determining {HEAD ~ concentration} of carbon dioxide flowing through {TAIL ~ at least one airflow control device}; and 
determining { HEAD ~ concentration } of carbon dioxide flowing through { TAIL ~ at least one airflow control device } ; and
OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH