Managing Airborne Pathogens in HVAC Applications
History of Indoor Air Quality (IAQ)
Indoor air quality (IAQ) involves a broad category of factors that evaluate how the air in and around a building impacts human performance in that space. Due to COVID-19 triggering a global pandemic, society shows a heightened desire to reduce airborne pathogens like viruses, bacteria, and mold to prevent the spread of illnesses and make indoor spaces safer. An array of systems, both active and passive, is proven to improve air quality by reducing pathogens. One specific technology, UVC, works by using a photolytic effect wherein light in the UV spectrum destroys or inactivates the micro-organism’s ability to reproduce (multiply), thereby sterilizing the air.1
The following evaluation of the Genteq™ UVantage™ air treatment system shows that the use of UVC illumination in HVAC systems provides effective, safe, and efficient sterilization and, when used with appropriate filtration and ventilation, can quickly reduce pathogens indoors.
In the mid to late 1800s, scientists observed that sunlight, specifically UV irradiation, effectively neutralized bacteria. UV dosing, measured in joules per square meter (J/m2), consistently showed germicidal action against bacteria, yeasts, mold, algae, and protozoa. These micro-organisms have different sensitivities to UV, and therefore laboratory research has determined different dosing intensities for effective sterilization of pathogens. Additionally, empirical evidence found variations in the UV spectrum (UVA, UVB, and UVC) have varying impacts as well, where the peak germicidal action occurs near 265nm, in the UVC range.2
See the following chart published by the Illuminating Engineering Society of North America (IESNA) for germicidal efficiency over 230 to 310nm.3
Figure 1 - UVC Germicidal Efficacy Curve (peak 270 nm)4
Therefore, sources that emit light between 250 and 290nm (in the UVC spectrum) are preferred for germicidal applications. Mercury-based discharge lamps, available for decades, operating at 254nm, near the peak germicidal range, are still in use for sterilization. Newer LED technology can be manufactured to specific wavelengths like 275nm, making them more efficient and easier to regulate.5
Between 1930 and 1955, a series of studies on the effectiveness of UV to sterilize air in clinical environments were conducted. In 1955 journal articles, “Air Contagion and Air Hygiene” by W. F. Wells and “Airborne Infection: Transmission and Control” by R. L. Riley document how managing airflow, the use of UV, and other approaches deliver effective disinfection. Wells and Riley determined that UVC delivers the best results in reducing airborne pathogens.6
This work has continued and expanded in recent years with further studies of air disinfection by M. W. First, C. B. Beggs, E. J. Avital and others.7The work of these and other scientists has matured the field such that there is a well-documented understanding of how pathogens, such as viruses, are reduced under specific dosing intensities. See following Table 1 for required UVC energy dosages for two common viruses.8
Historically, UVC source illumination was generated by mercury discharge lamps, manufactured in standard sizes and shapes (T5, T8, T12, PL-L, and others). Limitations with these lamps have been short useful lives (9,000 hours to 80% initial light) and a limited number of on/off cycles. Use in cold applications also reduced illuminance and, therefore, sterilization efficacy. Mercury discharge UV illumination systems require lamps, sockets (housing), and ballast (power conversion). The most common applications are water sterilization and upper air sterilization via purpose-build lighting and control systems.10, 11
Light-emitting diodes (LEDs) operating in the UVC spectrum are available, but until recently have not been widely available due to low market demand. With an increased focus on surface and air sterilization, UVC diode availability is improving and is now practical and cost-effective for commercial and residential applications Operating on a regulated class 2 power supply, LEDs can be installed in and on a variety of surfaces but require thermal management and a tuned power supply for proper operation. In Regal® laboratory testing, UVC LED systems, meeting UL8750 deliver 15,000 hours of useful operation, where turn on/turn off cycles have no impact on life of the product. In addition, UVC LEDs are manufactured, tested, and binned for operation at 275nm; eliminating undesirable wavelengths from the system.
Decades of research, field experience, and documentation prove the utility of UVC for air sterilization. Published standards by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the IESNA profile the efficacy of UVC for upper air, lower air, surface, air handler, ventilation, and other approaches. UVC sterilization operates to reduce pathogens, thereby reducing the probability of reproduction within a human host.12 Since airborne pathogens travel invisibly in air currents, system efficacy is based on probabilitic pathogen kill rates, with assumed room sizes, airflow rates, number of air cycles per hour, and differing UVC intensities.13 In summary, the best practical air quality is achieved with a powerful UVC source, appropriate filtration, adequate ventilation, and frequently recirculated air.14
Genteq UVantage Air Treatment System sterilization efficacy was simulated using practical residential and commercial configurations to provide guidance on application design. Figure 2 shows CFD simulation of radiation intensity in various sections of a blower, reflective materials located on the housing surfaces inside the blower are valuable to increase radiation intensity. The low-profile installation of the LEDs creates negligible turbulence and no static pressure increase within the system.
Figure 2 - Radiation Intensity in W/cm2 Inside Blower
UVantage is suitable for residential and commercial applications up to 2,400 CFM and 1” static pressure. In residential applications, UVantage’s sterilization efficacy was recorded when configured with and without a 10% filter in systems operating at 400 and 1,200 CFM. The results of these studies are shown in Figure 3 and Figure 4 respectively. The Y axis shows percentage of pathogen reduction and the X axis shows hours of operations at a specified airflow. Equations on the right of Figures 3 and 4, show the mathematical models describing the operation, where Vblower is the volume of blower, Vspaceis the volume of the conditioned space, N0 is the initial amount of virus particles inside the space, K is the virus standard rate constant, I is the UVC average intensity.
Figure 3 - Pathogen Reduction Over Time in a 400 CFM System in a 1,800 ft2 Residence
Figure 4 -Pathogen Reduction Over Time in a 1,200 CFM System in a 1,800 ft2 Residence
In classroom applications, the Regal study of UVantage shows that with filtration and fresh air, pathogen reduction of SARS-CoV-2 exceeded 82% within the first hour.
Figure 5 - Classroom Air Sterilization Model with Regal UVC
Figure 6 - Small 1,200 ft2 Commercial Installation with 200 CFM Fresh Air Exchange Rate
In other commercial applications, where the air-conditioned space is small and external fresh air ventilation is low (i.e.; 5 CFM/person), the Genteq UVantage air treatment system can provide significant pathogen reduction. Figure 6 shows a simulated case of a 1,200 ft2 commercial space with 200 CFM external fresh air exchange rate.
Application In Modern HVAC Systems
UVC sterilization is an effective means of improving indoor air, leading to healthier indoor environments. Incorporating UVC into residential and commercial HVAC systems seems a logical choice for reliable, consistent sterilization. However, installing into existing and new systems has practical challenges like powering the UV LED system (requires line or low voltage), making certain the system is off when not required (conserving energy and extending life of the UVC LEDs), and installing the system in a place and in a manner that ensures that accidental radiation exposure to humans will not occur.
Installing UVC LEDs within a blower housing (as UVantage is constructed) has several benefits:
- Efficacy: based on the airflow expected by the blower/motor combination, UVC LEDs are selected and tuned for optimal sterilization.16 Increased filtration, a common approach to reducing pathogens, causes increased system static pressure and can impact overall system design and efficiency.
- Lifetime: mounted in the thermally conductive metal housing (keeps LEDs cool), the UVC LEDs will run cooler than in free or static air, extended expected life, allowing for less maintenance and lower power operation.
- No static pressure increase; LEDs are placed in manner that air flows without resistance.
- Safety: enclosed space to minimize UVC light-leak; when attached to the motor and a technician uses a service disconnect/HVAC system circuit breaker, the LEDs will be off and pose no additional health risk.
- Access to power; by sharing the AC power input from the blower-motor, the LED system is powered and easily installed by an OEM or trained technician. This means no other system modifications, including increased filtration, ductwork, and/or finished ceiling/vents, are required.
- Centralized; sterilization in the air handling system simplifies system operation and becomes a simple, single-point of investigation for any service concerns.
- Clean; mold and bacteria will not grow within the blower housing or blower wheel due to the UVC LEDs.
Lifetime and maintenance remain a concern for HVAC applications where homeowners may not regularly follow recommended filter replacement or cleaning schedules. In practical applications, with 3 hours of operation in a 24-hour cycle, the UVC LED solution is expected to last over 10 years. For applications or customers with a desire to improve air quality without the use of air conditioning or heating, the “fan” option in most residential thermostats will ensure airflow and sterilization in a lower power mode of operation. Figure 8 shows the first generation of UVantage; incorporating .48 Watts of optical power, high reflectivity internal material and an average UV-C intensity of 6.13W/cm2. Third-party laboratory validation on simulation effectiveness is in process. Results will be presented in a future paper.
Figure 8 - First Generation Genteq UVantage Air Treatment System
There are three classifications of UV light, each with its own impact on human health:
- UVA (long-wave): from 315 to 400nm
- UVB (medium-wave): from 280 to 315nm
- UVC (short-wave): from 100 to 280nm17
Figure 7 - UV, Visible Light, and Infrared Spectrum18
The light emitted in the short-wave band, UVC, can cause erythema (reddening of the skin) and conjunctivitis (inflammation of the mucous membranes of the eye), and therefore care should be taken to avoid direct human exposure to incident irradiation. Most materials absorb UVC, including standard glass, and most UVC is absorbed by dead skin, not living cells (like UVA and UVB). However, risks to the eyesremain.19 Therefore, with adequate warning labels and appropriate disconnects such that service technicians should never operate with UVC present, the system should pose no risks to humans. Finally, in normal operation, the LEDs are housed within the blower, allowing no UV light leak. Further research on the biological effects of UVC radiation through the US Environmental Protection Agency (EPA) and the European Commission Scientific Committee on Health, Environment, and Emerging Risks (SCHEER) Scientific papers note that UVC at wavelengths below 240nm generates ozone (O3) from ambient air. Ozone is reactive and toxic and should be avoided.20 Therefore, Regal has selected UVC LEDs specifically tuned to operate in a very narrow band centered at 275nm, in order to avoid ozone risk.
1. Reed, Nicholas: “The History of Ultraviolet Germicidal Irradiation for Air Disinfection”, Public Health Reports, Jan-Feb 2020, Vol 125
2. American Society of Heating, Refrigerating and Air-Conditioning Engineers: 2019 ASHRAE Handbook “Chapter 62 Ultraviolet Air and Surface Treatment”
3. Illuminating Engineering Society of North America: IES CR-2-20-V1 “Germicidal Ultraviolet (GUV) – Frequently Asked Questions”
4. American Society of Heating, Refrigerating and Air-Conditioning Engineers: 2019 ASHRAE Handbook “Chapter 62 Ultraviolet Air and Surface Treatment
5. Krames, Mike: LEDs Magazine “The Rise of UV-C LEDs, July 2020
6., 7. Reed, Nicholas: “The History of Ultraviolet Germicidal Irradiation for Air Disinfection”, Public Health Reports, Jan-Feb 2020, Vol 125
8., 9. 10. Kowalski, W. J. Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection. New York: Springer, 2009
11. American Society of Heating, Refrigerating and Air-Conditioning Engineers: 2019 ASHRAE Handbook “Chapter 62 Ultraviolet Air and Surface Treatment
12. Kowalski, W. J. Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection. New York: Springer, 2009
13. Reed, Nicholas: “The History of Ultraviolet Germicidal Irradiation for Air Disinfection”, Public Health Reports, Jan-Feb 2020, Vol 125
14. ARTI HVAC&R Research for the 21st Century “Defining the Effectiveness of UV Lamps Installed in Circulating Air Ductwork” ARTI-21CR/610-40030-01
15. Centers for Disease Control and Prevention “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission”
16. Krames, Mike: LEDs Magazine “The Rise of UV-C LEDs, July 2020
17.,18. Philips Lighting BV UV Health and Wellness: PHI5998 UV Purification Tech Upd.qxd (signify.com)
19. American Society of Heating, Refrigerating and Air-Conditioning Engineers: 2019 ASHRAE Handbook “Chapter 62 Ultraviolet Air and Surface Treatment”
20. “Indoor Air Quality, “What are ionizers and other ozone generating air cleaners?”