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