Upper Room UV-C Keeps Air Cleaner

2020 saw the world rocked by widespread turmoil, as a virulent new pathogen started claiming lives around the globe. The COVID-19 pandemic saw a rush on masks, air filtration systems, and hand sanitizer, as terrified populations sought to stave off the deadly virus by any means possible.

Despite the fresh attention given to indoor air quality and airborne disease transmission, there remains one technology that was largely overlooked. It’s the concept of upper-room UV sterilization—a remarkably simple way of tackling biological nastiness in the air.

Warm Glowing Killing Glow

Upper-room UV systems sound kind of like science-fiction technology. They nuke nasty pathogens in the air, and do it while emitting a faint and weird-colored glow. In reality, they’ve been quietly hanging around for about 80 years. The idea is straightforward enough—you just shine UV-C light in the unused overhead zone of a room to zap airborne pathogens before they get inhaled by fragile humans!

Upper-room UV sterilization keeps the harmful UV-C light away from the occupants in the room. Credit: CDC

The concept came about as a direct result of 19th-century research that determined sunlight inhibited the growth of undesirable microbes and pathogens. Later work determined that light in the UV-C range of wavelengths is remarkably good at killing both bacteria and viruses, making it ideal for sterilizing purposes. The UV-C range is from 100 to 280 nanometers, but peak sterilizing action occurs around the 250-270 nm range. The primary method of action is that the UV-C light creates defects in DNA molecules that kill or inactivate microscopic organisms, including bacteria and viruses. It’s perfect for tackling all sorts of nasties, from measles to SARS to TB.

Unfortunately, that also means that UV-C light isn’t always safe to use around humans. Just as it hurts microbes, this light is also harmful to our skin and eyes in exactly the same way. Indeed, a prime example of this was a 2023 event that allegedly accidentally used UV sterilization lamps as decorative blacklights. While UV-C light is used in a wide range of sterilization applications, most keep the light hidden or localized to avoid direct human exposure.

Upper-room UV-C installations are particularly interesting, though, for their simplicity. To avoid dangerous exposures, these installations simply place the sterilizing lamps up high in a room and direct their light into the upper level of air. As long as the UV light output is directed into the top level of the room, well above the heads of any occupants, it can sterilize the air effectively with little risk of harm.

UV-C lamps typically have peak output at 254 nm, but they also output some light in the visual spectrum that gives them a characteristic green glow—as seen on this Phillips wall-mount unit. Credit: Phillips

For this reason, these systems are typically installed in places like schools, hospitals and other public buildings, where ceiling heights are high enough to make such installations safe. CDC guidelines suggest minimum ceiling heights must be at least 8 feet for these installations, though 8.5 feet is preferred. For most people, that’s high enough not to cause trouble, but if you’re one of the taller players in the NBA, you might want to take note.

Upper-room UV systems treat a massive volume of air simply by sectioning the room into a germ-killing overhead zone and a safer lower zone where people breathe. Natural convection, HVAC currents, and even a simple ceiling fan help keep the air circulating upward, doused by the UV field, and then returned to the lower portion, scrubbed clean. At least a minimal level of circulation is required in order to ensure all the air in a given room is being treated. Power levels required are relatively low. A 2015 study suggested a total output of just 15-20 milliwatts is enough per cubic meter of room volume, assuming adequate air mixing in the space.

Ultimately, though, proper sizing and safe installation are critical for creating an effective and safe sterilization system. UV-C is safe enough when used properly, but get it wrong, and you’ll see plenty of sore eyes and red skin almost immediately. The key is blocking direct and reflected UV light from reaching the lower zone of the room. Louvered fixtures with carefully aimed beams are necessary in rooms with lower ceilings, while more open fixtures are more for lofty spaces where they can blast UV upward without frying everyone’s eyeballs. Proper metering must be done at the time of installation to ensure light concentration is high enough in the sterilization region, and below safety limits in the occupied region. It’s also important to ensure the lamps are switched off for maintenance or if anyone is entering the upper zone of the room for any reason.

It might sound high-tech, but this approach predates modern pandemics by decades. It was already showing its effectiveness against diseases like measles and tuberculosis as far back as the 1930s and 1940s. Early success was found at Duke University in 1936, where post-surgical infections were cut from 11.62% to just 0.24% with the use of an operating room eqiupped with UV-C equipment. Later, a 1941 study determined that UV-C delivered by mercury-vapor lamps had drastically reduced measles transmission in classrooms.

This installation used bare UV-C bulbs, directing them with louvers or hiding them in a perforated sub-ceiling. Note the eerie green glow. Credit: research paper

Given its efficacy, you might think that upper room UV-C installations would be everywhere. Subways, airports, schools, hospitals, and malls could all benefit from this technology. However, it would require some investment and ongoing maintenance, and it seems that simple cost is too much to bear.

For whatever reason, upper-room UV remains an obscure technology, seldom discussed and rarely used. Here we are, after countless deadly airborne disease outbreaks, still largely ignoring a century-old technology that actually works. The simplicity is staggering—slap a UV fixture on the wall near the ceiling, tune it so that zero harmful light hits the occupants, and let it run. It’s not a silver bullet for all air quality concerns, and you still need ventilation, of course. But for dealing with nasty airborne pathogens? It’s hard to imagine an easier solution than upper-room UV. The only real question left is, why aren’t we using it everywhere?

Featured image by the US CDC.

12 thoughts on “Upper Room UV-C Keeps Air Cleaner

  1. “No, we won’t be filling this workspace with all day long with low-level UV spill because light never reflects or diffuses, right?. Besides, what could possibly go wrong, after all, it’s not like UV light exposure is very strongly correlated with developing cataracts or anything”

    1. Light follows the “inverse square law,” so every time the distance from the source doubles, the intensity is a quarter. If the source and the first reflective area is large enough, only a tiny fraction will reach the surface.

      And the surface will never be 100% reflective, because a large part of the incident radiation will be absorbed, and part will be reflected away from the occupants of the room. The inverse square law is applied again, this time over a very weak light.

      I doubt a well designed UVGI installation would increase the UV exposure enough to cause any detectable health effect. And when in doubt, there are commercial UV measuring devices that can tell exact how much UV is on the environment, and the installation can be tuned if the exposure is above limits.

      1. this time over a very weak light.

        True, but if you’re working in an office or retail space with extra UV, “over time” means 2000 hours a year.

        Depending on the hazard, exposure time can be as significant as exposure intensity.

        If you’re cutting the intensity of the noise from a loud machine by a factor of 100 you might go from being able to be around the source for a few minutes without causing damage to being around it indefinitely.

        But if you’re working around a reasonably tolerable level of, say, dioxin or radiation, prolonged exposure still adds up and sooner or later you start growing tentacles in inconvenient places.

  2. “It’s also important to ensure the lamps are switched off for maintenance or if anyone is entering the upper zone of the room for any reason.”

    This. Like when it’s time to change a (regular) ceiling-mounted light fixture.

  3. Yeah, there’s a reason this isn’t common despite UV-C light sources being available for over 50 years. Firstly, the safety aspect makes this impractical in an open areas – reflections aren’t easy to “eliminate” unless one has complete control over the structures size and shape. And would have to ensure there are no protruding parts in the ceiling – you know like, light fittings, or fans, which would clearly reflect significant UV. I think people often underestimate the damage UVC can do to the eyes. And whilst “low-level” chronic exposure isn’t well studied, and obviously won’t be as bad as acute high level exposure, I’m going to say it probably isn’t great. Even if workers in the areas chronically exposed wear clothes that cover most of the skin – their face will be exposed at all times.

    Secondly, air absorbs UVC so its range is limited. This is generally a positive since it limits exposure distance, but it also means large rooms may need emitters on opposite walls. It’s why UVC sterilisation is best done in enclosed spaces – air ducts, water pipes etc.. where flow can be controlled, and the UV can be fully contained.

    Thirdly, it’s not like you can just wave a beam of UVC over a surface and its germ-free. It takes time, and only works where there are no shadows caused by uneven surfaces, for say, oblique exposure.

    Honestly, doing this all in aircon would be easier, cheaper, less dangerous and arguably much more effective – which is exactly why it is used in this application.

    1. Forgot to add: this only affects airbourne virus/fungal particles. It does nothing for the surfaces on the lower part of the room covered in droplets inevitably expelled by people talking, which is the more common route for many viruses. Had to add this in case we get folks suggesting this technology somehow reduces the need for basic sanitation – it doesn’t.

  4. What about ozone production? my little lamp for pacifier disinfection produces ozone (I can tell from the smell)
    “they also output some light in the visual spectrum that gives them a characteristic green glow” It is bluish or perhaps cyan to me, not green. Who is color-blind?

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