UV-C and HVAC: in-duct UV light - a layer of protection

February 4, 2021

The most common question that illumiPure scientists and engineers are asked is how Air Guardian® differs from other air purification and disinfection systems.

For example, a common question is:

If we plan to put UV-C disinfecting lights inside our HVAC ducts, why do we need your solution?

Naturally, the thinking is that if UV-C lights can kill most microbes and pathogens, and if all of the air vented from the central HVAC ducts into the room has been treated with an “effective dose” of UV-C energy needed to kill those pathogens, why would additional air treatment - within any building space - be required?

The answer may surprise you.

First, some history on ultraviolet light.

Early recognition of ultraviolet light’s disinfection properties (in visible sunlight) was identified in experiments conducted in the late 19th century by researchers Arthur Downes and Thomas Porter Blunt between 1877-1878 and subsequently by Theodor Geisler in 1890.[1]

Also, in 1877, Downes and Blunt made a discovery using UV light spectra that was termed “one of the most influential discoveries in all of photobiology.”[2]:

“…Downes and Blunt observed that exposing test tubes containing Pasteur’s solution to sunlight prevented the growth of microorganisms inside the tube and, upon increased exposure durations, the test tubes remained bacteria-free for several months. Downes and Blunt went on to demonstrate that the ability of sunlight to neutralize bacteria was dependent on intensity, duration, and wavelength, with the shorter wavelengths of the solar spectrum being the most effective.”[3] [4] [5]

Later, in 1894, Percy F. Frankland and H. Marshall Ward demonstrated specific disinfecting properties of light on Bacillus anthracis. These early studies were conducted using sunlight. We now understand that sunlight contains various forms of ultraviolet radiation, most notably UV-A and UV-B. While UV-C radiation wavelengths are produced by the sun, they don’t enter the earth’s atmosphere because of the protective ozone layer.

H. Marshall Ward found that the spectral band of UV electromagnetic wavelength was related to the bactericidal effect - and that it was more robust in the shorter wavelengths of the UV spectrum.

Today we still know this to be true - that the shorter the UV wavelength, the stronger the destructive effect on microbial species.

Despite these discoveries in the 19th century, it wasn’t until 1929-1930 that a researcher named Gate published an action spectrum of the bactericidal effect of ultraviolet light.[6] [7] [8] That action spectrum is shown below and has changed little since modern science has validated its accuracy:

Since UV-C wavelengths are not found in the visible light spectrum (they are blocked by the ozone layer), it was found that the shorter UV wavelengths could be artificially generated using Hg lamps.

Exposure to UV light can be harmful to humans and animals, especially UV-B and UV-C. UV-A light is not naturally absorbed by DNA but can induce other kinds of damage. UV-B and UV-C wavelengths can induce "a variety of mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs), 6-4 photoproducts (6-4PPs), and their Dewar valence isomers as well as DNA strand breaks by interfering the genome integrity." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3010660/ .

Harvard scientists William F. Wells and Richard L Riley are well known for their work in advancing UV-C disinfection in upper room air - most notably in experiments showing reduced infections for measles in Philadelphia Day Schools. The name for the device was “UVGI.”

Well’s Air Contagion and Air Hygiene[9] was called a “landmark monograph on air hygiene” by Edward Nardell.[10] Riley followed with his Airborne Infection: Transmission and Control.[11]

These two scientists pioneered what is now considered a mainstream strategy for safely using UV-C light to reduce airborne disease transmission - using UVGI.

Also, they postulated the famous Wells-Riley Equation[12], which, to this day, helps researchers and scientists assess the risk of infection in certain environments.

A look at the timeline of UV-C and UVGI (UV Germicidal Irradiation)


The use of UV-C lamps as an in-duct source of air disinfection within HVAC systems has been used for decades. Numerous studies have been performed on the germicidal efficacy of UV-C lamps in duct systems, and the results can either be simplistic or highly complex.

The following variables affect the efficacy (measured in the Log reductions of an individual microbe or ALL microbes contained in the source air-release at any given vent):

Complex fluid dynamics[13] behavior of air within ducts - as measured in Langragian or Eulerian methods

Dose, as measured by:

Yes, it’s complicated, but that’s because the efficacy of UV-C lamps in HVAC systems depends not just on the UV-C dose and the pathogenic species but, also, the duct itself, complex fluid dynamics, speed, distance, spatial irradiance, and diffuse irradiance.

Therefore, one can conclude that each HVAC system - including air handling and ductwork - will present different conditions and variables. Thus, the HVAC UV-C in-duct system's overall efficiency and ability to vent disinfected, pathogen-free air will absolutely vary.

For example:

From an EPA study on HVAC and UV-C, “EPA Technology Evaluation Report: Biological Inactivation Efficiency by HVAC In-Duct Ultraviolet Light Systems”[14]

“The bioaerosol inactivation efficiencies calculated for the three organisms were 9% for B. atrophaeus, 99.96% for S. marcescens, and 75% for MS2. The irradiance was measured as 1190 uW/cm2 at 161 cm (63 in.) upstream from the lamps with an airflow of 0.93 m3/sec (1970 cfm). The system had four lamps that were burned in for 100 hours prior to measurements. The spore form of the bacteria B. atrophaeus is more resistant to being killed by UV than the vegetative bacteria S. marcescens.”

The EPA test above was performed under specific conditions that may vary from current technology and related solutions. However, there are many forms and products within the in-duct UV-C industry, and technology is improving rapidly.

Notwithstanding the above, even if one assumes that the UV-C lamps within a given HVAC system reduce the total pathogen count vented from ALL ducts in a given building by the most optimistic average Log reduction, such as 2 Log (99%), there are still issues with HVAC as the primary mechanism of disinfection.

1. Cross-contamination of air caused by air circulation originating from vented air - notably studied in the well-known case in Guangzhou, China[15]

2. Active pathogen removal is limited to HVAC-facilitated room air changes, subject to complex airflow dynamics, pressure, and venting outside room space, including temperature convection, duct placement, fan speed, etc.

3. While UV-C processed air may be 2 Log reduced, contamination is continuous within a given space. Disease transmission can occur within the room - even if it receives air from clean HVAC systems. The pathogenic reservoirs are room-based and continuously changing (viral shedding, coughing, breathing, surface aerosolization, movement, procedure-related aerosolization, etc.)  

4. Built-environment changes - which may be recommended for optimal duct placement to prevent cross-contamination by HVAC airflow, may be expensive or impossible to implement.

5. Acute infectious events, such as toilet plumes, procedure-produced pathogens, sneezing, coughing, fecal material, etc., can overwhelm the more passive process of HVAC room air changes AND create much higher particle counts within the HVAC system as room air is eventually passed to intake ducts.

In summary, while clean HVAC systems, facilitated by UV-C lamps, are a good part of an overall infection prevention strategy, the HVAC/UV-C system's inability to provide active, continuous elimination and removal of infectious pathogens within a space (consistent with the CDC hierarchy of controls https://www.cdc.gov/niosh/topics/hierarchy/default.html) means that in-duct UV-C falls short of being a primary solution for infection prevention.

Air Guardian®

While no one system will make for a “safe” space, Air Guardian® adds effective pathogen removal to any area, any room, in any environment - to help remove and eliminate harmful airborne microbes before they can be transmitted within a space.

Regardless of the quality, purity, or disinfection level of air vented into a space from the HVAC system - or in spaces with little or no ventilation - Air Guardian produces purified, disinfected air (between 2 Log and 6 Log reductions) in a protective displacement fashion (at pressure), and rapidly replaces all room air.

Thus, while in-duct UV-C can be important to ensure cleaner HVAC systems and higher-quality airflow, Air Guardian® provides the highest level of occupant safety, infection protection, and air quality within a given space.

Specific elements of the Air Guardian® solution:

  1. Filtration and purification are active, not passive. This active ingestion and downward airflow displacement process conforms to optimal airflow and venting recommendations that help prevent disease transmission and facilitate more complete room air changes
  2. UV-A photo-catalyzed nanoparticle oxidation - Ingested air is initially passed through a localized, intense ROS (reactive oxygen species) cloud. The ROS cloud and extreme UV-A irradiance combine for high initial levels of destruction/inactivation
  3. Intense time-dose UV-C irradiance - ingested air is forced through a series of channels, continuously exposed to UV-C irradiance, for as long as 45 seconds, depending on device models and options
  4. UV-C photo catalyzed nanoparticle oxidation - Air Guardian's internal channels  are flooded with electromagnetic UV-C radiation wavelengths, which a) inactivate and destroy pathogens and particles and b) trigger photo-activation of the channel surfaces, which creates continuous clouds of reactive ion species, which form within every channel and along every surface inside the Air Guardian device
  5. Air Guardian's UV-C dose energy is > 200% of the required levels for the  inactivation / removal of most pathogens, with few exceptions  (proven in numerous studies).
  6. Air Guardian's extreme UV-C dose is highly effective at removing pathogens and particles, yet safe in any setting because no UV energy is emitted from the sealed device
  7. Air is filtered using HEPA-like super-micron and carbon filters and released at precise pressures and distances to create constant downward displacement into breathing zones
  8. Complete room air changes can be performed as frequently as every few minutes, depending on device selection

For more on Air Guardian® and Immaculight™ and the associated science and research, see https://www.illumipure.com/science


[1] www.ncbi.nlm.nih.gov/pmc/articles/PMC2789813/[2] Hockberger PE. A history of ultraviolet photobiology for humans, animals and microorganisms. Photochem Photobiol. 2002;76:561–79. [PubMed] [Google Scholar][3] Downes A, Blunt TP. The influence of light upon the development of bacteria. Nature. 1877;16:218.[Google Scholar][4] Downes A, Blunt TP. Research on the effect of light upon bacteria and other organisms. Proc R Soc Lond. 1877;26:488–500. [Google Scholar][5] Downes A, Blunt TP. On the influence of light upon protoplasm. Proc R Soc Lond. 1878;28:199–212.[Google Scholar][6]Gates FL. A study of the bactericidal action of ultra violet light: I. The reaction to monochromatic radiations. J Gen Physiol. 1929;13:231–48. [PMC free article] [PubMed] [Google Scholar][7] Gates FL. A study of the bactericidal action of ultra violet light: II. The effect of various environmental factors and conditions. J Gen Physiol. 1929;13:249–60. [PMC free article] [PubMed] [Google Scholar][8] Gates FL. A study of the bactericidal action of ultra violet light: III. The absorption of ultra violet light by bacteria. J Gen Physiol. 1930;14:31–42. [PMC free article] [PubMed] [Google Scholar][9] Wells WF.  Airborne contagion and air hygiene: an ecological study of droplet infections. Cambridge (MA): Harvard University Press; 1955.  [Google Scholar][10]  Hockberger PE. A history of ultraviolet photobiology for humans, animals and microorganisms. Photochem Photobiol. 2002;76:561–79. [PubMed] [Google Scholar][11] Riley RL, O'Grady F.  Airborne infection: transmission and control. New York: The Macmillan Company; 1961.  [Google Scholar][12] Sze To GN, Chao CY. Review and comparison between the Wells-Riley and dose-response approaches to risk assessment of infectious respiratory diseases. Indoor Air. 2010;20(1):2-16. doi:10.1111/j.1600-0668.2009.00621.x[13] Yang Y, Zhang H, Lai AC. Lagrangian modeling of inactivation of airborne microorganisms by in-duct ultraviolet lamps. Build Environ. 2021;188:107465. doi:10.1016/j.buildenv.2020.107465[14] EPA - Biological Inactivation Efficiency by HVAC In-Duct Ultraviolet Light Systems - American Ultraviolet Corporation ACP-24/HO-4[15] Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerg Infect Dis. 2020;26(7):1628-1631. https://dx.doi.org/10.3201/eid2607.200764