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Shown in Figure S1, large particles (diameter  2.5 m) concentrations, unlike small particles (diameter between 0.5 -2.5 m), are strongly associated with SAP values (Spearman Rank test ρ value of 0.74 when compared to 0.03 for small particles). Among all sites, Sites 5 and 12 have the highest large particle concentrations and SAP percentages. The high concentrations at Site 12 could be explained by the construction activities inside and right adjacent to the furnace during the investigation. Small particle concentrations for Sites 5 and 12 did not stand out like for large

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ventilation grills to filter the air passing to recovery was the best solution for our case. We designed a housing for the filters to ensure a good seal. After installation, theatre ventilation function testing was repeated. In view of the increased resistance to air flow created by the filter, we assessed for a potential air leak, which was found to be significant. The smoke seal on the theatre doors was refitted. Further testing showed that the theatre air changes remained above the recommended minimum 25 per hour with negligible leak and contamination of adjacent areas. It is important to highlight that in a modified airborne infection isolation room such as this, the mechanical elements degenerate over time, rendering them inefficient and placing staff and patients at risk. They therefore require regular testing and maintenance to ensure they are operating effectively, the frequency of which should be guided by the manufacturer. Usual precautions required in respiratory isolation, such as keeping doors closed and using appropriate protective equipment, must also be adhered to. As our obstetric theatre setup is not unusual, we considered that we may not be alone in facing this infection control challenge. With the COVID-19 crisis potentially continuing for some time, we wished to share our experience in the hope that this will prove useful to colleagues in other hospitals and trusts.

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