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Reducing Hospital-Acquired Infections
By Arthur D. Hallstrom, MBA, BSME, PE, FASHRAE, BEMP

Correspondence concerning this article should be addressed to Arthur Hallstrom, AD Hall and Associates, 500 Grove Lane, Lexington, KY 40517. Contact: ahallstrom@ad-hall.net

Abstract: Infections are transferred from a source to receiver through surfaces and the air. Airborne transfer is harder to measure and control with the HVAC system. Recent advances in air cleaning technology are helping experts understand how the pathogens are transferred and how they can be controlled. The result is generally safer, healthier environments. Basic HVAC filters have a limited ability to capture and hold airborne pathogens. A new technology called Passive Photo Catalytic Oxidation (PCO) is able to neutralize a wide range of airborne carbonbased pathogens and VOC’s. Forensic experts looking into Hospital Acquired Infections (HAI), particularly among health-care workers and immunocompromised patients, will find this paper interesting. This paper looks at all air cleaning options with a focus on passive conversion application guidance, test results, maintenance, and energy use. Buyers beware. There are wide variations in HVAC filtration and air cleaning performance. This paper gives the latest on what seems to be most effective and why the airborne transfer path may be playing a bigger role in spreading infections than commonly expected.

Keywords: IAQ, Infection Control, Passive PCO, Photo Catalytic Oxidation, PCO, UV, UVGI

Learning Objectives:

  1. How conversion technology can reduce pathogens
  2. Best placement/location of Passive PCO air cleaning devices
  3. Current reduction capabilities and limits of passive conversion (PCO/UVc) technologies
  4. Comparative performance of other types of air cleaning technologies

To some extent, controlling infection transfer is similar to the field of noise control. In both, it is hard to control the source and very hard to control the receiver. Some receivers are more sensitive than others. So the focus in both disciplines is on path control. For noise, you path attenuate the noise to the desired level. For infections, you clean the air to so that the airborne path does not contribute to creating infections.

Recent advances in products, application knowledge, space performance measurement standards, and acceptable limit standards are contributing to better infection control. These advancements are timely. In today’s world, energy-efficient high-performance buildings may have windows that do not open; the outside air may be worse then what is inside the building or pollutant sources inside the building may be hazardous to the occupants—a particular concern in infection control.

An air-cleaning system is defined as a device or combination of devices applied to reduce the concentration of airborne contaminants such as microorganisms, dusts, fumes, respirable particles, other particulate matter, gases, and/or vapors in air (ASHRAE 62.1-2013).

Air cleaning devices can be grouped into three families and five technology types. See Table 1.

Table 1. Building-Related Air Cleaning Technologies

From an engineering perspective, creating acceptable IAQ requires:

  1. An understanding of the current IAQ in existing buildings or the expected indoor air in new buildings.
  2. 2. A determination if the outside ventilation air and particulate filtration will create acceptable IAQ at all times. Generally, this step includes space temperature and humidity control, which is primarily for occupant comfort and productivity and does have an impact on the growth of pathogens.
  3. 3. If ventilation and filtration are not adequate and the IAQ is not acceptable, supplemental cleaning and control options should be used.
    a. Source control (isolation). An example of source control is patient identification and isolation. Find the sources and remove or isolate them (CDC, 2007).
    b. Surface Cleaning. Assuming the sources cannot be totally removed, surface cleaning reduces transfer. Many cleaning methods exist including anti-microbial coatings and UVGI radiation. (CDC 2007)
    c. Additional Air Cleaning. Specifically the use of conversion technologies. The latest option is passive PCO. It involves the oxidation of carbon based chemicals and organisms into water vapor and CO2 and is superior to UVGI alone (Luna, VA et al 2008).

Photocatalysis Conversion

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis (PCO), light is absorbed by an adsorbed substrate. In photo generated catalysis, the photo catalytic activity (PCA) depends on the ability of the catalyst to create electron-hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions.

The passive PCO product configuration uses an inert screen or media coated with TiO2 (e.g. anatase). Light (typically UVc) illuminates the media, creating the hydroxyl radical OH. An important step of photoreaction is the formation of hole-electron pairs, which need energy to overcome the band gap between valence band (VB) and conduction band (CB). When the energy provided (photon) is larger than the band gap, the pairs of electron-holes are created in the semiconductor, and the charge will transfer between electron–hole pairs and adsorbed species (reactants) on the semiconductor surface, then photo-oxidation happens. In the presence of air or oxygen, UV-irradiated TiO2 is capable of destroying many organic contaminants completely. In the degradation of organic compounds, the hydroxyl radical (OH), which comes from the oxidation of adsorbed water or adsorbed OH−, is the primary oxidant; and the presence of oxygen can prevent the re-combination of hole-electron pairs. For a complete PCO reaction, the products of reactions are CO2 and H2O. OH∗ + pollutant + O2 → products (CO2; H2O; etc :) (Juan Zhao, Xudong Yang, 2002).

TiO2 is widely used as a photocatalyst due to its superior characteristics: (a) it is inexpensive, safe, and very stable, showing high photo catalytic efficiency; (b) it promotes ambient temperature oxidation of the major classes of indoor air pollutants; (c) complete degradation of a broad range of pollutants can be achieved under certain operating conditions; and (d) no chemical additives are required. The crystal structure of TiO2 and the choice of heterogeneous support materials have an effect on the PCO efficiency (Juan Zhao, Xudong Yang, 2002).

Passive PCO

The goal for Passive PCO is to create only the short-lived hydroxyl radical OH. See Table 2. The short life of this radical keeps the conversion process within a confined space (i.e. inside the cleaner in the air handler). It has a very high oxidation capability. The ideal process should not create any significant byproducts like ozone or formaldehyde. Passive PCO is designed for air cleaning only as a supplemental air-cleaning measure.

Applications now investigating or using passive PCO for infection control include hospitals, cancer centers, urgent care centers, pet hospitals, farms, airplanes, vehicles, and homes. VOC reduction applications include casinos, airports, and military installations (Genesis Air, 2013).

Table 2. ROS Life and Oxidation Potential

The process goal is complete reduction and conversion. This is product design and application issue. Partial conversion may create intermediate and undesired products. “Some PCO cleaners fail to destroy pollutants completely and instead produce new indoor pollutants that may cause irritation of the eyes, throat, and nose” (EPA, 2012). The solution to this issue may involve the purity of the catalyst and the total illumination of the TiO2 surface at the right intensity and right wavelength UV light. Figure 1 shows how UV bulb placement can impact the effective area. All media should be active with no bypass. Recent third-party product tests of specific designs have either negligible or significant byproducts – specifically ozone and formaldehyde. For passive PCO, the goal is little or no ozone production. (Glenn Morrison, et. al., Sept 2013). The California Environmental Protection Agency, Air Resources Board, lists products that have passed ozone level tests and are certified by their Air Resources Board (ARB). PCO products are not listed seperately but are lumped in with the “other” non-mechanical ccategory that includes ionizers, electrostatic precipitators, other electronic filtration devices, and other air cleaners using new technologies.

Figure 1. Partial and complete (right) illumination

Deactivation of the catalyst is a concern. Possible reasons of deactivation include (a) generation of reaction residues which cause the loss of active sites on the surface, and (b) fouling, which changes the catalyst surface by blocking pores. Some product designs have overcome this issue by managing TiO2 depth with photo-reactivity and continually renewing of the TIO2 with time. Current catalyst life performance guarantees are in the 10-15 year range if located downstream of a MERV 6 or better particulate filter (Genesis Air, 2014).

Passive PCO products including the wiring for the UVc light should be UL certified for safety. Not rated by or in accordance with. The word is certified.

For a fixed product configuration, the extent of passive PCO reduction conversion depends on the contaminant resistance, any biofilm covering (can increase resistance), the time of exposure (depth of the media, air speed thru the media), and relative humidity of the air.

Active Oxidation Type

The active type uses Photohydroionization with various noble metals to create ROS radicals with varying and longer half-lives (e.g. Ozone, hydrogen peroxide). The advantage of active PCO is it can clean the air and surfaces in the space. Surface cleaning occurs by the air transportation of the longer life radials. One application is in food storage preservation by extending shelf life of unwrapped food. The disadvantage and open question regarding active oxidation is the impact of the longer life ROS on human lungs and sometimes surfaces. “Constant exposure to ambient air containing toxic articulates and oxidant gases such as nitrogen oxide and ozone, the lungs are more susceptible to oxidant injury than any other organ in the body” (Val Vallyathan and Xianglin Shi, 1997). The impact of the longer life radicals on people is unknown but a potential safety concern (Kosuke Kawamoto, et. al., 2010). Because of these concerns, Active Oxidation types are not recommended for occupied spaces.

Ultra Violet (UVc)

Ultra violet, mainly UVc, uses direct radiation to clean air, surfaces, and water. This is sometimes referred to as UVGI (Ultraviolet Germicidal Irradiation). UVc is seen as an enhancement to, but not a replacement for, HEPA filtration. As a supplemental air-cleaning measure, UVGI is effective in reducing the transmission of airborne bacterial and viral infections in hospitals, military housing, and classrooms, but it has only a minimal inactivating effect on fungal spores (Siegel JD, et al, 2007). The rate of reduction is a function of energy intensity and resistance of biological. UV can be compared with other conversion technologies by doing comparison reduction tests. Currently no building industry wide test standard for UV currently exists. ASHRAE is currently developing a UV method of test (ASHRAE Standard 185.1P) and an application standard (ASHRAE Standard 185.2P).

Another safety concern with UV in general relates to occupant exposure to direct UV light. At the UV power levels typically used commercially, a fairly short direct exposure to UV light can cause temporary loss of eyesight (ASHRAE Applications Handbook, 2011). Care should also be taking in disposing of the bulbs as they contain a small amount of mercury.

Particulate Filters

Traditionally, particulates are removed with particulate filters rated by the ASHRAE MERV method. A MERV 1 filter is very coarse while a MERV 16 captures finer particle sizes. HEPA filters are at least 99.97% efficient for removing particles >0.3 μm in diameter. (As a reference, Aspergillus spores are 2.5–3.0 μm in diameter.) Filter limitations relate to air bypass around the filter, pass through of small particles like viruses and odors, issues with destroying captured viable containments (requiring filter bagging) and high pressure drops which use significant fan energy. In many infection control HVAC systems, filters are the largest pressure drop device in the system, particularly if multiple banks of increasing MERV levels are used to reduce filter change out costs. Multiple filter banks do lower filter replacement costs but increase the fan energy and operating costs significantly. HVAC system operating costs are area of owner interest.

Performance Comparisons

Air-cleaning products can be compared by using their reduction rates against specific particles, biologicals or chemicals, the amount of energy power required (direct light power), and indirect (increased fan) energy, first cost (including installation), maintenance cost over life of the product (for maintaining desired reduction), replacement cost, disposal costs, application limits, and safety considerations.

As mentioned, no industry-wide product test standards exist for the conversion family products. Benefits in infection control can be impressive but there are risks. Users are recommended to proceed with caution using the following approach:

  1. Review the product performance tests from third parties like government agencies and large end-users interested in indoor air quality (e.g. Trane, building owners). Many of the tests for various reasons are classified or propriety. Qualified third party tests are critical, reviewed by your in-house experts and/or outside consultants.
  2. Next step - Test the product in a test room with the building.
  3. If test is successful, increase the test to a larger area (one terminal, wing, floor or building).
  4. If that test is successful, can be used in all buildings where enhanced infection control and/or VOC reduction is desired.

Sample Passive PCO Air Cleaner Performance

Installed in air handler (like in-duct but at air velocities between 300-500 fpm).

  • Pressure Drop: .05-.2” (Function of design and velocity thru media)
  • Life: UVc bulbs – 18 months, TiO2 media – up to 15 years
  • Energy Use: 0.1 amp per square foot of media, UL certified
  • Configuration: Requires MERV 13 Filter upstream. UV light screens are used if UV light can be seen by occupants.
  • Best location: downstream of space cooling coil (higher RH area causes better PCO reduction), smooth airflow profile, air velocities at 500 fpm or less, and the UV light helps keep upstream coil clean.
  • Second best location: in a room recirculation unit downstream of the particulate filter and UV light screen and upstream of the fan. Typically sized for 6 space air changes an hour.
  • Other locations: outside air intake duct, exhaust ducts.

Sample Passive PCO Reduction

Performance can be tested in single pass or recirculated tests. The following are summaries of the test procedures used by RTI recently on two pathogens of common interest. The tests focus on the incremental performance of a passive conversion technology.

Bacillus atrophaeus (Bg) is a spore-forming bacterium with spore size ranging from 0.7-0.8 x 1-1.5μm. The organism is a ubiquitous environmental bacterium, found at high levels in soil and highly associated with indoor dust. Bg is generally recognized by the scientific and testing communities as one of the simulants for B. anthracis in numerous bio warfare agent testing scenarios. Accordingly, Bg has value as a historically used simulant and permits comparison to past testing and studies.

Staphylococcus epidermidis (S. epidermidis) is a common gram-positive human shedding organism, but can be a pyogenic (fever-causing and pus-forming) pathogen. It is also closely related to Staphylococcus aureus, the species responsible for Methicillin-resistant Staphylococcus aureus (MRSA) infections. S. epidermidis can be viewed more broadly as a representative of vegetative bacteria which are generally more susceptible to neutralization than bacterial spores.

Single Pass In-duct Test Method

The [single pass] testing was conducted in the test duct shown schematically in Figure 2. The test section of the duct is 0.61 m by 0.61 m (24 in. by 24 in.). The locations of the major components, including the sampling probes, the device section (where the device is installed), and the aerosol generator (site of bioaerosol injection) are shown. The test duct is operated following Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size (ANSI/ASHRAE Standard 52.2-2007).

Figure 2. Schematic of Test Duct. UV-PCO device is placed in device section.

Test Protocol

The test protocol was as follows:

  1. Turn on the test duct blower and adjust flow to 2000 CFM.
  2. Supply power to the ballast assembly and switch lamps on.
  3. Turn on the Collison nebulizer and drying air and run for at least 8 minutes.
  4. Collect upstream and downstream bioaerosol samples.
  5. Turn off Collison and UV light ballasts.
  6. For the “no-device” test, the test unit was removed and step 2 was omitted.


    The efficiency of the device for inactivating airborne bioaerosols was calculated as: Airborne Inactivation Efficiency (%) = 100 (1- Corrected Survival Rate) Equation 1. The calculation of the test organism survival rate (culturable transmission) was based on the ratio of the downstream to upstream culturable organism counts. To remove system bias, the Survival Rate was corrected by the results of the no-device transmission test. The no-device transmission rate was calculated in the same manner as the survival rate test, but using the culturable organism counts from the no-device tests.

    Single Pass In-duct Test Results

    Figure 3. Inactivation efficiencies for introduced bioaerosols

    Multi Pass Tests Method

    [Multiple Pass] test for each organism included a natural decay measurement and an air cleaner decay measurement. Both measurements are performed after filling the chamber with challenge bioaerosol. The natural decay is defined as the decay of the test bioaerosols in the chamber (Figure 4) with the air cleaner off. The air cleaner decay measurement is defined as the decay while the air cleaner is running.

    Figure 4. Dynamic Microbiological Test Chamber (DMTC)

    The test method has been described in depth by Foarde et al. (1999). As an overview, the paper describes a test method to determine a Clean Air Delivery Rate (CADR) type measurement for a device when challenged with microbiological aerosols. The method is a modification of the Association of Home Appliance Manufacturers (AHAM) Standard AC-1, “Standard Method for Measuring Performance of Portable Household Electric Cord-Connected Room Aircleaner” which determines the CADR for three different particulate matter challenges (smoke, dust, and pollen). This extension of the AHAM method to microbial aerosols follows the tradition of the AHAM test of using realistic particle challenges and provides a means to compare and evaluate different brands of room air cleaning devices regarding characteristics significant to product use. This is a useful approach for evaluating a wide range of devices.

    Sampling of B. atrophaeus and S. epidermidis was accomplished using one-stage Andersen viable bioaerosol samplers loaded with Petri dishes containing growth media. The one-stage Andersen sampler is a multiple-jet impactor. After sampling, the Petri dishes were removed from the sampler and incubated overnight at 37°C for both organisms. Colony forming units (CFUs) were then enumerated and their identity confirmed.

    Test Protocol

    The test protocol for B. atrophaeus and S. epidermidis was as follows:

    1. Turn on the chamber AHU and circulating fan.
    2. Allow the HEPA to clean the chamber air for at least 1 hour.
    3. Turn off AHU and turn on the unit for 10 minutes to let the UV lamps warm up.
    4. Turn off the unit and turn on the Collison nebulizer and run for 5 minutes with HEPA-filtered drying air. Turn off Collison nebulizer, and continue to allow the drying air to flow through the drying tower and into the chamber for another 8 minutes.
    5. One minute prior to the start of collection for the “0 min” sample, turn off the drying air, close the valve between the drying tower and the chamber to prevent backflow, and turn off the circulating fan in the chamber.
    6. Switch on the air cleaner at the start of collection for the “0 min” sample.
    7. Collect triplicate bioaerosol measurements at appropriate intervals (usually 0, 4, 8 and 12 minutes, or 0, 5, 10 and 15 minutes).

    Figure 5. Decay curves for S. epidermidis

    Figure 6. Decay curves for B. atrophaeus


    Passive PCO type products are starting to attain recognition as a viable, safe method for helping controlling airborne infections. It seems prudent for infection-control professionals and building designers consider this option as an additional control measures for airborne transmission, in addition to the precautions already in use when designing new healthcare facilities and updating existing facilities.

    An interesting unanswered question relates to air contamination of surfaces. Surface cleaning is critical to infection control. Air cleaning is important when the infection is passed directly to the receiver. An open question is, “Could airborne transmission of infections be recharging cleaned surfaces?” If yes, air cleaning would grow in importance as an infection control method. Research is needed in this area.


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    Arthur D. Hallstrom, MBA, BSME, PE, FASHRAE, BEMP, has a BSME from the University of Illinois and a MBA with Trane/Ingersoll Rand for 38 years in Applications, Engineering and Product Support Management Positions. As Trane’s Airside Applications Manager, he focused on management of Indoor Air Quality in standard and process applications, ranging from office buildings to hospitals to military hardened command centers. He was involved in the effect of temperature and humidity control, particulate filters and conversion technologies in creating better environments. Mr. Hallstrom is a registered Professional Engineer, ASHRAE Fellow, ASHRAE Certified Energy Modeler, and an AFCEI and AEE member. He is a former ASHRAE Director, a past President of the ASHRAE College of Fellows, and a US Army LTC (retired). Currently, he is President of AD Hall and Associates, Lexington, Kentucky and Executive Director of ae3, LLC, Tampa, Fla.

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