Carbon Dioxide

CO₂  

The primary source for CO₂ indoors is human exhalation.  This compound is a byproduct of human metabolic activity and might be more likely to build up in areas with a lot of people or poor ventilation.  Other sources include those resulting in combustion byproducts: car exhaust, fossil fuel power plants, leaking furnaces, cigarette smoke, etc.  CO₂ can be considered one of the primary products of combustion; complete (or ideal) combustion results in CO₂ and H2O and energy. In very high concentrations it may cause health effects in humans, but is generally not considered to be dangerous. CO₂ is a greenhouse gas and a major contributor to climate change.

Learn more about the carbon cycle

Sources: 

• Illinois Department of Public Health
• Department of Health