Carbon dioxide

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Carbon dioxide
Ball-and-stick model of carbon dioxide Carbon dioxide 3D ball.png
Ball-and-stick model of carbon dioxide
Space-filling model of carbon dioxide Carbon dioxide 3D spacefill.png
Space-filling model of carbon dioxide
Other names
  • Carbonic acid gas
  • Carbonic anhydride
  • Carbonic oxide
  • Carbon oxide
  • Carbon(IV) oxide
  • Dry ice (solid phase)
3D model (JSmol)
3DMet B01131
ECHA InfoCard 100.004.271
EC Number 204-696-9
E number E290 (preservatives)
MeSH Carbon+dioxide
PubChem CID
RTECS number FF6400000
UN number 1013 (gas), 1845 (solid)
Molar mass 44.009 g·mol−1
AppearanceColorless gas
  • Low concentrations: none
  • High concentrations: sharp; acidic [1]
  • 1562 kg/m3(solid at 1 atm and −78.5 °C)
  • 1101 kg/m3(liquid at saturation −37°C)
  • 1.977 kg/m3(gas at 1 atm and 0 °C)
Melting point −56.6 °C; −69.8 °F; 216.6 K(Triple point at 5.1 atm)
Critical point (T, P)31.1 °C (304.2 K), 7.38 megapascals (73.8 bar)
−78.5 °C; −109.2 °F; 194.7 K (1 atm)
1.45 g/L at 25 °C (77 °F), 100 kPa
Vapor pressure 5.73 MPa (20 °C)
Acidity (pKa)6.35, 10.33
−20.5·10−6 cm3/mol
Viscosity 0.07 cP at −78.5 °C
0 D
37.135 J/K mol
214 J·mol−1·K−1
−393.5 kJ·mol−1
V03AN02 ( WHO )
Safety data sheet See: data page
NFPA 704
Lethal dose or concentration (LD, LC):
90,000 ppm (human, 5 min) [4]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 5000 ppm (9000 mg/m3) [5]
REL (Recommended)
TWA 5000 ppm (9000 mg/m3) ST 30,000 ppm (54,000 mg/m3) [5]
IDLH (Immediate danger)
40,000 ppm [5]
Related compounds
Other anions
Other cations
Related carbon oxides
Related compounds
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Phase behaviour
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Infobox references

Carbon dioxide (chemical formula CO
) is a colorless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth's atmosphere as a trace gas. The current concentration is about 0.04% (410  ppm) by volume, having risen from pre-industrial levels of 280 ppm. [6] Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater. It is present in deposits of petroleum and natural gas. Carbon dioxide is odorless at normally encountered concentrations. However, at high concentrations, it has a sharp and acidic odor. [1]

A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and plus (+) and minus (−) signs. These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a chemical name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, and are generally more limited in power than are chemical names and structural formulas.

Gas One of the four fundamental states of matter

Gas is one of the four fundamental states of matter. A pure gas may be made up of individual atoms, elemental molecules made from one type of atom, or compound molecules made from a variety of atoms. A gas mixture, such as air, contains a variety of pure gases. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.

Carbon Chemical element with atomic number 6

Carbon is a chemical element with symbol C and atomic number 6. It is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Three isotopes occur naturally, 12C and 13C being stable, while 14C is a radionuclide, decaying with a half-life of about 5,730 years. Carbon is one of the few elements known since antiquity.


As the source of available carbon in the carbon cycle, atmospheric carbon dioxide is the primary carbon source for life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by photosynthetic organisms and geological phenomena. Plants, algae and cyanobacteria use light energy to photosynthesize carbohydrate from carbon dioxide and water, with oxygen produced as a waste product. [7]

Carbon cycle Biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere,

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration to and release from carbon sinks.

Life Characteristic that distinguishes physical entities having biological processes

Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential synthetic life as "living". Biology is the science concerned with the study of life.

The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

is produced by all aerobic organisms when they metabolize carbohydrates and lipids to produce energy by respiration. [8] It is returned to water via the gills of fish and to the air via the lungs of air-breathing land animals, including humans. Carbon dioxide is produced during the processes of decay of organic materials and the fermentation of sugars in bread, beer and wine making. It is produced by combustion of wood and other organic materials and fossil fuels such as coal, peat, petroleum and natural gas. It is an unwanted byproduct in many large scale oxidation processes, for example, in the production of acrylic acid (over 5 million tons/year). [9] [10] [11] [12]

Cellular respiration Cellular enzymatic release of energy from compounds

Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller ones, releasing energy in the process, as weak so-called "high-energy" bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction which releases heat. The overall reaction occurs in a series of biochemical steps, most of which are redox reactions themselves. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a living cell because of the slow release of energy from the series of reactions.

Decomposition The process in which organic substances are broken down into simpler organic matter

Decomposition is the process by which organic substances are broken down into simpler organic matter. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Bodies of living organisms begin to decompose shortly after death. Animals, such as worms, also help decompose the organic materials. Organisms that do this are known as decomposers. Although no two organisms decompose in the same way, they all undergo the same sequential stages of decomposition. The science which studies decomposition is generally referred to as taphonomy from the Greek word taphos, meaning tomb.

Fermentation Anaerobic enzymatic conversion of organic compounds

Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In the context of food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

It is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, as a chemical feedstock and as a supercritical fluid solvent in decaffeination of coffee [13] and supercritical drying. It is added to drinking water and carbonated beverages including beer and sparkling wine to add effervescence. The frozen solid form of CO
, known as dry ice is used as a refrigerant and as an abrasive in dry-ice blasting.

Supercritical drying

Supercritical drying, also known as critical point drying, is a process to remove liquid in a precise and controlled way. It is useful in the production of microelectromechanical systems (MEMS), the drying of spices, the production of aerogel, the decaffeination of coffee and in the preparation of biological specimens for scanning electron microscopy.

Beer alcoholic drink

Beer is one of the oldest and most widely consumed alcoholic drinks in the world, and the third most popular drink overall after water and tea. Beer is brewed from cereal grains—most commonly from malted barley, though wheat, maize (corn), and rice are also used. During the brewing process, fermentation of the starch sugars in the wort produces ethanol and carbonation in the resulting beer. Most modern beer is brewed with hops, which add bitterness and other flavours and act as a natural preservative and stabilizing agent. Other flavouring agents such as gruit, herbs, or fruits may be included or used instead of hops. In commercial brewing, the natural carbonation effect is often removed during processing and replaced with forced carbonation.

Sparkling wine

Sparkling wine is a wine with significant levels of carbon dioxide in it, making it fizzy. While the phrase commonly refers to champagne, EU countries legally reserve that term for products exclusively produced in the Champagne region of France. Sparkling wine is usually either white or rosé, but there are examples of red sparkling wines such as the Italian Brachetto, Bonarda and Lambrusco, Spanish wine cava, Australian sparkling Shiraz, and Azerbaijani "Pearl of Azerbaijan" made from Madrasa grapes. The sweetness of sparkling wine can range from very dry brut styles to sweeter doux varieties.

Carbon dioxide is the most significant long-lived greenhouse gas in Earth's atmosphere. Since the Industrial Revolution anthropogenic emissions – primarily from use of fossil fuels and deforestation – have rapidly increased its concentration in the atmosphere, leading to global warming. Carbon dioxide also causes ocean acidification because it dissolves in water to form carbonic acid. [14]

Greenhouse gas gas in an atmosphere that absorbs and emits radiation within the thermal infrared range

A greenhouse gas is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide and ozone. Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Carbon dioxide in Earths atmosphere Atmospheric constituent; greenhouse gas

Carbon dioxide is an important trace gas in Earth's atmosphere. It is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds. CO
absorbs and emits infrared radiation at wavelengths of 4.26 µm and 14.99 µm and consequently is a greenhouse gas that plays a significant role in influencing Earth's surface temperature through the greenhouse effect.

Industrial Revolution mid 18th – early 19th century period; First Industrial Revolution evolved into the Second Industrial Revolution in the transition years between 1840 and 1870

The Industrial Revolution, now also known as the First Industrial Revolution, was the transition to new manufacturing processes in Europe and the US, in the period from about 1760 to sometime between 1820 and 1840. This transition included going from hand production methods to machines, new chemical manufacturing and iron production processes, the increasing use of steam power and water power, the development of machine tools and the rise of the mechanized factory system. The Industrial Revolution also led to an unprecedented rise in the rate of population growth.


Crystal structure of dry ice Carbon-dioxide-crystal-3D-vdW.png
Crystal structure of dry ice

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640, [15] the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestris). [16]

The Flemish or Flemings are a Germanic ethnic group native to Flanders, in modern Belgium, who speak Flemish, but mostly use the Dutch written language. They are one of two principal ethnic groups in Belgium, the other being the French-speaking Walloons. Flemish people make up the majority of the Belgian population. Historically, all inhabitants of the medieval County of Flanders were referred to as "Flemings", irrespective of the language spoken. The contemporary region of Flanders comprises a part of this historical county, as well as parts of the medieval duchy of Brabant and the medieval county of Loon.

Jan Baptist van Helmont Flemish chemist, physiologist, and physician

Jan Baptist van Helmont was a Flemish chemist, physiologist, and physician. He worked during the years just after Paracelsus and the rise of iatrochemistry, and is sometimes considered to be "the founder of pneumatic chemistry". Van Helmont is remembered today largely for his ideas on spontaneous generation, his 5-year tree experiment, and his introduction of the word "gas" into the vocabulary of science.

Charcoal Lightweight black residue, made of carbon and ashes, after pyrolysis of animal or vegetal substances

Charcoal is a lightweight black carbon residue produced by removing water and other volatile constituents from animal and plant materials. Charcoal is usually produced by slow pyrolysis — the heating of wood or other organic materials in the absence of oxygen. This process is called charcoal burning. The finished charcoal consists largely of carbon.

The properties of carbon dioxide were further studied in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through limewater (a saturated aqueous solution of calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas. [17]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday. [18] The earliest description of solid carbon dioxide was given by Adrien-Jean-Pierre Thilorier, who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO
. [19] [20]

Chemical and physical properties

Stretching and bending oscillations of the CO
2 carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes. Co2 vibrations.svg
Stretching and bending oscillations of the CO
carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

Structure and bonding

The carbon dioxide molecule is linear and centrosymmetric. The carbon–oxygen bond length is 116.3  pm, noticeably shorter than the bond length of a C–O single bond and even shorter than most other C–O multiply-bonded functional groups. [21] Since it is centrosymmetric, the molecule has no electrical dipole. Consequently, only two vibrational bands are observed in the IR spectrum – an antisymmetric stretching mode at 2349 cm−1 and a degenerate pair of bending modes at 667 cm−1. There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum. [22]

In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms H
(carbonic acid), which is a weak acid since its ionization in water is incomplete.

+ H

The hydration equilibrium constant of carbonic acid is (at 25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO
molecules, not affecting the pH.

The relative concentrations of CO
, H
, and the deprotonated forms HCO
(bicarbonate) and CO2−
(carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.

Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3):

H2CO3 HCO3 + H+
Ka1 = 2.5×10−4 mol/L; pKa1 = 3.6 at 25 °C. [21]

This is the true first acid dissociation constant, defined as , where the denominator includes only covalently bound H2CO3 and does not include hydrated CO
(aq). The much smaller and often-quoted value near 4.16×10−7 is an apparent value calculated on the (incorrect) assumption that all dissolved CO
is present as carbonic acid, so that . Since most of the dissolved CO
remains as CO
molecules, Ka1(apparent) has a much larger denominator and a much smaller value than the true Ka1. [23]

The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion (CO32−):

HCO3 CO32− + H+
Ka2 = 4.69×10−11 mol/L; pKa2 = 10.329

In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

Chemical reactions of CO2

is a weak electrophile. Its reaction with basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with CO
to give carboxylates:

where M = Li or Mg Br and R = alkyl or aryl.

In metal carbon dioxide complexes, CO
serves as a ligand, which can facilitate the conversion of CO
to other chemicals. [24]

The reduction of CO
to CO is ordinarily a difficult and slow reaction:

+ 2 e + 2H+ → CO + H2O

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from CO
absorbed from the air and water:

+ nH
+ nO

The redox potential for this reaction near pH 7 is about −0.53 V versus the standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process. [25]

Physical properties

Pellets of "dry ice", a common form of solid carbon dioxide Dry Ice Pellets Subliming.jpg
Pellets of "dry ice", a common form of solid carbon dioxide

Carbon dioxide is colorless. At low concentrations the gas is odorless; however, at sufficiently-high concentrations, it has a sharp, acidic odor. [1] At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.67 times that of air.

Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78.5 °C (−109.3 °F; 194.7 K) and the solid sublimes directly to a gas above −78.5 °C. In its solid state, carbon dioxide is commonly called dry ice.

Pressure-temperature phase diagram of carbon dioxide Carbon dioxide pressure-temperature phase diagram.svg
Pressure–temperature phase diagram of carbon dioxide

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 5.1 bar (517 kPa) at 217 K (see phase diagram). The critical point is 7.38 MPa at 31.1 °C. [26] [27] Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid. [28] This form of glass, called carbonia , is produced by supercooling heated CO
at extreme pressure (40–48 GPa or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.

At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluid known as supercritical carbon dioxide. In this state it is starting (as of 2018) to be used for power generation.

Isolation and production

Carbon dioxide can be obtained by distillation from air, but the method is inefficient. Industrially, carbon dioxide is predominantly an unrecovered waste product, produced by several methods which may be practiced at various scales. [29]

The combustion of all carbon-based fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane and oxygen:

+ 2 O
→ CO
+ 2 H

It is produced by thermal decomposition of limestone, CaCO
by heating (calcining) at about 850 °C (1,560 °F), in the manufacture of quicklime (calcium oxide, CaO), a compound that has many industrial uses:

→ CaO + CO

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide: [30]

Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane). [31] This is a major source of food-grade carbon dioxide for use in carbonation of beer and soft drinks, and is also used for stunning animals such as poultry. In the summer of 2018 a shortage of carbon dioxide for these purposes arose in Europe due to the temporary shut-down of several ammonia plants for maintenance. [32]

Acids liberate CO
from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite. The reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is shown below:

+ 2 HCl → CaCl
+ H

The carbonic acid (H
) then decomposes to water and CO

→ CO
+ H

Such reactions are accompanied by foaming or bubbling, or both, as the gas is released. They have widespread uses in industry because they can be used to neutralize waste acid streams.

Carbon dioxide is a by-product of the fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages and in the production of bioethanol. Yeast metabolizes sugar to produce CO
and ethanol, also known as alcohol, as follows:

2 CO
+ 2 C

All aerobic organisms produce CO
when they oxidize carbohydrates, fatty acids, and proteins. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). The equation for the respiration of glucose and other monosaccharides is:

+ 6 O
6 CO
+ 6 H

Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds. [33] Regardless of the type of organic material, the production of gases follows well defined kinetic pattern. Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "landfill gas"). Most of the remaining 50–55% is methane. [34]


Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. [29] The compound has varied commercial uses but one of its greatest use as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer and sparkling wine.

Precursor to chemicals

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other products, [35] such as metal carbonates and bicarbonates.[ citation needed ] Some carboxylic acid derivatives such as sodium salicylate are prepared using CO
by the Kolbe-Schmitt reaction. [36]

In addition to conventional processes using CO
for chemical production, electrochemical methods are also being explored at a research level. In particular, the use of renewable energy for production of fuels from CO
(such as methanol) is attractive as this could result in fuels that could be easily transported and used within conventional combustion technologies but have no net CO
emissions. [37]


Carbon dioxide bubbles in a soft drink. Soda bubbles macro.jpg
Carbon dioxide bubbles in a soft drink.

Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU [38] (listed as E number E290), US [39] and Australia and New Zealand [40] (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas [41] at about 4 x 106 Pa (40 bar, 580 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents cause dough to rise by producing carbon dioxide. [42] Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or if exposed to acids.


Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation of beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, the most common method used is carbonation with recycled carbon dioxide. With the exception of British Real Ale, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

Wine making

Dry ice used to preserve grapes after harvest. Dry ice used to preserve grapes after harvest.jpg
Dry ice used to preserve grapes after harvest.

Carbon dioxide in the form of dry ice is often used during the cold soak phase in wine making to cool clusters of grapes quickly after picking to help prevent spontaneous fermentation by wild yeast. The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the sugar concentration in the grape must, and thus the alcohol concentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process used to produce Beaujolais wine.

Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this reason, other gases such as nitrogen or argon are preferred for this process by professional wine makers.

Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter. [43] "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress. [44] [45]

Inert gas

It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide is also used as an atmosphere for welding, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more brittle than those made in more inert atmospheres. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium.[ citation needed ] When used for MIG welding, CO
use is sometimes referred to as MAG welding, for Metal Active Gas, as CO
can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.

It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 bar (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminium capsules of CO
are also sold as supplies of compressed gas for air guns, paintball markers/guns, inflating bicycle tires, and for making carbonated water. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines.[ citation needed ] High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying of some food products and technological materials, in the preparation of specimens for scanning electron microscopy [ citation needed ] and in the decaffeination of coffee beans.

Fire extinguisher

Use of a CO
2 fire extinguisher. US Army 53023 Fire Prevention Week.jpg
Use of a CO
fire extinguisher.

Carbon dioxide can be used to extinguish flames by flooding the environment around the flame with the gas. It does not itself react to extinguish the flame, but starves the flame of oxygen by displacing it. Some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, because although it excludes oxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses they are free to catch fire upon exposure to atmospheric oxygen. Their desirability in electrical fire stems from the fact that, unlike water or other chemical based methods, Carbon dioxide will not cause short circuits, leading to even more damage to equipment. Because it is a gas, it is also easy to dispense large amounts of the gas automatically in IT infrastructure rooms, where the fire itself might be hard to reach with more immediate methods because it is behind rack doors and inside of cases. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of a protected space. [46] International Maritime Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it can cause suffocation in sufficiently high concentrations. A review of CO
systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries. [47]

Supercritical CO2 as solvent

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine from coffee. Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. Supercritical CO2 extraction is commonly used in the cannabis industry to extract cannabinoids and terpenes, as it allows for selective extraction of certain compounds without residual solvents in the product. [48] It is also used by some dry cleaners for this reason (see green chemistry). It is used in the preparation of some aerogels because of the properties of supercritical carbon dioxide.

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO
to sustain and increase the rate of plant growth. [49] [50] At very high concentrations (100 times atmospheric concentration, or greater), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies and spider mites in a greenhouse. [51]

It has been proposed that CO
from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into biodiesel fuel. [52]

Medical and pharmacological uses

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to oxygen for stimulation of breathing after apnea and to stabilize the O
balance in blood.

Carbon dioxide can be mixed with up to 50% oxygen, forming an inhalable gas; this is known as Carbogen and has a variety of medical and research uses.

Oil recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions, when it becomes miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by between 7% to 23% additional to primary extraction. [53] It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well. [54] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Bio transformation into fuel

A strain of the cyanobacterium Synechococcus elongatus has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO
using photosynthesis. [55]


Comparison of phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere Comparison carbon dioxide water phase diagrams.svg
Comparison of phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere

Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below −78.5 °C at regular atmospheric pressure, regardless of the air temperature.

Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery of R-12 and may enjoy a renaissance due to the fact that R134a contributes to climate change more than CO
does. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to the need to operate at pressures of up to 130 bar (1880 psi), CO
systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others. Coca-Cola has fielded CO
-based beverage coolers and the U.S. Army is interested in CO
refrigeration and heating technology. [56] [57]

The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO
is one discussed option.(see Sustainable automotive air conditioning)

Coal bed methane recovery

In enhanced coal bed methane recovery, carbon dioxide would be pumped into the coal seam to displace methane, as opposed to current methods which primarily rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane. [58]

Minor uses

A carbon dioxide laser. Carbon Dioxide Laser At The Laser Effects Test Facility.jpg
A carbon dioxide laser.

Carbon dioxide is the lasing medium in a carbon dioxide laser, which is one of the earliest type of lasers.

Carbon dioxide can be used as a means of controlling the pH of swimming pools, [59] by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over calcium carbonate in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some corals to build their skeleton.

Used as the primary coolant in the British advanced gas-cooled reactor for nuclear power generation.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO
include placing animals directly into a closed, prefilled chamber containing CO
, or exposure to a gradually increasing concentration of CO
. In 2013, the American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a displacement rate of 10% to 30% of the gas chamber volume per minute is optimal for the humane euthanization of small rodents. [60] However, there is opposition to the practice of using carbon dioxide for this, on the grounds that it is cruel. [45]

Carbon dioxide is also used in several related cleaning and surface preparation techniques.

In Earth's atmosphere

The Keeling Curve of atmospheric CO
2 concentrations measured at Mauna Loa Observatory Mauna Loa CO2 monthly mean concentration.svg
The Keeling Curve of atmospheric CO
concentrations measured at Mauna Loa Observatory

Carbon dioxide in Earth's atmosphere is a trace gas, currently (mid 2018) having a global average concentration of 409 parts per million by volume [61] [62] [63] (or 622 parts per million by mass). Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higher [64] and indoors they can reach 10 times background levels.

Yearly increase of atmospheric CO
2: In the 1960s, the average annual increase was 35% of the 2009-2018 average. CO2 increase rate.png
Yearly increase of atmospheric CO
: In the 1960s, the average annual increase was 35% of the 2009-2018 average.

The concentration of carbon dioxide has risen due to human activities. [66] Combustion of fossil fuels and deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 43% since the beginning of the age of industrialization. [67] Most carbon dioxide from human activities is released from burning coal and other fossil fuels. Other human activities, including deforestation, biomass burning, and cement production also produce carbon dioxide. Human activities emit about 29 billion tons of carbon dioxide per year, while volcanoes emit between 0.2 and 0.3 billion tons. [68] [69] Human activities have caused CO
to increase above levels not seen in hundreds of thousands of years. Currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans. [70] [71] [72] [73]

While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies (see the section "Structure and bonding" above). Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm−1, [74] as opposed to light emission from the much hotter sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric carbon dioxide traps energy near the surface, warming the surface and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption. [75] [76] Increases in atmospheric concentrations of CO
and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century. Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined and because it has a long atmospheric lifetime (hundreds to thousands of years).

2 in Earth's atmosphere if half of global-warming emissions are not absorbed.
(NASA computer simulation). M15-162b-EarthAtmosphere-CarbonDioxide-FutureRoleInGlobalWarming-Simulation-20151109.jpg
in Earth's atmosphere if half of global-warming emissions are not absorbed.
(NASA computer simulation).

Not only do increasing carbon dioxide concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide. This produces a positive feedback for changes induced by other processes such as orbital cycles. [77] Five hundred million years ago the carbon dioxide concentration was 20 times greater than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago. [78] [79]

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO
rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection. [80] High concentrations of CO
produced by disturbance of deep lake water saturated with CO
are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986. [81]

In the oceans

Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100. Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.jpg
Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.

Carbon dioxide dissolves in the ocean to form carbonic acid (H2CO3), bicarbonate (HCO3) and carbonate (CO32−). There is about fifty times as much carbon dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO
emitted by human activity. [82]

As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans, which is referred to as ocean acidification. This reduction in pH affects biological systems in the oceans, primarily oceanic calcifying organisms. These effects span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and mollusks. Under normal conditions, calcium carbonate is stable in surface waters since the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. [83] Corals, [84] [85] [86] coccolithophore algae, [87] [88] [89] [90] coralline algae, [91] foraminifera, [92] shellfish [93] and pteropods [94] experience reduced calcification or enhanced dissolution when exposed to elevated CO

Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents) [95] and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise.

Most of the CO
taken up by the ocean, which is about 30% of the total released into the atmosphere, [96] forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed by photosynthetic organisms that remove carbon from the cycle. Increased CO
in the atmosphere has led to decreasing alkalinity of seawater, and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases, [97] although there's evidence of increased shell production by certain species under increased CO
content. [98]

NOAA states in their May 2008 "State of the science fact sheet for ocean acidification" that:
"The oceans have absorbed about 50% of the carbon dioxide (CO
) released from the burning of fossil fuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about 30% since the start of the industrial age through a process known as "ocean acidification." A growing number of studies have demonstrated adverse impacts on marine organisms, including:

Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007: Synthesis Report: [99]
"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO
concentrations lead to further acidification ... While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively. [100]

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano in the Marianas Trench, produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in the Okinawa Trough. [101] The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006. [102]

Biological role

Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breaking down sugars, fats and amino acids with oxygen as part of their metabolism. This includes all plants, algae and animals and aerobic fungi and bacteria. In vertebrates, the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., amphibians) or the gills (e.g., fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis, plants can absorb more carbon dioxide from the atmosphere than they release in respiration.

Photosynthesis and carbon fixation

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO
2). Auto-and heterotrophs.png
Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO
Overview of the Calvin cycle and carbon fixation Calvin-cycle4.svg
Overview of the Calvin cycle and carbon fixation

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and (cyanobacteria) into energy-rich organic molecules such as glucose, thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars from which other organic compounds can be constructed, and oxygen is produced as a by-product.

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the enzyme involved in the first major step of carbon fixation, the production of two molecules of 3-phosphoglycerate from CO
and ribulose bisphosphate, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth. [103]

Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the biosynthesis of more complex organic molecules, such as polysaccharides, nucleic acids and proteins. These are used for their own growth, and also as the basis of the food chains and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the coccolithophores synthesise hard calcium carbonate scales. [104] A globally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed the basis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.

Plants can grow as much as 50 percent faster in concentrations of 1,000 ppm CO
when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients. [105] Elevated CO
levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated CO
in FACE experiments. [106] [107]

Increased atmospheric CO
concentrations result in fewer stomata developing on plants [108] which leads to reduced water usage and increased water-use efficiency. [109] Studies using FACE have shown that CO
enrichment leads to decreased concentrations of micronutrients in crop plants. [110] This may have knock-on effects on other parts of ecosystems as herbivores will need to eat more food to gain the same amount of protein. [111]

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO
. [112] [113]

Plants also emit CO
during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO
each year, a mature forest will produce as much CO
from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants. [114] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon [115] and remain valuable carbon sinks, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO
in the upper ocean and thereby promotes the absorption of CO
from the atmosphere. [116]


Main symptoms of carbon dioxide toxicity, by increasing volume percent in air. Main symptoms of carbon dioxide toxicity.svg
Main symptoms of carbon dioxide toxicity, by increasing volume percent in air.

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km (19 mi) altitude) varies between 0.036% (360 ppm) and 0.041% (410 ppm), depending on the location. [118] [ clarification needed ]

is an asphyxiant gas and not classified as toxic or harmful in accordance with Globally Harmonized System of Classification and Labelling of Chemicals standards of United Nations Economic Commission for Europe by using the OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling. [117] Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. [119] The physiological effects of acute carbon dioxide exposure are grouped together under the term hypercapnia, a subset of asphyxiation.

Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of Goma by CO
emissions from the nearby volcano Mt. Nyiragongo. [120] The Swahili term for this phenomenon is 'mazuku'.

Rising levels of CO
2 threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the lunar module, which they used as a lifeboat. Apollo13 apparatus.jpg
Rising levels of CO
threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the lunar module, which they used as a lifeboat.

Adaptation to increased concentrations of CO
occurs in humans, including modified breathing and kidney bicarbonate production, in order to balance the effects of blood acidification (acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible, as decrement in performance or in normal physical activity does not happen at this level of exposure for five days. [121] [122] Yet, other studies show a decrease in cognitive function even at much lower levels. [123] [124] Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse such condition.

Below 1%

There are few studies of the health effects of long-term continuous CO
exposure on humans and animals at levels below 1%. Occupational CO
exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period. [125] At this CO
concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. [126] Studies in animals at 0.5% CO
have demonstrated kidney calcification and bone loss after eight weeks of exposure. [127] A study of humans exposed in 2.5 hour sessions demonstrated significant effects on cognitive abilities at concentrations as low as 0.1% (1000ppm) CO
likely due to CO
induced increases in cerebral blood flow. [123] Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm. [124]


2 concentration meter using a nondispersive infrared sensor CO2Mini monitor.jpg
concentration meter using a nondispersive infrared sensor

Poor ventilation is one of the main causes of excessive CO
concentrations in closed spaces. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO
concentration has stabilized) are sometimes used to estimate ventilation rates per person.[ citation needed ] Higher CO
concentrations are associated with occupant health, comfort and performance degradation.[ citation needed ] ASHRAE Standard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Concentrations in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000).

Miners, who are particularly vulnerable to gas exposure due to an insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "blackdamp," "choke damp" or "stythe." Before more effective technologies were developed, miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The Davy lamp could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk, would make the lamp burn more brightly.

Human physiology


Reference ranges or averages for partial pressures of carbon dioxide (abbreviated pCO
kPa mmHg
Venous blood carbon dioxide5.5–6.841–51 [128]
Alveolar pulmonary
gas pressures
Arterial blood carbon dioxide 4.7–6.035–45 [128]

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person, [129] containing 0.63 pounds (290 g) of carbon. In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs, resulting in lower concentrations in the arteries. The carbon dioxide content of the blood is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume. [130] In humans, the blood carbon dioxide contents is shown in the adjacent table:

Transport in the blood

is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO
bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO
decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO
or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.

Regulation of respiration

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its concentration is high, the capillaries expand to allow a greater blood flow to that tissue.

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO
in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which can cause respiratory alkalosis.

Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness. [131]

The respiratory centers try to maintain an arterial CO
pressure of 40 mm Hg. With intentional hyperventilation, the CO
content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.

See also

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Estuarine acidification happens when the pH balance of water in coastal marine ecosystems, specifically those of estuaries, decreases. Water, generally considered neutral on the pH scale, normally perfectly balanced between alkalinity and acidity.While ocean acidification occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO2) from the atmosphere, pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off, human impact, and coastal current dynamics. In the ocean, wave and wind movement allows carbon dioxide (CO2) to mixes with water (H2O) forming carbonic acid (H2CO3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming carbonate (CO3) which is basic and helps form shells for ocean creatures, and two hydron molecules. This creates the potential for acidic threat since hydron ions readily bond with any Lewis Structure to form an acidic bond. This is referred to as an oxidation-reduction reaction.

Impacts of ocean acidification on the Great Barrier Reef Threat to the reef which reduces the viability and strength of reef-building corals

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean.

Ocean storage of carbon dioxide (CO2) is a method of carbon sequestration. The concept of storing carbon dioxide in the ocean was first proposed by Italian physicist Cesare Marchetti in his 1976 paper "On Geoengineering and the carbon dioxide Problem." Since then, the concept of sequestering atmospheric carbon dioxide in the world's oceans has been investigated by scientists, engineers, and environmental activists. 39,000 GtC (gigatonnes of carbon) currently reside in the oceans while only 750 GtC are in the atmosphere. Of the 1300 Gt carbon dioxide from anthropogenic emissions over the last 200 years, about 38% of that has already gone into the oceans. Carbon dioxide is currently emitted at 10 GtC per year and the oceans currently absorb 2.4 Gt carbon dioxide per year. The ocean is an enormous carbon sink with the capacity to hold thousands more gigatons of carbon dioxide. Ocean sequestration has the potential to decrease atmospheric carbon dioxide concentrations according to some scientists.

Ocean acidification in the Arctic Ocean

The Arctic ocean has experienced drastic change over the years due to global warming. It has been known that the Arctic ocean acidity levels have been increasing and at twice the rate compared to the Pacific and Atlantic oceans. The loss of sea ice has been connected to a decrease in pH levels in the ocean water. Sea ice has experienced an extreme reduction over the past 30 years, forming a minimum area of 2.9×106 km2 at the end of the boreal summer of 2007, 47%, less than in 1980. Sea ice limits the air-sea gas exchange with carbon dioxide. With less water completely exposed to the atmosphere, the levels of carbon dioxide gas in the water remain low. The Arctic ocean should have low carbon dioxide levels due to intense cooling, run off of fresh water and photosynthesis from marine organisms. However, the decrease of sea ice over the years due to global warming has limited freshwater runoff and has exposed a higher percentage of the ocean surface to the atmosphere. The increase of carbon dioxide in the water decreases the pH of the ocean causing ocean acidification. The decrease in sea ice has also allowed more Pacific water to flow into in the Arctic ocean during the winter, this is called Pacific winter water. The Pacific water flows into the Arctic ocean carrying additional amounts of carbon dioxide by being exposed to the atmosphere and absorbing carbon dioxide from decaying organic matter and from sediments.


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