What is the impact of a poor ductwork airtightness on the SARS-CoV-2 infection risk?

REMARK: This Q&A was part of the AIVC special COVID-19 newsletter published in July 2021. To subscribe to the newsletter please click here.

1) Ductwork leakage, by decreasing ventilation flowrates at air terminals, increases contamination risk. When ductworks are leaky, and the fan does not fully compensate for leakages, part of the flow is not extracted (resp. supplied) from (resp. to) the occupied zone. As a consequence, the actual air renewal of the occupied zone with uncontaminated air is lower than foreseen. Therefore, if an infected person is present in the zone, the concentration of infectious aerosols in the zone may be higher than expected. 

2) Ductwork leakage may induce transfer of infectious aerosols between zones. Ductwork leakage may jeopardize the designed pressure balance between zones. As ductwork leakage repartition is impossible to assess, it makes it difficult to predict the exact supply and extract flowrate in a zone and thus the relative pressure between zones. This may induce unexpected air flowrate (and transfer of infectious aerosols) between zones. (see also FAQ ‘Does transport of air from one room to another room play a role in relation to COVID-19?’).

3) Ductwork leakage may induce recirculation of infectious aerosols. In some cases, the exhaust ductwork (which is the part of the extract ductwork that is downstream the fan and then overpressurised)  is located inside the conditioned space, its leakage may induce the diffusion of infected aerosol in the conditioned space. The potential impact of ductwork leakage with respect to the three phenomena discussed above increases when the ductwork airtightness class is poorer, the operational pressure in the ducts is larger, and the size of the ductwork is bigger. Improving ductwork airtightness hence helps to limit airborne transmission of SARS-CoV-2. 

For more information on ductwork airtightness, see VIP 40: Ductwork airtightness - A review.

Authors

Valérie Leprince, INIVE