What is CO2? (Video 2)

Carbon dioxide is a colourless, odourless gas which is a natural by-product of human respiration. CO2 tends to build up in confined or poorly-ventilated areas; in these places, CO₂ levels provide a good indicator of air quality, as its concentration rises proportionally with all other substances, making it an efficient monitor to quantify the environmental burden of the air and evaluate the need for ventilation.