Dangers of Toxic Fumes from Blasting 

By Richard J. Mainiero, Marcia L. Harris, 

and James H. Rowland III 

Disclaimer: The findings and conclusions in this report are those of the author(s) and do not 

necessarily represent the views of the National Institute for Occupational Safety and Health.

Abstract

This paper reviews the potential hazards posed by the toxic fumes produced by detonating explosives in 

surface mining and construction operations. Blasting operations produce both toxic and nontoxic 

gaseous products; the toxic being mainly carbon monoxide (CO) and the oxides of nitrogen (NOx). The 

quantity of toxic gases produced by an explosive is affected by formulation, confinement, age of the 

explosive, and contamination of the explosive with water or drill cuttings, among others. Techniques to 

protect workers and the public from the potential hazards of explosive-related toxic fumes are 

discussed. These include: 

• Minimizing the quantity of toxic fumes produced. 

• Determining where the fumes may go so workers and neighbors can be moved out of 

harm’s way. 

• Preventing the fumes from moving towards workers and neighbors. 

• Monitoring the air near workers and neighbors so they can be relocated if fumes appear.

• Ventilating structures or confined spaces until CO falls below a hazardous concentration.
Background 

Ideally, the gaseous detonation products of explosives would consist of water (H2O), carbon dioxide 

(CO2), and nitrogen (N2). Due to the kinetics of the chemical reaction, the detonation of explosives in a 

blasting operation also produces toxic nitrogen dioxide (NO2), nitric oxide (NO), and carbon monoxide 

(CO) (ISEE, 1998). The concentrations Immediately Dangerous to Life or Health (IDLH) for NO2, NO, 

and CO are 20, 100, and 1,200 ppm, respectively (NIOSH, 1994). Blasters working in underground or 

confined environments have long been aware of the hazards of these gases and must ensure adequate 

ventilation to quickly dilute them below harmful levels. In an effort to protect workers, extensive 

research has been done on the toxic fumes generated by the detonation of high explosives and many 

countries have test procedures and formal or informal requirements in place for the maximum permitted 

fumes production by a given amount of explosives (Streng, 1971, Karmakar and Banerjee, 1984, and 

International Society of Explosives Engineers, 1998). 

Blasters at surface mines and construction operations have not been as concerned about blasting fumes 

as their counterparts in underground mines, believing that fumes would disperse in the open air (ISEE, 

1998). Surface blasters, however, must be aware that toxic fumes have the potential to create hazards in 

their operations. Large surface mines may detonate up to two million pounds of blasting agent in a 

single shot. Some of the shots produce a product cloud colored red or orange by the presence of NO2

(Barnhart, 2004), (Barnhart, 2003), and (Lawrence, 1995). At present it is not known whether the 

orange cloud contains toxic levels of NO2 since there have been no published reports of direct 

measurements. However, in the interest of safety every blaster should assume that any blasting product 

cloud is unsafe to breathe. 

For surface blasting operations, the CO in the gaseous products released immediately after a blast is not 

of great concern since CO is much less toxic than NO2; the IDLH for CO is 1,200 ppm compared to 20 

ppm for NO2. For CO, the danger lies with the gas that remains in the ground after the blast. This CO 

will be released during loading operations or may migrate hundreds of feet through the ground and 

collect in confined spaces. Since 1988, there have been eighteen documented incidents of CO migration 

in the United States and Canada; the confined space typically being a home and in one case a sewer 

manhole vault (NIOSH, 1998), (Eltschlager, Schuss, Kovalchuk, 2001), (NIOSH, 2001), and (Santis, 

2001). There have been thirty-nine suspected or medically verified carbon monoxide poisonings, with 

one fatality. In one incident in Kittanning, Pennsylvania, blasting fumes traveled 450 feet from a coal 

strip mine into a home, poisoning a couple and their baby. Fortunately, all three recovered following 

treatment in a hyperbaric chamber (Eltschlager et al. 2001) and (NIOSH, 2001). 

Protecting Personnel 

There are a number of ways to protect workers and neighbors from toxic fumes produced by blasting 

operations. Several of these are: 

1. Minimize the quantity of toxic fumes produced, 

2. Determine where the fumes may go so workers and neighbors may be moved out of the way, 

3. Prevent the fumes from moving towards workers and neighbors, 

4. Monitor the air near workers and neighbors so they can be relocated if fumes appear, and 

5. Ventilating structures or confined spaces until CO falls below a hazardous concentration. 

Each of these items will be discussed. 
1. Minimize the quantity of toxic fumes produced.

Due to expansion and subsequent cooling of detonation product gasses, the combustion reactions are 

quenched before they can go to completion. The quenching freezes out CO and NOx at concentrations 

higher than those expected for equilibrium. It is not possible to entirely prevent the release of CO and 

nitrogen oxides (NOx) in blasting, but the quantities can be minimized. Some factors that lead to 

excessive CO and NOx production are incorrectly formulated explosives, use of deteriorated explosives, 

reaction in diameters below the critical diameter, loading wet boreholes with explosives that are not 

water resistant, mixing of explosive with drill cuttings at the top and bottom of the hole, and poor 

confinement (ISEE, 1998), (Rowland III and Mainiero, 2000), (Roberts, Katsabanis, and deSouza, 

1992), and (Engsbraten, 1980). 

An explosive containing a stoichiometric mix of fuel and oxidizer minimizes the production of CO and 

NOx. If there is an excess of fuel, detonation of the explosive or blasting agent will generate increased 

quantities of CO. If there is not enough fuel, detonation of the explosive or blasting agent will generate 

increased quantities of NOx. Figures 1 and 2 illustrate the effect of ANFO fuel oil content on CO and 

NOx production. 

Explosive manufacturers are careful to balance the oxidizer and fuel in their explosive formulations to 

minimize fumes production. Blasters must insure the proper compositions for explosives and blasting 

agents mixed in the field. The performance of modern explosives is controlled by both the composition 

and the physical structure of the chemical mix. Explosives that are beyond the manufacturerrecommended shelf life or visibly deteriorated should not be used. As some explosives age, ingredients 

may leak out of the packaging, changing their compositions or their physical structure may break down. 

Either of these will result in an explosive that may not function as intended by the manufacturer and may 

produce excessive fumes. 

Proper use of explosives and blasting agents is also very important in minimizing toxic fume production. 

For every explosive or blasting agent there is a minimum charge diameter, commonly referred to as 

critical diameter, below which it will not detonate properly. Below this critical diameter, the 

surroundings absorb sufficient energy from the explosion front to quench the detonation. Bulk-loaded 

blasting agents used in large-scale surface mine blasting do not detonate properly in boreholes of 1-inch 

diameter or less (ISEE, 1998). If the blasting agent is diluted by mixing with drill cuttings at the top or 

bottom of the borehole it may not detonate properly and excessive quantities of toxic fumes may be 

produced (Sapko, 2002). Similarly, the blasting agent may flow into cracks and crevices around the 

borehole where it may not detonate properly because the width of the cracks and crevices may be below 

the critical diameter. Incomplete detonation of the blasting agent leads to excessive toxic fumes (ISEE, 

1998). Stemming plugs may be placed in the top and bottom of the blasthole to prevent mixture of the 

blasting agent with drill cuttings or rocks. Flow of the blasting agent into cracks and crevices may be 

prevented through the use of packaged product or borehole liners. 

Production of excessive NOx during blasting may also be caused by incomplete detonation as a result of 

loading wet boreholes with an explosive that is not water resistant. When wet boreholes are 

encountered, the water must be removed or they must be loaded with explosives or blasting agents that 

are packaged to keep out the water or with a product that is designed to be water resistant. ANFO is not 
water resistant and will not shoot properly in wet holes unless it is packaged to resist the water. 

Emulsion blasting agents are water resistant and may be loaded in bulk in wet boreholes. 

ANFO/emulsion blends exhibit water resistance to varying degrees depending on the ratio of ANFO to 

emulsion. The explosive supplier can recommend a mix ratio that is appropriate for a given application. 

2. Determine where the blasting fumes are likely to go.

For surface blasting, much of the detonation products can be seen as a cloud of gas and dust coming off 

the blast. When a surface blast is initiated all workers should be positioned at locations outside of the 

likely path of the product cloud. Monitoring the wind direction immediately prior to the blast can be 

useful in accomplishing this. Some mines also have blasting plans that specify a blast should not be 

initiated if the wind will carry the cloud in the direction of neighbors off mine property. In addition, 

detonation product gases may be present in the muck pile and may also move into cracks and fissures in 

the ground. The gases move through the ground and may collect in a nearby confined space such as 

underground sewers, pipeline trenches, or basements of homes and businesses. As the gases move, CO 

will be the toxic gas of main interest since NO2 and indirectly NO are absorbed by the soil. (NO 

oxidizes to NO2 which is readily absorbed by the soil.) 

In most cases the fumes will spread slowly through the ground in all directions. However, in some 

cases, pathways exist that allow the gases to move preferentially in one direction. Such pathways may 

be created by broken rock from an earlier blast (ISEE, 1998), a hill seam (a pathway caused by the 

movement of rock layers on a hillside) (Eltschlager et. al. 2001) and (NIOSH, 2001), underground utility 

lines, a French drain, or fractures in the ground (Harris, Sapko, and Mainiero, 2005). A review of 

available maps and examination of nearby structures should reveal utility lines or French drains that may 

serve as pathways. Identifying naturally occurring pathways would be much more difficult and it would 

be impractical to do this for every blast. However, once CO migration has been identified as a problem 

at a blast site, the blaster may want to consult a geologist for aid in identifying the pathway. Knowledge 

of the probable pathway will be useful in deciding how to minimize the likelihood of CO migration 

problems in future blasts. 

3. Prevent the fumes from moving towards workers and neighbors.

For surface blasts there is no practical way to change the direction in which the product cloud will move;

all a blaster can do is try to ensure that no one will be in the cloud’s path. This is not the case for 

blasting fumes moving through the ground. 

Techniques for mitigating the migration of CO were evaluated during blasting research conducted at the 

NIOSH Pittsburgh Research Laboratory (PRL) (Harris et al. 2005). When no actions were taken to 

prevent or mitigate CO in the ground, CO was measured for several days in monitoring boreholes after a 

blast. This has been demonstrated at the PRL site and also during reported incidents in the field. 

However, when the muck pile is immediately excavated after the shot, the levels of CO measured in 

monitoring holes are orders of magnitude lower and do not last for a long duration. When negative 

pressure was applied to a monitoring hole close to the blast location after a blast that was not excavated, 

the levels of CO measured were comparable with immediate excavation and were of a short duration as 

well. A reasonable and immediate source of negative pressure is the vacuum from the dust collection 

system of a drill rig. If a hole is drilled in the near proximity of the blast, the end of the drill boom can 
be located on top of the drilled hole and the dust collection system turned on for a period of time. A 

more extensive system may be constructed using several holes connected to a fan. These techniques 

need not be applied to every shot but rather only when a problem with CO migration is encountered. 

Mucking will remove some gas that is trapped in the muck pile (Harris et al. 2005). Over time CO may 

migrate beyond the rubble zone and mucking will not remove any CO that has migrated beyond the 

rubble area. To be effective, mucking should be carried out as soon after the blast as possible. 

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. 

4. Monitor the air near workers and neighbors so they can be relocated if fumes appear.

Studies at blasting sites in Amherst, New York (Harris and Mainiero, 2004) and Bristow, Virginia 

(Harris, Rowland III, and Mainiero, 2004) identified ways to protect people from the CO that may 

migrate from a blast into nearby homes or other confined spaces. Based on these studies, it was 

recommended that the blaster place CO monitors in occupied parts of nearby homes and businesses. CO 

monitors of the type sold in department and hardware stores for home use should be adequate if the 

instructions on the packaging are followed. These detectors are designed and tested to protect people in 

their homes from CO poisoning, whatever the source. 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 homes or businesses may continue for an extended period of time, from 

several hours to a few days. Monitoring should continue until CO from the blasting operation no longer 

enters the home or business. In recent years CO poisonings were most likely prevented by the early 

warning of a homeowner-installed CO detector. Because of early warning, the source of CO was 

determined and affected homes were evacuated and closely monitored before anyone could become ill. 

To the best of our knowledge, no one has had to be treated for blasting-related CO poisoning since the 

western Pennsylvania incident in April, 2000 (Eltschlager et al. 2001) and (NIOSH, 2001). 

It is important that workers follow the confined space requirements of the Occupational Safety and 

Health Administration when entering a manhole vault, trench, or other confined space near a blasting 

site (OSHA, 2005). In 1998 a worker was killed and two injured when they entered a manhole vault 45 

minutes after a nearby blast. No one had checked the vault for toxic gases prior to entry. The vault 

contained toxic levels of CO (NIOSH, 1998). 

5. Ventilate structures or confined spaces until CO falls below a hazardous concentration.

Once CO is detected in a confined space near a blast site, no one should reenter until safety personnel 

have stated that it is safe to do so. Local firefighters and other emergency response personnel may be 

called to assist. These people have been trained and are equipped to deal with toxic atmospheres in 

homes, businesses, and other confined spaces, and will take appropriate action. 

Conclusion 
The major toxic gases produced by detonation of commercial explosives and blasting agents are CO and 

NOx. These gases may migrate through the ground into the basements of nearby homes and businesses, 

trenches, manhole vaults, and other confined spaces. NOx does not migrate through the ground because 

it is absorbed by the soil as the gases travel. However, NOx is a concern in surface blasting because it is 

very toxic; much more toxic that CO. Excessive NOx production at a blasting site may be evidenced by 

the presence of an orange or red cloud produced by the blast. The boreholes must be properly loaded to 

minimize the production of NOx. To the best of the authors’ knowledge, no one knows the 

concentrations of NOx in a blasting product cloud but it is best to err on the side of safety and assume 

the cloud is toxic. People should be kept out of contact with the product cloud. Carbon monoxide is a 

serious concern because it is not absorbed on passage through the ground. Carbon monoxide may travel 

up to several hundred feet and collect at toxic levels in a confined space. Carbon monoxide is odorless 

so there is no obvious indication that a hazard exists. This hazard may be dealt with at several levels. A 

blaster should use explosives and blasting agents in the manner specified by the manufacturer to 

minimize the quantity of CO produced. The blaster should attempt to identify any pathways by which 

gases produced by the detonation may travel from the blast site into homes, businesses, or other 

confined spaces. If a blaster is aware that there is a likelihood of CO migrating into occupied spaces 

he/she may minimize the hazard by excavating the blasted rock soon after the blast or may connect a fan 

to a borehole near the blast to pull the CO out of the ground. The blaster may place home-type CO 

monitors in homes or businesses near the blast site so occupants will be alerted if CO concentrations rise 

to unsafe levels. OHSA’s confined space regulations must be followed when a worker enters a trench, 

manhole vault, or other confined space. Firefighters or other emergency personnel may be called in to 

ventilate any homes or businesses where CO has been detected and determine when a CO hazard no 

longer exists. 

It is very difficult to predict when CO produced by a blast will migrate into homes, businesses, and other 

confined spaces. It would be impractical to do this for every blast. At present the best defense is to 

ensure that people are alerted if the air they are breathing contains toxic levels of CO. 
Figure 1. The effect of fuel oil content on the quantity of carbon monoxide produced by 

detonating ANFO. (Rowland III and Mainiero, 2000) 

Figure 2. The effect of fuel oil content on the quantity of nitrogen oxides produced by detonating 

ANFO. (Rowland III and Mainiero, 2000) 

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