So, why is HCEA concerned?    It is simple, 2% = 2000ppm; IAQ issues arise from poor building ventilation, which is measured by comparing inside air and outside air.  Consequences of Carbon Dioxide Concentrations are shown in charts below....

Additional sites on Carbon Dioxide are available on Chemical Fact Sheets (Click here), Modest Levels of Carbon Dioxide (Click here), and more facts on Carbon Dioxide (click here)...Please note above charts are from EPA Gov. Site (Click here)