Affordable measures to monitor and alarm nosocomial SARS-CoV-2 infection due to poor ventilation.

Since the coronavirus disease 2019 (COVID-19) outbreak, the nosocomial infection rate worldwide has been reported high. It is urgent to figure out an affordable way to monitor and alarm nosocomial infection. Carbon dioxide (CO ) concentration can reflect the ventilation performance and crowdedness, so CO sensors were placed in Beijing Tsinghua Changgung Hospital's fever clinic and emergency department where the nosocomial infection risk was high. Patients' medical records were extracted to figure out their timelines and whereabouts. Based on these, site-specific CO concentration thresholds were calculated by the dilution equation and sites' risk ratios were determined to evaluate ventilation performance. CO concentration successfully revealed that the expiratory tracer was poorly diluted in the mechanically ventilated inner spaces, compared to naturally ventilated outer spaces, among all of the monitoring sites that COVID-19 patients visited. Sufficient ventilation, personal protection, and disinfection measures led to no nosocomial infection in this hospital. The actual outdoor airflow rate per person (Q ) during the COVID-19 patients' presence was estimated for reference using equilibrium analysis. During the stay of single COVID-19 patient wearing a mask, the minimum Q value was 15-18 L/(s·person). When the patient was given throat swab sampling, the minimum Q value was 21 L/(s·person). The Q value reached 36-42 L/(s·person) thanks to window-inducted natural ventilation, when two COVID-19 patients wearing masks shared the same space with other patients or healthcare workers. The CO concentration monitoring system proved to be effective in assessing nosocomial infection risk by reflecting real-time dilution of patients' exhalation.