Carbon dioxide is an odourless, colourless gas produced from human respiration.  While it is a natural component of air, it is present in air in a fixed proportion.  Many office and residential spaces are subject to increased respiration from human activity during the day, and insufficient air exchange with outdoor air.  As a result, indoor carbon dioxide levels are often elevated, even more than doubling levels found in outdoor air (1). 

Elevated levels of carbon dioxide are associated with a phenomenon termed sick building syndrome (SBS).  SBS describes a set of symptoms with unknown etiology that are associated with prolonged time spent indoors.  These symptoms include headache, fatigue, nausea, and irritation to the eyes, sinuses, and respiratory tract.  Several studies have demonstrated a positive correlation between the occurence of SBS symptoms and elevated indoor carbon dioxide levels (1,2,3). In order to prevent SBS and offer a healthy environment for building occupants, carbon dioxide levels should be monitored and ventilation adjusted as needed to maintain healthy levels.

Carbon dioxide levels within a building will vary based on factors such as time of day (end of day levels are highest), occupant density, and distance from vents.  As a result, measurments should be taken at various locations around the building throughout the day.  Measurements can be taken as spot readings, which only indicate the carbon dioxide level at a specific location and point in time, or on a continuous basis, which will provide a more comprehensive picture of levels and how they fluctuate throughout the day.  The two major instrument categories for testing indoor carbon dioxide levels are single-use direct reading tubes suitable for spot testing, and more permanent infrared analyzers that provide direct, continuous readings.

Sources:
¹ Erdmann, C.A., Steiner, K.C. and Apte, M.G., 2002.  Indoor Carbon Dioxide Concentrations and Sick Building Syndrome Symptoms in the BASE Study Revisited: Analyses of the 100 Building Dataset. Indoor Environment Department, Lawrence Berkeley National Laboratory, Berkeley, CA, LBNL-49584.
² Apte M.G., Fisk W.J., and Daisey J.M., 2000. Associations Between Indoor CO2 Concentrations and Sick Building Syndrome Symptoms in U.S. Office Buildings: An Analysis of the 1994-1996 BASE Study Data.  Indoor Air 10(4), pp. 246-257.
³ Seppänen, O.A., Fisk, W.J., and Mendell, M.J., 1999. Association of ventilation rates and CO2 concentrations with health and other responses in commercial and institutional buildings. Indoor Air 9(4), pp. 226-252.