Exposure of health care workers and occupants to coughed air in a hospital room with displacement air distribution: impact of ventilation rate and distance from coughing patient

The exposure of a doctor and a second patient to coughed air by an infected patient was studied in a simulated two-bed hospital patient room. The air temperature in the room, ventilated at two air change rates (3 h-1 and 6 h-1) was kept 22 oC. Thermal manikin with realistic body shape and surface temperature distribution was used as the doctor standing 0.55, 1.1 or 2.8 m downstream the cough.

A coughing thermal dummy, lying in one bed and a second thermal manikin in the other bed (1.3 m away), were used as the “sick” and the “exposed” patients. The cough consisted of 100% CO2. The doctor and the coughing patient faced each other. The Peak Cough Time (PCT) was around 6 s, when the doctor was 0.55 m downstream the cough and increased when the distance between the sick patient and the doctor increased.

The highest Peak Concentration Level (PCL) for the doctor, i.e. excess of CO2 level in inhaled air above background concentration (ppm), for the three measured distances was at 6 h-1. PCL decreased with distance. The exposure of the second patient was low when the doctor was at 0.55 or 1.1 m downstream the cough (blocking effect), but was quite high when at 2.8 m. 6 h-1, recommended in present hospital standards as minimum ventilation rate in hospital patient rooms, resulted in elevated exposure to coughed air for the doctor, suggesting increased risk from airborne cross-infection.

Displacement air distribution does not reduce the risk from cross-infection.