Extraction ducts become dirty more quickly, as the air that flows through the duct hasn’t been filtered.

Therefore, if you are experiencing odour nuisance, arrange for your ventilation system to be inspected and, if necessary, thoroughly cleaned. This applies, of course, to system D installations, but definitely also to system C installations (mechanical air exhaust, natural air supply). Extraction ducting also becomes dirty in system Cs, therefore impeding optimum ventilation.