This　study　presents　that　increases　in　atmospheric　CO2　levels　impact　the　ventilation　and　energy　in　buildings.　The　most　significant　impact　of　ventilation　occurs　during　use,　because　of　the　indoor　air　quality.　According　to　the　2014　IPCC　report,　observed　atmospheric　CO2　levels　increased　at　a　rate　of　1.9–2.1　ppm　per　year.　This　study　measured　CO2　levels　in　a　building　in　an　urban　area　in　Shanghai,　China,　and　analyzed　existing　ventilation　performance　and　energy　simulation　in　relation　to　the　impact　of　increased　atmospheric　CO2　concentrations.　As　long　as　the　atmospheric　CO2　level　increased,　the　outdoor　airflow　rate　had　to　increase　also　to　maintain　an　acceptable　indoor　CO2　level.　The　ventilation　rate　was　also　considered　in　relation　to　the　surrounding　environment.　In　Shanghai,　the　average　value　of　CO2　concentrations　on　an　auto　road　was　550　±　20　ppm　and　the　average　value　on　a　pedestrian　road　was　435　±　15　ppm.　A　building　that　was　surrounded　by　high　traffic　roads　needed　20–30%　more　outdoor　airflow　rate　than　a　building　surrounded　by　pedestrian　roads.　Accordingly,　energy　loads　in　buildings,　for　heating　and　cooling　were　also　affected.　In　order　to　dilute　indoor　CO2　level,　the　thermal　ventilation　energy　and　the　ventilation　energy　uniformly　provided　a　20%　energy　load　increase　during　the　12　months,　while　each　monthly　energy　load　varied,　depending　on　the　season　and/or　month.　By　analyzing　indoor　CO2　levels　and　the　rise　in　atmospheric　CO2　levels,　this　study　determined　how　these　rises　impacted　the　energy　consumption　of　the　building,　and　how　to　design　a　new　outdoor　airflow　supply　rate　to　adapt　to　a　changed　environmental　condition.　©　2019　Elsevier　Ltd