What is Carbon Dioxide (CO2)?

CO2 Molecule

Carbon dioxide is a natural, colorless, odorless gas in our atmosphere. The CO2 molecule is made of 1 atom of carbon and 2 atoms of oxygen.

At normal temperatures, CO2 it exists as a gas. Under pressure it can become a liquid, or at below -109 degrees Fahrenheit (-78 C) it becomes a solid (dry ice).

Carbon dioxide is one of the more commonly found gases on the earth. While global carbon dioxide levels are predominately controlled by air-sea gas exchange, indoors carbon dioxide levels are mostly effected by human respiration.

Is Carbon Dioxide Important?

CO2 in air

While carbon dioxide is just as critical as oxygen for life on earth, surprisingly it only makes up about 0.04% of air.

The reason carbon dioxide is important is because it is necessary for plant respiration, the process where plants use sugars and oxygen to create energy for plant growth. Humans and animals breathe in oxygen and exhale CO2. Plants take in CO2 and give off oxygen. This exchange is part of the carbon cycle which is the foundation for all life on earth.

All animals and humans inhale oxygen and exhale carbon dioxide. The carbon dioxide level in exhaled air about 3.8%, or 38,000ppm (parts per million). When carbon dioxide is exhaled it is quickly mixed with the surrounding air and, if the ventilation is good, the concentration is quickly reduced to harmless levels.

Indoor CO2 levels usually vary between 400 ppm (0.04% or fresh air) and 2,000 ppm (parts per million). Outdoor CO2 levels are about 400 ppm, although power plants, cars and trucks can raise the outdoor level of CO2 in populated areas. In fact, studies show CO2 levels can temporarily increase by 5-15% in urban areas depending on the time of day or time of year.

What is Carbon Dioxide mainly used for?

Carbon dioxide gas can be removed from fresh air and compressed into a liquid at 5.1 atmospheres pressure (5.2 bar; 75 psi) and under 31.1 °C (88.0 °F) (temperature of critical point) and above −56.6 °C (−69.9 °F).

Learn more about the chemical properties of carbon dioxide.

Once in a liquid state, tanks or cylinders of CO2 are used in many industries.

The largest user of carbon dioxide is the fertilizer industry, which uses CO2 for urea manufacturing, which is a required step in the creation of nitrogen-based fertilizer. The next largest is the oil and gas industry which uses CO2 for enhanced oil recovery. Other commercial applications include food and beverage production, metal fabrication, HVAC cooling and refrigeration, fire suppression and stimulating plant growth in greenhouses.

Is CO2 a Major Pollutant?

Carbon dioxide is not considered a pollutant. CO2 is critical for life.

However, while CO2 is not considered an indoor air pollutant, it is considered a suitable tracer gas for indicating high levels of dust, pollen, mold, VOCs and airborne micro-organisms like germs and viruses that contribute to poor air quality. The more CO2 in a room, the less fresh air, and therefore the more likely other particles are in the air. With high CO2 levels in a room, occupants are more likely to complain of tiredness, headache, and sometimes a feeling of sickness. Carbon dioxide itself does not present these problems until levels approach 2,000ppm.

Learn more about indoor air quality here.

Organizations and authorities all over the world have established recommendations for the maximum permitted concentration of carbon dioxide and/or permitted minimum air flow in occupied buildings:

50,000 ppm - Level at which CO2 is hazardous to human and animal life.

5.000 ppm - Maximum concentration during an 8-hour working-day according to OSHA, ASHRAE and many EU countries.

2.000 ppm - According to many investigations this level of CO2 produces a significant increase in drowsiness, tiredness, headache and a common discomfort

1.000 ppm - ASHRAE recommended maximum carbon dioxide concentration in a closed room. It is also a recommended as the maximum comfort level in many other countries, i.e. Sweden and Japan. It corresponds to an airflow (a need of fresh air) of approx 7 litres/second per person.

800 ppm - Target maximum carbon dioxide level by commercial HVAC companies. It is also a maximum permitted concentration for offices in California. It corresponds to an airflow (a need of fresh air) of about 10 litres/second per person.

400–800 ppm - Risk for over-ventilation (too much fresh air = energy wasted)

400-420 ppm - Common outdoor concentration in fresh air worldwide.

Is Carbon Dioxide the same as Carbon Monoxide?

CO2 must not be confused with carbon monoxide (CO), a very toxic gas that is a by-product from poor combustion (cars or fireplaces, for example). Carbon monoxide is dangerous at very low concentrations (25 to 50 ppm).

Learn more about the difference between carbon dioxide and carbon monoxide here.


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