VENTILATION
St Luke’s Hospital case study In a major US study recently published in the journal, Surgery, over 1000 patients spread across three wards at St Luke’s University Hospital in Bethlehem, Pennsylvania, were tracked over a 12-month period to determine the effects of UV airstream disinfection on pathogen levels and hospital-acquired infections. The patients were separated into three different zones (Figure 5), with all zones having HEPA filtration of recirculated air, but with two of the three also benefiting from ‘advanced air treatment’ technology involving a combination of UVGI and activated carbon filtration. The study found that the advanced air
UV treatment applied to both Zone A and Zone B patients resulted in: n >90% reduction in airborne and surface pathogen levels in UV-treated zones compared with non-UV treated zones. n A 39% reduction in length of stay in the
‘‘ Outside Air St Luke’s Hospital (PA) Air-Handling Unit HEPA Filter UVGI + Advanced Filtration Zone A 278 patients Zone B 343 patients Zone A
381 patients (Control Group)
6.2 cfu
Prevalance of Fungi and Bacteria 23.0 cfu
Surface and Air Surface and Air Length of Stay in Hospital
60.5% of average Control Group stay
62.4% of average Control Group stay
Cost of Hospital Stay
77.7% of average Control Group charges
UVGI of air resulted in:
• >90% Reduction in Airborne and Surface Pathogen levels • 39% Reduction in Hospital Length of Stay • 20% Reduction in Hospital Charges
Figure 5: Summary of St Luke’s Hospital UV airstream disinfection study. 24 Health Estate Journal August 2022
80.5% of average Control Group charges
100% of average hospital charges for Zone C
100% of average hospital stay for Zone C
Outside Air Supply
A 2013 study by P. Azimi and B. Stephens showed the relative costs of filter operations plotted against the relative risk of infection. This suggests that MERV13 filters provide the optimal filter performance in terms of pathogen removal rates
hospital for patients occupying UV- treated zones compared with non-UV treated zones.
n An associated 20% reduction in hospital fees and charges incurred by patients in the UV- treated areas. This study is one of the first large-scale
field tests investigating the effectiveness of UV air disinfection on clinical patients in a hospital setting, and can help the decision of many healthcare facilities to install UV air disinfection systems in their buildings. The air treatment system employed also
Outside Air
Outside Air Supply
Air-Handling Unit HEPA Filter
used activate carbon filters to reduce volatile organic compounds (VOCs).
Implications for HEPA filter performance on filtering pathogens High Efficiency Particulate Arrest (HEPA) filters have typically been used in high- risk settings such as hospitals and fertility clinics. While it is known that HEPA filters do not screen all particle sizes, with some 0.1-0.3 µm particles able to penetrate through the filter, there is a widely held perception in the industry that HEPA filters work as well as UV air disinfection. The results from this study, particularly in terms of the prevalence of bacteria and fungi, indicate that HEPA filtration alone does not achieve the necessary removal of pathogens so as to limit the spread of hospital-acquired infections. The size of viruses and bacteria ranges
from 0.1-5 µm (SARS-CoV-2 is 0.12 µm, while Tuberculosis can range in size from 0.2-0.5 µm in width), and it would theoretically be expected that HEPA filters would capture most of these particle sizes. A recent study of the penetration of small particle sizes through MERV filters (which offer less filter efficiency than HEPA filters) has highlighted the potential for HVAC filtration to perform well below modelled performance standards for aerosols in the 0.5-4 µm range when tested in a central ventilation system. While the study did not test HEPA filters, the results of the St Luke’s study suggest this question also needs to be examined.
183.7 cfu Surface and Air
Cost of operating HEPA filtration systems Consideration also needs to be given the to the cost of operating HEPA filtration systems. A 2013 study by P. Azimi and B. Stephens (see 8 in Further reading) showed the relative costs of filter operations plotted against the relative risk of infection (Figure 6). This suggests that MERV13 filters provide the optimal filter performance in terms of pathogen removal rates. ASHRAE’s COVID-19 guidance also recommends a minimum filtration standard of MERV13. Installation of UV air disinfection does
not remove the need for air filtration, but rather complements it. UV lamps can become fouled by airborne particulates, and air filtration is required to minimise this effect. The effectiveness of UV also relies on direct irradiation of pathogens in the air stream. An unfiltered airstream can result
Return air
Return Air
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