Adding Ultraviolet Disinfection Systems to Schools’ COVID Defenses: A Background Briefing for School Districts
With billions in federal aid available for school facility and HVAC improvements, districts are getting inundated with pitches from suppliers offering ways to strengthen their COVID defenses. The proposals about which we receive the most questions are for ultraviolet air disinfection systems.
Up to now, most districts have focused on minimizing COVID risk by using two broad approaches.
The first is source reduction measures like masking, social distancing, and limiting class sizes and/or activities (e.g., banning choir practice).
This background briefing will cover four critical issues:
1. The Science Behind UV Air Disinfection Systems
2. Engineering Issues
3. Operational and Safety Issues
4. Financial Considerations
The Science Behind UV Air Disinfection Systems
In contrast to increased ventilation (which reduces the number of viral particles within a volume of air) and filtration (which traps viral particles), ultraviolet light is used to inactivate a virus by destroying or damaging enough of its genetic material (RNA or DNA) to prevent it from replicating inside a person who inhales it.
Utraviolet or UV light is classified according to its wavelength, measured in nanometers (nm). The wavelength of visible light is between 400 and 750nm. Ultraviolet light is between 10 and 400nm.
UV light in wavelengths between 100 and 280nm is used in surface, liquid, and air disinfection applications. This range is known as UV-C. Within UV-C, a particular spectrum of UV radiation between 200 and 280nm, has been employed extensively as the germicidal range of UV light.
Maximum absorption of UV-C light by viral RNA/DNA occurs at a wavelength of 265nm. In addition to the wavelength of the UV-C radiation, the extent of the damage done to a viral particle’s RNA or DNA also depends on a number of factors.
The effective germicidal radiation level, or fluence rate, is measured in units of watts per square meter (W/m2). This is a function of UV lamp efficiency. The dose of UV radiation delivered, (fluence) is calculated by multiplying the fluence rate by the amount of time a virus is exposed to the UV-C light. Fluence describes the UV energy per unit area that an airborne pathogen receives (measured in joules per square meter (J/m2).
Note that the required UV dose and the corresponding level of SARS-CoV-2 inactivation has yet to be determined by any regulatory health authority.
Many factors can reduce the amount of radiation level a virus received, including:
- The length of the chamber in which flowing air is exposed to UV-C light;
- The number, type, power, and configuration of UV lights in the chamber;
- Reduction in a UV lamp’s output of UV-C radiation due to degradation over time, variation in temperature, or the presence of dust and humidity on the lamp;
- The speed at which air containing viral particles is flowing through the chamber;
- The turbulence of the airflow; and
- The amount of dust and humidity in the airflow (more of either reduces the UV radiation dose that viruses receive).
Reduced UV radiation exposure as viral particles flow through the disinfection chamber is, potentially, not without consequence.
“The lethal effect of the UV-induced nucleic acid (DNA or RNA) damage depends on the location of changes within the viral genome. In addition, many mutations will not have any discernible effect on the virus, as they are repaired by the host nucleic acid repair mechanism.
“The majority of the mutations diminish the infectivity of the viruses since most viral genes have a specific role to perform. However, some mutations may lead to the evolution of more pathogenic viruses. For instance, a novel receptor-binding protein can be synthesized within the virus structure that enables the virus to infect a different cell type in host.
“It is also likely that some UV-resistant strains of viruses
will emerge, perhaps as a result of evolving a thicker capsid
structure to protect the nucleic acid from UVC damage” (reference: “A Critical Review on Ultraviolet Disinfection Systems against COVID-19 Outbreak: Applicability, Validation, and Safety Considerations” by Milad Raeiszadeh and Babak Adeli).
In this regard, it is critical to note that coronaviruses (including SARS-CoV-2 which causes COVID) have some of the largest genomes among RNA viruses. Thus more UV-C power and/or longer exposure time of the viruses to UV radiation is required to destroy a sufficient amount of genomic material to inactivate the SARS-CoV-2 virus.
Two types of UV-C configurations are used for air disinfection. The first (which we don’t discuss here) uses wall-mounted fixtures that aim a cone of UV-C light across the top of a room. This is often used to passively treat air in rooms where a large number of people congregate (e.g., hospital cafeterias).
The second type of UV-C disinfection configuration is the installation of UV-C lamps in some part of a building’s HVAC system. The most common installation is in the air handling unit (AHU), where UV-C lights are installed near the cooling coil to prevent the buildup of bacteria and fungi on it.
A less common approach, but the one that is now most often being pitched to school districts, is the installation of UV-C disinfection equivalent in an air duct (ideally after the AHU before the main duct branches), which enables longer exposure of airflow containing viral particles to UV radiation, to ensure they receive a sufficient dose to achieve the virus deactivation target, given the velocity at which the air is moving.
However, as noted above, a critical issue is that what constitutes a sufficient dose has not yet been established by any regulatory health authority. Hence any district investing in a UV-C system today runs the risk that it will not meet future regulatory requirements for such a system (or, alternatively, for overall indoor air quality) when and if they are established.
Assuming that eventually happens, districts will still have to confront a range of engineering issues. These include:
- How long a duct will be required to install sufficient UV-C lamps to achieve our target incremental reduction in viral particles (this will vary with lamp power and other design factors)?
- Which UV lamps will we install (e.g., widely used tubular lamps versus waiting for LED lamps that are still in development)?
- How to measure and track these lamps’ actual radiation output (their visible light can remain constant even as their radiation output decreases)?
- How to size UV radiation exposure area given the inevitable degradation of UV lamp performance over time?
- How to control vibration caused by other elements in the HVAC system that can degrade UV lamp performance?
- How to control temperature, dust, and humidity levels in the duct section where UV lights are installed, all of which can degrade system performance and cause disinfection results to fall below the indoor air quality target?
- How to take the impact of turbulence into account when designing the installation (there are some examples of computational fluid dynamics models being used for this purpose)?
- How to control ozone generation (and stay within regulatory limits for exposure to it), which can be a by product of UV disinfection, depending on the system configuration?
- How to control the degradation of materials (e.g., polymers) that can occur due to exposure to UV radiation?
In sum, in-duct UV-C disinfection systems present a very complex engineering design problem.
Operational and Safety Issues
Use of UV-C air disinfection systems will require rewriting operational procedures to manage complex operating parameters (e.g., temperature, humidity, dust, airflow velocity, and bulb output and replacement) and retraining employees to follow them.
UV-C disinfection systems also require rewriting safety policies and procedures (and delivery of associated training), as exposure to UV-C radiation can cause severe skin burns and eye injuries.
Introduction of UV-C systems in a district also requires the redesign of Indoor Air Quality policies, processes, and procedures. In system that are based only on the use of ventilation, filtration, and varying the number of air changes per hour (ACH), the level of CO2 in a classroom has been found to be a effective proxy for the level of viral particles.
Other research has found that managing CO2 levels in classrooms (through increased ventilation) also has clear benefits for academic achievement and other important results (e.g., see, “The Ventilation Problem in Schools” by William Fisk from the Indoor Air Quality Science Group at Lawrence Berkeley National Laboratory).
However, the installation of a UV-C disinfection system in an air duct will reduce the correlation between the level of CO2 and the level of viral particles in a classroom. This can have unintended consequences.
For example, in the absence of a detailed understanding of UV-C energy costs, an operations team whose performance is primarily measured by energy use and cost, might see the use of UV-C as a way to reduce the need for ventilation, and its impact on higher seasonal energy use for air heating and cooling. An unintended consequence could easily be higher levels of classroom CO2.
Most districts have already implemented new ventilation, filtration, and air changes per hour protocols to reduce COVID infection risk in their classrooms.
When analyzing a proposed investment in UV-C based in-duct disinfection systems, districts must take an incremental analysis that considers:
- Incremental capital costs for UV-C systems in schools. This includes engineering, UV lamps and reflectors and their associated installation costs, new sensors (to measure lamp radiation output), safety equipment and new procedures, and additional engineering, equipment, and installation costs needed to address issues related to airflow, temperature, dust, humidity, vibration, and ozone generation.
- Having weakened the correlation between the level of CO2 and viral particles in a classroom, new equipment and/or third services and procedures will be required to measure and monitor the level of viral particles in classrooms, and therefore infection risk levels.
- Incremental operating costs include consumables such as UV lamps, incremental energy costs, incremental cleaning of the duct and lamps, incremental training, and the cost impact on maintenance routines (including the possible cost of outsourcing UV-C maintenance, and enabling those service providers to access school premises).
- The economic cost of higher CO2 levels in classrooms and its impact on student achievement and other results (assuming the installation of UV-C disinfection results in a reduction in ventilation rates).
- The economic value of the incremental reduction in COVID infection risk due to the incremental reduction in the level viral particles in classrooms.
It is also critical to note that investing federal aid to install UV-C air disinfection systems in schools is unlikely to be the only incremental indoor air quality improvement option under consideration by a district.
For example, another option might be incremental improvements to air handling units to enable them to use MERV-13 filters.
In sum, the financial analysis that districts should undertake should focus on simultaneously comparing the costs, benefits, and uncertainties for multiple options that can be used to improve indoor air quality and reduce the risk of COVID infection.
An unprecedented amount of federal and state aid is now flowing to school districts across the United States to help them achieve multiple goals, from recovering COVID learning losses to improving indoor air quality in order to minimize the risk of infection by SARS-CoV-2 or other respiratory viruses.
As a result, district administrators and boards are increasingly inundated with pitches from a wide range of suppliers, many of whose offerings are backed by very uncertain evidence.
The risk of disappointing investments is high, as is the associated risk of a negative public reaction if that occurs.
To mitigate both risks, districts should take a rigorous approach to assessing and comparing the incremental value of vendors’ proposals, such as the one we have outlined here for UV-C air disinfection systems.
Susan Miller advises school districts around the country on enrollment, facilities, financing, and other issues. She is also a member of the Board of Education for Jefferson County (CO) Schools, the nation’s thirty-sixth largest district. In October 2020, her article “Are We Ready To Close Schools’ Windows?” warned of the need to improve schools’ indoor air quality to enable them to stay open during the pandemic. These are her personal views.