Applying UVGI in health care HVAC systems
The importance of and attention to cleaner air for patients and staff is ever-increasing. This reviews UVGI system effectiveness, capabilities and application to provide cleaner and safer air in the health care environment
- Understand how ultraviolet germicidal irradiation systems operate.
- Learn the limitations and considerations of specifying UVGI systems.
- Know where to use UVGI systems based on desired criteria.
Health care facilities require that clean air be provided to all spaces to reduce the spread of disease and the risk of patients developing new infections. Surgical rooms require clean air to protect both the doctors and the patient during the operation. The World Health Organization has determined that 1 billion people contract diseases through airborne transmission each year while in the U.S. roughly 275,000 surgery patients develop surgical site infections each year.
The mechanical engineer’s duty is to design the heating, ventilation and air conditioning system to provide clean air to the surgical environment, maintain proper pressure relationships and remove or neutralize any harmful particulates in the air to help prevent surgical site infections. Traditional methods incorporate high-efficiency particulate filter media (MERV 13, 14 or high-efficiency particulate air filters based on the type of procedure) to catch particulates, including microbes, in the air moving through the air handling unit.
Ultraviolet germicidal irradiation systems are gaining popularity due to their ability to kill harmful drug-resistant bacteria such as Methicillin-resistant Staphylococcus aureus or tuberculosis, as well as inactivate viruses, including newer viruses such as COVID-19.
With viruses, UVGI systems use UV light to inactivate cells hindering their ability to replicate and multiply. UVGI systems can destroy or inactivate particulates regardless of size, thus not relying on the filter efficiency versus size of the particulate or virus for assistance.
How does UVGI work?
UVGI fixtures use mercury molecules to emit the UV light to inactivate cells by hindering their reproductive process effectively killing the bacteria or virus (see Figure 1).
UVGI lamps in the HVAC industry commonly use low-pressure mercury discharge to produce UV-C light of 253.7 nm wavelength. UV-C is used in air cleaning over UV-A or UV-B for risk and effectiveness reasons. While UV-C can cause more damage to skin or eyes than UV-A, it is 1,000 times more effective at deactivating viruses. UV-B penetrates deeper than UV-C causing more damage to skin and eyes and is actually less effective at inactivating viruses than UV-C.
The electrodes at each end of the lamp release an electrical discharge that collides with a noble gas (argon, neon or a combination of the two) inside the lamp. This excites the mercury atoms inside the lamp to emit the UV light from within the lamp. The electrode selected influences the intensity of the UV light emitted. Two types of electrodes generally used are:
- Cold cathodes require a high voltage potential to ionize the gas and cause current to flow through the lamp. Cold cathodes produce less UV intensity but use less energy. They can start up instantly and last thousands of hours longer than the alternative hot cathodes.
- Hot cathodes must be heated up before the discharge of electrons occurs. Once electrons are discharged, the UVGI intensity is much higher than that generated by cold cathodes. With more intense UV irradiance, hot cathode UVGI can kill more bacteria and inactivate more viruses.
UV transmitting glass or quartz makes up the lamp. Both glass and quartz allow the transmission of 253.7 nm wavelength, but quartz can transmit 185 nm along with the 253.7 if needed. The 185 nm wavelength UV lamps are used to produce ozone, which can be used to further fight bacteria in the air stream or captured and used in water systems. The mercury inside can be either pure metal or an alloy of mercury. Three types of ballasts can be used for UVGI lamps:
- Preheat ballasts heat the electrodes before current flows into the lamp.
- Rapid start ballasts heat the electrodes before and while the lamp is on. Rapid start ballasts allow for dimming capabilities.
- Instant-start ballasts do not heat the electrodes before the lamp is turned on.
UV-C LED is an emerging technology with similar characteristics to disinfect the airstream by providing light at a 265 nm wavelength. It is more energy efficient and safer due to the lack of mercury present in the lamp, but comes with reduced efficacy and increased initial equipment cost when compared to cold cathode mercury lamps. As the technology improves, it will become more viable as a replacement for mercury lamps.
Understanding the construction, the next step is understanding how effective UVGI is at inactivating harmful bacteria and viruses.
How effective is UVGI?
The engineer must select the best system for a project for the health, safety and welfare of the occupants of a space. A good design will use the most effective method to remove the harmful contaminants from the air before entering a space or those created from a space before being recirculated into the same or another space. Harmful contaminants in surgical room HVAC systems should be removed or rendered inactive before the air is introduced into the space to limit the particulates with which the patient may come in contact, hence limiting the chance for infection.
Particulate’s susceptibility to UV depends on a microbe’s individual properties (see Figure 2).
Vegetative bacteria and mycobacteria are particularly susceptible to UV light. Staphylococcus aureus, the bacteria that causes staph infections, fall under vegetative bacteria, while mycobacteria claim Mycobacteria tuberculosis, the cause of tuberculosis. Staph infections and tuberculosis are major threats in health care facilities where the close proximity of patients can cause the transmittance of these infections. These particulates are very small and require high-efficiency filters to stop their movement through the air.
UVGI fixtures can help minimize additional filters needed by effectively ridding the air of these bacteria. The susceptibility constant, Z, measures the resistance to UV light of a bacteria. The higher the Z value, the more susceptible to UV light the bacteria are allowing for less exposure required to kill the cell.
Mycobacteria tuberculosis has been found to have a susceptibility constant, Z, around 0.2 for one strain and 0.5 for another. An average UV irradiance of 1.086 eliminated almost all particles with a Z value of 0.5 and an irradiance of 2.135 watts/square meter eliminated almost all particles for Z = 0.2. A minimum average UV irradiance of above 2.135 watts/square meter is necessary to eliminate nearly all particles of both strands of tuberculosis with a single pass through the kill zone.
The Z value for Staphylococcus aureus, staph infection, was found to be Z = 0.946. A UVGI fixture will be able to eradicate nearly all staph-causing bacteria with a single pass at an average UV irradiance of a little more than 0.5.
Staph infections and tuberculosis have long been the main infections plaguing health care facilities that must be controlled for patient safety. Today’s focus is on limiting the transmission of the coronavirus or the next yet-to-be-known virus. Studies suggest that COVID-19 virus has a Z value of approximately 0.2 requiring an average UV irradiance of 2.1 watts/square meter to inactivate 99% of the coronavirus with a single pass through the kill zone at a negligible exposure time.
In-duct UVGI systems inactivate particulates by inactivating the DNA as it passes through the unit in the air distribution system creating the kill zone. They can be placed in ductwork or most commonly in the AHU. The overall effectiveness of a UVGI system is dependent on the particulate’s time in the kill zone. In an in-duct system, air moving too fast through the ductwork system will have a lower exposure duration to the UV light lowering the effectiveness of an in-duct system.
The Sheet Metal and Air Conditioning Contractors’ National Association recommends maintaining an air velocity at or below 500 feet/minute for maximum efficiency of an in-duct UVGI system while ASHRAE recommends an exposure time of 0.25 seconds and an average irradiance of 9.3 watts/square foot. This results in a minimum exposure distance or kill zone of approximately 2 feet.
The relative humidity of the air that the particulates are aerosolized in also affects the effectiveness of UVGI. High RH causes moisture to cover the particulates in the air stream creating a barrier that dilutes the intensity of the UVGI systems. It has been observed that the effectiveness of UVGI on particulates starts to decrease as the RH in the air exceeds 60%. A RH above this 60% threshold will require increased intensity.
These conditions of the ductwork system housing the UVGI systems should be considered in determining installed average irradiance. Increased intensity is required for velocities exceeding 500 feet/minute or when RH is higher than 60%. To increase the average irradiation, more lamps are required, which adds first cost, maintenance, extra required power and pressure drop to the system incurring additional penalties to the energy consumption for the system.
Where to use UVGI in HVAC systems
Knowing that the type of bacteria or virus and design of the ductwork system determines the intensity of UVGI required, the engineer must determine the target when specifying where in the HVAC system the UVGI system should be located.
In-duct UVGI can be placed in different parts of the ductwork system such as the AHU, main duct or branch duct (see Figure 3).
The mixed air UVGI system targets aerosolized particulates that can travel through the HVAC system and spread throughout the health care facility to infect patients. The mixed air temperature and humidity are generally below 60% RH and do not hinder the effectiveness of the UVGI system allowing the maximum capacity to kill or neutralize bacteria or viruses in a single pass. Maintaining the air velocity as low as possible at or below 500 feet/minute helps maximize the effectiveness in eliminating harmful particulates.
As the particulates are in the kill zone for a short period of time and a single pass, it is recommended to specify the irradiation for kill “on-the-fly.” An intensity of 20 to 27 watts/square foot is used to attack particulates as they pass the UVGI system in the main air stream. Kill on-the-fly can be achieved in the supply air, but it generally requires additional intensity to overcome the higher humidity in the supply air or higher velocity if placed further down in the duct system.
Conventional “coil cleaning” UVGI system is placed downstream of the cooling coil to prevent bacterial and fungal growth on the cooling coil. As the mixed air crosses the cooling coil, the air approaches the saturation curve and is then cooled even further to the target supply air temperature fully saturated. All moisture taken out of the air from the cooling process appears as condensate on the cooling coil. This makes for an excellent condition for bacteria and fungi to grow on the face of the cooling coil and drain pan.
The UVGI system downstream of the cooling coil irradiates the discharge side of the cooling coil and drain pan to help prevent microbial growth on the surfaces. Lower intensities can be used from 7 to 10 watts/square foot as the bacterial and fungal growth is constantly in the kill zone of the UVGI system. This approach does not kill or inactivate bacteria or viruses that are in the air as it passes through the coil.
Engineers should work with the facility to establish the target goal for the UVGI system to specify the correct placement and intensity.
Safety concerns with UVGI
A primary consideration when selecting any UVGI system should be the safety of the occupants and maintenance staff. UVGI fixtures and air handling systems in which they are provided must be properly designed and installed and staff must be trained in how to use and maintain them to avoid potential harmful effects.
The possible harmful effects require consideration by the design engineer to include warning signs, louvers, on/off switches, safety interlock switches on unit doors, UV protection on view windows, etc. Maintenance staff should use eye and skin protection whenever handling UVGI fixtures and be trained in the proper maintenance of these systems.
In-duct UVGI should be placed in accessible areas for maintenance staff to easily access them to perform required maintenance. The fixtures need to be replaced routinely because the intensity of the UV output degrades over time. Most manufacturer’s recommend replacement every 9,000 hours or yearly and provide guidance for replacement and handling of lamps. Proper care must be taken when handling the lamps to prevent breakage and ensure proper disposal.
UVGI systems can effectively disinfect the air supplied to operating rooms, patient rooms and other critical spaces for patient care in health care facilities. The engineer should discuss with the facility what are their target goals for the UVGI system are. Is minimizing the need for maintenance staff to clean the cooling coil for bacteria or fungal growth the desire or is the facility also wanting to look at UVGI to help limit the transmission of harmful particulates that spread infections? Proper design and implementation of a UVGI system can help achieving both or a combination of these goals. The key is assembling the team to establish the goals and together arrive at the optimal solution.