Carbon Dioxide and Indoor Air Quality
Indoor air quality (IAQ) is influenced by many pollutants from many different sources: indoor pollutants (e.g., volatile organic compounds), pollutants originating from outdoor sources (e.g., ozone), and pollutants generated by people (carbon dioxide). It would be impossible to monitor every potential pollutant because it would require too many sensors, some of which are not yet portable. However, there are strong correlations between CO2 levels and air exchange rates. The level of ventilation in a space is a major determining factor of IAQ, ventilation is responsible for removing harmful pollutants and ensuring good air quality. By relating CO2 levels, the number of people in a room, and the size of the room, researchers can determine the air flow rates per person and the air changes per hour (in a given room). Both the air flow rates and the air changes can give users an idea of their IAQ — lower air exchange rates suggest more pollutant exposure, while higher exchange rates suggests less pollutant exposure.
To add some context to CO2 levels and other IAQ measurements, the ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) limit for indoor CO2 levels is a maximum of 700 ppm above outdoor concentration levels. The accepted minimum for air changes per hour is .351 and the lower limit for air flow per second per person is 7.51 (these were also established by ASHRAE).
To learn more about ASHRAE's standards check out this link: http://www.ashrae.org/technology/page/548
During a user study, the MAQS research team analyzed correlations between the CO2 levels, air changes per hour, and air flow per person. The graphs below illustrate the results. (complete information on this study and data can be found in the paper "A Personalized Mobile Sensing System for Indoor Air Quality Monitoring")

The first graph shows the average CO2 levels experienced by each user, the second the average air changes per hour experienced by each user (in the same order as on the first graph), and the average air flow per person experienced by each user (also in the same order as the first graph). The data indicates that users with exposure to lower CO2 levels experienced higher air changes per hour and higher air flow rates per person, while the opposite is true for users exposed to high levels of CO2.
The research team plans to use a model that will calculate the air exchange information based on the CO2 levels, number of occupants, and the room volume — in turn this model will provide an indication of IAQ. There is currently not a option to view air exchange rates on the app or the network, but the teams hopes to add these features soon.
To learn more about Indoor Air Quality, follow this link to the EPA's website: http://www.epa.gov/iaq/ia-intro.html
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