Pathogen Control Engineering Institute

SCHOOL OF CIVIL ENGINEERING

 

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The Case Studies

Current Projects: We have several projects at the moment.

CFD Analysis of a TB ward: Redesigning ventilation in a TB ward in Peru.

Upper Room UV: Upper room ultra-violet light gives good coverage - how good is the protection?

Ionisers: There is a lot of myth surrounding ionisers - but they can be effective for infection control.

Hospital Monitoring: We regularly go into hospitals to monitor environmental contamination. What can be measured and what use can be made of findings?

Past Projects: Here you can see some of the completed projects.

CFD Analysis of a TB ward

The use of CFD analysis in modifying a TB ward in Lima, Peru

Tuberculosis (TB) is a major worldwide health threat and the leading cause of infectious death. Nosocomial TB transmission is a major problem, especially in resource-limited countries where over-crowding is common and the burden of TB is highest. This and the increase in multi-drug resistant strains (MDR-TB) have prompted increased interest in engineering control solutions such as ventilation system design and the use of ultraviolet germicidal irradiation.

Objectives

The objectives of this study were to

  • evaluate the contribution of the existing ventilation airflow to the possible transmission of TB
  • examine how changes in the design of a ward and its ventilation system, may influence the transmission of TB from patients to healthcare workers, visitors and other patients.
  • Recommend how the ward may be modified to minimise the potential risks to patients, healthcare workers and visitors

Methodology

The study considered a mechanically ventilated, two-bed room in a TB ward in Hospital Nacional Dos de Mayo, Lima , Peru . The analysis was carried out using Computational Fluid Dynamics (CFD) simulations, carried out with CFX 5.6 software (ANSYS-CFX), to model the airflow in the room and the distribution of infectious material generated by either of the two patients. The CFD simulations were carried out to examine the potential impact of changes to the ward design and ventilation system by considering the following cases:

  1. Existing ventilation system (single ceiling supply + single low level extract) and ward design
  2. Existing ventilation system with a full height partition between the two beds.
  3. New ventilation system with second ceiling supply, existing extract + new high level wall extracts above each bed, and a partition between the beds.
  4. New ventilation system with second ceiling supply, removal of existing extract, new high level wall extracts above each bed, and a partition between the beds.
  5. New ventilation system with second ceiling supply, removal of existing extract, new low level wall extracts by each bed, and a partition between the beds.

Key Results

The results for the original room layout in Figure 1 show that away from the high concentrations close to the sources, infectious particles from both patients may be spread throughout the room as a result of the airflow. The contour values, plotted on a horizontal plane at a height of 1.5m, are similar in both cases, suggesting there is significant mixing of the air throughout the space. In both cases the concentrations close to the bed of the source patient are much higher than the rest of the room, indicating the increased risk of transmission at close proximity to the patient. The results suggest that patient 1 is of greater risk of acquiring TB from patient 2 than the other way around as the infectious particles from patient 2 are transported towards patient 1 by the air movement.

CFD results images: Case 1, original room layout and ventilation system design

Figure 1: Case 1, original room layout and ventilation system design

Case 4 in contrast, shown in Figure 2, shows that the modification of the ventilation system as well as adding partitions leads to significant improvements, with much less transfer between the two zones of the room.

Figure 6: Case 4, addition of second ceiling inject, original extract removed, new high level extract behind each patient, partitions between the beds.

Figure 6: Case 4, addition of second ceiling inject, original extract removed, new high level extract behind each patient, partitions between the beds.

Figure 3 compares all five cases by considering the volume averaged concentration in two areas of the room, where bed 1 is considered to be the region to the left of the partition and bed 2 to the right. By weighting the concentration based on the duration of exposure for patients and staff/visitors, the figure estimates the relative risks of infection in each of the five cases.

Figure 3: Potential risk of infection for patients and staff/visitors. Assuming patients are exposed for 24 hours and staff/visitors for 2 hours over a 24 hour period.

Figure 3: Potential risk of infection for patients and staff/visitors. Assuming patients are exposed for 24 hours and staff/visitors for 2 hours over a 24 hour period.

The results of the study demonstrated that changes in the design of a hospital ward may potentially reduce the risk of transmission of airborne infections between patients and healthcare workers, visitors and other patients. In particular the study showed:

  • In a mechanically ventilated environment, placing partitions between beds may be a low cost way of reducing the transfer of airborne infectious particles between areas of the room.
  • Designing the ventilation system so that each area has a separate supply and extract further reduces the spread of bioaerosols between zones.

The results study were used as an aid to local architects and engineers in remodelling the TB ward. Partitions were added between the beds and the ventilation system was modified by adding a second ceiling supply diffuser at the foot of bed 2, providing an air supply inlet for each zone. The original extract was removed and new extracts were installed close to the head of each bed. To reduce costs the system was designed so that the extracted air from each bay was removed through a common duct, with the diffusers located at approximately the head height of the patient. In addition the room layout was arranged to offer the maximum protection for visitors, by locating the chairs on the opposite side of the bed to the extract duct. This ward now forms part of an on-going study on the dynamics of airborne TB transmission.

Further Reading

This study was undertaken with financial support from the Wellcome Trust for the ward redesign and the Engineering and Physical Sciences Research Council (EPSRC) for the CFD study. Further details of the study can be found in:

Noakes CJ, Sleigh PA, Escombe AR, Beggs CB [2005], Use of CFD analysis in modifying a TB ward in Lima, Peru, Indoor and Built Environment, 15(1):41-47