efficiency and products expected or 

predicted under ideal detonation condition. NIOSH is continuing to study this issue as part of its inhouse research program. 

EFFECTS OF EXCAVATION 

The effect of mucking on the Latrobe shots is illustrated in Figures 18 and 19. In Figure 18, the CO 

concentration in Hole 5 steadily increased for about 22 hours after shot 4 while the barometric pressure 

increased and decayed during this period. At about 22 hours after shot 6 (as indicated in Figure 19), 

mucking operations began nearby. At about this same time, the CO concentration started to decrease. 

In this case, the near surface mucking operations seems to play a key role in reducing the concentration 

of trapped CO whereas the barometric pressure shows little to none. The result can be contrasted with 

data shown in Figure 18, where no mucking occurred during the sampling period. The barometric 

pressure rose and leveled to form two plateaus at about 30.04 in Hg (763 mm Hg) and 30.05 in Hg 

(763.3 mm Hg) approximately 7 and 20 hours after the shot, respectively. Approximately 7 hours after 

the shot, CO appeared in the monitoring hole and steadily increased to almost 500 ppm despite the 

plateaus in barometric pressure. This rise in pressure may have caused by the redistribution of trapped 

gas, producing a rise in CO concentration. 

Figure 20 shows the CO accumulation after the first shot in the 10-ft (3.05-m) deep holes. The top of 

the muck pile was excavated overnight. Holes B and C were monitored and both reached a peak or 

plateau in CO during the night at about 22 hours. Prior to this peak being reached, the CO had increased 

from 0 to 2,500 ppm despite a decrease in barometric pressure. 

After the shot 8 (no excavation), the CO did not dissipate from the ground (see Figure 21). There was 

an initial dramatic increase until it leveled off at about 5,100 ppm. After it leveled, the CO curve 

followed the barometric pressure. When the barometric pressure increased 0.02 in Hg (0.5 mm Hg), the 
CO concentration increased from 5,100 ppm to 6,100 ppm. The excavation of the material may have 

removed all CO from the immediate rubble material but, as supported by the figures, did not 

significantly impact the CO that had already traveled into the cracks and fissures beyond the rubble 

zone. 

SUMMARY 

Preliminary results do not indicate a consistent relationship between measured PPV or borehole 

pressures at monitoring holes and the subsequent migration of CO to those locations. There are general 

trends in the data that suggest a relationship, but unknown variations in geology of rock formation from 

site to site, fracture systems, old trenches, or other anomalies make it impossible to predict or locate the 

potential migration paths away from the shot. 

Based on this study, changes in barometric pressure did not significantly influence the subsurface CO 

migration, whereas the application of suction on holes placed between the shot and monitoring point was 

very effective in reducing the concentration of trapped CO. Based on this result, applying negative 

pressure to a hole or series of holes shows promise as an effective mitigation technique. At the PRL 

site, the rate of CO disappearance with negative pressure application to sealed holes is much more 

effective than letting it naturally dissipate. The effectiveness of this technique was also demonstrated in 

the study of the Amherst, NY incident.6

 Also, the vacuum technique need not be applied to every shot 

but rather when a problem with CO migration is encountered. As in these studies, the CO can persist for 

several weeks if no action is taken to remove the trapped CO. Thus, the CO could remain trapped and 

continue to migrate along available pathways. 

Mucking does appear to help remove some gas but depending on when mucking is done relative to the 

shot, CO may migrate beyond the rubble zone. Mucking at a later stage will not completely remove the 

CO already migrated beyond the rubble area. Studies continue to better understand the effects of 

mucking. 

Clearly, blasters’ awareness is important in preventing future CO poisonings. Monitoring nearby 

enclosed spaces for toxic gases before and after blasting still remains the best recommendation for a first 

approach to intervention and triggering other actions. 

ACKNOWLEDGMENTS 

The work of Cindy Hollerich, Paul Kolarosky, Jerry Morrow, and Rich Sobeck, technicians, who 

instrumented and collected the data at both the NIOSH and Latrobe sites, is gratefully acknowledged. 

The assistance from Wampum Hardware and Atlas Construction was invaluable. Without their help, the 

data gathered at the Latrobe field location would not have been possible. 
A B C D E G F

H 

I 

J 

K 

L 

M 

P 

R 

S 

T 

N 

O 

N 

Pressure Transducer 

Pressure Transducer and Seismograph 

Detonation Hole 

Monitoring Point 5 ft 5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

5 ft 5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

5 ft 

Figure 1 - PRL Sample Hole Layout. 

2 in PVC pipe 

2 in PVC cap 

Cement 

2 1⁄2 in. 

10 ft. 

2 in. 

1/ 4 in. 

Gas Sampling Tube Pressure Monitoring Tube 

3 in. 

(Not drawn to scale) 

8 ft 

Figure 2 - PRL Borehole Liners. 

Shot 1 

Shot 2 

Shot 3 

Shot 4 

Shot 5 

Boreholes are spaced 15’x15’, 35’ - 40’ deep 

5 6 7 8 

9 

1 

2 

3 

4 

Shot 6 

Monitoring Hole 

Shot Hole N 

Figure 3 - Latrobe Field Site, Shots 1-6. 
4 in PVC pipe 

Toilet Flange 

4 in sleeve 

with screw plug Wood Supports 

Drill Cuttings 

Sleeve 

Fluted Skirt 

25 ft. 

Gas Sampling Tube Pressure Transducer Tube 

6 1⁄2 in Borehole 

Diameter 

5 ft 

Figure 4 - Latrobe Gas Sampling Borehole Configuration. 

Shot 7 

Shot 8 

B A

C 

D 

E 

F 

G 

N 

H Monitoring Hole 

Shot Hole 

Boreholes are spaced 12’x12’, 10’ deep 

Figure 5 - Latrobe Field Site, Shots 7-8. 
0.5 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Time, seconds 

Pressure 

PPV 

5 

0.4 4 

Pressure, psi 0.3 3 

0.2 2 

0.1 1 

0 0 

Particle Velocity, in/s 

-0.1 -1 

Figure 6 - Pressure and Particle Velocity vs. Time, Hole A. 

0 

2000 

4000 

6000 

8000 

10000 

12000 

14000 

16000 

0 5 10 15 20 25 30 35 40 45 50 55 

Time (hours elapsed since shot) 

CO Concentration, ppm 

14 

14.2 

14.4 

14.6 

14.8 

15 

15.2 

15.4 

15.6 

CO O2 

Oxygen, % 

Figure 7 – CO and O2 concentrations, Hole M after the shot. 

y = 3288.1x + 2631.4 

R2

 = 0.9904 

0 

2000 

4000 

6000 

8000 

10000 

12000 

14000 

16000 

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 

Time (hours elapsed since shot) 

CO Concentration (ppm) 

754 

755 

756 

757 

758 

759 

CO Barometric Pressure Linear (CO) 

Barometric Pressure (mm Hg) 

Figure 8 - Rise of CO concentration in Hole M on the Day of the shot. 
CO Concentration (ppm) 14000 763 

10000 

12000 y = -691.19x + 27602 

R2

 = 0.9671 762 

8000 761 

6000 760 

2000 

4000 

759 

0 

22.5 23.0 23.5 24.0 24.5 25.0 25.5 26.0 26.5 27.0 27.5 28.0 

758 

Time (hours elapsed since shot) 

CO Barometric Pressure Linear (CO) 

Figure 9 - Decline Of CO Concentration In Hole M The Day After The Shot. 

Barometric Pressure (mm Hg) 

 

2500 771 

Start vacuum on hole H (30') 

-233 ppm/hr Stop vacuum on hole H 

2000 770 

CO Concentration (ppm) 1500 

-580 ppm/hr Start vacuum on hole J (20') 

769 

1000 

-290 ppm/hr Stop vacuum on hole J 

-132 ppm/hr 

768 

500 767 

0 

70.5 71.0 71.5 72.0 72.5 73.0 73.5 74.0 74.5 

Time (hours elapsed since shot) 

CO Barometric Pressure 

75.0 75.5 76.0 76.5 

766 

Barometric Pressure (mm Hg) 

Figure 10 - Negative Pressure Applied as Mitigation Technique, Hole M. 

133 CFM vacuum flow rate, -12 in. W.C., 738.9 mm Hg barometric pressure 

25.0 250

Start vacuum Stop gas O2 

Measured Amount

20.0 200 Measure natural decline
 

R2 = 0.67, -7 slope
 

15.0 

CO Concentration (ppm) 

(% - O2) 10.0
 

Flow gas into ground
 Measure natural decline 

R2 = 0.97, -17 slope 

CO 

50 

Stop vacuum 

5.0 

0.0 0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 

Time (hours) Elapsed Since Start of Gas Injection 

150 

Negative pressure applied 

R2 = 0.98, -188 slope 100 

Figure 11 - Gas injection (Hole M) to evaluate negative pressure mitigation. Vacuum applied to Hole G, CO, O2 

concentration measured at Hole N. 
0 0.2 

Hole N 

Hole E 

0.4 0.6 0.8 

Hole C 

Hole G 

Hole J 

Hole A Hole H Hole T 

Hole pressure, psig

2 

1.5 

1 

0.5 

0 

-0.5 

1 

Time, seconds 

Figure 12 - Pressure History at Each Monitoring Hole, PRL Shot. 

-0.3 

-0.2 

-0.1 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

0 10 20 30 40 50 60 

Time, seconds 

Bore Hole Pressure,psig

Hole 5 

Hole 6 

Hole 7 

Hole 8 

Hole 9 

Figure 13 - Pressure History for Holes 5-9, Latrobe Field Site, Shot 5. 

-6.0 

-5.5 

-5.0 

-4.5 

-4.0 

-3.5 

-3.0 

-2.5 

-2.0 

-1.5 

-1.0 

-0.5 

0.0 

0.5 

1.0 

1.5 

0 5 10 15 20 25 30 

Time, seconds 

Bore hole pressure,psig

Hole 5 

Hole 6 

Hole 7 

Hole 8 Hole 9 

Figure 14 - Pressure History for Holes 5-9, Latrobe Field Site, Shot 6. 
 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

20 

0 2 4 6 8 10 12 

Distance/Weight1/3 (ft/lb1/3) 

Pressure Curve Area (psi-sec), PPV (in/sec)

0 

5 

10 

15 

20 

25 

30 

35 

40 

45 

Shot 1 Area Shot 2 Area Shot 1 PPV Shot 2 PPV Shot 1 CO Shot 2 CO 

CO Measurements (next-day, ppm) 

Figure 15 - Pressure Curve Area, Peak Particle Velocity, and Next-Day CO Measurements vs. Scaled Distance, Shots 

1 and 2. 

 

0 

2 

4 

6 

8 

10 

12 

14 

16 

18 

0 1 2 3 4 5 

Distance/Weight1/3 (ft/lb1/3) 

Pressure Curve Area (psi-sec), PPV (in/sec)

0 

1000 

2000 

3000 

4000 

5000 

6000 

7000 

Shot 7 Area Shot 8 Area Shot 7 PPV Shot 8 PPV Shot 7 CO Shot 8 CO 

CO Measurements (next-day, ppm) 

Figure 16 - Pressure Curve Area, Peak Particle Velocity, and Next-Day CO Measurements vs. Scaled Distance, Shots 

7 and 8. 

0 

2 

4 

6 

8 

10 

12 

4.33 4.34 4.35 4.36 4.37 4.38 4.39 4.40 4.41 4.42 

Distance/Weight1/3 (ft/lb1/3) 

Pressure Curve Area (psi-sec), PPV (in/sec) 

0 

50 

100 

150 

200 

250 

300 

Shot 5 Area Shot 5 PPV Shot 5 CO 

CO Measurements (next-day, ppm) 

Figure 17 - Pressure Curve Area and CO vs. Scaled Distance, Latrobe Field Site, Shot 5. 
 

0 

100 

200 

300 

400 

500 

600 

15 20 25 30 35 

Time (hours since shot) 

ppm CO 

759 

760 

761 

762 

763 

764 

765 

766 

767 

768 

CO Baro. Press. 

mm Hg 

07/20 - 07/21 

Figure 18 – CO Monitoring at Hole 5 After 

Shot 4 (No Excavation After the Shot). 

 

0 

200 

400 

600 

800 

1000 

1200 

1400 

1600 

1800 

2000 

15 20 25 30 35 

Time (hours since shot) 

ppm CO 

759 

760 

761 

762 

763 

764 

765 

766 

767 

768 

CO - Hole 7 - 43 ft Baro. Press. 

mm Hg 

07/29 - 07/30 

Figure 19 – CO Monitoring at Hole 7 After Shot 6 

(Previous Shot Behind It Was Excavated). 

 

0 

500 

1000 

1500 

2000 

2500 

3000 

12 17 22 27 32 37 

Time (hours since shot) 

ppm CO 

759 

760 

761 

762 

763 

764 

765 

766 

767 

768 

CO - Hole B - 30 ft CO - Hole C - 30 ft Baro. Press. 

mm Hg 

08/09 - 08/10 

Figure 20 – CO Monitoring at Hole B & C 

After Shot 7 (The Top Of The Muck Pile Was 

Excavated Overnight). 

0 

1000 

2000 

3000 

4000 

5000 

6000 

7000 

12 17 22 27 32 37 

Time (hours since shot) 

ppm CO 

759 

760 

761 

762 

763 

764 

765 

766 

767 

768 

CO Baro. Press. 

hPa 

08/11 - 08/12 

mm Hg 

Figure 21 – CO Monitoring At Hole G After 

Shot 8 (No Excavation Overnight). 

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2

 Karmakar, N. C. and S. P. Banerjee. A Review of Laboratory and Field Test Methods for Studying 

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pp. 398-402.

3

 Santis, Lon D., A Summary of Subsurface Carbon Monoxide Migration Incidents. Proceedings of the 

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Explosives Engineers, 2001 volume 2-AS, pp. 143-154, 2001. 

4

 Martel, R., L. Trépanier, L. Boutin, M. Lavigne, B. Lévesque, G. Sanfaçon, P. Auger, L. Galarneau, 

and P. Brousseau. Carbon Monoxide Poisoning From Blasting Operations in Construction Works, An 

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5

 Harris, M. L., J. H. Rowland, and R. J. Mainiero, Blasting Related Carbon Monoxide Incident in 

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6

 Harris, M. L., J. H. Rowland, and R. J. Mainiero, CO Migration from Trench Blasting in Amherst, 

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 Technology News 488. Mainiero RJ [2001]. Migration of blasting fumes into a western Pennsylvania 

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8

 Sapko, M. Rowland J. H., Mainiero, R. J., and Isaac Zlochower [2002]. Chemical and Physical Factors 

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9

 Carter HN, Durst CS [1955-1956]. The Influence of Barometric Changes on the Emission of 

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10 Füssell W. Hudewentz D [1974]. Die Barometrische Ausgasung-Beobachtungen Und 

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11 Eschenburg HM [1977]. Note on the Interaction Between Barometric Pressure and Methane Issuing 

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12 Stevenson, JW [1968]. Effects of Bleeder Entries During Atmospheric Pressure Changes. Mining 

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13 Blasters’ Handbook, 17th Edition, ed. Robert B. Hopler, International Society of Explosives Engineers, 

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