Blasting-Related Carbon Monoxide Incident in Bristow, Virginia 

By Marcia L. Harris, James H. Rowland III, and Richard J. Mainiero 

Abstract 

In the past several years, there have been a number of blasting-related carbon monoxide 

migration incidents. In each case, there have been some common factors that appear to be related 

to carbon monoxide migration. Geology, heavy confinement of the blast, close proximity to the 

affected structure, and open pathways into the affected structure are just a few. 

Prince William County, VA is an area undergoing a boom in suburban development. In the year 

2001, more than 4,050 new homes were constructed within the county.1 In the new housing 

development located in Bristow,VA, trench blasting was utilized to excavate proposed sewage 

and utility lines. On November 8, 2002, a trench blast was conducted for the installation of 

utility lines. Later that evening a carbon monoxide detector alarmed in a nearby house. The gas 

company and fire department sampled for carbon monoxide and measured 100 ppm and more 

than 200 ppm on the first floor and in the basement respectively. Subsequent efforts to prevent 

migration of CO into the nearby homes met with mixed results. The only reliable technique for 

protecting neighbors from CO appears to be the placement of CO monitors in the homes. The 

events are presented and discussed. 
2 

Introduction 

Toxic gases such as carbon monoxide (CO) and nitric oxide (NO) are produced by the 

detonation of explosives. The implications and minimization of such products have been studied 

for decades. Over the past decade there has been an increased interest in the toxic gases that are 

released during some large surface mine blasts. More recently, the mining and construction 

industries have been concerned with toxic detonation products that may travel laterally through 

the earth rather than vent to the atmosphere. Since 1988, there have been 17 reported incidents 

in which explosive-generated CO moved through the earth and accumulated in nearby 

underground enclosed spaces.2,3 The reported incidents involved 39 suspected or medically 

verified CO poisonings and one fatality. In each case, overburden heavily confined the 

explosive in the blasts, restricting the venting of explosive gases to the atmosphere. All the 

blasts were in or near residential areas and none of the areas was excavated immediately. Five of 

the blasts were within 20 to 50 ft (6 to 15 m) of the 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. Likely pathways 

included old trenches filled with porous material intersecting the blast site, horizontal 

sedimentary bedding, unconsolidated material in a horizontal plane, and hill seams together with 

sumps, drains, the gap between a basement floor and wall, and direct openings in structure walls. 

Background 

On Friday, November 8, 2002, in the late afternoon, a trench blast was shot by the development 

contractor for utility lines in a new housing development in Bristow, Virginia. For this particular 

shot, 3.5 in (8.9 centimeter [cm]) diameter boreholes were loaded with a bulk emulsion blasting 

agent to break diabase intrusive bedrock. Three-quarter pound (lb) (0.34 kilogram [kg]) boosters 

were used to initiate the blasting agent. A total of 2,581 lbs (1,170 kg) of bulk emulsion and 116 

lbs (52.6 kg) of primer were used in the blast. The trench was approximately 320 ft (97.5 m) long 

and the stone to be excavated, which consisted of diabase bedrock (hard limestone), ranged 

between 4 and 8 ft (1.2 and 2.4 m) in depth. 

During the evening of November 8, a CO monitor in a home located approximately 90 ft (27.4 

m) from the blast (see Figure 1) alarmed. The residents called the gas company and the local fire 

department of Prince William County. The gas company measured in excess of 800 parts per 

million (ppm) of CO near the water sump in the basement of the house. The fire department also 

took CO readings on the first floor of the house and in the basement. The CO readings were 100 

ppm on the first floor and more than 200 ppm in the basement. Exposures for long periods of 

time at these levels can be hazardous to a person=s health. The NIOSH recommended exposure 

limit (REL) is 35 ppm time weighted average (TWA) for an 8-hour period.4

 In the range of 200 

ppm, slight headaches, tiredness, dizziness, and nausea (after 2-3 hours) may be experienced. A 

person could experience frontal headaches within 1-2 hours when the CO is in the 400 ppm 

range. In the 800 ppm range, dizziness, nausea and convulsions may occur within 45 minutes of 

exposure. If exposed to 800 ppm for 2-3 hours, death will occur.5 

The house was ventilated (by opening doors and windows) to remove the CO. The homeowners 

were able to return to the house later that evening. The next morning, November 9, 2002, around 

7:00 A.M. the fire department returned to the house to take more CO measurements. In the 
3 

basement, the CO reading was greater than 300 ppm. At the water sump, the fire department 

measured 680 ppm. Three neighboring homes were checked for CO at this time. The CO 

readings in these homes ranged from 23 to 370 ppm. These homes were not equipped with their 

own CO detectors. 

It was determined that blasting was the source of CO. This conclusion was based on the number 

of homes affected, the distance between the homes and the blast, the apparent entry routes, and 

the short time delay between blast and first detection of CO. The gas company had initially 

responded and determined the utility was not the source. No other source for the CO was 

identified. 

The second trench blast (see Figure 1) occurred on November 14, 2002. The blast consisted of 

82.5 lbs (37.4 kg) of boosters and 770 lbs (349.3 kg) of packaged cap sensitive emulsion. To 

reduce the possibility of CO entering the four affected homes, the fire department covered the 

water sumps in the homes with plastic and sealed the plastic with duct tape. CO monitors were 

also purchased and placed in each of the homes. After the shot, the CO monitors did not alarm 

in any of the houses. 

The third trench blast (Figure 1) was shot on December 3, 2002, at 11:30 a.m. The trench was 

120 ft (36.6 m) long with boreholes 20 to 27 ft (6.1 to 8.2 m) deep. The shot consisted of 3,504 

lbs (1,605.7 kg) of bulk emulsion blasting agent primed with 167.75 lbs (76.1 kg) of booster. 

Immediately after the shot, four venting pits approximately 6 ft x 6 ft x 5 ft (1.8 m x 1.8 m x 1.5 

m) deep were excavated along the length of the shot to release the trapped gases into the 

atmosphere. Following the shot, the home CO monitor alarmed in house # 2. The fire 

department was called to take measurements. In the basement near the water sump, the CO 

reading was 370 ppm, while in the living area the CO reading was 55 ppm. The levels measured 

in the living areas of the other three homes ranged from 55 ppm to 182 ppm. The next day, CO 

readings taken in the living area of each home ranged from 27 ppm to 226 ppm. Every home 

except one was evacuated. Two days later on December 5, 2002, no CO could be detected at 

house #3. None of the other home owners were available for access to their homes. At this 

point, the fire department voided the blasting permit and would not reissue the permit until the 

department was confident that measures had been put in place to protect the homeowners from 

CO poisoning. 

Several parties agreed to meet on Friday, December 6, 2002, in Bristow to discuss possible 

actions to prevent CO migration into the homes in future shots. Also, by this time, concern had 

spread throughout the neighboring housing development and most residents purchased their own 

home CO monitor. 

All parties agreed that the best solution to prevent CO from migrating into nearby homes was to 

excavate the entire length of the trench down to and into the bedrock immediately after the blast. 

 It was agreed that the present utility line trench would be left open to allow for venting of future 

shots, as well as providing a barrier to the movement of gases toward the homes. The blaster 

also directed his contractor to note the existing sumps, type of foundation/concrete slabs, and 

existing CO monitors in each home in future pre-blast surveys. This was incorporated into the 

new blast plan with other precautions listed below: 
4 

1.	 All structures within 200 ft (60.96 m) of blasting must be surveyed, have CO monitors 

installed if they have open sumps or plumbing, and must be monitored for 48 hours after 

each shot. 

2.	 In front of each shot, approximately 6 to 8 ft (1.8 to 2.4 m) will be excavated all the way 

to grade and then back filled, to provide proper relief of rock and gas movement. 

3.	 The Fire Marshals= office will be notified 24 hours prior to each blast. 

After submitting a revised blasting plan, a blasting permit was reissued. On December 13, 2002, 

a shot was fired to break up rock for excavation of a house foundation pad (Figure 1). The blast 

pattern consisted of about 120 holes, each 7 to 8 ft (2.1 to 2.4 m) deep, loaded with 4.6 lb (2.1 

kg) of explosive primed with 3⁄4-lb (0.34 kg) boosters. The total 630 lbs (285.7 kg) of bulk 

emulsion blasting agent was primed with 101.5 lbs (46 kg) of boosters. To prevent migration of 

CO to the homes that were affected previously, the shot was excavated immediately following 

the blast as required in the new blasting permit. Additionally, the 15 to 25 ft (4.6 to 7.6 m) deep 

storm sewer trenches between the shot and the homes were left open to ventilate any CO to the 

atmosphere before it could migrate away from the blast. 

On Saturday, December 14, 2002, at 5:00 a.m. CO was detected in homes 1 and 2. This result 

was totally unexpected considering that there was an open 15 to 25 ft (4.6 to 7.6 m) deep trench 

between the pad and the homes and the shot was excavated immediately after the blast. After 

this incident, the county would not reissue a blasting permit until a guarantee could be made that 

CO would not enter the homes. No guarantee could be given with the current knowledge and 

information. The blasting company would no longer conduct blasting at this site. This decision 

was encouraged by the blaster=s insurance company. 

Discussion 

Several factors must be present for CO to migrate through the ground and enter an underground 

enclosed space. First, there must be a source. It was determined that blasting was the source of 

CO. This conclusion was based on the number of homes affected, the distance between the 

homes and the blast, the apparent entry route, and the short time delay between blast and first 

detection of CO. Nothing else was identified as a source for CO. 

The composition of explosive detonation gaseous products depends upon the formulation and the 

conditions surrounding the explosive’s use. Carbon dioxide, water vapor, and nitrogen are the 

main products and are always produced. In addition, CO, NO, methane (CH4), and hydrogen 

(H2) may form in large or small quantities. All explosives generate CO and NO.6,7 Using an 

oxygen balanced explosive is important. The less CO produced, the less CO there is to possibly 

migrate into underground spaces. Also, using the appropriate explosive in each blast for the 

conditions present is important. The explosives used were an emulsion blasting agent that was 

supplied in bulk and pumped into the blastholes and a packaged product. The bulk blasting 

agents were used on shots 1, 3, and 4 and were supplied by two different manufacturers, 

according to blasting reports. The explosive is assumed to be oxygen balanced in the production 

process by the manufacturers. If the explosive is used in the manufacturer-recommended 

fashion, then the CO generated should be minimized. 
5 

Another factor affecting CO migration is the available venting of the fumes to the atmosphere. 

In open-pit blasting operations, wind rapidly dilutes the vented gases during the fragmentation 

process and continues to dilute the gases slowly emanating from the muck pile. Generally, 

trench blasting near occupied dwellings is done with heavy explosive confinement, with a focus 

on preventing flyrock and minimizing air blast damage. Consequently, there is little or no 

surface heave or venting for most of the shots. The Institute of Makers of Explosives 

recommends the excavation of muck piles as soon as possible after the shot.8 Past CO incident 

investigations have determined that the muck piles were not immediately mucked out after the 

shot.2,3 

In this study, the first shot was not immediately mucked out, which may have contributed to CO 

migrating through the ground into the homes. Four excavated pits measuring 6 ft x 6 ft x 5 ft 

(1.8 m x 1.8 m x 1.5 m) were installed on the second and third shots to vent any generated 

fumes. No CO accumulated in the nearby homes after the second shot. However, the venting 

pits still were not sufficient to ventilate the detonation fumes of the third shot. CO appeared in 

the homes the same day as the shot. 

Obviously, a pathway must be present. However, the pathway may not be obvious. It is hard to 

discern or predict fractures in the material to be shot. Currently, there is no practical way to 

characterize the ground strata and be able to predict possible routes of migration. If knowledge 

is available on locations of current and previous utilities, trenches, drains, etc., then those may be 

considered as possible routes. Suspected pathways from blast sites to structures have been cited 

as old trenches, fracture zone, hill seams, horizontal bedding plane, unconsolidated material, and 

a water path.2,3 

Before the meeting on December 6, the blaster took NIOSH personnel to the construction site 

where the incidents occurred to assess the geology and their blasting operation. In the venting 

pits, fractures with an orientation of approximately 20 degrees east of north were observed in the 

bedrock; this orientation lines up perfectly to intercept the homes affected by the CO migration 

after the blast shots 1 and 3 (Figures 2 and 3). Another observation was that the fractures 

appeared to be naturally occurring and not caused by blasting. It was not determined how deep 

the fracture system may extend. It may be entirely possible the fracture system may extend 

below the blast site and trenches. Also, the elevation of the bedrock in the pits excavated after 

blast shot #3 visually appeared to be at the same elevation as the basements of the houses 

affected by the CO migration. It appears that a fracture system is in place that is positioned just 

right to result in a strong conduit from the blasted material to the homes. 

A route of entry into the enclosed space must be present. In the past, routes of entry have 

included sumps, drains, and cracks in the foundation.2,3 In the case discussed here, the existence 

of sumps in full basements was considered to be the route of entry into the homes. Most of the 

other homes surrounding the affected homes did not have full basements; they were constructed 

on a concrete slab without openings to the strata beneath them. CO monitors did not alarm in 

these homes. Even though the sumps were covered and taped, CO was still able to enter the 

homes with basements. It is not possible to completely seal the sump using plastic sheet and 

duct tape. It is also possible that a crack existed in the foundation that was not accounted for. 
6 

In past CO migration incidents, the people affected were victims of CO poisoning. In the 

published case summaries, there have been 39 suspected or medically verified CO poisonings (9 

with recuperation in a hyperbaric chamber) and 1 fatality. Fortunately, in this instance, no one 

experienced any symptoms or had to seek medical treatment because commercially available, 

home-type CO monitors had been purchased and installed by homeowners. The initial home that 

was affected had a home CO monitor installed. Once the problem was identified, the other 

affected homes were equipped with these inexpensive monitors. 

Even after the modified blast plan was put into effect, CO had appeared in the homes after the 

fourth shot. This is puzzling because there was an open trench 15 to 20 ft (4.6 to 7.6 m) deep 

between the affected homes and the location of the blast. One possible explanation is that the 

CO may have been present in the ground from previous blasts and pressure from the last blast 

forced the CO into the homes. Another explanation may be that the trench did not intersect the 

conduit of migration. 

In many past cases of blasting related CO migration incidents, the problem was not recognized 

until after symptoms of CO poisoning were experienced and reported.2,3 In some cases, recovery 

from CO poisoning required medical treatment and hyperbaric chambers. In this case, medical 

treatment was not required. CO poisonings were most likely prevented by early warning 

provided by a homeowner’s installed CO detector. 

Summary 

During pre-blast surveys, the blaster should determine where the likely or open pathways exist 

for potential gas entry into nearby homes. Possible entries may include, but are not limited to, 

sumps, open plumbing, an open wall (dirt floor or unsealed stone wall), an uncased well, or 

cracks in a basement floor or wall. The CO monitor should be located at the most likely point of 

entry. 

Each CO migration occurrence is unique and depends on the route of entry, distance of site from 

CO generation source, and geology. Therefore, possible monitoring of nearby underground 

enclosed spaces may continue for an extended period of time from several hours to a few days. 

It is imperative for construction personnel to coordinate their efforts closely with the blaster. 

Shots should be excavated as soon as possible following detonation. Venting is important for 

relief of potentially harmful toxic gases. 

CO poisonings were most likely prevented by early warning provided by a homeowner’s 

installed CO detector. Because of early warning, the source of CO was determined and affected 

homes were evacuated and closely monitored. Fortunately, no one had to be treated for CO 

poisoning. 

In any accident, there are multiple contributing factors. Any action to prevent or minimize 

should address each of these factors. A blast plan should be developed and implemented in 

situations where possible CO migration is anticipated. It should include methods, materials, and 
7 

sequence of blasting to complete the job safely. Everyone involved with the project should 

understand the plan. The blast plan should specify actions to take if CO migration should occur. 

It is important to understand that techniques may help reduce the chance for CO migration into a 

home, structure, or building, but they do not always work, as in these Prince William County 

events. 

Acknowledgements 

The invaluable assistance of Captain Ernie Little and Captain Gary Gesling of the Prince 

William County Fire Department are greatly appreciated. The cooperation of the blaster and 

contractor is greatly valued. 
Blast

1 

Blast 3 

House 1

House 2

House 3

House 4 

House 5 

Houses affected with CO 

Blasted areas 

Other homes unaffected 

Figure 1 - Locations of houses and blasts. 

Figure 2 - The first of four pits dug at the location of the trench blast in an effort to vent CO. 

Homes 1 and 2 can be seen in the background. 

8 
Figure 3 - A close-up of the bedrock seen in Fig. 2. Note the fractures are oriented towards 

nearby homes. 

9 

References 

1

 Prince William County Government “News from Prince William County Government,” 

http://www.co.prince-william.va.us/communications/pr/press-02/pr-047.htm accessed September 

19, 2003. 

2

 Santis, LD, A Summary of Subsurface Carbon Monoxide Migration Incidents, In: Proceedings 

of the 27th Annual Conference on Explosives and Blasting Technique. Orlando, FL: 

International Society of Explosives Engineers, pp. 133-141. 

3

 Mantel, 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 Earth Odyssey, 2001. 

4

 NIOSH Pocket Guide to Chemical Hazards “Carbon Monoxide,” 

http://www.cdc.gov/niosh/npg/npgd0105.html accessed June 27, 2003. 

5

 Health Affects of Carbon Monoxide Poisoning, http://www.bacharachtraining.com/CO%20Zone1/Health%20Effects.htm accessed June 26, 2003. 
10 

6

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

Engineers, 1998, pp. 48 – 49. 

7

 Harris, ML, M. Sapko, and R. Mainiero, Toxic Fume Comparison of a Few Explosives Used in 

Trench Blasting, , In: Proceedings of the 29th Annual Conference on Explosives and Blasting 

Technique. Nashville, TN: International Society of Explosives Engineers, 2003G Volume 2, pp. 

319-336. 

8

 Institute of Makers of Explosive (IME), IME Guidelines and Recommended Practices AFumes 

from Blasting Operations,@ http://www.ime.org/fumes.htm accessed September 2, 2003.