Intro to UV Light Against Diseases
Ultraviolet Germicidal Irradiation (UVGI), also known as Germicidal Ultraviolet (GUV) simply refers to using UV light to inactivate microbes like viruses, bacteria or fungi. They will still be floating in the air or found on surfaces and you will still touch them, ingest them or inhale them, but they become harmless.
There are many different ways UV light can be used and it often causes confusion. To know whether the UV system you are using is extremely effective, okay, useless or harmful, you need to be able to recognize the different types.
How is UV light created?
We have a few different methods to create UV light. Here is a review of the different methods. For GUV, the most common is low pressure mercury lamps which is how we make fluorescent light bulbs. They emit UVC light at 254 nm. The only difference is fluorescent bulbs have a phosphor coating which take the UV light and convert it into visible light. UV lamps have no coating and let the UV light pass through. They primarily emit UV light at 254 nm but also have a light blue glow in the visible spectrum.
This wavelength of light is very effective against microbes, but can also have bad effects when it is shone on people including erythema (skin reddening) and photokeratitis (like sand in your eyes). Unlike UV-B, it does not have a skin cancer risk because the wavelengths of light do not penetrate well into the deeper layers of skin.
These lamps are very cheap to make, so to use them against diseases, there needs to be a way to expose pathogens to UV light without exposing people. There are a few ways to do this.
Surface Cleaning
This is one of the most common ways UV light is used. If you see an exposed light bulb, it can’t be used for cleaning the air in occupied rooms. People would need to leave the room and then the light can be turned on to disinfect any surfaces the light irradiates.
It can also be used as a light bulb but only turned on when the room is empty.
UV lights can also be shone inside containers and items to be disinfected can be placed in there.
One other common method is UV wands, which are portable surface cleaning devices.
All these methods are for cleaning surfaces and have some advantages over cleaning surfaces with chemical disinfectants. However, respiratory viruses are airborne and spread primarily through the air. The current evidence about the spread of COVID-19 through surfaces indicates it is low risk. These technologies do not clean the air in occupied spaces and should not be considered as a method of mitigating COVID transmission or a primary mitigation tool for other respiratory viruses. They can be effective, especially in healthcare settings, against bacteria and other pathogens which spread through surfaces. For mitigating the spread of COVID and respiratory infections, the focus should be on cleaning air.
Air Handling Units
Air handling units (or AHUs) are what supply air to spaces. Sometimes they supply 100% outdoor air which is already virus free, so it does not require disinfection. Often, they have a lot of recirculated air, so instead of letting it recirculate microbes and resupply them to the space, you can insert UV lamps inside AHUs to disinfect all the air that is passing through.
The unit in the picture has UV lights in two separate locations. The lower place is by the cooling coil of the unit. When cooling the air is necessary, air is passed though the unit through a coil with either cold water or refrigerant inside the coil to cool down the air. When this happens, water condenses out of the air and ends up on the coil where it can cause microbial growth. Using UV lights by the coil can prevent this problem. When UV systems are designed for this, they are generally not sized with enough irradiance and contact time to effectively disinfect the air.
The UV lights above are installed parallel to the duct. This is to increase the amount of time the air is exposed to UV light. These should be sized to be effective at disinfecting the air.
UV lights are also designed to be placed inside your furnace to disinfect any microbes that could be recirculating.
One of the biggest problems with these systems is the contact time. For a pathogen to become inactivated, it requires a sufficient dose. The dose is the irradiance (power of the light over an area) and the exposure time. Since there is such a small exposure time, the irradiance must be very high. These systems can sometimes be installed with insufficient dose to be effective.
Another issue is that an alternative to these systems is improved filtration. Upgrading the filter in an HVAC unit to MERV-13 can also remove pathogens. Filtration has the benefit of also removing other particulate matter in the air. Improving filtration and installing in-duct UV at the same time would have diminishing returns, so there is little benefit of using them together. One of the issues with filtration is that some AHUs cannot handle better filters and filters can get loaded with dust and restrict ventilation, which is not a concern with UV lights.
UV in a Box
Instead of using an air handling unit, you can add devices to the room to disinfect the air. It requires a box, a fan and a UV light. The fan pulls the air into the box where it is exposed to UV light and then supplies the disinfected air back to the space. This can be mounted on the ceiling or wall.
It can also be done as a portable air cleaning device, just like portable HEPA filters.
The issue of contact time exists for these devices as well. Ones sold by reputable companies should provide the disinfection rate with lab tests. They can also have all the same disadvantages of in-room filtration: noise, air distribution and human operation.
Another consideration is these devices are often accompanied with a HEPA filter (the Galax UV air is). HEPA filters can remove more than 99.95% of particles that pass through them. There is little benefit to having UV in a box if there is already a HEPA filter. Similarly, if a HEPA filter is advertised as having UV as an addition, the UV likely provides little benefit.
Upper Room UV
A full introduction to upper room UV can be found here. Going back 80 years, a different method of using UV to disinfect the air was used. Divide the room into two parts: the upper room or disinfection zone is the space between the tops of people’s heads and the ceiling, while the lower room or breathing zone is where people reside. Fixtures are designed to emit UV light into the upper room without exposing people in the lower room to harmful levels of UV light.
Pathogens are constantly floating around the room. When an infectious person emits respiratory aerosols, they will eventually float into the upper room, where they will be disinfected and then will not infect others when they return to the lower room and are inhaled.
UV in an air handling unit can provide 2–4 air changes per hour if it is designed effectively. UV in a box can provide a similar rate. Upper room UV can provide 20–30 air changes per hour. It’s much more effective than the previous methods because it doesn’t rely on a fan to draw air. It constantly illuminates a significant percentage of the air in the room, providing a very high non-infectious air delivery rate.
To identify upper room UV fixtures, you can often see louvered fixtures mounted on the wall or ceiling.
Of all the methods we have to clean the air with a long historical evidence base, upper room UV is the most effective.
Far-UV
A full introduction to far-UV can be found here. Far-UV does not use the blue glowing mercury lamps. It uses a more recently discovered Krypton Chloride (KrCl) excimer lamp. It does not emit light at 254 nm, but around 222 nm. This has one significant difference.
UV light between 200 nm and 230 nm is known as far-UV and is really poor at penetrating anything. Your skin has an outer layer of dead skin called the stratum corneum and your eyes have a tear layer over them. Consequently, your body has a thin layer of protection around it which far-UV cannot penetrate well.
There is a limit to how much UV light people can be exposed to per day. For UV 254 nm, the limit is 6 mJ/cm². For UV 222 nm, the limit is 160 mJ/cm² for your eyes and 479 mJ/cm² for your skin. So the limit from far-UV is 27 x higher than UV 254 nm on your eyes and 80 x higher on your skin.
What this means is that you can design a far-UV system to shine in an occupied room and there will be no bad effects on the people while still being effective against airborne viruses. This cannot be done with other UV light.
While upper room UV can give 20–30 air changes per hour, far-UV can give greater than 100 air changes per hour. It is the most effective system that exists.
There are still some concerns about the use of far-UV as it does not have the same long history of use like upper room UV does. Nevertheless, its use is growing and is showing promise as a safe and effective tool to disinfect the air in occupied rooms and mitigate the spread of airborne diseases.
Photocatalytic Oxidation (PCO)
UV light can be used in a way that is not germicidal. The goal with PCO is not to use UV to directly inactivate pathogens. UV PCO shines UV light on a surface which is a catalyst for chemical reactions. That surface is generally titanium dioxide (TiO2) but can also be cobalt or other materials.
When the photocatalyst is irradiated with UV light, a chemical reaction occurs where hydroxyl or superoxide radicals are formed and then released into the space. The goal is for these to react with pollutants and render them harmless. PCO devices “are well established to produce an array of unintended byproducts depending on the composition of air flowing through the device and its design, including aldehydes, phosgene, and chlorinated VOCs.”
Companies are often very vague about this technology, so you might see the following items advertised with UV: catalyst, photocatalyst, PCO, TiO2, active or removes volatile organic compounds (VOCs). Some examples of how it can be difficult to determine if something has PCO:
- This Sanuvox device states it has a cobalt catalyst in the benefits
- ActivePure is not very clear about how their technology works on their website, but indicate here that it is PCO technology
- TrioPlus uses ProCell which uses an aluminum honeycomb as a catalyst for UV light.
- Germ Guardian “works with Titanium Dioxide to reduce volatile organic compounds”
There are many similar products.
PCO is a form of additive air cleaning, also known as electronic air cleaning. ASHRAE 241 has established a method to determine if they are safe and effective, but to my knowledge, no products have yet to undergo any tests.
The Ontario Society of Professional Engineers recommends these devices are not used until there is a standardized method to ensure safety and effectiveness.
Summary
Here’s a summary table with what we know about the different uses for UV:
