Indoor air co2 only lowers due to outdoor filtration. So, higher outdoor co2 will make us reach higher indoor levels faster. A change of outdoor air to 500 from

Indoor air co2 only lowers due to outdoor filtration. So, higher outdoor co2 will make us reach higher indoor levels faster. A change of outdoor air to 500 from 400 ppm, for example, means there's 16% less difference with indoor air at 1000 ppm.