lunar eclipse

Lunar phase

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Animation of lunar phases (in the northern hemisphere)

"Moon phase" redirects here. For the manga and anime series, see Tsukuyomi: Moon Phase.

Lunar phase (or Moon phase) refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun. One half of the lunar surface is always illuminated by the Sun (except during lunar eclipses), and is hence bright, but the portion of the illuminated hemisphere that is visible to an observer can vary from 100% (full moon) to 0% (new moon). The boundary between the illuminated and unilluminated hemispheres is called the terminator.

Contents [hide] 1 Overview 2 Names of lunar phases 3 Calendar 4 See also 5 External links 6 Educational aids

[edit] Overview

The lunar phase depends on the Moon's position in orbit around the Earth, and the Earth's position in orbit around the sun. This diagram looks down on Earth from north. Earth's rotation and the Moon's orbit are both counter-clockwise here. Sunlight is coming in from the right, as indicated by the yellow arrows. From this diagram we can see, for example, that the full moon will always rise at sunset, and that the waning crescent moon is high overhead around 9:00 AM local time.

Lunar phases are the result of looking at the illuminated half of the Moon from different viewing geometries; they are not caused by shadows of the Earth on the Moon that occur during a lunar eclipse. The Moon exhibits different phases as the relative geometry of the Sun, Earth, and Moon change, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth, and as a new moon (also named dark moon, as it is not visible at night) when they are on the same side. The phases of full moon and new moon are examples of syzygies, which occur when the Earth, Moon, and Sun lie (approximately) in a straight line. The time between two full moons (and between successive occurrences of the same phase) is about 29.53 days (29 days, 12 hours, 44 minutes) on average (hence, the concept of a timeframe of a period of time of an approximated month was derived). This synodic month is longer than the time it takes the Moon to make one orbit about the Earth with respect to the fixed stars (the sidereal month), which is about 27.32 days. This difference is caused by the fact that the Earth-Moon system is orbiting about the Sun at the same time the Moon is orbiting about the Earth. The actual time between two syzygies is variable because the orbit of the Moon is elliptic and subject to various periodic perturbations, which change the velocity of the Moon.

It might be expected that once every month when the Moon passes between Earth and the Sun during a new moon, its shadow would fall on Earth causing a solar eclipse. Likewise, during every full moon, we might expect the Earth's shadow to fall on the Moon, causing a lunar eclipse. We do not observe a solar and lunar eclipse every month because the plane of the Moon's orbit around the Earth is tilted by about 5 degrees with respect to the plane of Earth's orbit around the Sun. Thus, when new and full moons occur, the Moon usually lies to the north or south of a direct line through the Earth and Sun. Although an eclipse can only occur when the Moon is either new or full, it must also be positioned very near the intersection of Earth's orbit plane about the Sun and the Moon's orbit plane about the Earth (that is, at one of its nodes). This happens about twice per year, and so there are between 4 and 7 eclipses in a calendar year. Most of these are quite insignificant; major eclipses of the Moon or Sun are relatively rare.

[edit] Names of lunar phases

The phases of the Moon have been given the following names, which are listed in sequential order:

Phase	Northern Hemisphere	Southern Hemisphere
Darkened moon	Not visible	Not visible
New moon	Not visible, or traditionally, the first visible crescent of the Moon
Waxing Crescent moon .	Right 1-49% visible	Left 1-49% visible
First Quarter moon	Right 50% visible	Left 50% visible
Waxing gibbous moon	Right 51-99% visible	Left 51-99% visible
Full Moon	Fully visible	Fully visible
Waning gibbous Moon	Left 51-99% visible	Right 51-99% visible
Last Quarter Moon	Left 50% visible	Right 50% visible
Waning Crescent Moon	Left 1-49% visible	Right 1-49% visible

Phases of the Moon, as seen from the Northern Hemisphere.

Animation of the Moon as it cycles through its phases, as seen from the Northern Hemisphere. The apparent wobbling of the Moon is known as libration.

When the Sun and Moon are aligned on the same side of the Earth, the Moon is "new", and the side of the Moon visible from Earth is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress from new moon, crescent moon, first-quarter moon, gibbous moon and full moon phases, before returning through the gibbous moon, third-quarter moon, crescent moon and new moon phases. The terms old moon and new moon are interchangeable, although new moon is more common. Half moon is often used to mean the first- and third-quarter moons.

Gibbous (red) and crescent (blue) shapes.

When a sphere is illuminated on one hemisphere and viewed from a different angle, the portion of the illuminated area that is visible will have a two-dimensional shape defined by the intersection of an ellipse and circle (where the major axis of the ellipse coincides with a diameter of the circle). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards), whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent.

In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving towards a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (moving towards a new moon). Assuming that one is in the northern hemisphere, the right portion of the Moon is the part that is always growing.

A waxing crescent moon if viewed in the northern hemisphere or a waning crescent moon if viewed in the southern hemisphere.

[edit] Calendar

Main article: Lunar calendar

The average calendrical month, which is 1/12 of a year, is about 30.4 days, while the Moon's phase (synodic) cycle repeats every 29.53 days. Therefore the timing of the Moon's phases shifts by an average of about one day for each successive month. If you photographed the Moon's phase every day for a month, starting in the evening after sunset, and repeating approximately 25 minutes later each successive day, ending in the morning before sunrise, you could create a composite image like the example calendar below from May 8, 2005 to June 6, 2005. Note that there is no picture on May 20 since a picture would be taken before midnight on May 19, and after midnight on May 21. For a similar reason, if you look at a calendar listing moon rise or set times, there will be days where the moon neither rises nor sets.

dioxine

Dioxines have been a part of our environment for more than 60 million years, and there are more than 200 different types of them. The chemical connection is among the most poisonous group we know, both for man and nature. 12 of the dioxines are especially dangerous and hard for nature to break down. Dioxines are toxic material which is being formed as a by-product in consumption processes. Studies show that dioxines cause cancer, reduces immune-defence, gives gene- and hormone disturbances, embryo damage and other serious harm. In the USA the legal concentrationlevel can't exceed 0,001 pg/liter. The Dioxine molecule

GRENLAND AND HYDRO

Fish and shellfish from our area, Grenland, will not be edible until after the year 2000, and this is a huge problem for the fishermen. The content of dioxines in fish from the Frierfjord (in Grenland) was in 1986 from 1,5 to 12 ng TCCD per kilo fish. We still don`t know how much damage the dioxines do to our health.

There is a little mystery around this. Further out in the fjord the cod has a lower content, but the crabs has a higher content. The scientists can`t explain this, but they think it has to do with the the current in the area.

The highest concentration of dioxines that has been measured in Norway, came from the Hydro plant in Porsgrunn. After this they had to spend millions of kroner (Norwegian monetary unit) to clean the air and the water. This was an economic downer for Hydro.

RUSSIA & EASTERN EUROPE

A few years ago, the end of 1980, the problems about dioxins established attention in the media. Before that, the gouverment kept this as a secret. In opposition to the West, they had been aware of the dioxins since the end of the 60`s.

Even if all the coutries in the East have huge dioxin sources, they can hardly be compared to the enormous emission in Russia. These problems are to be changed.
There are two options to reduse dioxin pollution in Russia:

They have to focus on new tecnology and purify their own emissions.

The other solution demands that the cellulose idustry, must increase their acitivity because of their economic unstability

In 1991/92 was the first mapping of dioxin emission from cellulose industry accomplished in Russia. The main source to these polutions are the local paper- and cellulose industry, the constructions in Kotlas, Solobala and Arkhangelsk city. The dioxin contents were messured to 1825 times the Netherland bordervalues.

HEALTH DAMAGES

A lot of dioxines are extremely poisonous to people and mammals. The most spoken of and poisonous dioxine is TCDD. The most important health damages from TCDD on people is rash on their skin, enlargened liver and partial damaged liver functions. The exposed groups have been small, though, which has made it impossible to confirm the developing effect. In Sweden there has been proved an accelerated rate of cancer among the foresters who has sprayed trees with TCDD. Testing on animals have been made to find out the range of damage of the dioxines. There have been several accidents on places where dioxines are made as a bi-product, and there have been a total of 15 such incidents. A major one in Seveso, Italy 1976, where 187 people was injured.

ENVIRONMENTAL ORGANISATIONS & MEDIA

The environmental-organisation Bellona did not take action before the Norwegian Environmental Bureau (SFT) and Hydro had agreed on a measure against dioxines. The media twisted the case with fiction and biased comments. Following this, people believed living here in Grenland was a threat to their health.

Pollution in the Atlantic and the Mediterranean. Note the polluted Bay of Biscay!

carbon monoxide

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Carbon monoxide
IUPAC name	[show] Carbon monoxide
Other names	Carbonic oxide
Identifiers
CAS number	630-08-0
EC number	211-128-3
UN number	1016
RTECS number	FG3500000
ChemSpider ID	275
Properties
Molecular formula	CO
Molar mass	28.010 g/mol
Appearance	Colourless, odorless gas
Density	0.789 g/mL, liquid 1.250 g/L at 0 °C, 1 atm 1.145 g/L at 25 °C, 1 atm
Melting point	-205 °C (68 K)
Boiling point	-192 °C (81 K)
Solubility in water	0.0026 g/100 mL (20 °C)
Dipole moment	0.112 D
Hazards
MSDS	External MSDS
MSDS	ICSC 0023
EU classification	Highly flammable (F+) Repr. Cat. 1 Toxic (T)
EU Index	006-001-00-2
NFPA 704	4 4 0
R-phrases	R61, R12, R23, R48/23
S-phrases	S53, S45
Flash point	Flammable gas
Related compounds
Related carbon oxides	Carbon dioxide Carbon suboxide Dicarbon monoxide Carbon trioxide
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Carbon monoxide, with the chemical formula CO, is a colorless and odorless, tasteless, yet highly toxic gas. Its molecules consist of one carbon atom covalently bonded to one oxygen atom. There are two covalent bonds and a coordinate covalent bond between the oxygen and carbon atoms.

Carbon monoxide is produced from the partial oxidation of carbon-containing compounds, notably in internal-combustion engines. Carbon monoxide forms in preference to the more usual carbon dioxide when there is a reduced availability of oxygen present during the combustion process. Carbon monoxide has significant fuel value, burning in air with a characteristic blue flame, producing carbon dioxide. Despite its serious toxicity, CO plays a highly useful role in modern technology, being a precursor to myriad products.

Contents [hide] 1 Production 2 Structure 3 Principal chemical reactions 3.1 Industrial uses 3.2 Coordination chemistry 3.3 Organic and main group chemistry 4 Carbon monoxide in the atmosphere 5 Role in physiology and food 6 History 7 Source concentrations 8 Toxicity 9 See also 10 References 11 External links

[edit] Production

Carbon monoxide is so fundamentally important that many methods have been developed for its production.^[1]

Producer gas is formed by combustion of carbon in oxygen at high temperatures when there is an excess of carbon. In an oven, air is passed through a bed of coke. The initially produced CO₂ equilibrates with the remaining hot carbon to give CO. The reaction of O₂ with carbon to give CO is described as the Boudouard equilibrium. Above 800 °C, CO is the predominant product:

O₂ + 2 C → 2 CO

ΔH = -221 kJ/mol

The downside of this method is if done with air it leaves a mixture that is mostly nitrogen.

Synthesis gas or Water gas is produced via the endothermic reaction of steam and carbon:

H₂O + C → H₂ + CO

ΔH = 131 kJ/mol

CO also is a byproduct of the reduction of metal oxide ores with carbon, shown in a simplified form as follows:

MO + C → M + CO

ΔH = 131 kJ/mol

Since CO is a gas, the reduction process can be driven by heating, exploiting the positive (favorable) entropy of reaction. The Ellingham diagram shows that CO formation is favored over CO₂ in high temperatures.

CO is the anhydride of formic acid. As such it is conveniently produced by the dehydration of formic acid, for example with sulfuric acid. Another laboratory preparation for carbon monoxide entails heating an intimate mixture of powdered zinc metal and calcium carbonate.

Zn + CaCO₃ → ZnO + CaO + CO

Another laboratory method to generate carbon monoxide is reacting sucrose and sodium hydroxide in a closed system.

[edit] Structure

The CO molecule possesses a bond length of 0.1128 nm.^[2] Formal charge and electronegativity difference cancel each other out. The result is a small dipole moment with its negative end on the carbon atom^[3]. The reason for this, despite oxygen's greater electronegativity, is that the highest occupied molecular orbital has an energy much closer to that of carbon's p orbitals, meaning that greater electron density is found near the carbon. In addition, carbon's lower electronegativity creates a much more diffuse electron cloud, enhancing the dipole moment. This is also the reason that almost all chemistry involving carbon monoxide occurs through the carbon atom, and not the oxygen.

The molecule's bond length is consistent with a partial triple bond. The molecule has a small dipole moment and can be represented by three^{[citation needed]} resonance structures:

[edit] Principal chemical reactions

[edit] Industrial uses

Carbon monoxide is a major industrial gas that has many applications in bulk chemicals manufacturing.^[4]

High volume aldehydes are produced by the hydroformylation reaction of alkenes, CO, and H₂. In one of many applications of this technology, hydroformylation is coupled to the Shell Higher Olefin Process to give precursors to detergents.

Methanol is produced by the hydrogenation of CO. In a related reaction, the hydrogenation of CO is coupled to C-C bond formation, as in the Fischer-Tropsch process where CO is hydrogenated to liquid hydrocarbon fuels. This technology allows coal to be converted to petrol.

In the Monsanto process, carbon monoxide and methanol react in the presence of a homogeneous rhodium catalyst and HI to give acetic acid. This process is responsible for most of the industrial production of acetic acid.

Carbon monoxide is a principal component of syngas, which is often used for industrial power. Carbon monoxide(CO) is also used in industrial scale operations for purifying Nickel (Mond Process}.

[edit] Coordination chemistry

Main article: metal carbonyl

The HOMO of CO is a σ MO

The LUMO of CO is a π* antibonding MO

Most metals form coordination complexes containing covalently attached carbon monoxide. Only those in lower oxidation states will complex with carbon monoxide ligands. This is because there must be sufficient electron density to facilitate back donation from the metal d_xz-orbital, to the π* molecular orbital from CO. The lone pair on the carbon atom in CO, also donates electron density to the d_x²−y² on the metal to form a sigma bond. In nickel carbonyl, Ni(CO)₄ forms by the direct combination of carbon monoxide and nickel metal at room temperature. For this reason, nickel in any tubing or part must not come into prolonged contact with carbon monoxide (corrosion). Nickel carbonyl decomposes readily back to Ni and CO upon contact with hot surfaces, and this method was once used for the industrial purification of nickel in the Mond process.^[5]

In nickel carbonyl and other carbonyls, the electron pair on the carbon interacts with the metal; the carbon monoxide donates the electron pair to the metal. In these situations, carbon monoxide is called the carbonyl ligand. One of the most important metal carbonyls is iron pentacarbonyl, Fe(CO)₅:

Many metal-CO complexes are prepared by decarbonylation of organic solvents, not from CO. For instance, iridium trichloride and triphenylphosphine react in boiling 2-methoxyethanol or DMF) to afford IrCl(CO)(PPh₃)₂.

[edit] Organic and main group chemistry

In the presence of strong acids and water, carbon monoxide reacts with olefins to form carboxylic acids in a process known as the Koch-Haaf reaction.^[6] In the Gattermann-Koch reaction, arenes are converted to benzaldehyde derivatives in the presence of AlCl₃ and HCl.^[7] Organolithium compounds, e.g. butyl lithium react with CO, but this reaction enjoys little use.

Although CO reacts with carbocations and carbanions, it is relatively unreactive toward organic compounds without the intervention of metal catalysts.^[8]

With main group reagents, CO undergoes several noteworthy reactions. Chlorination of CO is the industrial route to the important compound phosgene. With borane CO forms an adduct, H₃BCO, which is isoelectronic with the acylium cation [H₃CCO]⁺. CO reacts with sodium to give products resulting from C-C coupling such as Na₂C₂O₂ (sodium acetylenediolate), and potassium to give K₂C₂O₂ (potassium acetylenediolate) and K₂C₆O₆ (potassium rhodizonate).

[edit] Carbon monoxide in the atmosphere

MOPITT 2000 global carbon monoxide

Carbon monoxide, though thought of as a pollutant today, has always been present in the atmosphere, chiefly as a product of volcanic activity. It occurs dissolved in molten volcanic rock at high pressures in the earth's mantle. Carbon monoxide contents of volcanic gases vary from less than 0.01% to as much as 2% depending on the volcano.^{[citation needed]} It also occurs naturally in bushfires. Because natural sources of carbon monoxide are so variable from year to year, it is extremely difficult to accurately measure natural emissions of the gas.

Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of methane and tropospheric ozone through chemical reactions with other atmospheric constituents (e.g., the hydroxyl radical, OH^.) that would otherwise destroy them. Through natural processes in the atmosphere, it is eventually oxidized to carbon dioxide. Carbon monoxide concentrations are both short-lived in the atmosphere and spatially variable.

In urban areas carbon monoxide, along with aldehydes, reacts photochemically to produce peroxy radicals. Peroxy radicals react with nitrogen oxide to increase the ratio of NO₂ to NO, which reduces the quantity of NO that is available to react with ozone. Carbon monoxide is also a constituent of tobacco smoke.

[edit] Role in physiology and food

Carbon monoxide is used in modified atmosphere packaging systems in the US, mainly with fresh meat products such as beef and pork. The CO combines with myoglobin to form carboxymyoglobin, a bright cherry red pigment. Carboxymyoglobin is more stable than the oxygenated form of myoglobin, oxymyoglobin, which can become oxidized to the brown pigment, metmyoglobin. This stable red colour can persist much longer than in normally packaged meat, giving the appearance of freshness.^[9] Typical levels of CO used are 0.4% to 0.5%. The technology was first given generally recognized as safe status by the U.S. Food and Drug Administration (FDA) in 2002 for use as a secondary packaging system. In 2004 the FDA approved CO as primary packaging method, declaring that CO does not mask spoilage odour.^[10] Despite this ruling, the technology remains controversial in the US for fears that it is deceptive and masks spoilage.^[11] Elsewhere, coloring meat to make it appear fresh is banned in many other countries.

One reaction in the body produces CO. Carbon monoxide is produced naturally as a breakdown of heme (which is one of hemoglobin moieties), a substrate for the enzyme heme oxygenase. The enzymatic reaction results in breakdown of heme to CO, biliverdin and Fe³⁺ radical. The endogenously produced CO may have important physiological roles in the body (eg as a neurotransmitter or a blood vessels relaxant). In addition CO regulates inflammatory reactions in a manner that prevents the development of several diseases such as atherosclerosis or severe malaria.

CO is a nutrient for methanogenic bacteria,^[12] a building block for acetylcoenzyme A. This theme is the subject for the emerging field of bioorganometallic chemistry. In bacteria, CO is produced via the reduction of carbon dioxide via the enzyme carbon monoxide dehydrogenase, an Fe-Ni-S-containing protein.^[13]

A haeme-based CO-sensor protein, CooA, is known.^[14] The scope of its biological role is still unclear, it is apparently part of a signaling pathway in bacteria and archaea, but its occurrence in mammals is not established.

CO is also currently being studied in several research laboratories throughout the world for its anti-inflammatory and cytoprotective properties that can be used therapeutically to prevent the development of a series of pathologic conditions such as ischemia reperfusion injury, transplant rejection, atherosclerosis, sepsis, severe malaria or autoimmunity. There are yet no clinical applications of CO in humans.

[edit] History

It was first described by the Spanish doctor Arnaldus de Villa Nova in the 11th century; but the creation of Carbon monoxide was first made by the French chemist de Lassone in 1776 by heating zinc oxide with coke. He mistakenly concluded that the gaseous product was hydrogen as it burned with a blue flame. The gas was identified as a compound containing carbon and oxygen by the English chemist William Cumberland Cruikshank in the year 1800.

The toxic properties of CO were first thoroughly investigated by the French physiologist Claude Bernard around 1846. He poisoned dogs with the gas, and noticed that their blood was more rutilant in all the vessels. 'Rutilant' is a French word, but also has an entry in English dictionaries, meaning ruddy, shimmering, or golden. However, it was translated at the time as crimson, scarlet, and now is famously known as 'cherry pink'.

During World War II, carbon monoxide was used to keep motor vehicles running in parts of the world where gasoline was scarce. External charcoal or wood burners were fitted, and the carbon monoxide produced by gasification was piped to the carburetor. The CO in this case is known as "wood gas". Carbon monoxide was also reportedly used on a small scale during the Holocaust at some Nazi extermination camps (Most notably by gas vans in Chelmno), and in the Action T4 "euthanasia" program.

[edit] Source concentrations

• 0.1 ppm - natural background atmosphere level (MOPITT)
• 0.5 to 5 ppm - average background level in homes^[15]
• 5 to 15 ppm - levels near properly adjusted gas stoves in homes^[15]
• 100-200 ppm - Mexico City central area from autos etc.^[16]
• 5,000 ppm - chimney of a home wood fire ^[17]
• 7,000 ppm - undiluted warm car exhaust - without catalytic converter^[17]
• 30,000 ppm - undiluted cigarette smoke^[17]

[edit] Toxicity

Main article: Carbon monoxide poisoning

Carbon monoxide is a significantly toxic gas and has no odor or color. It is the most common type of fatal poisoning in many countries.^[18] Exposures can lead to significant toxicity of the central nervous system and heart. Following poisoning, long-term sequelae often occurs. Carbon monoxide can also have severe effects on the fetus of a pregnant woman. Symptoms of mild poisoning include headaches and dizziness at concentrations less than 100 ppm. Concentrations as low as 667 ppm can cause up to 50% of the body's haemoglobin to be converted to carboxy-haemoglobin (HbCO). Carboxy-haemoglobin is quite stable but this change is reversible. Carboxy-haemoglobin is ineffective for delivering oxygen, resulting in some body parts not receiving oxygen needed. As a result, exposures of this level can be life-threatening. In the United States, OSHA limits long-term workplace exposure levels to 50 ppm.

The mechanisms by which carbon monoxide produces toxic effects are not yet fully understood, but haemoglobin, myoglobin, and mitochondrial cytochrome oxidase are thought to be compromised. Treatment largely consists of administering 100% oxygen or hyperbaric oxygen therapy, although the optimum treatment remains controversial.^[19] Domestic carbon monoxide poisoning can be prevented by the use of household carbon monoxide detectors.

carbon dioxide

"CO2" redirects here. For the postal district, see CO postcode area.

Carbon dioxide
IUPAC name	[show] Carbon dioxide
Other names	Carbonic acid gas Carbonic anhydride Dry ice (solid)
Identifiers
CAS number	124-38-9
PubChem	280
EC number	204-696-9
UN number	1013 (cylinder) 1845 (solid) 2187 (liquid)
RTECS number	FF6400000
SMILES	[show] O=C=O
InChI	[show] 1/CO2/c2-1-3
ChemSpider ID	274
Properties
Molecular formula	CO2
Molar mass	44.0095(14) g/mol
Appearance	colorless gas
Density	1,600 g/L (solid) 771 g/L (liquid) 1.98 g/L (gas)
Melting point	−56.6 °C (216.6 K) −69.9 °F (at 5.185 bar)
Boiling point	−78.5 °C (194.7 K) −109.3 °F (subl.)
Solubility in water	1.45 g/L (25°C, 100kPa)
Acidity (pKa)	6.35 and 10.33
Viscosity	0.07 cP at −78 °C
Dipole moment	zero
Structure
Molecular shape	linear
Hazards
MSDS	External MSDS
MSDS	ICSC 0021
EU Index	Not listed
Flash point	Non-flammable
Related compounds
Related carbon oxides	carbon suboxide dicarbon monoxide carbon monoxide carbon trioxide
Related compounds	Carbonyl sulfide Carbon disulfide Carbonic acid
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Carbon dioxide (chemical formula: CO₂) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure. Carbon dioxide exists in Earth's atmosphere currently at a globally averaged concentration of approximately 385 parts per million by volume.^[1] Carbon dioxide is a greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared.

Carbon dioxide is used by plants during photosynthesis to make sugars which may either be consumed again in respiration or used as the raw material to produce polysaccharides such as starch and cellulose, proteins and the wide variety of other organic compounds required for plant growth and development. It is produced during respiration by all animals, fungi and microorganisms that depend on living and decaying plants for food, either directly or indirectly. It is, therefore, a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of fossil fuels or the burning of vegetable matter, among other chemical processes. Over very long time scales (thousands to millions of years), concentrations are influenced by emissions from volcanoes and other geothermal processes such as hot springs and geysers and by the dissolution of carbonates in crustal rocks.

Carbon dioxide has no liquid state at pressures below 5.1 atm. At 1 atm it is a solid at temperatures below −78 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO₂ is an acidic oxide: an aqueous solution turns litmus from blue to pink.

CO₂ in concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour^[2].

Contents [hide] 1 Chemical and physical properties 2 History of human understanding 3 Isolation and production 4 Uses 4.1 Drinks 4.2 Foods 4.3 Pneumatic systems 4.4 Fire extinguisher 4.5 Welding 4.6 Caffeine removal 4.7 Pharmaceutical and other chemical processing 4.8 Biological applications 4.9 Lasers 4.10 Polymers and plastics 4.11 Oil recovery 4.12 As refrigerants 4.13 Coal bed methane recovery 4.14 Wine making 4.15 Dry Cleaning 5 In the Earth's atmosphere 6 In the oceans 7 Biological role 7.1 Role in photosynthesis 7.2 Toxicity 7.3 Human physiology 8 See also 9 References 10 External links

[edit] Chemical and physical properties

Carbon dioxide pressure-temperature phase diagram showing the triple point and critical point of carbon dioxide

For more details on this topic, see Carbon dioxide (data page).

Carbon dioxide is a colourless, odorless gas. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.^[3]

At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole, and as it is fully oxidized, it is moderately reactive and is non-flammable, but will support the combustion of metals such as magnesium.

Small pellets of dry ice subliming in air.

Crystal structure of dry ice

At −78.51 °C or −109.3 °F, carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Solid carbon dioxide is normally called "dry ice", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase leaving no liquid. It can often be found in grocery stores and laboratories, and it is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning.

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6 °C (See phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.^[4]

An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure.^[5] This form of glass, called carbonia, was produced by supercooling heated CO₂ 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. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

See also: Supercritical carbon dioxide and dry ice

[edit] History of human understanding

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, 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 sylvestre).

The properties of carbon dioxide were studied more thoroughly 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 did not support either flame or animal life. Black also found that when bubbled through an aqueous solution of lime (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.^[6]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.^[7] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 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 CO₂.^[8]

[edit] Isolation and production

Carbon dioxide may be obtained from air distillation. However, this yields only very small quantities of CO₂. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below:

2 HCl + CaCO₃ → CaCl₂ + H₂CO₃

The H₂CO₃ then decomposes to water and CO₂. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

The production of quicklime (CaO) a chemical that has widespread use, from limestone by heating at about 850 °C also produces CO₂:

CaCO₃ → CaO + CO₂

The combustion of all carbon containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.

CH₄ + 2 O₂ → CO₂ + 2 H₂O

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

2 Fe₂O₃ + 3 C → 4 Fe + 3 CO₂

Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits, but also in the production of bioethanol:

C₆H₁₂O₆ → 2 CO₂ + 2 C₂H₅OH

All aerobic organisms produce CO₂ when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use another modus operandi: Plants absorb CO₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH₂O → (CH₂O)_n + nO₂

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H₂CO₃ (carbonic acid). The relative concentrations of CO₂, H₂CO₃, and the deprotonated forms HCO₃⁻ (bicarbonate) and CO₃²⁻(carbonate) depend on the pH. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

[edit] Uses

Carbon dioxide bubbles in a soft drink.

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.^[9] 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. Aluminum capsules are also sold as supplies of compressed gas for airguns, paintball markers, for inflating bicycle tires, and for making seltzer. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests, such as the Common Clothes Moth.

[edit] Drinks

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these drinks artificially.

[edit] Foods

A candy called Pop Rocks is pressurized with carbon dioxide gas at about 40 bar (600 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents produce carbon dioxide to cause dough to rise. 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.

A carbon dioxide laser.

[edit] Pneumatic systems

Carbon dioxide is the most commonly used compressed gas for pneumatic systems in portable pressure tools and combat robots.

[edit] Fire extinguisher

Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for total flooding of a protected space, (National Fire Protection Association Code 12). International Maritime Organisation standards also recognise carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths. A review of CO2 systems (Carbon Dioxide as a Fire Suppressant: Examining the Risks, US EPA) identified 51 incidents between 1975 and the date of the report, causing 72 deaths and 145 injuries.

[edit] Welding

Carbon dioxide also finds use 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 brittler than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium.

[edit] Caffeine removal

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds, and is used to remove caffeine from coffee. First, the green coffee beans are soaked in water. The beans are placed in the top of a column seventy feet (21 m) high. Then super-pressurized carbon dioxide in fluid form at about 93 degrees Celsius enters at the bottom of the column. The caffeine diffuses out of the beans and into the carbon dioxide.

[edit] Pharmaceutical and other chemical processing

Carbon dioxide has begun to attract attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It's used by some dry cleaners for this reason. (See green chemistry.)

In the chemical industry, carbon dioxide is used for the production of urea, carbonates and bicarbonates, and sodium salicylate.

[edit] Biological applications

Plants require carbon dioxide to conduct photosynthesis, and greenhouses may enrich their atmospheres with additional CO₂ to boost plant growth, since its low present-day atmosphere concentration is just above the "suffocation" level for green plants. A photosynthesis-related drop in carbon dioxide concentration in a greenhouse compartment can kill green plants. At high concentrations, carbon dioxide is toxic to animal life, so raising the concentration to 10,000 ppm (1%) for several hours can eliminate pests such as whiteflies and spider mites in a greenhouse.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel fuel.^[10] Carbon dioxide is already increasingly used in greenhouses as the main carbon source for Spirulina algae. In medicine, up to 5% carbon dioxide is added to pure oxygen for stimulation of breathing after apnea and to stabilize the O₂/CO₂ balance in blood.

[edit] Lasers

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A common type of industrial gas laser is the carbon dioxide laser. The electronic transitions available in carbon dioxide produce laser light relatively efficiently. A working laser can be made with a variety of gas pressures and compositions, even with pure gas, allowing students to experiment with the effect of gas mixtures on laser performance. This same property makes commercially-made carbon dioxide lasers relatively inexpensive for the power available.

[edit] Polymers and plastics

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Carbon dioxide can also be combined with limonene oxide from orange peels or other epoxides to create polymers and plastics.^[11]

[edit] Oil recovery

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Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions. It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well.^[12] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

[edit] As refrigerants

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.

Liquid carbon dioxide (industry nomenclature R744 / R-744) was used as a refrigerant prior to the discovery of R-12. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to their operation at pressures of up to 130 bars, CO₂ systems require highly resistant components that have been already developed to serial production in many sectors.

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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, hot water heat pumps, among others. Some applications: Coca-Cola has fielded CO₂-based beverage coolers and the US Army is interested in CO₂ refrigeration and heating technology.^[13][14]

By the end of 2007, the global car industry is expected to decide on the next-generation refrigerant in car air conditioning. CO₂ is one discussed option.(see The Cool War)

[edit] Coal bed methane recovery

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In enhanced coal bed methane recovery, carbon dioxide is pumped into the coal seam to displace methane.^[15]

[edit] Wine making

Carbon dioxide in the form of dry ice is often used in the wine making process to cool down bunches of grapes quickly after picking to help prevent spontaneous fermentation by wild yeasts. The advantage of using dry ice over regular water ice is that it cools the grapes without adding any additional water that may decrease the sugar concentration in the grape must, and therefore also decrease the alcohol concentration in the finished wine.

Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. The carbon dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is heavier than regular air. The settled carbon dioxide gas creates an hyoxic environment which helps to prevent bacteria from growing on the grapes until it is time to start the fermentation with the desired strain of yeast.

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 gasses such as nitrogen or argon are preferred for this process by professional wine makers.

[edit] Dry Cleaning

Carbon dioxide in its liquid state is increasingly being used as a more environmentally responsible solvent for dry cleaning.^[16] It is also used as a rinse and drying agent in dry cleaning processes that use a more traditional solvent for the cleaning step.

[edit] In the Earth's atmosphere

Atmospheric CO₂ concentrations measured at Mauna Loa Observatory.

Main article: Carbon dioxide in the Earth's atmosphere

Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 385 parts per million by volume or 582 parts per million by mass.^[17] The mass of the Earth atmosphere is 5.14×10¹⁸ kg ^[18], so the total mass of atmospheric carbon dioxide is 3.0×10¹⁵ kg (3,000 gigatonnes). Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay (see graph at right). Concentrations also vary considerably on a regional basis: in urban areas it is generally higher and indoors it can reach 10 times the background atmospheric concentration.

Carbon dioxide is a greenhouse gas. See greenhouse effect for more.

Yearly increase of atmospheric CO₂: In the 1960s, the average annual increase was 37% of the 2000–2007 average.^[19]

Due to human activities such as the combustion of fossil fuels and deforestation, the concentration of atmospheric carbon dioxide has increased by about 35% since the beginning of the age of industrialization.^[20] In 1999, 2,244,804,000 (=~2.2×10⁹) metric tons of CO₂ were produced in the U.S. as a result of electric energy generation. This is an output rate of 0.6083 kg (1.341 pounds) per kWh.^[21]

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic period and then maintained a slow decline until the industrial revolution, with a particularly swift reduction occurring 49 million years ago.^[22][23]

Up to 40% of the gas emitted by some volcanoes during subaerial volcanic eruptions is carbon dioxide.^[24] According to the best estimates, volcanoes release about 130-230 million tonnes (145-255 million tons) of CO₂ into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of CO₂ rise to above 75% overnight, sufficient to kill insects and small animals, but warm rapidly when sunlit and disperse by convection during the day^[25] Locally high concentrations of CO₂, produced by disturbance of deep lake water saturated with CO₂ are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.^[26] However, emissions of CO₂ by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year.^[27]

[edit] In the oceans

There is about 50 times as much carbon dissolved in the oceans in the form of CO₂ and CO₂ hydration products as exists in the atmosphere. The oceans act as an enormous carbon sink, having "absorbed about one-third of all human-generated CO₂ emissions to date."^[28] Generally, gas solubility decreases as water temperature increases. Accordingly the ability of the oceans to absorb carbon dioxide from the atmosphere decreases as ocean temperatures rise.

Most of the CO₂ taken up by the ocean forms carbonic acid. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon cycle. There is considerable concern that as a result of increased CO₂ in the atmosphere the acidity of seawater has been increasing and may adversely affect organisms living in the water. In particular, with increasing acidity, the availability of carbonates for forming shells decreases.^[29]

[edit] Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

[edit] Role in photosynthesis

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials (cellulose) by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration.

Even when vented, carbon dioxide must be introduced into greenhouses to maintain plant growth, as the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm. Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm CO₂ when compared with ambient conditions.^[30]

Plants also emit CO₂ during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tons of CO₂ each year, however a mature forest will produce as much CO₂ from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants.^[31] Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved CO₂ in the upper ocean and thereby promotes the absorption of CO₂ from the atmosphere.^[32]

[edit] Toxicity

Carbon dioxide content in fresh air (averaged between sea-level and 10 hPa level, i.e. about 30 km altitude) varies between 0.036% (360 ppm) and 0.039% (390 ppm), depending on the location (see graphical map of CO₂).

According to the Australian Maritime Safety Authority, "Prolonged exposure to moderate concentrations can cause acidosis and adverse effects on calcium phosphorus metabolism resulting in increased calcium deposits in soft tissue. Carbon dioxide is toxic to the heart and causes diminished contractile force. At concentrations of three per cent by volume in air, it is mildly narcotic and causes increased blood pressure and pulse rate, and causes reduced hearing. At concentrations of about five per cent by volume it causes stimulation of the respiratory centre, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath. At about eight per cent concentration it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes." ^[33]

A natural disaster linked to CO₂ intoxication occurred during the limnic eruptions in the CO₂-rich lakes of Monoun and Nyos in the Okun range of North-West Cameroon: the gas was brutally expelled from the mountain lakes and leaked into the surrounding valleys, killing most animal forms. During the Lake Nyos tragedy of 1986, 1700 villagers and 3500 livestock died.^[34]

Due to the health risks associated with carbon dioxide exposure, the U.S. Occupational Safety and Health Administration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less. For short-term (under ten minutes) exposure, the U.S. National Institute for Occupational Safety and Health (NIOSH) and American Conference of Government Industrial Hygienists (ACGIH) limit is 30,000 ppm (3%). NIOSH also states that carbon dioxide concentrations exceeding 4% are immediately dangerous to life and health. ^[35]

Adaptation to increased levels of CO₂ occurs in humans. Continuous inhalation of CO₂ can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (ex. Submarine) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level.^[36][37]

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO₂ concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO₂ alone. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO₂ concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known by the names given to it by miners: blackdamp (also called choke damp or stythe). Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before CO₂ reached levels toxic to people. (The canary would also indicate dangerous levels of methane and other gases by the same principle.)

Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space.

[edit] Human physiology

See also: Arterial blood gas

CO₂ is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).

• Most of it (about 70% – 80%) is converted to bicarbonate ions HCO₃⁻ by the enzyme carbonic anhydrase in the red blood cells,^[38] by the reaction CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻.

• 5% – 10% is dissolved in the plasma^[38]

• 5% – 10% is bound to hemoglobin as carbamino compounds^[38]

Haemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO₂ 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 CO₂ decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels 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 CO₂ or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr Effect.

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its levels are 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 CO₂ in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which may cause respiratory alkalosis.

Although the body requires oxygen for metabolism, low oxygen levels 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 going unconscious.^[38]

Typically the gas we exhale is about 4% to 5% carbon dioxide and 4% to 5% less oxygen than was inhaled.

According to a study by the United States Department of Agriculture, an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day