Building Heating Ventilation and Air-conditioning System (HVAC) regulates indoor air for the comfort of occupants and with energy saving in perspective. 
Indoor air is exchanged through recirculation and replacement. Indoor air that is high in CO2 is exhausted while diluted with outdoor fresh O2.

Indoor combustion appliances and building occupants consume O2 and release CO2. Indoor CO2 is presumably higher than natural outdoor.
However, in an urban environment, congested with industries, vehicles, and sources of combustion, CO2 in the outdoor air is possibly higher.
Which calls for a greener environment that replenishes and balances the O2 density.

Higher CO2 relative to O2 in confined buildings with high occupancies such as schools, industrial workplaces, and offices is correlated to a reduction in productivity.
People become lethargic, suffer difficulty focusing, and sense the discomfort of stale air, known as Sick Building Syndrome.
 

ASHRAE 62 provides a general guideline of recommended outside air per person (CFM per person) measured relative to CO2 differential (inside/outside).
CO2 will likely increase in an occupied space by the end of the day in a low ventilation environment. 

Measuring CO2 is complicated but thanks to technology, CO2 monitoring devices, built-in or separate have now been widely applied in buildings.
These devices tend to measure also other important parameters related to IAQ such as temperature, relative humidity (RH), other PM2.5, and TVOC.

Solutions to Sick Building Syndrome are, to remove pollutant sources, by increasing ventilation, performing air cleaning (purification), and creating Occupational Safety and Health (OSH) awareness.