STALE INDOOR AIR—led to a Dutch nursing home #COVID19 outbreak infecting 34 in 1 of 7 wards. But nobody in other 6 wards affected. Why? An air system that recycled & did not refresh air—airborne aerosols. Other 6 wards had AC that refreshed outside air!
https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciaa1270/5898577

2) Netherlands was experiencing a low prevalence of COVID-19 the week of the outbreak, with only 493 of that country's residents testing positive, compared with 8,391 cases during the most intense week of the outbreak in April.

3) To prevent transmission, all healthcare workers were assigned to specific wards and required to wear surgical masks during patient care starting Apr 26. Residents lived in individual rooms and spent part of each day in shared living rooms; some residents were mobile.

4) Suspecting that the ventilation system of the affected ward could have contributed to the outbreak, investigators found that an energy-efficient system had been installed in which indoor air was refreshed only when indoor CO2 concentrations detected elevated levels.

5) If CO2 levels didn't exceed a certain threshold, unfiltered indoor air was simply recirculated throughout the ward. In contrast, the six unaffected wards were refreshed regularly with outside air.

6) low CO2 levels produced by inactive patients may have led to stale air in the affected ward, which was cooled by 2 AC units that also recirculated the air. Virus was found in dust on the mesh dust filter of living room AC and in four filters from 3 of 8 ventilation units.

7) Authors: "We advise that prevention of COVID-19 transmission should take into account the possibility of aerosol transmission in healthcare facilities and other buildings where ventilation systems recirculate unfiltered inside air”

8) Craziest thing is that despite 7 wards of the nursing home being connected together, their segregated AC systems made all the difference. Key: None of the 95 residents or 106 healthcare staff in the other six wards tested positive!!! Wow.

9) Ironically, these 6 wards had older less energy efficient AC system that constantly pulled in new outdoor air. While the one infected ward with the outbreak had a new energy efficient one that only pulled in new air if CO2 high (less often w/ sedentary nursing home patients)