Intro to Upper-Room UVGI
UV-C light can be used to disinfect air and prevent the transmission of airborne diseases, such as COVID-19. When used in this way, it is referred to as ultraviolet germicidal irradiation (UVGI) or germicidal ultraviolet (GUV). The most commonly used type of UV light for UVGI is from low-pressure mercury lamps, which emit UV-C light at 254 nm. These lamps have been in use for over 150 years and are not expensive, but they can have harmful effects on people when they are directly exposed to high doses. So here’s the conundrum:
With people around, air must be clear,
Disinfected and pure, free from all fear.
How can we do this, with UV light shining bright,
Without exposing people, day and night?
A riddle by ChatGPT
A newer solution to the problem is to directly expose people with far-UV, but it is still relatively new, expensive, and much is still unknown, so caution is warranted. We have developed three ways to disinfect air in occupied spaces using UVGI without exposing people:
In-duct UV — place a UV light inside an air handling unit to disinfect the recirculated air, and then supply the disinfected air to the space.
In-room air cleaners with UV — these are units that consist of a box with a fan and a UV light inside. These units can also be portable. Air is drawn into the box, where it is exposed to UV light, and then resupplied to the space.
Both of these methods have two significant limitations. First, to disinfect the air, pathogens need to be exposed to UV light for a sufficient amount of time. Air passing through a duct or box has a small amount of contact time with the light, so improperly designed units may not deliver a sufficient dose and may not properly disinfect the air. Second, the air disinfection rate is limited by the disinfected air delivery rate. With mechanical ventilation systems, there is a limited amount of recirculated air supplied to the space. For in-room air cleaners, the same disadvantages as HEPA filters apply: fan noise, air distribution, and human operation.
Furthermore, both of these methods can be replaced by filtration. While filters have some disadvantages, they are also usually less expensive than UV devices and have the added benefit of removing other particulate matter pollutants.
However, there is a third way of using UV in occupied rooms that can be much more effective than the other methods. It is called upper-room UV.
How does it work?
Upper-room UV, sometimes called upper-air UV, works by dividing the room into two parts: the lower-room or breathing zone, where all the people are, and the upper-room or disinfection zone, which is the empty space between the top of everyone’s head and the ceiling. Typically, this space is around 3 feet or 1 meter in many commercial spaces. To ensure minimal amounts of UV light leak into the lower room, it is important to set up the UV lights correctly.
Air circulates from the lower room into the upper-room, gets disinfected by the UV light, and then returns to the lower room. By using the entire upper-room as a place to expose UV light, a very high disinfection rate can be achieved.
When the lights are turned off in the room, it is often noticeable how the UV light is concentrated into the upper-room. However, it is not easily observable when the lights are on.
History
There’s a long history of upper-room UVGI that is discussed here, but I’ll point out some of the more important events. They are summarized in this table from the publication:
Between 1937 and 1941 Wells et al. ran a study of upper-room UV in classrooms showing it reduced measles transmission by 75%.
Between 1957 and 1958, upper-room UV was installed in one wing of Livermore VA Hospital. The published study shows that there was a 90% reduction in influenza transmission in the hospital wing with upper-room UV compared to the wing without upper-room UV.
The realization dawned on medical professionals in Infection Prevention and Control (IPAC) that utilizing air cleaning technology, such as upper-room UV, could greatly curb the spread of airborne diseases like influenza. This led to widespread implementation of this technology in hospital settings, safeguarding vulnerable patients and preventing billions of dollars in healthcare costs from being lost due to healthcare-acquired infections.
Just kidding. That never happened. It’s barely used.
Upper-room UV has been used over the past few decades to combat tuberculosis in developing countries. There is currently one study of upper-room UV being used against COVID. In a Russian hospital, when upper-room UV was used, no cases of transmission between patients and healthcare workers were found in rooms with the upper-room UV system installed.
For over 80 years now, we have strong evidence that upper-room UV is a very effective tool against the transmission of airborne diseases.
Types of fixtures
Upper-room UV fixtures can be categorized in two different ways: wall-mounted versus ceiling-mounted and open versus louvered. The choice to mount them on the wall or ceiling can depend on the size and layout of the room, as well as what is easier for electrical installation.
When light hits the wall, it is stopped, and this reduces the volume of the air that can be disinfected. Consequently, wall-mounted fixtures are preferable, as they provide the longest line of sight between the fixture and the opposing wall. However, ceiling-mounted fixtures might be optimal in very large rooms or when wall mounting is not practical.
Upper-room UV fixtures can also be categorized as louvered or open. Louvered fixtures tightly control the direction of the light beam, prevent light from either leaking into the lower room or reflecting off the ceiling, and are required for places with ceilings between 9 and 12 feet high. Much of the light is lost as the louvers absorb or scatter it, so only 10–20% of the power is actually supplied to the space.
Open fixtures have a reflective surface beneath the light that prevents light from leaking into the lower room while reflecting as much light as possible upwards. These fixtures are only usable in places with high ceilings. If used in a place with low ceilings, too much light would reflect off the ceiling and enter the lower room. Open fixtures are much more powerful than louvered fixtures, allowing upper-room UV to be more effective in places with high ceilings.
Ceilings below around 8.5 feet are too low to safely use upper-room UV. This is why upper-room UV generally cannot be used in residential settings unless there are specific locations with high ceilings. For much higher clean air delivery rates than can be attained with ventilation or filtration, far-UV is the best option where upper-room UV cannot be used.
Dosing
To be effective, you need sufficient UV light supplied to the upper-room and the air to be mixed well between the upper and lower room. The power that is supplied to the space is measured in Watts (W). Research based on upper-room use with TB indicates a dosing formula of 13–17 mW/m³ = 16–24 equivalent air changes per hour (eACH). For reference:
- Minimum ventilation requirements are usually 2–3 ACH
- Improved filtration can add 2–3 eACH
- Ideal air quality would have non-infectious air supplied at minimum of six eACH
The 16–24 eACH is an extremely high rate which exceeds any other widely available methods.
The equivalent air change rates are dependent on the pathogen. A recent publication calculated a dosing formula for SARS-CoV-2 of 12mW/m³= 30 (eACH). Some IAQ/UV experts have estimated that a lower dose would be okay (6mW/m³ = 20 eACH), but I haven’t found any studies confirming this yet.
Assuming you want 30 eACH for COVID in a typical classroom of 200 m³, the dose would be: 12mW/m³ x 200m³ = 2.4 W. If you were using one of the more well known fixtures — Aeromed Lexus L3.1 which outputs 1.2 W, you would need 2 of them.
The other concern is air mixing. The more the air is travelling between the lower room and upper-room, the more effective this system will be. This is one way that good ventilation complements upper-room UV — properly ventilated spaces have good air mixing. Adding in ceiling fans can increase the effectiveness by more than 60%.
Placement
From an effectiveness perspective, it is best to expose the light to as much air as possible, which means you want to have the furthest distance between the fixture and any surface (such as a wall) that the light will hit. The intensity of the light is also higher closer to the fixture, so it is better to have multiple fixtures spread out throughout the room.
From a safety perspective, the most sensitive part of your body to UV light is your eyes, so it is best not to have the light shining in the direction people are looking throughout the day. With this in mind, it is good to install the fixtures on the side walls where people are not generally facing most of the time.
Nevertheless, it still needs to be safe even if looking at it all day long. That’s why putting it at the front of the classroom would still be safe if installed properly.
Safety
Upper-room UV has been available for over 80 years. During this time, numerous deadly airborne diseases, such as smallpox, tuberculosis, COVID-19, influenza, measles, pertussis, and many others, have taken the lives of hundreds of millions of people. Despite this, the use of upper-room UV has never caused permanent harm to anyone.
The threshold limit value (TLV) for exposure to UV 254 nm is 6 mJ/cm² per day. Therefore, over 8 hours (28,800 seconds), the maximum constant irradiance should be 0.2 µW/cm². Once these systems are installed, an experienced professional should commission the system by using a UV-C meter and scanning the room to ensure the irradiance is below that limit.
When exposed to UV light below the limit, it will have unobservable effects on skin cells or eye cells that shed every few days anyway. Therefore, there will be no long-term effects. However, there have been cases where people have been harmed due to improper installation, followed by a lack of commissioning to verify proper installation. Without proper commissioning, these systems should not be used.
If people do get overexposed to UV 254, the two most common symptoms are photokeratitis (eye redness, like sand in the eyes) and erythema (skin reddening, like a mild sunburn). With UV-C, it does not penetrate the skin that well and can only affect the upper layers of the skin, which shed every few days anyway. Unlike UVA and UVB, the risk of any long-term effects on the skin and eyes (including cancer) from UV 254 nm is very low.
A double-blind, placebo-controlled field trial of upper-room UV at 14 homeless shelters found no observable adverse effects on occupant comfort or safety, with the exception of one case where a bunk bed replaced a single bed without consideration to the upper-room UV fixture.
A recent publication showed that upper-room UV can create byproducts in the air and possibly have detrimental effects. When used in a place with ventilation, it is unclear if there are any concerns. More research is needed. UV can be used to mitigate transmission of diseases but is not a way to address indoor air quality in general. Ventilation is required for adequate indoor air quality.
Low-pressure mercury lamps should have a filter for very short wavelengths and should not produce any ozone. Ozone generation should not be a concern with devices purchased from reputable companies. UL1598 is the electrical safety standard. UL2998 is the ozone safety standard. Devices should comply with these standards.
When installing, there should be training for building occupants and employees to know to shut off the system if they will stand on ladders or tables (such as to change a light bulb). Ideally, this should also be combined with proper labeling.
Where and how to buy upper-room UV
The power indicated in the specification sheet could represent one of three things: the total power consumed by the entire fixture, the UV power provided by the light bulb before passing through the louvers, or the power delivered to the room after passing through the louvers. To accurately determine the appropriate system size, it is necessary to determine the actual power delivered to the room through independent laboratory testing. Without this information, it is impossible to accurately size the system and determine its effectiveness.
While an electrician can install the fixture, it is not sufficient. To purchase an upper-room UV product, it is essential to obtain two things: laboratory results verifying the power delivered and assistance from the company in commissioning the system, or at least a referral to a commissioning agent.
I am not endorsing any specific suppliers, but I am familiar with the following ones:
Aeromed, Atlantic UV, Signify, Sanilume, UV Can Sanitize, UV Resources, Light Progress, Trinity, ELED Lights, ON-LIGHT, Lightdis.
Installation and maintenance
This is just a light fixture, and you need an electrician to mount it and connect power to it, as you would with any other light mounted on a wall or hung from the ceiling. An experienced professional should direct the electrician on where and how to install the fixture for safety and effectiveness. Commissioning should be done after installation.
Fixtures range from $800 to $2000. Electrical fees would depend on the existing layout but could cost between $500 and $1000. It’s possible there is already a receptacle close by, and you just need to mount the fixture and plug it in. According to the CDC, the cost to install it in a 500 ft² area would be between $1500 and $2000.
A significant advantage of these systems is the ease of maintenance. It requires dusting the system on a specific schedule (between 2 and 4 times a year) and changing light bulbs every 1 to 2 years, depending on how often it is used. Energy consumption is extremely low, as it is just the equivalent of running a fluorescent light bulb.
Don’t confuse it with other UV products
Although upper-room UV is the most effective way to utilize UV 254 nm, most UV products on the market are not designed as upper-room UV systems. UV can be used in various ways, and even some sellers of UV products may confuse their various applications (I’m speaking from experience).
If the UV light is directed to cover the entire room, it is used for surface disinfection in unoccupied rooms and cannot be used for air disinfection in occupied rooms. Because these fixtures are typically installed on or near the ceiling, people often mistake them for upper-room UV systems.
If the upper-room is not exposed to the UV light but instead contains a unit with a fan, it is a form of UV-in-a-box that can be used to disinfect the air. However, it is generally much less effective than a properly sized upper-room UV system.
Where and how to use UV
The Ontario Society of Professional Engineers (OSPE) recommends prioritizing upper-room UV in healthcare and congregate living settings, areas with high density and a high risk of airborne disease transmission. The Centers for Disease Control and Prevention (CDC) recommends that it can be used in any indoor space (see FAQ 7). However, ceilings need to be above a minimum height for it to be used safely.
While far-UV systems have the potential to be more effective than upper-room UV, they still have some disadvantages. Upper-room UV remains the most effective method for cleaning the air without directly exposing occupants to UV light.
OSPE states that in-duct UV or UV-in-a-box can also be used, but they do not prioritize these methods as highly as upper-room UV. UV for surface cleaning can help reduce the spread of certain pathogens, but it is not effective in stopping airborne pathogens such as COVID, where surface transmission is low risk.
UV can also be used with a catalyst like TiO2 (titanium dioxide) through a process called UV photocatalytic oxidation (PCO), which is a type of electronic air cleaning. However, OSPE recommends against its use until the safety and effectiveness of these systems have been standardized. The CDC also urges caution with these devices (see FAQ #8).
Other Resources
If you are interested in listening to leading experts in upper-room UV, please watch this video — it gives you all the necessary background. Experts include:
Dr. David Sliney, Ph.D. — UV safety
Dr. Don Milton, MD, DrPH — Airborne disease transmission
Dr. Edward Nardell, MD — Upper-room UV theory and application
Dr. William Bahnfleth, Ph.D., PE — Integrating upper-room UV as part of a total building infection control program
Dr. Paul Jensen, Ph.D., PE, CIH — Practical applications/commissioning upper-room UV
