How to use UVGI to mitigate airborne pathogens
UVGI can be an effective means to mitigate spread of airborne pathogens if selected and installed correctly
- Understand the constraints of incorporating UVC into a mechanical system.
- Identify the impact on mechanical design.
- Be aware of the applicable HVAC safety standards.
Concerns about safety within the workplace have arisen because of the coronavirus outbreak. Questions such as “What methods can owners take to keep a workplace safe?” and “Can heating, ventilation and air conditioning systems help reduce or eliminate the airborne spread of the virus throughout the building?” have been asked of the HVAC design industry.
When considering using the HVAC system with ultraviolet light to combat airborne pathogens, the engineer needs to coordinate the UV product constraints into the mechanical system design.
Ultraviolet germicidal irradiation effectiveness has been studied as early as 1877 for the inactivation of microorganism. Past researchers have made significant strides and advancements in showing that UVGI can be an effective approach for disinfecting surfaces and airstreams.
The realization that tuberculosis was an airborne pathogen and that UVGI could counteract the spread of the tuberculosis bacterium led to the installation of UV light fixtures installed on room walls (“upper room UV”) in which lamps would shine above the occupied zone to disinfect the slow convective currents in a room. The efficacy of UV on microorganisms was then incorporated into air conditioning units to mitigate organic growth in cooling coil drain pans.
This was followed using UV to inactivate airborne pathogens in an air stream. In response to many different research efforts, ASHRAE Standard 185.1: Method of Testing UV-C Lights for Use in Air-Handling Units or Air Ducts to Inactivate Airborne Microorganisms was developed to standardize UVC light application.
Incorporating an UV section that provides the intensity and exposure time that results in sufficient dosage to be effective is much easier when designing and installing a new air handling unit. But what about retrofit applications to existing HVAC systems?
Adding a UV section to an existing AHU would involve significant rework and modification of the unit. It would be likely be easier to insert a UV section somewhere in the ductwork, which has additional constraints. To obtain sufficient dosage requires intensity and exposure time; whereas air velocity in AHUs is in the range of 500 feet per minute, ductwork velocities are most likely higher. This means less exposure time and/or greater intensity.
Some manufacturers may have limits on the available intensity of their product; to obtain a sufficient dose, the AHU will likely need to be longer for air path length and/or have wider casing to obtain the required slower velocity to provide sufficient exposure time. This longer AHU then affects mechanical room size. A UV section for cooling coil cleanliness can be added relatively easily but adding a section for UV disinfection of a moving air stream has a bigger impact.
UV lighting can produce ozone. Manufacturers should produce their systems to operate with no ozone production. Equipment provided should come with documentation that the products comply with UL 2998: Environmental Claim Validation Procedure for Zero Ozone Emissions from Air Cleaners. There are other safety aspects in the application of UV systems, such as safety disconnect interlocks with access doors that need to be incorporated. The manufacturer’s installation guidelines are the starting point for safe installation.
Almost every system installed in a building has parts that wear out or degrade based on time and/or usage and UV lamps are no different. ASHRAE notes that UVC lamps can be expected to last about 9,000 hours if constantly energized, so building owners should expect to change the lamps annually.
Upper air germicidal irradiation
While not exactly part of HVAC systems, upper room UV does focus on the room air distribution. UV lamp fixtures can be mounted on walls above the occupied zone (i.e., at least seven feet or higher) as a means of room disinfection. Upper room UV has been used extensively for more than 70 years. It was primarily focused on mitigation of tuberculosis bacteria. The natural convection currents slowly rise where the airborne pathogens are killed by exposure to the lamps; the slow movement helps provide the exposure time for the dose needed for disinfection. Forced air movement enhances the efficacy of upper air UV systems.
Although the exact effectiveness is dependent on room-specific conditions including room size and arrangement, air distribution patterns, air temperature, relative humidity and occupancy patterns, ASHRAE notes that an installed capacity of 30 to 50 microwatts/square centimeter is effective in disinfecting room air.(
The fixtures are located so that the lamp output is not in the line of sight of occupants. Installation should follow the manufacturer’s guidelines.
The units should comply with UL 2998 to document that they do not produce ozone. Consistent with other UV lamps, plan to replace them every year.
Photo catalytic oxidation
UV lights can also be used indirectly by photo catalytic oxidation. When UV light hits certain surfaces (titanium dioxide), it starts a catalytic reaction that releases hydroxyl ions into the air stream. The ions are distributed by the supply air into the spaces where they “seek and destroy” airborne pathogens; these ions combine with airborne pathogens to change the biological structure and, in effect, neutralize them into benign, larger particles that are more effectively filtered out of the air stream.
It is relatively straightforward to incorporate PCO systems into an AHU — they can work at normal AHU velocities of 500 feet per minute and add low pressure drop (about 0.1 inch w.c.). They can tolerate saturated air from a cooling coil but should not be used immediately downstream of a humidifier. To prolong effectiveness, an upstream filter (MERV-8 as a minimum, but preferably MERV-13) is needed to keep the unit clean, like keeping a coil clean to maintain effective heat transfer.
Since PCO is based on using a UV light, the same safeguards apply. The lamps supplied for the PCO product should be UL 2998 compliant. Safety disconnects with access doors prevent maintenance personnel from exposure to the energized lamps. Warning lights that indicate the operation of UV lights provides an additional safety measure.
One safety aspect of PCO is the use of titanium dioxide as the catalyst surface. TiO2 has been mentioned as “carcinogenic” in some published articles. As a material, TiO2 is used in many everyday products — toothpaste, cosmetics, sunscreen — and the Food and Drug Administration has approved it for food additives.
Some of the concerns about the harmful effects are focused on small, very fine powders or even nano-sized particles. Research was conducted by the National Institute for Occupational Safety & Health on the exposure of workers in TiO2 plants, where concentrations would be expected to be much higher than in common situations.
But for environments other than TiO2 factories, manufacturers indicate the TiO2 used in PCO products is bonded to the product so that particles are not released into the airstream. The National Institutes of Health published a study in February 2021 of PCO systems, noting in the abstract that “UV and TiO2 based disinfection technologies may represent a valuable tool to mitigate the spread of airborne pathogens.”
One benefit of PCO, depending on the actual product selected, is that some PCO products can be more easily retrofitted to existing AHUs or duct systems. PCO products typically do not require additional air path length and can be added to an existing AHU without significant rework or modifications to the AHU.
Another benefit of PCO is that PCO can help in odor control. The oxidation process that works to break down and neutralize airborne pathogens has the same effect on other organic particulates and volatile organic compounds, many of which have unpleasant odors.
UV radiation is invisible to the human eye. It lies on the electromagnetic spectrum between visible light and X-rays. The most common source of UV radiation is sunlight, and it is divided into three types based on wavelength: UVA, UVB and UVC.
UVA wavelengths are from 315 to 400 nanometers; UVB is from 280 to 315 nanometers; and UVC is from 200 to 280 nanometers. The longer wavelengths have greater penetrating power. The earth’s ozone layer absorbs all UVC radiation and most UVB radiation, so the UV radiation that reaches earth’s surface is primarily UVA, with some UVB. The only way objects on the earth’s surface can be exposed to UVC radiation is from an artificial source.
Low-pressure mercury discharge lamps are commonly used in commercial applications to emit UVC radiation for the purpose of disinfection, known as UVGI. UVC radiation can kill or disable a wide range of microbes by damaging the DNA, making it incapable of replication. Each microorganism species (bacteria, virus or fungi) has a unique susceptibility to UV light.
However, pathogens consistently show a peak DNA absorption near 265 nanometers. For most pathogens, there is a steep drop in sensitivity below 250 nanometers. Low-pressure mercury discharge lamps emit UVC radiation at a wavelength of 253.7 nanometers, very close to the peak absorption point.
UVGI effectiveness depends on the UV dose, which is calculated as the product of the average irradiance, or intensity of the UV radiation, and exposure time. The effectiveness also depends on a species-dependent inactivation rate constant, k. Measured k-values for many species of virus, bacteria and fungi have been published in scientific literature.
However, these values can vary widely between sources depending on the conditions and methods used. Viruses and vegetative bacteria (such as E. coli and the bacteria responsible for staph infections and strep throat) are generally the most susceptible to UVC inactivation, followed by Mycobacteria (including Mycobacterium tuberculosis), bacterial spores and fungal spores.
Even though UVC rays have less penetrating power than UVA or UVB lights, exposure can still cause severe burns of the skin and injury to the eye. You should never look directly at a UVC light source, even briefly. The injuries from UVC exposure generally resolve within a week with no known long-term damage, but the injuries are delayed, and can cause severe pain even after only seconds of exposure.
Some UVC lamps may emit small amounts of UVB radiation, so prolonged exposure can potentially lead to cataracts or skin cancer. Additionally, some UVC lamps generate ozone, which is irritating to breathing passages, and can worsen chronic respiratory diseases or increase vulnerability to infection.
Many manufacturers and users have voluntarily developed safeguards against accidental exposure to UV radiation, including signage, personal protective gear and training for installers or maintenance personnel.
However, these measures were not mandated by code and not applied consistently. In addition, the International Commission on Non-Ionizing Radiation Protection notes that “engineering control measures are preferable to protection clothing, googles and procedural safety measures.”
The fifth edition of UL 1995: Heating and Cooling Equipment, which covers safety requirements for a broad range of equipment, includes safety requirements that seek to prevent accidental exposure to UV lamps. UV lamps that are factory or field installed in HVAC equipment after November 2019 must comply. UL 1995 prescribes the maximum leakage to surroundings, and requires interlocking mechanisms for access doors, panels or covers that will de-energize the UV source when the UVC irradiance exceeds 1.7 µW/square centimeter. In addition, standardization of warnings and signage are detailed.
Manufacturers should have their products tested and labeled as “zero ozone emissions” per UL 2998: Environmental Claim Validation Procedure for Zero Ozone Emissions from Air Cleaners.
Applying UV lighting to HVAC systems can be done in several ways. UV lamps can be installed to shine directly into the moving air stream in an AHU or in ductwork. Another way to use UV directly is upper air or upper room germicidal irradiation in which UV lamp fixtures are mounted on a wall above the occupied zone. UV lamps mounted on pedestals can be used if the room is unoccupied when in operation.
There are also products that use UV light indirectly in a photo catalytic oxidation, or PCO, process that can also be installed in an AHU or in ductwork. The efficacy of each type of system depends on how closely the installation adheres to the manufacturer’s guidelines and constraints.
Direct exposure with UV
When applying direct UV lighting in an AHU or duct, it is important to understand the dose required to achieve the goals and the factors that impact the delivery of the dose. The effectiveness of UV is dependent on the dose of UV; the dose of UV is directly proportional to both exposure time and intensity.
For example, the dose required to mitigate microbial growth on a cooling coil is significantly less than to kill airborne pathogens in a moving air stream. ASHRAE notes that 50 to 100 microwatts/square centimeter is a typical intensity to inhibit microbial growth on cooling coils; this relatively low intensity is sufficient because the lamp shines on a stationary coil so there is almost unlimited exposure time.
Conversely, to determine the intensity required for a 90% kill rate of the COVID-19 virus in a moving airstream, we have to consider the dose required, which is 611 microwatt-seconds/square centimeter, and the exposure time available. For an airstream moving at 500 feet per minute and 2 feet air path length, the required intensity is 2,444 microwatts/square centimeter, which is 24 times the intensity of that required to mitigate growth on a cooling coil (see Figure 2). This is, however, the first approximation of intensity required.
Manufacturers will take into account other factors, such as the temperature and relative humidity of the air, the quantity and location of lamps and the reflectiveness of the inside AHU or duct surface, to determine the actual required intensity for their product to be effective.
The design engineer has three primary tasks in applying UVC to HVAC systems:
- Understanding the goals of incorporating UVC into a mechanical system.
- Identifying the impact on mechanical and architectural design.
- Specifying equipment that adheres to applicable safety standards.
It is important to realize the capacities and limitations of UV disinfection of air streams. Different organisms have differing resistances or susceptibilities to UV light and in a building where doors open/close, people come and go, etc., no product or system will provide a 100% mitigation rate.
The engineer needs to advise the owner of not only the capabilities but the limitations of the UVC system so that the owner has realistic expectations. Is the owner expecting to have a facility “free of all airborne pathogens?” Is reducing the presence of seasonal flu the main concern or of some bacteria or mold? The manufacturer of the UV system should include information on the intensity provided, the air path length required to obtain the necessary exposure time and any other requirements for their equipment to work according to ratings and to address the goals of the owner.
Meeting project goals
UVGI can be an effective means to mitigate spread of airborne pathogens if selected and installed correctly. To ensure that a solution meets the project goals:
- Be specific in setting the goal of the project. Understand the owner’s expectations; advise them of not only the capacities but also the limitations of the proposed solution. Educate them in the meaning of advertised “kill rates” and how they differ from real world scenarios. Explain the concept of “doses” required to kill different types of pathogens.
- Decide how much impact is allowable for modification to the HVAC system — is this a new project where a separate UV light section can be added to the layout of a new AHU or are there project constraints where a solution with less interruption to the current operations is needed?
- Once a solution is determined, get specific information and guidance from the manufacturer for the specific project application.
- Ask if their product complies with applicable standards such as UL 2998.
- Determine how incorporating a product affects HVAC system design (air velocity, air path length, equipment location and arrangement, filtration protection, safeties, etc.)
- Have the manufacturer review the proposed installation and respond that it includes the necessary constraints for their product to be effective on that specific project. A manufacturer will likely not respond with a “guaranteed number” for effectiveness for a particular project. However, the engineer should obtain some degree of assurance that the installation of a particular system will deliver the expected benefits.
- Acknowledge and identify the maintenance effort to keep the system operating at desired effectiveness and confirm that the owner can provide that effort and expense.