Carbon dioxide is a simple molecule made of 1 atom of carbon and 2 atoms of oxygen. At normal temperatures, it exists as a gas. Under pressure it can become a liquid, or at very cold temperatures it can be a solid (dry ice).
Carbon dioxide is one of the more frequently found gases on the earth. While global carbon dioxide levels are predominately controlled by air-sea gas exchange, in homes and offices carbon dioxide levels are mostly effected by human respiration.
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 and 2,000 ppm (parts per million). Outdoor CO2 levels are usually 350 - 450 ppm. Power plants, cars and trucks all tend to raise the levels of outdoor CO2 in populated areas.
Note that 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).
While carbon dioxide is not indoor air pollution, 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 deteriorated comfort. The more CO2 in a room, the less fresh air, and therefore the more likely other particles are in the air. This is why higher levels of CO2 are permitted in industrial environments than in buildings where people-generated micro-organisms is the principal concern.
Although CO2 itself is not dangerous in normal concentrations, it is frequently used as an indicator of indoor air quality. 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 give these problems until levels approach 2,000ppm.
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:
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)
350-450 ppm - Common outdoor concentration