Distribution of droplets/droplet nuclei from coughing and breathing of patients with different postures in a hospital isolation ward

Abstract

Suspected and confirmed cases of infectious diseases such as COVID-19 are diagnosed and treated in specific hospital isolation wards, posing a challenge to preventing cross-infection between patients and healthcare workers. In this study, the Euler-Lagrange method was used to simulate the evaporation and dispersion of droplets with full-size distribution produced by fluctuating coughing and breathing activities in an isolation ward. The effects of supply air temperature and relative humidity, ventilation rates and patient postures on droplet distribution were investigated. The numerical models were validated by an aerosol experiment with an artificial saliva solution containing E. coli bacteria conducted in a typical isolation ward. The results showed that the small size group of droplets (initial size ≤87.5 μm) exhibited airborne transmission in the isolation ward, while the large size group (initial size ≥112.5 μm) were rapidly deposited by gravitational effects. The ventilation rate had a greater effect on the diffusion of droplet nuclei than the supply air temperature and relative humidity. As the air changes per hour (ACH) increased from 8 to 16, the number fraction of suspended droplet nuclei reduced by 14.2% and 6.4% in the lying and sitting cases, respectively, while the number fraction of escaped droplet nuclei increased by 16.2% and 14.6%. Regardless of whether the patient was lying or sitting, the amount of droplet nuclei deposited on the ceiling was highest at lower ventilation rates. These results may provide some guidance for routine disinfection and ventilation strategies in hospital isolation wards.

Keywords: Droplet; Isolation ward; Patient posture; Relative humidity; Ventilation rate.

Fig. 1

Computational models of isolation wards: (a) with a patient lying; (b) with a patient sitting.

Fig. 2

Unstructured grids of domain, prism layers around the patients, mouth and nose, (a) the patient lying case; (b) the patient sitting case.

Fig. 3

Boundary condition for coughing and breathing (a) cough velocity, (b) breath velocity, and (c) size distribution for cough droplets.

Fig. 4

(a) The configuration of the isolation ward experimental cabin and (b) the computational model and locations of sampling points.

Fig. 5

Comparison of simulated and experimental airflow velocity.

Fig. 6

Comparison of simulated and experimental normalized bioaerosol concentration.

Fig. 7

Airflow distribution above the lying patient in Case 2 (23 °C, 50%, ACH 12, Lying).

Fig. 8

Airflow distribution around the sitting patient in Case 9 (23 °C, 50%, ACH 12, Sitting).

Fig. 9

Spatial distribution of droplets/droplet nuclei from a single cough and continuous breathing in Case 2 (23 °C, 50%, ACH 12, Lying).

Fig. 10

Spatial distribution of droplets/droplet nuclei from a single cough and continuous breathing in Case 9 (23 °C, 50%, ACH 12, Sitting).

Fig. 11

Number fraction of suspended droplet groups with different initial sizes: (a) in case 2; (b) in case 9.

Fig. 12

Distribution of droplet nuclei: (a) suspension, (b) escape, (c) deposition, and (d) number fraction of droplet nuclei at 250s. All cases have the same ACH of 12, but different supply air temperature and relative humidity. For lying cases, Case 1: 18 °C, 50%, Case 2: 23 °C, 50%, Case 3: 28 °C, 50%, Case 4: 23 °C, 30%, and Case 5: 23 °C, 70%; for sitting cases, Case 8: 18 °C, 50%, Case 9: 23 °C, 50%, Case 10: 28 °C, 50%, Case 11: 23 °C, 30%, and Case 12: 23 °C, 70%.

Fig. 13

Number of droplet nuclei deposited at various locations in the isolation ward at 250s. All cases have the same ACH of 12, but different supply air temperature and relative humidity. For lying cases, Case 1: 18 °C, 50%, Case 2: 23 °C, 50%, Case 3: 28 °C, 50%, Case 4: 23 °C, 30%, and Case 5: 23 °C, 70%; for sitting cases, Case 8: 18 °C, 50%, Case 9: 23 °C, 50%, Case 10: 28 °C, 50%, Case 11: 23 °C, 30%, and Case 12: 23 °C, 70%.

Fig. 14

Distribution of droplet nuclei: (a) suspension, (b) escape, (c) deposition, and (d) number fraction of droplet nuclei at 250s. All cases have the same supply air temperature of 23 °C and relative humidity of 50%, but different ACH. For lying cases, Case 2: ACH 12, Case 6: ACH 8, and Case 7: ACH 16; for sitting cases, Case 9: ACH 12, Case 13: ACH 8, and Case 14: ACH 16.

Fig. 15

Number of droplet nuclei deposited at various locations in the isolation ward at 250s. All cases have the same supply air temperature of 23 °C and relative humidity of 50%, but different ACH. For lying cases, Case 2: ACH 12, Case 6: ACH 8, and Case 7: ACH 16; for sitting cases, Case 9: ACH 12, Case 13: ACH 8, and Case 14: ACH 16.