A search for “ultraviolet light” on Google will yield no less than 91 million search results. UV light sanitizers, industrial UV sanitizers, and thousands of other products show up. These results are an impressive show of our society’s ability to provide millions of pieces of information, all in our living room or office, in just under a half second.
If you narrow your search to “HVAC ultraviolet light”, the results are a much more manageable 667,000 results in 0.57 seconds. Still, an impressive number. Still, far too much to filter through to make an educated decision. So perhaps Google is not the place to get reliable, peer-reviewed information on Ultraviolet Germicidal Irradiation (UVGI), which is the technical description for using UV lighting to mitigate viruses and bacteria.
The reason that the internet search is so unreliable, and in many cases dangerous, is that there are 3 different bands of UV lighting, UV-A, UV-B, and UV-C. Each can be harmful to the skin and eyes of humans (photo-keratitis, conjunctivitis) with surprisingly little exposure. Each can degrade materials such as wire jacketing, filter material, and so on, paint, down to your carpet through a process called photodegradation. Only UV-C, which produces a wavelength of light between 200 and 280 nanometers (nm), is effective in disrupting the DNA of a virus, effectively stopping its ability to spread. This is not new technology, having been discovered in 1877 according to a recent IES report.
UVGI can be applied in air-handling units, ductwork, room direct, and even in portable and handheld applications. When a lamp providing light wavelength of 254 nm irradiates a surface or airstream at the proper intensity, it can be extremely effective in eradicating such harmful substances as anthrax, legionella, tuberculosis, flu viruses and the common cold. As you may have guessed, it also appears to be effective on the SARS family of viruses, and likely SARS-COV2.
UV-C lighting, which is typically produced at 254nm, is not safe for exposure to humans for any length of time. According to the ASHRAE presentation on UVGI, the National Institute for Occupational Safety and Health (NIOSH) exposure limits for 253.7nm lighting, based on intensities between 0.2 watts per square centimeter (W/cm²) and 600 W/cm² can be as long as 8 hours to as short as 1 second, respectively. The problem of sanitizing spaces that people inhabit, but not allowing it to occur when they are present is why upper room UVGI and HVAC system UVGI have been such a popular application. Unfortunately, it has also spawned the handheld market available to the public, which should only be utilized by trained personnel in a controlled environment. I feel I would be remiss if I failed to mention that watching a YouTube™ video on UV light use and closing the bathroom door does not qualify as training and controlled environment.
HVAC system UVGI, when used in conjunction with mechanical filtration, is typically designed to target an 85% reduction in airborne viruses in a single pass. It can also be designed to perform as high as 99.98% efficacy for a single pass, which requires higher intensity, but will approach HEPA filtration levels.
These systems, given the appropriate space, may be applied in either ductwork or in the air-handling unit. The ancillary benefits of the air-handling unit application make it the most attractive from an investment standpoint. Studies have shown up to a 15% energy efficiency savings based on coil heat transfer effectiveness, when compare to an untreated coil, when UV lighting is positioned to irradiate a cooling coil. The light significantly inhibits biological growth, and therefor prevents an insulative film from forming on the coil. The result is a system that not only provides a safer environment but will ultimately save energy.
When seeing numbers like 85% and 99.98% effectiveness, you may be wondering why not just install these systems and invite everyone back to the office? If it is really that effective, can’t we use this and get back to normal? Unfortunately, most air systems in office spaces, classrooms, and other general use spaces only provide 2-3 air changes per hour. That means it takes 20-30 minutes just to supply enough conditioned air to the space to displace the existing air in the space. Traditional HVAC systems are designed for comfort cooling and heating, and they do not just replace the air, they mix the air, providing a uniform temperature to avoid drafts. As you may have guessed, that provides a uniform air quality as well. So instead of thinking of this treated airstream as a fresh coat of paint every time new air is added, it is more like adding clear water to a muddy stream. It reduces the relative number of contaminants, but it cannot remove them all, particularly when occupants add more with every breath.
UVGI systems are simply another tool, another strategy in the overall effort to provide clean, comfortable, safe spaces for occupant. If you have any questions, please do not hesitate to contact Stephen E. Oskin, PE, LEED AP at (814) 880-6070 or firstname.lastname@example.org.