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When is CO2 more hazardous than H2S

01.01.2011  |  Beitler, C.,  Trimeric Corp., Buda, TexasFisher, K.,  Trimeric Corp., Buda, TexasMcIntush, K. E.,  Trimeric Corp., Buda, TexasTyndall, K. ,  Pastor, Behling & Wheeler LLC, Round Rock, TexasLundeen, J.,  Trimeric Corp., Buda, Texas

Data shows potential harmful effects to workers due to acid gas exposure

Keywords: [carbon dioxide] [hydrogen sulfide] [acid gas] [safety]

Many different types of facilities produce or use streams containing a high carbon dioxide (CO2) content (98+%) with low hydrogen sulfide (H2S) concentrations, e.g., a few parts per million by volume (ppmv) to a few volume percent (vol%). Examples include CO2-flood enhanced oil recovery, pre-combustion carbon capture (from fossil fuel-fired power plants and industrial facilities) and sequestration, natural gas conditioning, and agricultural manufacturing, among others. In all of these industries, the potential for a release in a processing step or during transmission through a pipeline exists.

The health effects and dangers of H2S are well known, but those of CO2 are not as commonly understood. It is uncertain if industry realizes that CO2 is a mildly toxic gas and not just a simple asphyxiant like nitrogen. Because CO2 itself is toxic at higher concentrations, the high-purity CO2 streams can actually be more hazardous than the H2S and they are the subject of discussion in this article. In such cases, the presence of H2S may actually allow easier detection of the CO2 danger.

This article reviews the hazards of H2S and CO2 and compares the effects from these acid gases on humans. Concentration levels corresponding to the immediately dangerous to life and health (IDLH) levels of the two gases are used to illustrate conditions where both H2S and CO2 are present, and the CO2 (not the H2S) is the predominant concern. A goal is to educate readers to think of CO2 as a mildly toxic gas and not just an asphyxiant, and to recognize conditions where it can represent the more significant hazard, even if small concentrations of H2S are also present.

Toxicity of H2S.

Hydrogen sulfide is an intensely hazardous, toxic compound.1 It is a colorless, flammable gas that can be identified in relatively low concentrations by a characteristic rotten egg odor. This acid gas is naturally occurring and is in the gases from volcanoes, sulfur springs, undersea vents, swamps and stagnant bodies of water and in crude petroleum and natural gas. Hydrogen sulfide is produced when bacteria break down sulfur-containing proteins, and it is a component of decomposing materials. In addition, H2S is also produced from man-made operations and processes such as petroleum refineries, food processing plants, tanneries, municipal sewers, sewage treatment plants, landfills, swine containment and manure-handling operations, and pulp and paper mills.

Hydrogen sulfide has a very low odor threshold, with its smell being easily detected by most people in the range of 0.0005 ppmv to 0.3 ppmv.2 As the gas becomes more concentrated, the odor increases with a strong rotten egg smell identifiable up to 30 ppmv. From about 30 ppmv to 100 ppmv, the gas is stated to have a sickeningly sweet odor. However, at concentrations above 100 ppmv, a person’s ability to detect the gas decreases due to a rapid temporary paralysis of the olfactory nerves in the nose that leads to a loss of the sense of smell. This means that the gas can be present in the environment at extremely high concentrations with no noticeable odor. This unusual property of H2S makes it very dangerous to depend solely on the sense of smell as a warning sign of the gas.3

Once H2S is released as a gas, it remains in the atmosphere for an average of 18 hours, after which it changes to sulfur dioxide and sulfuric acid.2 It is water-soluble and, therefore, it may partition to surface water or adsorb onto moist soil, plant foliage, or other organic material where it loses much of its toxic properties.

Hydrogen sulfide is classified as a chemical asphyxiant, similar to carbon monoxide (CO) and cyanide gases. It interferes with nerve cell function, putting certain nerves to sleep, including olfactory (as discussed previously) and the ones necessary for breathing. Table 1 shows the typical exposure symptoms of H2S.


It is important to note that while most chemicals are toxic, exposure has to occur (at a level that is considered toxic) before adverse health effects are observed. Most, if not all, of the irreversible health outcomes including death have occurred due to overexposure to H2S in confined areas.

Toxicity of CO2.

Carbon dioxide is a slightly toxic, odorless and colorless gas. It is typically found in air at around 360 ppmv (0.036 vol%) while exhaled air may contain as much as 40,000 ppmv (4 vol%). Table 2 shows the general affects of CO2 over different ranges of exposure.

At lower concentrations, CO2 affects the respiratory system and central nervous system. Too much CO2 also acts as a simple asphyxiant by reducing the amount of oxygen available for respiration.6 At higher concentrations, too, the ability to eliminate CO2 decreases and it can accumulate in the body. In this way, CO2 differs from some other asphyxiants, such as nitrogen (N2). Unlike CO2, N2 does not get distributed throughout the body to cause an adverse health effect; rather, N2 acts simply by displacing oxygen from the air and, thereby, decreasing the amount of oxygen available for respiration. Result: CO2 is dangerous at a much lower level than some other asphyxiants, such as N2.


Nitrogen is discussed here because it is a common potential asphyxiant in industrial settings. The following example illustrates the differences between CO2 and N2. Consider a hypothetical example where 90 parts of atmospheric air (normally 21% O2 and 79% N2) are mixed with 10 parts of either pure CO2 or N2. The resulting mixture compositions are shown in Fig. 1.

  Fig. 1. Mixture compositions with 90 parts air   
  and 10 parts CO2 or N2.     

As shown in Fig. 1, the resulting mixture with CO2 addition contains 18.9% O2, 71.1% N2, and 10% CO2. As discussed previously, such a mixture could potentially kill a person. Conversely, the mixture with N2 contains 18.9% O2 and 81.1% N2; while this mixture is lower in oxygen than normal air and below the recommended O2 % for workers, it is not likely to cause irreversible health effects. The effect of going from a 21% oxygen atmosphere to an 18.9% oxygen atmosphere is similar to going from sea level to about 3,000 ft in elevation (roughly the elevation of Midland, Texas), as far as the oxygen partial pressure is concerned. Most people who are acclimated to sea level would have no trouble going to 3,000 ft in elevation.

In summary, mixing 10 parts CO2 with 90 parts air can possibly cause a person breathing the mixture to die if exposed long enough. In contrast, mixing 10% N2 with air probably has little effect on a person. Clearly, it is very important to recognize that CO2 is not the same simple asphyxiant as N2.

Occupational exposure limits for H2S and CO2.

Table 3 provides a summary of occupational exposure limits for H2S and CO2. Occupational exposure limits are typically designed to protect health and to provide for the safety of employees for up to a 40-hour work week, over a working lifetime. The threshold limit value (TLV) was developed by the American Conference of Governmental Industrial Hygienists (ACGIH) while the permissible exposure limit (PEL) is an enforceable standard developed by the Occupational Safety and Health Administration (OSHA). The short-term exposure limit (STEL) was developed by ACGIH and represents a 15-minute time-weighted average exposure that should not be exceeded at any time during the workday. The IDLH value was developed by the National Institute for Occupational Safety and Health (NIOSH) to provide a level at which a worker could escape without injury or irreversible health effects.

IDLH values are conservatively established by NIOSH to give a worker approximately 30 minutes to evacuate an area. The IDLH for both H2S and CO2 are purposefully established below levels at which adverse and irreversible health effects would be seen following 30 minutes of exposure. The IDLH for H2S was developed based on human data (and supplemented with information from laboratory animals) that showed that between 170 ppmv and 300 ppmv, a person can be exposed for one hour without serious health effects and that 400 ppmv to 700 ppmv can be dangerous if exposure is greater than 30 minutes. A person can be exposed to H2S at 800 ppmv for approximately 5 minutes before unconsciousness occurs, while exposure at 1,000 ppmv or greater can cause immediate respiratory arrest, unconsciousness and possibly death.

For CO2, a person can sustain exposure to the IDLH of 40,000 ppmv for 30 minutes with minimal signs of intoxication (e.g., changes in breathing rate, headache and fatigue). At 30 minutes of exposure to 50,000 ppmv CO2, signs of intoxication become more pronounced. A person can sustain exposure to 70,000 ppmv to 100,000 ppmv CO2 for about 5 minutes and signs of intoxication become intense with very labored breathing, visual impairment, headache, ringing in the ears and potentially impaired judgment. Air containing CO2 at a concentration greater than 100,000 ppmv (i.e., 10 vol%) can produce extreme discomfort and, as indicated above, can be life-threatening.

Table 4 shows an example of how a gas stream containing initial concentrations of H2S of 2,000 ppmv and of CO2 of 98 vol% would change assuming a uniform dispersion in air for both compounds. As shown in the table, when the IDLH of H2S (100 ppmv) is reached, the CO2 content is still above the IDLH level of 40,000 ppmv. Even more dramatic are the 5-minute exposure levels; when the H2S exposure level is at the 5-minute limit of 800 ppmv, the CO2 concentration is at 392,000 ppmv, which is far above the level a person can survive for 5 minutes. Thus, given the much higher percentage of the CO2 in this gas stream, the danger from CO2 is higher than the danger posed by H2S.


Potential exposure scenarios to H2S and CO2.

 In actuality, it is difficult to determine the likelihood of a release and the potential concentration a person may encounter following a release. A release could occur at any point in the processing unit or transfer pipeline depending on the source of the stream (see Fig. 2). Atmospheric conditions, such as the wind or physical location of the release (low lying area), can greatly affect the dispersion rate and exposure concentrations of the two compounds. Some potential exposure scenarios are discussed here.

  Fig. 2A. Example sources of high-purity 
  CO2 and low H2S streams—CO2 
  dehydration unit.     

If there is wind, a small release (i.e., not a catastrophic event) would most likely disperse relatively quickly. Under this scenario, a person downwind (unless they were within close proximity to the release) would probably not be exposed to a harmful concentration of either compound. In fact, the presence of H2S (which has an odor at very low concentrations) may actually provide an early indicator of a CO2 release that would otherwise go undetected. Although H2S may provide an early indicator of a release in certain situations, this should not be relied upon because H2S deadens the sense of smell at higher concentrations. Exposure should be kept to a minimum by applying sufficient engineering controls and safe work practices. Appropriate monitoring and personal protective equipment should always be used.

  Fig. 2B.  Example sources of high-purity CO2
  and low H2S streams—CO2 piping.     

Because both compounds are heavier than air (the specific gravity for H2S and CO2 is 1.192 and 1.52, respectively), the most likely place to encounter harmful levels of either compound would be in a low-lying area or depression. This is currently an issue for CO2 pipelines in which harmful levels of CO2 can accumulate in these areas, regardless of the presence of H2S. The presence of H2S increases concerns due to its more insidious toxicity (i.e., it can render a person incapable of escape at sufficiently high concentrations). However, levels above the IDLH could occur in a confined space or depression for either compound. As indicated earlier, the presence of H2S may provide a warning that a release has occurred and prevent a person from entering the area where potentially dangerous levels of CO2 or H2S may be present. Note: The use of direct reading gas detection instrumentation and other protective measures should be required before entering confined spaces such as manholes, tanks, pits and vessels that could contain a buildup of these gases.

Potential synergistic effects of concurrent exposure.

Since the mechanisms of action for CO2 and H2S are very different, it is unlikely that exposure to both compounds will be worse than exposure to only one compound. Most occupational exposure limits are based on exposure to single compounds, even though it is recognized that multiple compounds may be encountered, and Environmental Protection Agency only considers compounds additive if they affect the same target organ or act by the same mechanism. Moreover, industries such as swine production, where both CO2 and H2S are measured in the air, do not adjust occupational exposure limits for added worker safety nor have synergist effects (i.e., effects that are worse when in combination than when exposure is to a single compound) been noted for industries where exposure to both compounds occur.8

Evaluation of risk.

Based on the general qualitative analysis of exposure to both H2S and CO2 discussed here, it appears that there is no increased risk from the presence of H2S at low levels (e.g., up to perhaps 2,000 ppmv or higher) in high-purity CO2 gas. In fact, in these types of gas streams, the potential exposure to high CO2 concentrations during a release event could be as dangerous, or more dangerous, than exposure to lower concentrations of the more toxic H2S. At high concentrations, CO2 may accumulate in the body, which is different than some other asphyxiants (i.e., N2). It is most important to recognize the difference between CO2 and other common asphyxiants. In some cases, the H2S in the gas may serve as a warning for the more hazardous CO2 environment. Dispersion modeling for specific release scenarios should be conducted to better understand possible exposure limits and impacts on human health for both compounds. Appropriate safety precautions should be implemented including monitoring (both fixed and personal detection systems) and training on chemical hazards, personal protection equipment and safety rescue procedures. HP


1 US Environmental Protection Agency, Integrated Risk Information System (IRIS), Profile for Hydrogen Sulfide (CASRN 7783-06-4). Online database, http://www.epa.gov/iris/subst/0061.htm, August 2010.
2 “Hydrogen Sulfide Fact Sheet,” August 2004, SafetyDirectory.com; http://www.safetydirectory.com/hazardous_substances/hydrogen_sulfide/fact_sheet.htm
Agency for Toxic Substances and Disease Registry (ATSDR), Draft Toxicological Profiles for Hydrogen Sulfide, US Department of Health and Human Services. Public Health Service, September 2004.
4 Gossel, T. A. and J. D. Bricker, Principles of Clinical Toxicology, Third Edition, Raven Press, New York, New York, 1994.
5 Goodman Gilman, A., L.S. Goodman, T.W. Rall and R. Murad, The Pharmacological Basis of Therapeutics, Seventh Edition, MacMillan Publishing Co., New York, New York, 1985.
6 Klaassen, C. D., Cassarett and Doull’s Toxicology—The Basic Science of Poisons, Seventh Edition, McGraw-Hill Publishing Co., New York, New York, 2008.
7 National Institute for Occupational Safety and Health, NIOSH Pocket Guide to Chemical Hazards, US Department of Health and Human Services. Public Health Service. Centers for Disease Control and Prevention. February 2004.
8 Lemay, S., L. Chenard and R. MacDonald, “Indoor Air Quality in Pig Buildings: Why Is It Important And How Is It Managed?,” London Swine Conference—Conquering the Challenges, April 11–12, 2004.

The authors

  Kirby Tyndall, PhD, DABT, is a senior consulting toxicologist with Pastor, Behling, & Wheeler, LLC. She is a board certified toxicologist with over 19 years of experience in the fields of toxicology, risk assessment and risk management. Dr. Tyndall has worked in both the environmental consulting and government sectors, and has significant experience evaluating potential human health and ecological risks associated with exposure to contaminants in environmental media (air, water, soil, sediment and biota including fish, etc.). 

  Ken McIntush, PE is a practicing chemical engineer and president of Trimeric Corp., a small company based in Buda, Texas, that is focused on chemical/process engineering. He has about 21 years of varied process engineering experience, serving clients in oil refining, oil and gas processing, silicon refining and several other industries. Mr. McIntush performs troubleshooting, debottlenecking and other projects for the company. He holds a BS degree in chemical engineering from Texas A&M University, College Station.  


Joe Lundeen is a principal engineer at Trimeric Corp. in Buda, Texas. He has 21 years of experience in process engineering, process troubleshooting, and facility installation for oil and gas production and CO2 processing clients. His recent experience has been focused on dehydration, contaminant removal, and transport of super-critical CO2. He holds BS and MS degrees in chemical engineering from the University of Missouri, Rolla. 


Kevin Fisher, PE is a principal engineer at Trimeric Corp. in Buda, Texas. He has over 20 years of experience in process engineering, research and development, and troubleshooting for oil and gas production and oil refining clients, as well as for private and government-sponsored research programs. He holds an MS degree in chemical engineering from the University of Texas, and BS degrees in chemical engineering and chemistry from Texas A&M and Sam Houston State University, respectively. 


Carrie Beitler is a senior engineer at Trimeric Corp. in Buda, Texas. She has over 15 years of experience in process engineering, process modeling and optimization of unit operations in the natural gas, petroleum refining and CO2 processing areas. She also specializes in the development of process design packages for the fabrication of open-art technology such as caustic scrubbers, acid-gas injection units, glycol dehydrators and amine treaters. She graduated with a BS degree in chemical engineering from Purdue University. 


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Abdul Rasak

good way to undastand if you add some video clip all so
thyank you


It is very good article. I would like to take opinion / comments of experts about the case where fuel gas contains Co2 more than one lakh ppm and H2S is less than 90 ppm, In such case, what precautions will be required for using this fuel gas in boiler and power turbine which are proposed to be used in enclosed area at FPSO? I would be grateful if expert give comments with supporting codes / class reference for handling about 0.15MMSCMD natural gas as fuel gas.


Excellent assessment.The same opinion with the precedent comments on H2S. I am working as corrosion engineer. My point is that as both together are the chlorides- not enough considered from environmental and chemical agressivity point of view. I recommend studies on the last- chlorides, present also in our potable (!?) water. Thank you for your efforts. Trimeric is not a "small" Company, having so good specialists. I would like to be in contact with them.


The authors have performed a valuable service by developing this well written article describing the insidious but harmful nature of high concentration CO2 releases into ambient air.

I do take exception to the comparison with H2S however. H2S is arguably the most LETHAL chemical commonly existing. A single breath of 10% H2S can be fatal with a 2nd breath not ever taken. 'I thought I smelled H2S, but it was just a little bit' is a common exclamation from survivors of single breath incapacitation--who were rescued and resuscitated. Such in the unforgiving nature of H2S --as long as you can smell it--you are going to be able to get out to fresh air--when you cannot smell it--you MAY have taken your last breath of life.

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