Effectiveness of the new ambulance air exhaust system in reducing aerosol particle concentration

Abstract

The objective of this work is to assess the effectiveness in reducing airborne particles of the Camfil® CC410-concealed air exhaust system, that is installed in the ER-1 ambulance (a King Chulalongkorn Memorial Hospital ambulance). This is done by diffusing aerosols into the cabin at the patient’s face position for an injection period (IP) of 1 minute and measuring the aerosol concentration inside the cabin over time. The experiment is conducted for 2 ventilation conditions, when the exhaust system is off (Minimal Ventilation: MV) and at maximum speed (High Ventilation: HV). Density of the aerosol is interpreted in terms of its volume concentration. Time period is counted at the end of the IP.

Aerosol is found to disperse toward the cabin’s frontal part and then throughout the entire cabin shortly after IP. Its spatial distribution is non-uniform with multiple local peaks, differs between the two cases. Particles of different size range is removed from the cabin with different rates. Without an aid from the exhaust system (MV case), larger particles tend to decay relatively faster. To recover aerosol concentration to that ‘comparable’ to the initial (or background) state for the MV case, it takes 16 minutes for 0.5-1 microns, 12 minutes for 1-2.5 microns, and 9 minutes for 2.5-5 microns particles. The background state is reached after 18 minutes for all size ranges. With the aid in the HV case, the decaying period is shortened to about 5 minutes for all aerosol size ranges. During the passive decay (MV), small aerosols tend to aggregate in the rear cabin and larger aerosols in the front (close to the diffusion nozzle), reflecting their “native” dispersion ability (or inability) with respect to their sizes. Aerosol of medium-to-large 1-5 microns range is relatively large at the cabin frontal part (seat 1) but 0.5-1 microns range is large at the cabin rear (seat 5). During the active decay (HV), on the other hand, larger particle is advected toward the middle and rear parts. Small-to-medium-size aerosols at 0.5-2.5 microns is concentrated at the cabin frontal part (seat 2) while that of 2.5-5 microns concentration is peaked in the middle (seat 3) and on average in the rear (seat 5). The air exhaust system is proven satisfactory effective in reducing aerosol concentration in the ambulance cabin. Considering among seats and particle size in this experiment, the system is able to reduce peak aerosol concentrations by 25% and temporal-averaged concentration by 70%.