Carbon Dioxide (CO2) is often used within the indoor environment as a way to determine the ventilation rate. When an indoor environment is unoccupied and there is air entering from outside the concentration of carbon dioxide will eventually reach the same as the outdoor levels. When people enter the space, as a result of breathing they produce carbon dioxide (amongst other things) and over time the concentrations increase .
Whilst many buildings are designed to have specific ventilation rates, often the Heating, Ventilation and Cooling (HVAC) systems also contain Carbon Dioxide sensors to establish if additional ventilation (i.e. external air) is required.
Assuming there are no other Carbon Dioxide sources, understanding that people are the main contributor to the indoor concentration allows for the understanding of if ventilation rates are sufficient within the indoor environment. This ventilation assists in reducing Carbon Dioxide concentrations within the indoor environment, but also has a secondary impact of flushing / diluing other airborne pollutants.
As with many airborne pollutants there are health effects associated with Carbon Dioxide however, within most typical indoor environments you would be unlikely to reach the threshold values.
Within a typical rural environment, outdoor carbon dioxide levels would usually be in the range of 400 – 500ppm, and within urban area(s) it may be around 600 – 900ppm .
Within an occupational setting, the Australia sets an eight (8) hour exposure limit of 5000ppm for Carbon Dioxide (not in mines), and a short term exposure limit of 30,000ppm . The United States specify that the eight hour exposure limit for Carbon Dioxide is 5,000 ppm with a 15 minute short term exposure limit of 30,000 ppm .
From a health perspective, the symptoms may differ depending on the person, concentrations and a range of factors. Generally speaking though the potential health problems are ;
Cognative impacts may be seen at concentrations far lower than the time weighted averages.
Numerous studies have been carried out that have identified a significant link between Carbon Dioxide concentration and cognative function. In some cases these links may be due to the buildup of other airborne pollutants (i.e. it is showing the ventilation rate is low) whilst in other cases studies have looked at the direct impact of Carbon Dixiode concentration.
A study from Joseph Allen, Piers MacNaughton et al  identified that in a controlled environment, average cognitive function scores decreased at each higher level of Carbon Dioxide. Cognitive function scores were 15% lower for the when the average Carbon Dioxide levels were around 945 ppm, and 50% lower on the day with average Carbon Dioxide Concentrations of 1400 ppm.
Interesting research has also been conducted which had a look at Carbon Dioxide and its impact on decision making. The researchers commented that impact of 2,500 parts per million of carbon dioxide was roughly equivalent to a .08 blood alcohol concentration .
Australia typically follows ASHRAE (ANSI/ASHRAE Standard 62.1-2016) with respect to acceptable Carbon Dioxide levels within indoor environments. This standard recommends that Carbon Dioxide concentrations should be maintained at, or below 1000ppm in schools and 800ppm in offices .
Carbon Dioxide concentration varies significantly based on the number of ocucpants, type (and presence) of ventilation in addition to a significant number of factors.
Within Australia, I have conducted testing within a range of environments. Typically office levels have been in the range of 600 – 1200ppm (in well controlled offices) whilst School concentrations vary wildly. Some schools may be in the range of 600ppm however, other schools I have tested have been closer to 3000ppm (in a closed portable building in the middle of winter, with little ventilation and a room full of kids).
These values are not unusual, David A Coley et al  measured CO2 levels in seven classrooms across four schools. The results showed the average concentration during the occupied period was around 1957ppm.