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, 9 (10), 3685-710

Modelling Mass Casualty Decontamination Systems Informed by Field Exercise Data

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Modelling Mass Casualty Decontamination Systems Informed by Field Exercise Data

Joseph R Egan et al. Int J Environ Res Public Health.

Abstract

In the event of a large-scale chemical release in the UK decontamination of ambulant casualties would be undertaken by the Fire and Rescue Service (FRS). The aim of this study was to track the movement of volunteer casualties at two mass decontamination field exercises using passive Radio Frequency Identification tags and detection mats that were placed at pre-defined locations. The exercise data were then used to inform a computer model of the FRS component of the mass decontamination process. Having removed all clothing and having showered, the re-dressing (termed re-robing) of casualties was found to be a bottleneck in the mass decontamination process during both exercises. Computer simulations showed that increasing the capacity of each lane of the re-robe section to accommodate 10 rather than five casualties would be optimal in general, but that a capacity of 15 might be required to accommodate vulnerable individuals. If the duration of the shower was decreased from three minutes to one minute then a per lane re-robe capacity of 20 might be necessary to maximise the throughput of casualties. In conclusion, one practical enhancement to the FRS response may be to provide at least one additional re-robe section per mass decontamination unit.

Figures

Figure 1
Figure 1
Schematic of casualty movement through an idealised mass decontamination response. The Police Service inner cordon runs around the Hot and Warm Zones. Emergency decontamination/interim mass decontamination for ambulant casualties may take place between the Scene and the casualty collection point (CCP). Ambulant casualties progress to the Fire and Rescue Service (FRS) mass decontamination unit (termed MD1) in groups of five males and five females; FRS personnel patrol the middle lane removing disrobe clothing and providing re-robe packs. Non-ambulant casualties progress to the Ambulance Service (AS) clinical decontamination unit; two lanes are available to decontaminate individual casualties. The middle lane of the MD1 can also support clinical decontamination should it be needed but only when specifically configured and not simultaneously with mass decontamination. All casualties finally enter the casualty collection station (CCS) and are assessed for hospital (Hosp.) treatment or otherwise (Home). Radio Frequency Identification (RFID) detection mats were placed at shown locations in Exercise 1. The lights of the MD1 flow control system are located above the four RFID detection mats positioned at the MD1.
Figure 2
Figure 2
Histograms of the final disrobe and shower durations for both exercises.
Figure 3
Figure 3
Current (left) and proposed (right) flow control system. The light and dark grey sections represent the time when the lights are illuminated red and green, respectively. “Disrobe” refers to the final disrobe section as described in the main text.
Figure 4
Figure 4
Histograms of the re-robe durations for both exercises. The solid black lines show the baseline (middle) and worst-case (top) re-robe distribution fits to the data. The dashed black line overlays the baseline fit on the female data in order to highlight their similarity. The worst-case fit is based on a combination of male and female data from Exercise 1 whereas the baseline fit is based only on the male data from Exercise 2.
Figure 5
Figure 5
Average simulated results per lane with a three minute shower duration.
Figure 6
Figure 6
Average simulated results per lane with the baseline re-robe distribution and the exit light included.
Figure A1
Figure A1
For the purposes of this analysis the warm zone extended to the exits of both the MD1 and clinical decontamination units. The casualty collection point (CCP) extended to the entrance of both decontamination units making the dashed radio frequency identification (RFID) detection mat redundant. The outer lanes of the MD1 were used to process (fe)male ambulant casualties whereas the middle lane was for non-ambulant casualties if required but only when specifically configured and not simultaneously with mass decontamination. Human evaluators captured the disrobe, shower and re-robe timings within the clinical decontamination unit. The exit to the casualty clearing station (CCS) was considered the exit from the system although the police did perform subsequent interviews.
Figure A2
Figure A2
There were a total of 50 casualties (29 males, 21 females) in the system but due to the first 45 minutes ‘notional play’ the exercise started with 20 casualties (12 males, 8 females) in the hot zone and 30 casualties (17 males, 13 females) in the warm zone. There was no casualty movement between the hot and warm zones or the warm and cold zones in the first 30 minutes of the exercise. The “V” shape of the warm zone graph is a result of the flow of ambulant casualties through to the cold zone followed by the bottle-neck of non-ambulant casualties in the warm zone. At the end of the exercise (total duration 3 hours 21 minutes) all 20 casualties had been removed from the hot zone, 15 casualties (10 males, 5 females) were in the warm zone and 35 casualties (19 males, 16 females) were in the cold zone.
Figure A3
Figure A3
This graph represents the same information as in Figure A2 but with the data stratified by severity of injury (i.e., P1, P2, and P3). Due to the first 45 minutes “notional play” there were no P3 casualties in the hot zone at the beginning of the exercise. The first P2 casualty entered the cold zone after 1 hour (having walked unaided from the hot zone to the warm zone) and the first P1 casualty entered the cold zone after 2 hours. The final P1 casualty was removed from the hot zone after 2 hours 30 minutes and the final P2 casualty was removed from the hot zone after 3 hours 15 minutes. The four P3 casualties that were still in the warm zone at the end of the exercise were those with non-waterproof electronic prosthetic legs.
Figure A4
Figure A4
13 female ambulant casualties passed through the disrobe, shower and re-robe sections of the MD1 before a transit to the casualty clearing station. The times on the x-axis correspond to the approximate timing of the start of the flow control system’s green light; the graph represents 10 second time slices. Only 1 group of 3 casualties spent more than 30 seconds in the shower but this group didn’t stay in for the full 3 minute duration. For a period of approximately 30 seconds the re-robe section contained 6 casualties, 1 greater than the intended capacity of 5. For a number of casualties a significant amount of time was taken in transit, a distance of approximately 30 metres.
Figure A5
Figure A5
11 male ambulant casualties passed through as described in the caption of Figure A4 but 1 of these casualties was untracked through the entire process. 1 group of 2 casualties and 1 group of 3 casualties were in the shower section for the full 3 minute duration. The re-robe section was at the intended capacity of 5 for approximately 6 minutes despite groups of less than 5 entering the MD1. The casualty clearing station never exceeded the intended capacity of 12 when combining both male and female casualties. The final male casualty to go through the system was a member of the film crew having re-entered the hot zone; this was also the case for the two final females in Figure A4.
Figure A6
Figure A6
For the purposes of this analysis the warm zone extended to the exits of the three decontamination units (2 × MD1, 1 × Clinical). The initial disrobe was actually performed on the line of 6 radio frequency identification (RFID) detection mats at the edge of the casualty collection point (CCP).The outer lanes of the MD1 were used to process (fe)male ambulant casualties whereas the middle lane was for non-ambulant casualties if required but only when specifically configured and not simultaneously with mass decontamination. Human evaluators captured the disrobe, shower and re-robe timings within the clinical decontamination unit. The exits from the three decontamination units were considered the exits from the system although casualty clearing notionally took place at the survivor centre.
Figure A7
Figure A7
There were 130 casualties (92 males, 38 females) but 14 casualties (4 males, 10 females) were instructed to by-pass decontamination by exercise players leaving a total of 116 casualties (88 males, 28 females) in the system. 20 ambulant casualties (13 males, 7 females) removed their tags at the initial disrobe following player instruction meaning that they were untracked through the MD1s. There was no casualty movement between the hot and warm zones in the first 30 minutes of the exercise despite the decontamination units being ready to process casualties. The “Λ” shape of the warm zone graph is primarily a result of the increasing numbers of casualties that performed their initial disrobe followed by their subsequent decontamination. It took 2 hours 11 minutes to process 116 casualties through the decontamination process although it is impossible to know exactly when the untracked casualties were processed.
Figure A8
Figure A8
The time taken to perform the initial disrobe at the casualty collection point was captured for 72 males and 20 females. To complete their initial disrobe both males and females took on average 10.5 minutes, and no males nor females took less than 5 minutes or more than 16 minutes.
Figure A9
Figure A9
18 female ambulant casualties passed through the casualty collection point where they performed their initial disrobe, and then waited in the MD1 queue before entering the final disrobe, shower and re-robe sections of the MD1. 10 different groups entered the MD1 giving an average group size of 1.8 (range = {1,3}). 10 casualties (56%) spent less than 30 seconds in the shower. The re-robe section was briefly at the intended capacity of 5 despite the shower section having at most 3 casualties at any one time.
Figure A10
Figure A10
25 male ambulant casualties passed through as described in the caption of Figure A9. 11 different groups entered the MD1 giving an average group size of 2.3 (range = {1,5}). 9 casualties (36%) spent less than 30 seconds in the shower. For a period of approximately 3 minutes the re-robe section contained 9 casualties, 4 greater than the intended capacity of 5. At approximately 12:30 the flow control system stopped working which may have resulted in the approximate 20 minute time gap in processing casualties.
Figure A11
Figure A11
Only 2 female ambulant casualties passed through as described in the caption of Figure A9 both of whom spent more time in the re-robe section than in the final disrobe and shower sections combined.
Figure A12
Figure A12
In contrast to the female ambulant timeline in Figure A11, 47 male ambulant casualties passed through as described in the caption of Figure A9. 13 different groups entered the MD1 giving an average group size of 3.6 (range = {1,8}). 14 casualties (32% as 3 casualties were untracked) spent less than 30 seconds in the shower. At times there were 8 casualties in the final disrobe, shower and re-robe sections, 3 more than the intended capacity of 5 casualties per section.

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