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Understanding Carbon Dioxide in the Home

Carbon dioxide (CO2) is a normal constituent of our atmosphere with outdoor concentrations ranging from about 250 parts per million (ppm) to 600ppm depending on geographical location. We exhale about 38,000ppm of CO2 with each breath.

The proliferation of animal and plant life is based on the exchange of oxygen and CO2 gases. Humans inhale oxygen and exhale CO2; plants take in CO2 and give off oxygen. Thus, CO2 is one of the most commonly found gases on Earth. Carbon dioxide is also a product of burning fuel such as oil, natural gas, LP gas, wood, coal, alcohol, etc. as well as burning of other products. Indoor concentrations of CO2 have been and continue to be one of the most widely used indicators of indoor air quality. However, the concentration measured has often lead to a number of misunderstandings due to inappropriate sampling methods, etc. that have resulted in an interpretation suggesting there may be health implications as a direct result of the amount of CO2. Carbon dioxide is a simple asphyxiate. Although electrolyte imbalances and other metabolic changes have been associated with prolonged exposures of 10,000 to 20,000ppm of carbon dioxide, studies have shown that there are no toxicological effects associated with indoor concentrations of CO2 routinely found in non-industrial buildings where the range is from 500 to 4,000ppm. Therefore, CO2 should not be considered an indoor air contaminant.

In addition to CO2, humans exhale bioeffluents (pollutants) which include bacteria, gases, odors, pherons, viruses, and particulate matter. When bioeffluents accumulate in an enclosed space such as a tightly sealed, energy efficient home or office, the humans occupying that space experience discomfort such as headaches and fatigue. Most often, high concentrations of CO2 occur at the same time as high concentrations of bioeffluents. However, bioeffluents are not easily measured, which makes measuring CO2 a practical way of indicating indoor air quality.

Measuring the amount of CO2 indoors also provides an indication of how often the air changes (ventilation rate) inside a home or other building. While outdoor CO2 levels remain fairly constant, indoor levels are defined by the number of people occupying the space. Since most humans produce a consistent and predictable amount of carbon dioxide, the more people in the space, the more CO2 is produced. Indoor CO2 is also related to the amount of outdoor air that is introduced either by natural infiltration or by design and circulated throughout the space. The fresh air dilutes the CO2 concentration.

By adjusting or controlling the amount of ventilation, comfort can be maintained for the occupants of the space. Energy costs can also be dramatically affected by increasing or decreasing the amount of ventilation based on the number of occupants. Typically, an indoor CO2 concentration in a range near 1,000ppm would be considered reasonable to maintain comfort levels. This is about equivalent to 700ppm above the lower outdoor concentration.