Carbon Dioxide in Human Physiology and Plants

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 80% 90%) is converted to bicarbonate ions HCO3- by the enzyme carbonic anhydrase in the red blood cells.

5% 10% is dissolved in the plasma

5% 10% is bound to hemoglobin as carbamino compounds.

The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather 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 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. 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. This is known as the Bohr Effect.

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

Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.

It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others otherwise one risks going unconscious without being aware of the imminent peril.

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

Use in plants
Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials 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. Carbon dioxide gas must be introduced into greenhouses to maintain plant growth, as even in vented greenhouses the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm, at which level photosynthesis is significantly retarded. Venting can help offset the drop in carbon dioxide, but will never raise it back to ambient levels of 340ppm. Carbon dioxide supplementation is the only known method to overcome this deficiency. Direct introduction of pure carbon dioxide is ideal, but rarely done because of cost constraints. Most greenhouses burn methane or propane to supply the additional CO2, but care must be taken to have a clean burning system as increased levels of NO2 result in reduced plant growth. Sensors for SO2 and NO2 are expensive and difficult to maintain, accordingly most systems come with a carbon monoxide (CO) sensor under the assumption that high levels of carbon monoxide mean that significant amounts of NO2 are being produced. Plants can potentially grow up to 50 percent faster in concentrations of 1000ppm CO2 when compared with ambient conditions.

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 tonnes 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. Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere.

Pollution and toxicity
Carbon dioxide content in fresh air varies and is between 0.03% (300 ppm) and 0.06% (600 ppm), depending on location and in exhaled air approximately 4.5%. When inhaled in high concentrations (greater than 5% by volume), it is immediately dangerous to the life and health of humans and other animals. The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an 8-hour work day is 0.5% (5000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues would be significantly less.

These figures are valid for carbon dioxide supplied in "pure" form. In indoor spaces occupied by humans the carbon dioxide concentration will also reach a level higher than in pure outdoor air. Concentrations higher than 1000 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 itself. At 2000 ppm will the majority of occupants feel a significant degree of discomfort, and many will develop nausea and headache. The CO2 concentration between 300 and 2500 ppm is used as an indicator of indoor air quality in spaces polluted by human occupation.

Acute carbon dioxide toxicity is sometimes known as Choke damp, an old mining industry term, and was the cause of death at Lake Nyos in Cameroon, where an upwelling of CO2-laden lake water in 1986 covered a wide area in a blanket of the gas, killing nearly 2000. The lowering of carbon dioxide in the atmosphere is largely due to absorption by plants, which convert it to sugars through photosynthesis. Phytoplankton photosynthesis absorbs dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.

Carbon dioxide is a surrogate for 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 carbon dioxide must be reduced to below 600 ppm. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1000 ppm are a marker suggesting inadequate ventilation (1,000 ppm equals 0.1%). ASHRAE recommends that CO2 levels not exceed 1000 ppm inside a space. OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 3% and considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7 10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.
06 July 2007

Articles on Science Matters:

Dry Ice - Solid Carbon Dioxide

Solid carbon dioxide, often known by the genericized trademark "dry ice", is a versatile cooling agent. Unlike water ice at atmospheric pressure it sublimes, changing from a solid directly to a gas. Its sublimation point is -78.5C (-109.3F). A combination of its low temperature, solid phase and direct sublimation to gas makes it a simple and effective coolant. Dry ice is also inexpensive; it costs about US$2 per Kg (US$1 per lb).

Dry ice was first observed in 1825 by the French chemist Charles Thilorier. Upon opening the lid of a large cylinder of liquid carbon dioxide he noted much of the carbon dioxide rapidly evaporated leaving solid dry ice in container. Throughout the next 60 years, dry ice was observed and tested by many scientists.

Dry ice is readily manufactured;
Carbon dioxide is obtained in any of the ways listed above.

It is pressurized and refrigerated, until it changes into its liquid form.

The pressure is reduced. When this occurs some liquid carbon dioxide vaporises, and this causes a rapid lowering of temperature of the remaining liquid carbon dioxide. The extreme cold makes the liquid solidify into a snow-like consistency.

The snow-like solid carbon dioxide is compressed into either small pellets or larger blocks of dry ice.

Dry ice is typically produced in two standard sizes; solid blocks and cylindrical pellets. A standard block is most common and will normally weigh about 30 kg (60 lb). These are largely used in the shipping industry because they sublime slowly due to a relatively small surface area. The pellets are around 1 cm ( inch) in diameter and can be bagged easily. This form of dry ice is more suited to small scale use, for example at grocery stores and laboratories.

Dry ice can be a dangerous substance. It must be handled using protective insulated gloves. Direct contact with the skin can freeze it in seconds, causing a burn-like injury. Dry ice must not be stored in a sealed container, since its sublimation produces massive volumes of gaseous carbon dioxide. A sealed container can fail explosively from the pressure, sufficient to cause shrapnel injuries and hearing loss. Furthermore, dry ice should never be stored in a standard freezer or refrigerator. The dry ice is so cold that it can freeze and disable the thermostat of the unit. It can also cause problems through thermal contraction. Dry ice should never be left on brittle surfaces or in glass containers. The contraction caused by cooling can result in cracking.

Aside from obvious uses in cooling and shipping, dry ice has many other applications:
In medicine, dry ice can freeze warts and other similar skin conditions, easing their removal.
Dry ice blast cleaning.

Removal of floor tiles - the low temperature makes the tiles shrink and crack. This will loosen them so that they can be removed.

Carbonation of water (and other liquids). This is widely used in the carbonated drinks industry.

Reduces the effect of dents on cars, the temperature puts pressure on the dent to align with the rest of the metal.

Dry ice can repel mosquitoes and other insects due to its low temperature.

Rapid sublimation of dry ice caused by putting it in water produces a dense fog of water vapour. This is a popular dramatic effect, common as a stage effect or in recreation at Halloween. The fog produced, being largely dense carbon dioxide, sinks to the floor.

Dry ice blast cleaning
One of the most important alternative uses of dry ice around the world is dry ice blast cleaning. Dry ice pellets are shot out of a jet nozzle with compressed air. This can remove residues from industrial equipment, for example ink, glue, oil, paint, mould and rubber, replacing sandblasting, steam blasting, water blasting or other (potentially environmentally damaging) solvent blasting.

Dry ice blasting involves three factors:
kinetic energy
thermal shock
thermal kinetic energy.

The kinetic energy of the dry ice pellets is transferred when it hits the surface, directly dislodging residues, as in other blasting methods. The thermal shock effect occurs when the cold dry ice hits a much warmer surface and rapid sublimation occurs. The thermal kinetic effect is the result of the rapid sublimation of the dry ice hitting the surface. These factors combine cause small "micro-explosions" of gaseous carbon dioxide where each pellet of dry ice impacts, dislodging the residue.

Solid amorphous CO2
An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure. 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 germanium oxide glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.
06 July 2007

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