Using Carbon Dioxide as a Tracer Gas to Determine Air Changes Per Hour in Ambulances

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      Emergency Medical Services often transport patients with suspected/confirmed airborne diseases such as Mycobacterium tuberculosis. These pathogens are contagious and have potential to remain suspended in the air for extended periods of time. The timeframe for pathogen removal by airing out ambulances is not well defined. This study evaluates the time required to effectively remove these airborne pathogens from ambulances by determining the air changes per hour (ACH).


      We measured the ACH in twenty ambulances of various models using the concentration decay method with carbon dioxide (CO2) as a tracer gas. Individually, we evaluated the relationship between temperature, humidity, volume, and air exhaust velocity with ACH. Linear regression was used to measure the line of best-fit for CO2 concentration over time, where the gradient of the line represented ACH for each ambulance tested. Environmental factor influence was examined by correlation coefficients with an alphaapt-ent-b 202F apt-ent-e   apt-ent-uapt-ent-b 003D apt-ent-e = apt-ent-uapt-ent-b 202F apt-ent-e   apt-ent-u0.05. Statistical analyses were performed using R version 3.6.1.


      Under the conditions studied, the measured air change per hour ranged from 14.62 to 40.9 ACH. Using the Centers for Disease Control and Prevention guidelines for airborne contaminant removal by 99.9apt-ent-b 0025 apt-ent-e % apt-ent-u efficiency, the amount of time required to air out ambulances based on maximum and minimum measured ACH is 21 and 35 minutes respectively. Changes in humidity, volume, temperature, and exhaust velocity all demonstrated significantly weak relationships with ACH (r < 0.2, p < 0.001).


      This study found that the CO2 concentration decay method can be used to determine ACH, which is necessary to determine the amount of time required to effectively remove airborne pathogens in an ambulance. This study shows that 21-35 minutes is an effective timeframe to air out the ambulances. Environmental factors have a weak relationship with ACH and therefore is a relatively unimportant factor in air out timeframe.
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