CO 2 : An Underrecognized and Underappreciated Threat to Worker Safety during Construction Activity

: Many fatal inhalational accidents occurring during construction typically involving confined spaces and structures that confine the atmosphere continue to defy identification. Very little information is available, principally from accident summaries and government reports. Increasingly, these identify CO 2 (carbon dioxide) as a probable cause. This article discusses situations identified in accident summaries and worldwide databases. CO 2 lacks an odor and other means of identification without the use of monitoring instruments. Emissions typically involve exhaust gases; aerobic and anaerobic respiration in microbiological systems in wastewater and landfills; geological deposits capable of chemical reaction to produce CO 2 ; and unintended discharge from pressurized systems. Emissions can occur continuously or abruptly subject to the type of system and conditions involved. Anaerobic systems that behave as shear-thinning, pseudoplastic, non-Newtonian fluids emit abruptly on the application of a shear force. A lethal concentration can develop almost instantaneously. Upon cessation of the stress, the ambient condition restores rapidly. Chemical and physical processes provide reservoirs for the storage of gas. Very limited methods are available for the prevention of these accidents because of the infrequency and unpredictability of the emission. Preventive measures include mandatory atmospheric monitoring and ventilation at all times, where hazardous conditions can develop, and sometimes the use of high-level respiratory protection.


Introduction
Atmospheric emissions of hazardous gases or gas mixtures are the leading cause of fatal events in confined spaces and atmospheres confined in partially enclosed structures [1].Accident records over the years routinely reported on fatal inhalation events [2][3][4][5].
Accidents reported in these resources present common elements including rapid collapse of the victim typically without a warning odor prior to the event or evidence of atmospheric contamination at the time of emergency response and follow-up investigation.The cause typically was attributed to methane (CH 4 ), carbon dioxide (CO 2 ), or oxygen (O 2 ) deficiency but without supporting evidence.Given the advances in instrumentation over the years, this situation should be resolved.Resolving this situation is essential for implementing protective and preventive measures.
A companion article in this series examined the situation involving hydrogen sulfide (H 2 S) [6].H 2 S often occurs in shear-thinning, pseudoplastic, non-Newtonian fluids.Shearthinning, pseudoplastic, non-Newtonian fluids are produced during anaerobic respiration involving organic materials.Sudden disturbance of shear-thinning, pseudoplastic, non-Newtonian fluids causes rapid high-level emission sometimes to lethal levels followed by a rapid return to a potential zero background when the disturbance ceases.The net impact is that the change can occur almost instantly without warning and without any indication of what occurred.
McManus and Henderson [7] documented one of the few available examples of this phenomenon in the workplace.This occurred during the excavation of soil containing H 2 S from an ocean shoreline.
A recent fatal accident [8] prompted this article and a further opportunity to investigate and determine the identity of another of the causative agents of the unresolved fatal accidents mentioned above.The accident mentioned here is very typical of fatal atmospheric events involving exposure to sewer gases.Two men died in a manhole containing an opening in the top and water in the bottom.The manhole was part of a newly installed sewer pipeline that paralleled a poorly maintained in-service sewer pipeline.Both pipelines were positioned close to and parallel to a watercourse.Flow in the watercourse depended on the time of year.Following entry down a ladder to the bottom without ventilation and confirmatory air monitoring, both men collapsed into the water.A firefighter later measured 18.5% O 2 near the surface of the water and 19.5% halfway to the top (about 3 m above the water) prior to removing the bodies of the victims during the emergency response.The 4-gas monitoring instrument did not detect H 2 S.This situation is quite typical.The discussion that follows later in the manuscript indicates that the concentration of CO 2 corresponding to 18.5% of O 2 is lethal.
Excluding CO, H 2 S is the most toxic gas followed by CO 2 and ammonia (NH 3 ) [6].Rapid, olfactory fatigue caused by high levels of H 2 S is a recognized issue.CO 2 and NH 3 have received little formal attention in the literature concerning accident causation compared to H 2 S. NH 3 has an easily recognizable odor and occurs infrequently.CO 2 has no odor, color, or other distinguishing characteristics and is undetectable without the use of technology containing specific sensors.CO 2 is both dense relative to air and physiologically active.
NIOSH [9] was among the first of the modern-day references to warn about the hazardous properties of CO 2 with a focus on workplace activity.This reference provided information about the toxicological behavior of CO 2 and was freely available.
FACE 86-13 reported on the investigation of a fatal accident in a fermentation tank attributed to inhalation of a high concentration of CO 2 and a low level of O 2 [10].
FACE 86-13 commented on CO 2 as being a "simple asphyxiant, which will displace O 2 in a confined space".Simple asphyxiants include nitrogen, water vapor, and the inert gases of the Periodic Table .They displace/dilute O 2 and do not have a regulatory exposure limit.CO 2 has had a regulatory exposure limit of 0.5% (5000 parts per million) averaging over 8 h since 1948 [11].Substances that do have regulatory exposure limits are considered to be chemical asphyxiants and not simple asphyxiants.
The idea expressed in the comment in FACE 86-13 [10] has considerable importance.The comment was part of misinformation about CO 2 and its toxic properties that persisted until recently and perhaps still continues to do so.This misinformation stifled recognition of the need to assess exposure to CO 2 at the time of entry into confined spaces where the level of oxygen was less than the normal of 20.9%, even while above the regulatory limit of 19.5% when CO 2 was the reason for the depression (as will be explained later).Reliance on readings from the O 2 sensor alone is insufficient for determining the hazard posed by CO 2 .
The OSHA database containing summaries of recent fatal events [5] is searchable according to the type of work location.A review of recent documents and summaries of fatal events in comparison to historic ones summarized by McManus [4] indicates ongoing repetition of fatal events occurring in almost the same manner under the same circumstances despite the effort expended by regulators, employers, workers, and worker representatives, and practitioners in occupational health and safety to prevent their reoccurrence.
Overexposure to CO 2 is causing fatal events in many areas of the industry.Mc-Manus [6] lists some examples involving winemaking.This review focuses on exposure to potentially hazardous atmospheres containing CO 2 during construction activities.Fatal events involving CO 2 have received almost no attention in the literature despite the fact that they occur regularly.
Construction encompasses many activities and environments and is widely acknowledged as posing hazardous conditions.The construction environment is diverse and often rapidly changing.This reality forces emphasis on the rapid identification of problems and prompt response to improve conditions rather than studying them for the purpose of categorizing the magnitude of exposures.This approach is especially important for exposures involving CO and CO 2 , where there are no warning properties compared to H 2 S and NH 3 that announce their presence through readily detectable odors.This review will focus on the construction of in-ground structures of the type found in pipelines with the intent to: • identify serious and fatal events caused by the production of CO 2 and uncontrolled release into the airspace of structures in the infrastructure; • identify and discuss potentiating mechanisms involved in these releases; • alert designers, in particular those involved in the design of "green" and "sustainable" structures, about the implications of choices made in design; • identify and discuss precautionary measures for minimizing the occurrence of these events.

Methods and Methodology
The basis for this investigation is the availability of information and data concerning fatal atmospheric accidents.Information contained in accident summaries and investigative reports is often sparse.McManus and Haddad [12] developed techniques for maximizing output from these documents.
Information about this subject in the worldwide literature is also sparse and reports on fatal accidents or near-miss situations.
Records of exposure preceding and during fatal events are highly infrequent.Air monitoring for CO 2 has lagged behind because of a lack of emphasis by regulators, researchers, and practitioners based on the mistaken belief that CO 2 is just another inert gas.Recognition of the role of CO 2 in fatal accidents depends on understanding possible sources and records of monitoring by workers affected by the event and first responders such as firefighters.
Given these realities, the approach taken in this investigation was heavily based on personal observation of sources of emission in construction, technical reports published by NIOSH and OSHA in the US, and databases containing summaries of fatal accidents published by regulators and research organizations.More focused sources included articles in medical databases (PubMedCentral, National Library of Medicine), articles in health, safety, and environmental journals, engineering journals, and reports in the news media.
The situations identified in these examples are rare events (or point sources of information).There is every reason to believe that multiple examples of these events exist but are not reported.Sometimes there are repeating themes to the point where subsequent events are highly predictive to situations where only a single event has occurred.

Production and Emission of CO 2 by Bacteria
Bacteria are major sources of production of CO 2 [13,14].Some types of bacteria respire aerobically and obtain O 2 from the water in which they live.Other types of bacteria respire anaerobically and do not require oxygen for respiration.Wastewater treatment plants process wastewater in separated chambers containing aerobic or anaerobic conditions.Still, other bacteria (facultative anaerobes) can live under both conditions.Anaerobic microorganisms differ from aerobic microorganisms.The two types of respiration occur through different metabolic pathways.Facultative anaerobes contain the enzymes needed for both pathways.As a result, these microorganisms can participate in both pathways and emission of CO 2 will occur at all times.
In aerobic situations, microbial activity can produce a stable depression of the O 2 level in the airspace above the water [13].Aerobic respiration requires oxygen obtained from the water.Microorganisms that respire only aerobically continue to do so as long as the oxygen level meets or exceeds a minimum value.Aerobic respiration occurs in water that is flowing and/or turbulent.Turbulence induced by natural or mechanical means aerates the water and maintains the concentration of O 2 above the level needed for aerobic respiration.Production of CO 2 occurs continuously provided that requirements for respiration are met [14].
Anaerobic respiration occurs in water having a level of O 2 insufficient to support aerobic respiration [13].The level of O 2 in such water could be zero.In anaerobic situations, production of CO 2 occurs and emission also occurs from the surface of the water.Some microorganisms respire only anaerobically.The introduction of oxygen into this environment could stop anaerobic respiration.This would stimulate competition from aerobic microorganisms already in this environment.Facultative anaerobes considerably complicate this situation as they can exist under all possible conditions [14].

Sources of CO 2 in Construction 4.1. Exhaust and Combustion Gases
The exhaust emitted by engines in vehicles and in mobile and portable equipment is the most obvious source of CO and CO 2 outdoors and in some workplaces.Engines are present in equipment on the vast majority of construction sites.With the emphasis on exposure to carbon monoxide (CO), the hazard posed by CO 2 in the same emissions has largely escaped attention.Exhaust systems in small engines do not normally contain catalytic converters.Catalytic converters reduce exposure to CO emitted by engines in vehicles through conversion to CO 2 .Depending on temperature, small engines can produce >800 of CO when first started [15].Carbon dioxide produced by the engine in this study did not pose an undue hazard when operated in open air under the same conditions.
Additional sources of CO and CO 2 on construction sites include open-flame torches used for heating and flame cutting, some shield gases used in arc welding, and infra-red catalytic heaters.Heaters used in construction combust propane or diesel.The literature contains no information concerning exposure to CO 2 from the sources mentioned above despite the fact that considerably more CO 2 than CO is produced during combustion.This could outweigh some of the perceptions that exposure to CO 2 during workplace activities is not a concern.An additional consideration is that ventilation measures undertaken to eliminate or control exposure to CO will considerably reduce exposure to CO 2 .

Compressed CO 2
Compressed CO 2 is used to pressurize pneumatic cylinders to exert force against panels of wood used for shoring (air shores) to prevent the collapse of trenches [16].These devices are precision-engineered pneumatic cylinders and structures.The only gases recommended for pressurizing the cylinders are compressed CO 2 and compressed air.Compressed CO 2 is an unusual product on a construction site and may have specialized applications, where dry gas is required to protect the interior of the pneumatic cylinder against condensation and corrosion.
The use of compressed CO 2 to pressurize pneumatic cylinders creates the risk of leakage and accumulation in the bottom of the trench because of density compared to air at the same temperature.Regulatory authorities such as WorkSafeBC require the supply tank to remain outside the trench as a precaution against entry of leaked gas [17].
The parallel situation in industry is the large-scale escape of pressurized CO 2 .The main example is unintended discharge from fire protection systems during repair and modification [18].A recent article examined CO 2 safety and summarized occurrences involving pipeline and tank failure [19].A major concern with tank failure is a Boiling Liquid, Expanding Vapor Explosion (BLEVE).
Carbon (dioxide) capture and storage installations are an emerging area of concern because of leakage from pipelines and associated processing and containment equipment and tank failures.A companion concern is the failure of geological storage formations.These projects receive the benefit of considerable engineering and health and safety input during design, construction, and operation.These installations are well served compared to other activities in construction that are the focus of this article where seemingly small emissions of CO 2 can be surprisingly deadly.Exposure due to leakage from pressurized systems is inevitable as these systems age and are dismantled for maintenance.

Abrasive Blasting with Dry Ice Pellets
One of the most recent applications of CO 2 in construction activities is abrasive blasting using pellets of dry ice propelled by a stream of compressed air [20,21].Abrasive blasting using pellets of dry ice minimizes the risk of musculoskeletal injury compared to sanding and abrading in overhead and other awkward positions.A blasting medium that leaves no residue and can penetrate into inaccessible areas holds considerable promise.Dry ice, the solid form of CO 2 gas, readily sublimes from the solid to the gaseous form at normally encountered temperatures.The fracturing of the pellets into small fragments during a collision with surfaces in their path considerably promotes sublimation.
The compressed air used in the process pushes the pellet stream considerably away from the operator and, at the same time, supplies large quantities of air containing the normal level of oxygen (20.9%).This supply of air near the breathing zone of the worker ensures that oxygen deficiency (defined in regulatory terms as being less than 19.5% in most jurisdictions) is unlikely to occur.(Refer to a discussion about O 2 in a later section.) While the disappearance of dry ice due to sublimation during abrasive blasting is its greatest strength, this property is also its greatest limitation.Overexposure to CO 2 is a serious possibility.Continuous monitoring to determine exposure to carbon dioxide during abrasive blasting is essential and poses a difficult technical challenge.

In-Service Infrastructure
In-service infrastructure has been and usually continues to be exposed to the environment in which it is located and utilized.Following consideration of engines and the sources mentioned above, the breadth of sources of CO 2 narrows considerably.The relevant work occurs in partially or fully enclosed structures.Experience has shown that out-leakage of wastewater into the ground in-service infrastructure and in-leakage of rainwater, groundwater, and tidewater can occur.Out-leakage can occur through release points deliberately intended to prevent overpressure in trunk sewers and following damage or deterioration involving the structure.In-leakage of groundwater and tidewater can occur because of deteriorated sealing surfaces.

Isolated Structures
These structures can contain standing water from surface drainage that entered through opening(s) in access covers; from leakage through seams at the point of contact between prefabricated sections and between the side wall and the base; and through floors containing dirt or porous rock.The upward and downward movement of the water table creates a piston that can inhale moisture and gases from the subsurface soil.
The presence of even a small amount of water in an enclosed structure combined with entry from above leaves, the carcass of a small animal, or other organic matter has led to lethal atmospheric conditions [2,4,5].These conditions typically are expressed in terms of O 2 deficiency and lack of identification and discussion about the gas that replaced the oxygen in the atmosphere in the airspace above the water.

Infrastructure Networks
Considerably more common in the causation of hazardous atmospheres are infrastructure systems, in particular stormwater and wastewater networks.These systems typically contain flowing or stagnant water or a combination of both.Water level often varies with weather conditions and the magnitude of discharge from inputs.These considerations apply equally to CO 2 and H 2 S when anaerobic respiration is occurring.These considerations also apply when aerobic and anaerobic respiration occurs in flowing water with or without simultaneous development of sludge.
These systems share a common atmosphere in the airspace above the water within the individual network.Accident records show that this work is especially hazardous when contact with the atmosphere in the existing system occurs [2][3][4][5].In order to understand this reality further, additional information is needed about how these conditions can develop.

Unusual Emissions into the Airspace of Drain Piping and Collecting Structures
Some systems create unusual conditions for drainage involving the airspace in the drainage network.One example investigated by the author as yet unpublished involved a cemetery.Immediately below ground in today's cemeteries is a network of concrete vaults that sit on coarse drain rock.The drain rock encloses perforated drain piping.This configuration is completely hidden from view in the cemetery.All that a visitor sees is a lawn.This design considerably simplifies the operation of the cemetery because it eliminates the need to dig deep holes for a burial.
Decay of the body produces CO 2 .The drainage network underlying the vaults collects the CO 2 as well as drainage and conveys both to the pump station.As with other situations in this discussion, there is no odor to provide a warning about contamination in the airspace of the pump station by CO 2 .In this situation, the concentration of O 2 was 5.5% at ground level at the opening of the access manhole and 4.5% about 4 m down just above the pumps.A gas sample showed that there was no methane.The cemetery operators had believed prior to the visit that methane had depressed the oxygen level measured by their instrument.(Had methane been present, it would have registered on the ignitable sensor of the air monitoring instrument.) This situation is possible in any system involved in the collection of wastewater into a pump station or other structure where there is no trap and vent to the atmosphere to prevent migration of the atmosphere into the airspace of the collecting structure.Negative pressure is applied to the interior of an isolated manhole by an external vacuum pump during integrity testing (vacuum testing).Piping entering the manhole is isolated using a mechanical or inflatable plug(s).A vacuum to a predetermined level is applied, and the vacuum system is isolated from the manhole.The ability of the structure to maintain the vacuum at a constant level for a specified period of time determines success or failure relative to the criteria of the test.
Application of internal negative pressure in a manhole promotes in-leakage of soil gas, groundwater, and water contained in the pipeline.All of these sources can contain and exhale CO 2. The emission of CO 2 to lethal levels has occurred with tragic consequences following the enrichment of the internal atmosphere in this manner [22].This situation can apply to in-service and newly constructed manholes where wastewater is present.

Emission of CO 2 from Fluids
Fluid flow at low velocity encourages the growth of anaerobic bacteria in a biofilm that coats the wetted surface of the sewer.To illustrate, a layer of debris develops at the bottom of a structure unless water flow is sufficient to prevent accumulation.The flow velocity needed to maintain this condition is estimated to be >2 m/s [23].Collection systems in warm climates that have a flat grade and/or flow-through velocities <0.6 m/s (<2 ft/s) enable the development of anaerobic conditions.
The development of a layer of debris creates conditions for stagnation and the development of an anaerobic condition.Hence, an aerobic condition can exist in water flowing above an anaerobic layer.An anaerobic layer having a semi-fluid consistency is known as sludge.This situation suggests that the production of CO 2 can occur simultaneously and independently in both layers of water in a pipeline and in support structures such as manholes.
CO 2 can also participate in rapid release by shear-thinning, pseudoplastic, non-Newtonian fluids because it is produced in the same environments as H 2 S [6].Production and emission of CO 2 are more broad-spectrum than that of H 2 S because of involvement by facultative anaerobes [14].
Emission of CO 2 can occur continuously or abruptly subject to the type of system and conditions involved.Aerobic systems and non-provoked anaerobic systems emit continuously with reasonable predictability.In some situations, these emissions can reach lethal levels.Upon cessation of the stress, the ambient condition restores rapidly.
Where emission to lethal levels can occur, very likely this has occurred and has caused one or more fatal events [2,4,5].In the absence of warning properties (odor, color, taste, condensation to form a visual cloud), the only way to detect CO 2 is to measure its level in the airspace of the structure.Structures involved in the formation of hazardous atmospheres include those that meet regulatory requirements for classification as confined spaces as well as those that are sufficiently enclosed to enable accumulation to the level needed to constitute a hazardous atmosphere.Such structures can be inside buildings as well as outdoors.

New Construction
New construction describes structures never exposed to the in-service environment.Situations in which structures under construction have experienced the in-service environment no longer can be considered under the description of "new construction".This situation arises when premature tie-in or unintended entry of contaminated groundwater has occurred.Tie-in is the activity involved in connecting the new pipeline to the existing network.
Activities of new construction include excavation; placement and connection of sections of pipe; installation of pre-cast and cast-in-place structures; backfilling; entry into existing systems at manholes; and tie-in of the new pipeline to the existing one.Tie-in can occur at a manhole or at the end of the new and existing pipeline.
Despite expectations to the contrary, excavation and installation of pipe and pre-cast and cast-in-place structures are not without risk of exposure to atmospheric hazards containing CO 2 .The obvious ones are the gases produced in the engines of small portable equipment operated inside the structure or external to it.These occur following inappropriate or careless placement of exhaust piping at ground level and/or in the zone of capture of the fan used to provide air into the structure.Additional sources of CO 2 also affect this work.

External Hydraulic Pressure
The previous discussion has highlighted the transport and emission of CO 2 gas in water within sewer pipelines.Leakage of wastewater from failed sewer pipelines into the surrounding ground can contaminate groundwater.Migration of contaminated groundwater through soil and entry into the airspace of newly installed pipeline and manhole(s) can and do occur.Entry of CO 2 dissolved in water occurs through leakage paths in seams between sections of pipe and precast sections in manholes as well as through substandard pipe walls.This situation can be especially problematic when the water table is above the depth of the pipeline coupled with extensive rainfall or other source of subsurface water.

Sources of CO 2 in Soil
The soil offers pathways for the migration of gases, including, when present, CO 2 .The existence of such pathways in the soil is demonstrable by observation in excavations where the emission of H 2 S occurs [7].
Anecdotal experience similarly has demonstrated on several occasions the horizontal travel of propane and related gases contained in fuel following spillage onto asphalt pavement in filling stations.The liquid passes through cracks and gaps in the pavement and vaporizes.The gases migrate in a direction that offers escape to the atmosphere.This escape sometimes occurs in unrelated excavations.Asphalt and concrete pavement on top of the soil provide a barrier that prevents escape to the atmosphere.Gases can migrate considerable distances in this manner.

Mineralogical Sources
In a highly unusual situation, acid naturally occurring in soil attacked carbonates in rock located in a lower region of the soil profile and crushed rock used for setting pipe and precast manhole sections during excavation and backfilling associated with the installation of a sewer pipeline [23].The CO 2 produced by this action passed through the soil and entered the airspace of a pipeline and a newly installed precast manhole.The resulting hazardous atmosphere killed entrants.
A second situation concerned chemical reactions occurring in a pile of waste rock at a former mine [24].The waste rock contained sulfides and carbonates.The pile of rocks was covered by 1 m of soil to prevent entry of water and further formation of acid drainage.Drainage that did occur was collected into a pipeline that ran to a treatment plant.The pipeline passed through an inspection station that housed an in-ground pit used for the collection of water samples.
Air passed up the airspace in the collection pipe and other leakage paths into the pile of rocks.The reaction between the O 2 in the air and the sulfide in the rock produced sulfuric acid.The sulfuric acid reacted with the carbonates to produce CO 2 .The CO 2 (8% of the resulting atmosphere) and highly oxygen-deficient air (2% of the resulting atmosphere) migrated along the airspace of the collecting pipe and replaced the atmosphere in the inspection station.Four people perished in the inspection station.

Landfill Gas
Landfilling as a means of disposal of household and industrial waste has created immense problems because of contamination of groundwater by breakdown products.Landfilling has also created major opportunities for recovery of methane produced during anaerobic digestion.Anaerobic digestion also creates CO 2 and where molecules containing sulfur atoms are present, H 2 S.
Migration of landfill gas to distant points has occurred with the same fatal outcome as recorded above [25].Entry into the pipeline and manholes can occur through the open end.The open end poses the least resistance to inflow.Inflow could also occur through leaking joints in precast sections of the manhole.This can occur as gas, as mentioned, or dissolved in water.

Sources of CO 2 in Groundwater
A possible source of entry of CO 2 into the airspace of sewer pipelines and manholes (new construction and in-service) is groundwater.These distinctions are important.Normally during new construction, the sewer line and new manholes are physically separated from the existing, in-service line.Contact with wastewater in the existing line and mixing with the in-service atmosphere only occur during tie-in.

Solution Effects
CO 2 molecules dissolve in water and either remain intact or react chemically with water or something dissolved in water to form an acid and one or more anions.CO 2 both remains intact and reacts with water.This characteristic enhances the solubility of CO 2 in water.Given the opportunity, CO 2 molecules dissolved in water establish an equilibrium with CO 2 molecules in the air when in enclosed airspace at a particular temperature and pressure (Equation ( 1)).CO (2, gas in air) ←→ CO (2, gas in water) (1) CO 2 molecules dissolved in the water constitute the reservoir available to emit into the air above the water should conditions for dissolving become less favorable.The ratio of the two concentrations is the Henry's Law constant [26].A large Henry's constant as in the case of methane (Table 1) indicates the low solubility of the gas in the liquid.Table 1 indicates that CO 2 is less soluble than H 2 S and especially, NH 3 and more soluble than CH 4 in water at the same temperature and pressure.For methane to be a causative agent in fatal events, large-scale displacement from the liquid into the airspace must occur in order to lower the concentration of oxygen sufficiently to become oxygen-deficient.McManus [4,6] provided further information considering methane.Methane is toxicologically inert as are nitrogen, water vapor, mist and steam, and the inert gases of the Periodic Table.These gases do not cause oxygen deprivation until the concentration of O 2 is around 14%.The gas density of methane is considerably less than that of air [27].Hence, given the opportunity to escape from structures from any available opening at the top, the ability of methane to depress the level of O 2 is small.CO 2 on the contrary is both dense and physiologically active [9].It will remain near the surface of the water.The recent fatal accident that prompted this manuscript [8] illustrates the point.The attributed causes are not consistent with the physical properties and physiological actions of the gases mentioned.
Equation ( 2) introduces the concept of reservoirs.To illustrate their importance, the introduction of CO 2 into the air above the liquid will shift the equilibrium toward more CO 2 dissolved in the liquid.Similarly, the introduction of CO 2 gas into the liquid will shift the equilibrium toward more reaction with H 2 O to form more H 2 CO 3 .More H 2 CO 3 will promote more ionization to HCO 3 − and finally to CO 3 2− .

Role of pH
The second important mechanism concerns the control of pH.Acidifying the solution (lowering the pH) will shift the equilibrium toward the formation of more CO 2 molecules dissolved in the liquid.This, in turn, promotes the conversion of CO 3 2− to HCO 3 − and HCO 3 − to H 2 CO 3 and H 2 CO 3 to CO 2 and H 2 O and the emission of CO 2 molecules into the airspace above the liquid.
An extremely useful tool for visualizing the situation is the Henderson-Hasselbalch equation taught in chemistry courses (Equation 3) [28].All that is needed to produce a graph from Equation (3) is the value of pK A .pK A for a given acid is available from chemistry reference sources [28].The value of [A − ]/[HA] is arbitrary.The fraction is directly convertible to percent ionization.
pH is a measure of the acidity or alkalinity of the liquid.pK A is a value measured for the substance.Reference textbooks contain tables of pK A values [29].HA is the molecular form, in this example, H 2 CO 3 .A − is the ionized form, in this example, HCO 3 − or CO 3 2− .The value of pK A1 for the first anion from H 2 CO 3 is 6.4 (Table 1).At pH = 6.4,half of the H 2 CO 3 in the water is present as HCO 3 − ions (Figure 1).At this pH, the concentration of A − equals the concentration of HA because the logarithmic term is zero.(The log of 1 is zero.)Lowering the pH to 4.5 converts all of the bicarbonate ions to H 2 CO 3 molecules.The presence of greater numbers of H 2 CO 3 molecules in the water displaces the equilibrium with the result that CO 2 molecules emit from the water into the airspace above the water.

Role of pH
The second important mechanism concerns the control of pH.Acidifying the solution (lowering the pH) will shift the equilibrium toward the formation of more CO2 molecules dissolved in the liquid.This, in turn, promotes the conversion of CO3 2− to HCO3 − and HCO3 − to H2CO3 and H2CO3 to CO2 and H2O and the emission of CO2 molecules into the airspace above the liquid.
An extremely useful tool for visualizing the situation is the Henderson-Hasselbalch equation taught in chemistry courses (Equation 3) [28].All that is needed to produce a graph from Equation (3) is the value of pKA.pKA for a given acid is available from chemistry reference sources [28].The value of [A − ]/[HA] is arbitrary.The fraction is directly convertible to percent ionization.
pH is a measure of the acidity or alkalinity of the liquid.pKA is a value measured for the substance.Reference textbooks contain tables of pKA values [29].HA is the molecular form, in this example, H2CO3.A − is the ionized form, in this example, HCO3 − or CO3 2− .
The value of pKA1 for the first anion from H2CO3 is 6.4 (Table 1).At pH = 6.4,half of the H2CO3 in the water is present as HCO3 − ions (Figure 1).At this pH, the concentration of A − equals the concentration of HA because the logarithmic term is zero.(The log of 1 is zero.)Lowering the pH to 4.5 converts all of the bicarbonate ions to H2CO3 molecules.The presence of greater numbers of H2CO3 molecules in the water displaces the equilibrium with the result that CO2 molecules emit from the water into the airspace above the water.As pH decreases, the % of sulfide present as H2S molecules increases.

CO2 in Fatal Atmospheric Events
Taken together, the events summarized in the previous discussion indicated a rapid collapse of individuals exposed to a highly toxic, rapid-acting, sometimes constant, sometimes transient atmospheric agent.This agent was able to kill one or more individuals As pH decreases, the % of sulfide present as H 2 S molecules increases.

CO 2 in Fatal Atmospheric Events
Taken together, the events summarized in the previous discussion indicated a rapid collapse of individuals exposed to a highly toxic, rapid-acting, sometimes constant, sometimes transient atmospheric agent.This agent was able to kill one or more individuals during these events and was present in open areas, buildings, and confined spaces [1,2,4,5].
Oxygen deficiency is a major concern in the occupational setting as a cause of death and a topic of discussion in many standards and regulations [4].
Table 2 provides relevant information concerning the concentration of a gas in the airspace of a structure in the context of the level detected by the oxygen sensor in a monitoring instrument.This calculation applies to all gases and vapors that dilute or displace the atmosphere in the airspace and do not react chemically with it.The concentration of a contaminant predicted by this method (Equation ( 4)) is as follows: The oxygen sensor in the instrument detects one molecule out of every 5 in the atmosphere.The molecules not detected are nitrogen and the traces of other substances.Hence, the concentration of contaminant is considerably greater than suggested by the instrument reading alone.
In situations where depression of oxygen occurs, the employer is responsible for determining the identity and physiological impact of the substance.The physiological impact of the diluting or displacing gas or vapor could far outweigh that of the depression of oxygen.This is the case where H 2 S and/or CO 2 and/or NH 3 is/are the diluting gas(es) [6].
A fundamentally important concept is the set-point for the O 2 alarm in these situations in context with the information provided for CO 2 in Table 2.The set point adopted by regulators for situations where the diluting or displacing gas or vapor is not physiologically active is 19.5% [30].Table 3 provides the concentrations of relevant benchmarks of exposure to CO 2 that has diluted or displaced the atmosphere in a structure and not reacted with it.Comparison with the benchmarks for CO 2 provides a starting point for assessing exposure.Notes: TLV is the threshold limit value published by the American Conference of Governmental Industrial Hygienists (ACGIH) [30].The regulatory exposure limit would also apply.These values typically refer to an 8 h workshift.IDLH is the concentration immediately dangerous to life or health published by the US National Institute for Occupational Safety and Health.IDLH applies to a period of 30 min [31].
Where employers, regulators, and first responders have relied on readings provided by the O 2 sensor in a 4-gas tester and not a separate sensor for CO 2 to estimate the level of concern in a situation involving an unknown, the alarm point should be set as high as possible.Experience has shown that the readings of oxygen sensors in field monitoring instruments above 20.5% are affected by workplace conditions including humidity level and possibly other issues including altitude relative to that of the calibration.These become most problematic when the instrument is operated for long periods as would occur during a workshift.Long-period exposure is necessary where the TLV or regulatory exposure limit of 8 h, for example, for CO 2 .
Field instruments used for oxygen monitoring in this manner are most reliable for sensing other gases and vapors by difference at an O 2 level of 20.5% or less.This is the suggested alarm point.This should be confirmed in discussion with the manufacturer.The TLV/regulatory exposure limit for CO 2 (0.5%) occurs at an O 2 reading of 20.8% [29].The oxygen sensor detects one molecule in every five in the atmosphere.This is in the region of uncertainty during long-period O 2 monitoring and likely will not provide a warning about overexposure.
A comparison of response to reduced concentration of oxygen [4] and Table 3 shows that due only to oxygen, increased breathing and heart rate occur around 16% O 2 .Due only to CO 2 , the same effects occur at a concentration of 2% to 4% CO 2 , which corresponds to an oxygen concentration of 20.1% to 20.5%.Hence, a small increase in the concentration of CO 2 produces a considerably larger impact than a small decrease in the concentration of oxygen.This comparison indicates that the possible presence of CO 2 at a worksite demands considerable caution especially when confinement can occur.This comparison also indicates that testing for CO 2 using suitable equipment also must occur.
Intuition would suggest that the composition of the atmosphere in the new sewer pipeline during construction is the same as that of the external atmosphere.Experience has shown, as illustrated above in many circumstances, that the development of a hazardous atmosphere in sewer lines under construction can and does occur.
This situation becomes particularly problematic when the hazardous situation is undistinguishable from the nonhazardous situation because of the absence of warning properties such as odor.Mandatory air monitoring using suitable instruments is the only way to assess the safety of a situation.
Carbon dioxide is about 1.5 times as dense as air at the same temperature and pressure based on molecular weight [9].CO 2 could accumulate in partially or fully enclosed structures for this reason.CO 2 is a product of human and animal respiration and is tolerated at concentrations considerably above normal atmospheric levels.CO 2 acts as a respiratory stimulant at elevated levels [9].At concentrations at which CO 2 poisoning would occur, oxygen deficiency would also occur because dilution of the atmosphere would accompany carbon dioxide poisoning as a cause of death at high levels.
Table 4 summarizes the toxicological properties of the most important and prevalent substances potentially involved in accident causation in construction.[30] and LC (1, 0.5 h) (Lethal Concentration to 1% in 0.5 h) [32].IDLH [31] is not a usual working condition.IDLH requires a high level of respiratory protection and possibly chemical protective clothing in order to be able to work safely.LC (1, 0.5 h) refers to the lethal concentration for 1% of a group of animals projected to a duration of exposure of 0.5 h [32].
The values of IDLH and LC (1, 0.5 h) reported in Table 4 are useful for comparison of relative toxicity between individual substances.While CO 2 lacks the acute toxicity of H 2 S and the other gases mentioned in Table 4, it presents no warning properties and can accumulate without warning and detection without the use of monitoring instruments.Rapid collapse, unconsciousness, and death can occur after one or two breaths at concentrations exceeding 5% to 10% (19.9% to 18.8% O 2 on the monitoring instrument).These conditions can arise in structures containing continuously emitting sources as well as shear-thinning, pseudoplastic, non-Newtonian fluids.
This information, confirmed by workplace experience [8], suggests that an episode involving toxic exposure to CO 2 can occur very quickly without warning from the surface of the water in a standard manhole.This reality also suggests that the episode could affect more than one person, including would-be rescuers.As well, the atmosphere could disperse rapidly before responders equipped appropriately to effect rescue could arrive on the scene.This supposition reflects information from several sources [1][2][3][4][5] supplemented by the discussion presented here concerning the behavior of continuous emitters and shear-thinning, pseudoplastic, non-Newtonian fluids.

Elimination and Control of Emission of CO 2 in Construction
This article has presented considerable, hard-to-obtain information concerning the uses of CO 2 in construction and man-made and natural sources of emission and their potential role in the causation of fatal accidents.The information presented indicates that fatal accidents involving CO 2 in construction situations are highly consequential rare events (point sources).
One factor not mentioned in the previous discussion is predictability.Construction often involves repetitive tasks on a particular job site and tasks that repeat from one location to another.Prediction, a natural human characteristic, has led to deadly consequences during this type of activity [1][2][3][4][5].Basing technical decisions on this type of logic is highly flawed as shown by experience.What worked during repetitive activity on one site will not necessarily apply to all repetitions on that site or repetitions on other sites.Experience has confirmed this tragic reality.
The absence of warning signs for CO 2 factored into this reality indicates that the options available for worker protection are extremely limited.The focus of these actions must be protection and prevention rather than control.The focus of protection and prevention is to ensure that a deleterious event cannot occur and that conditions are maintained the same as ambient.The focus of control is to ensure that the conditions that precipitate these events cannot occur but in a less rigorous manner.Control is more forgiving than protection and prevention.To illustrate, protection and prevention demand that the concentration of CO 2 cannot increase beyond the ambient level.Control allows the concentration of CO 2 to exceed the regulatory limit of 0.5% (5000 ppm), provided that this is compensated by levels <0.5% so that the time-weighted average during the workshift is <0.5% [29].The inability to maintain the concentration of CO 2 < 0.5% using portable ventilation equipment is indicative of a major source of CO 2 .This situation would require input from a well-experienced professional.
Insights gained from this analysis suggest that investigators have failed to identify the mechanism and agent(s) of the causation of these accidents and underestimated the frequency of their occurrence.
Regulators worldwide have addressed these accidents through requirements applicable at the time of entry into structures regulated as confined spaces.Confined spaces are structures into which entry for the purpose of performing work normally does not occur [17].These workspaces pose elevated levels of risk.
This approach operates on the premise that only the owner of the equipment or structure or employer of workers who must enter and work in them has responsibility for conditions created by choices made during design.This responsibility also extends to design.Engineering design clearly influences the conditions of work to be performed at a later time, yet the literature contains little information concerning obligations voluntary or otherwise imposed on engineering designers to reduce to the extent possible the risk involved in accessing, entering, and working in these structures and providing appurtenances needed for efficient emergency response.
The points raised above further highlight this concern, especially in regard to the changes occurring because of emphasis on "green" and "sustainable" engineering.These concepts lead to the adoption of unusual measures that can considerably increase the risk of work in these structures.These measures include the capture and storage of rainwater for reuse and recovery of heat from wastewater.These types of water contain organic matter on which aerobic and anaerobic respiration can occur as reported and discussed in this article.Maintenance activities involving these structures are subject to hazardous conditions created through the emission of CO 2 into the airspace above the water.Entry to perform maintenance could be delayed by decades.
This concern extends to respiratory protection.As mentioned in the previous section, NIOSH [30] and other agencies mandate the use of Self-Contained Breathing Apparatus (SCBAs) during work in atmospheres posing unquantifiable inhalation hazards.This choice of respirator introduces additional complexity regarding emergency response.
A possible means to overcome this problem is to wash the interior of the equipment and the structure with high-pressure jets of water to expel toxic gases from the fluid and surface sludges [4].This concept is implemented most efficiently and safely when the means to do so are incorporated by design into the equipment of the structure.
The reality for almost all of the work occurring in construction is that options for control of exposure to CO 2 are limited or non-existent.Efforts must focus on the prevention of overexposure through mandatory ventilation using portable equipment and mandatory routine testing using air monitoring instruments of conditions prior to entry into structures once the identification of actual or potential sources of CO 2 occurs.
This article highlights the occurrence of mostly rare events.In order to eliminate/reduce the occurrence of these events, worksite managers and the technical people who control activity must considerably increase routine air monitoring and ventilation of workspaces.Regulators already mandate such practices.Workers and supervisors are informed about them in training sessions.Once we can identify and catalog what has happened and is continuing to happen and what can happen, point-source rare events become preventable and not simply exotic or as some have said, acts of God.Failure to prevent is punishable by regulators and in some jurisdictions, the courts.Failure to prevent is devastating to all concerned.

Conclusions
This article identifies and discusses sources of emission of CO 2 that can occur during construction activities.Construction activities are highly varied, as are possible scenarios of emission of CO 2 .Fatal events are rare.Accident summaries prepared by regulators and technical reports prepared by research organizations provide little, if any monitoring data.Monitoring data obtained by employers is totally absent.Data, if available at all, originate only from first responders or accident investigators following the event.Hence, the data that are available may not reflect the conditions of work at the time of the event.Since little monitoring data are available and the breadth of possible scenarios is limited, this type of article is restricted to discussion about the anticipation and recognition steps in the accident prevention paradigm (anticipation, recognition, evaluation, control).
Emission of CO 2 can occur from continuous sources as well as those that behave as shear-thinning, pseudoplastic, non-Newtonian fluids.Both aerobic and anaerobic sources can emit CO 2 .Shear-thinning, pseudoplastic, non-Newtonian fluids can form highly constrained systems related to the storage of highly toxic gases (H 2 S, CO 2 , and NH 3 , among others) and unexpected rapid release into the air during the application of a shear force.This characteristic can change atmospheric conditions from ambient to potentially lethal almost instantaneously.Conditions return to ambient rapidly following cessation of application of the shear force.Investigation of fatal atmospheric events occurring in confined spaces indicates that in some cases the causative agent had disappeared at the time of investigation and was otherwise not identifiable.
Well-known chemical and physical processes not widely appreciated or considered in this context contribute to the severity of the situation.The chemical reaction between water and CO 2 considerably increases the quantity available for emission into the air through the creation of reservoir(s) of anions or cations.Conversion of the ionic reservoir through a change in pH to the molecular form can rapidly exceed the solubility of the gas molecules in the water.The more viscous form of a shear-thinning, pseudo-plastic, non-Newtonian fluid can store considerably more molecules of gas than the less viscous form already subjected to the shear force.

4. 4 . 4 .
Conditions That Enhance Emission of CO 2 from Water in In-Service Systems Internal Negative Pressure (Vacuum Testing)

Figure 1 .
Figure 1.Plot of the H2CO3 molecule-HCO3 − ion equilibrium expressed as a fraction of dissolved bicarbonate ion versus pH.This graph visualizes the relationship between pH and H2S molecules.As pH decreases, the % of sulfide present as H2S molecules increases.

Figure 1 .
Figure 1.Plot of the H 2 CO 3 molecule-HCO 3 − ion equilibrium expressed as a fraction of dissolved bicarbonate ion versus pH.This graph visualizes the relationship between pH and H 2 S molecules.As pH decreases, the % of sulfide present as H 2 S molecules increases.

Table 1 .
Physical and chemical properties of gases potentially involved in the causation of fatal events.

Table 2 .
Concentration of gas in the airspace corresponding to a measured level of oxygen.

Table 3 .
Benchmarks for CO 2 compared to a measured level of O 2 in the airspace of a structure.

Table 4 .
Toxicological properties of gases potentially involved in the causation of fatal events.

Table 4
provides values for IDLH (Immediately Dangerous to Life and Health)