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Determining UV-C Irradiance for Disinfection Applications
With the rapid growth of UV-C LED applications in germicidal and disinfection applications, there is a growing need to quantify and determine whether or not a particular UV-C lamp or setup will accomplish the disinfection objectives. Unlike regular lighting products that we can visually confirm if it’s “bright enough,” UV-C is invisible to the naked eye, and this can make it especially challenging from a measurement perspective.
In this article, we will go over the primary measurement methods and principles, and then work through a few examples that will help work through the steps needed in determining the irradiance requirements.
We will show that the 15W UV-C light module from illumenai is an effective solution for disinfecting the workplace.
Illnesses from the work environment
While COVID-19 is very much in people’s minds at the moment, 20 million days of work are lost worldwide every year just due to the common cold.
Viruses, bacteria and molds are an everyday problem that causes significant lost production for businesses worldwide.
There are hundreds of bacteria varieties found in workplaces, and most of them come from humans, whether they are from our skin, nasal, oral or intestinal cavities. Conventional wisdom would suggest that most bacteria can be found on our mouse and keyboard, but while their levels are high, the biggest offenders are chairs and telephones.
Some example bacteria.
Streptococcus pyogenes is a highly contagious bacteria that causes strep throat.
Staphylococcus (staph) is a group of bacteria. There are more than 30 types. A type called Staphylococcus aureus causes most infections, such as staph infections, food poisoning, sepsis and pneumonia.
Diphtheroids are a commonplace bacterium in most office buildings and can be especially dangerous to those with weak immune systems, or those whose systems are weak already due to an existing illness.
Pseudomonas aeruginosa is another variety of bacteria that is particularly common on keyboards, even after a sanitary wiping process is completed. They are one of the leading causes of pneumonia and urinary tract infections.
Dr. Gerba, from the University of Arizona, recently shared that the desktop is one of the germiest places in the office and that 20-30% of handbags and knap sacks have fecal bacteria underneath! They discovered that the average office toilet seat had 49 germs per square inch, but desktops had almost 21,000 germs per square inch, and phones had more than 25,000 germs per square inch! Oh dear.
Desks, phones, computer keyboards and your mouse are key germ transfer points because people touch them so often, Coughing and sneezing can leave behind “a minefield of viruses” that can live on a surface for up to three days.
Then, there are the viruses.
Covid-19. We all know about this one. However, it is just one of many viruses found in the workplace. This is a particular problem right now that is causing untold disaster around the world in human life and economic losses.
The common cold is really not so common and is caused by a number of different viruses. In fact, there are more than 200 that can lay you low.
It’s likely that someday you’ll have a close encounter with one of these types:
Cause 10%-40% of colds. You’ll feel miserable when you catch one, but the good news is they rarely make you seriously sick.
- Coronavirus (yes, there are many variants other than COVID-19)
Cause of about 20% of colds. There are more than 30 kinds, but only three or four affect people.
- RSV and parainfluenza
Cause 20% of colds. They sometimes lead to severe infections, like pneumonia, in young children
There are also a lot of viruses that doctors haven’t identified. About 20%-30% of colds in adults are caused by these “unknown” bugs.
Keeping the office surfaces germ and virus free will pay huge dividends in the wellness of your employees.
The technical section
Why UV light?
UV-C LED technology works by directly targeting the DNA and RNA molecules of bacteria, viruses, and molds.
Research has shown that thymine (in DNA) and uracil (in RNA) absorb ultraviolet radiation most readily at wavelengths between 255 and 275 nanometers (the best wavelength being 265.
When the DNA and RNA of these pathogens are exposed to 265 nanometer UV-C radiation, a chemical change occurs in the nucleic acids, resulting in a corruption in the genetic code.
Once the genetic code of viral and bacterial colonies has been corrupted, the result is an altered genetic code that inactivates the pathogen and stops replication. This alteration is called dimerization.
Note that wavelengths higher than 300 nanometers (UV-B and UV-A) fail to cause dimerization and are therefore ineffective for sterilization applications.
Dosage = UV Amount + Time
When designing for UV-C disinfection, we need to start with UV-C dosage, because the ultimate goal is to achieve a certain UV-C dosage needed to inactivate the pathogen.
But first, what exactly is dosage, and how is it measured?
UV-C dosage, also called exposure dosage or fluence, is a way to measure how much total UV-C energy has irradiated a surface. This is the most crucial element in UV-C system design, because UV-C dosage is the primary determinant in whether or not we have successfully achieved pathogen inactivation.
Dosage is determined not only by the strength of the UV-C that falls on a surface, but also how long that surface is exposed to the UV-C radiation for. In other words, a UV-C lamp with half the strength can achieve just as much UV-C dosage if it is used for twice the amount of time.
The strength of the UV-C that falls on a surface is called irradiance, and is measured in W/m² (or some variant of power per surface area). The exposure time is measured in seconds.
The simple form of the formula is shown here:
Exposure Dosage (J/m²) = UV Irradiance (W/m²) x Time (seconds)
UVC radiation over distance is affected in the same manner as visible light using the inverse square law, where irradiance is proportional to the square of the distance between the LED and the disinfecting surface.
LEDs are angled in a manner that evenly distributes the UV radiation over the entire space, with fields of view overlapping at the surface level to account for the exposure reduction at a non-perpendicular surface.
There are no mechanical barriers obstructing the exposure of UVC to the bacteria, like dust or dirt on the disinfecting surface or the surface of the LED.
There is virtually no difference in effectiveness between the 254nm LED used in the benchmark and the 265nm LED used in the experiment. In fact, studies have shown that wavelengths from 260nm (Sagripanti) to 266nm (Soo-Ji Kim) are even more effective than 254nm.
Relative Humidity (RH) is approximately 55%. Note that it is harder to kill viruses at higher RH (Li, Inactivation of Viruses on Surfaces), so it is important to monitor the RH to adjust exposure dosage appropriately.
Powering one 265nm UVC LED at 1.75W, a UVC meter measured the irradiance, perpendicular to the LED, 0.33 meters away. Total irradiance measured 14uW/cm2. Extrapolating the irradiance when using 8 LEDs instead of just 1, we multiply the measured value of 1 LED by 8, giving us 112uW/cm2.
In a typical office space application, the distance from ceiling to floor is around 3 meters. So, the inverse square law was used to compensate for the extra distance in a worst-case scenario at floor level. Where distance from measured level to floor level is 2.66 meters (8.73 feet), the calculation is as follows:
(112uW/cm2) / (8.73ft2) = 1.47uW/cm2 at floor level
The target fluence (dosage) we needed to achieve for bacteria is referenced from a scientific paper published by American Society for Microbiology, titled “Using UVC Light-Emitting Diodes at Wavelengths of 266 to 279 Nanometers to Inactivate Foodborne Pathogens and Pasteurize Sliced Cheese”. Looking at Figure 2(c) on page 4 of 7, the chart illustrates that a fluence of 0.5mJ/cm2 is required to achieve a 4 log reduction (99.99% inactivation rate) of Listeria, when disinfecting with a 266nm UVC LED fixture (Soo-Ji Kim).
An assumption is made about relationship between the size of the bacteria/virus and the UVC fluence required for inactivation. Listeria is used as the reference since it requires the highest fluence for the same log reduction compared to Salmonella and E.coli (Soo-Ji Kim).
With the target fluence and the irradiance from the LEDs, we calculated how much exposure time would be required to achieve at least a log 4 reduction. Knowing that 1J/cm2 is equivalent to 1W-S/cm2, our target fluence can be translated to 500uW-S/cm2. The exposure time is then calculated by:
(500uW-S/cm2) / (1.47uW/cm2) = 340.1 seconds = 5.67 minutes
Fluence levels follow the Bunsen-Roscoe law, which states that the survival fraction of virus with UVGI irradiation being dependent on UV dose, is not affected by reciprocal changes in UV intensity or to exposure time (Li). This means the required fluence can be achieved by any amount of UVC intensity and exposure time (seconds) that multiply to 500uW-S/cm2. Therefore, we can theoretically inactivate at least 99.99% of equivalent bacteria from surfaces 3 meters away with 8 x 265nm UVC LEDs within 5.67 minutes.
Higher effectiveness can be achieved by lengthening the exposure time of UVC radiation. Increasing the exposure time to 1 hour instead of 5.67 minutes, the fluence increases to:
(60 / 5.67) * 500uW-S/cm2 = 5.29mW-S/cm2
A useful benchmark specifically regarding the family of coronaviruses was published by the Edgewood Chemical Biological Center, where the required fluence to achieve D37 (63% inactivation rate) using 254nm UVC was found to be 3.1J/m2, or 310uW-S/cm2 (Sagripanti). Knowing our fluence of 5.29mW-S/cm2 in 1 hour and the relationship between inactivation rate and fluence being logarithmic, we can predict the required exposure time to achieve a 4 log reduction by following the virus survival curve.
The most common survival curve for viruses is as follows:
n/no = e-kD (Sagripanti)
where n/no is the virus surviving fraction, D is the fluence and k is the slope of the survival curve when ln(n/no) is plotted versus D.
The D37 is the exposure level required to achieve a surviving fraction of 0.37 where D = 1/k (and k = 1/D)
Using the common survival curve equation, we can rearrange to get:
ln(n/no) = -k*D à ln(0.37) = -k*310u à k = 3225.8
Substituting a 4 log reduction (0.0001 surviving fraction) to find the required dosage, we can say:
ln(0.0001) = -3225.8*D à ln(0.0001) / -3225.8 = D à D = 2.85mW-S/cm2
With the required dosage, we can calculate the required exposure time:
(2.85mW-S/cm2) / (1.47uW/cm2) = 1938.8 seconds à 32.2 minutes
Therefore, we can theoretically inactivate 99.99% of exposed virus within 32.2 minutes.
It is then possible to have the UVC exposure to run for 32 minutes within common rooms and board rooms in between uses to eliminate 99.99% of any traces of COVID from the air, surrounding surfaces, and the floor.
If it is not possible to wait the 32 minutes in between space uses, a 99.99% inactivation rate can be achieved at the desk level (6.3 feet away) in 18.2 minutes.
Exposure time will be configurable, but the recommended time should be 35 minutes to overshoot the 99.99% Inactivation rate.
We can also calculate what inactivation rate we can achieve with 1 hour of exposure time:
Substituting our 1-hour fluence level of 5.29mW-S/cm2 to find the surviving fraction, we can say:
n/no = e-3225.8*5.29m à n/no = 0.0000000388
Which means our inactivation rate (%) is:
(1 – 0.0000000388) * 100 = 99.99999%
This goes above and beyond the minimum requirement to achieve a 4 log reduction (99.99%) and is expected to inactivate virtually all traces of COVID that is exposed to the UV-C radiation.
D37 is defined as The UV exposure that produces an average of one lethal hit per virion. This is the minimum level required to inactivate a virus on average (UV wavelength used is 265nm).
Using our UV-C light module, the exposure times required for various virus families are shown below in order of easiest to hardest to kill:
Coronaviridae are large, enveloped, single-stranded RNA viruses. They are the largest known RNA viruses. Includes COVID-19, SARS-CoV, MERS-CoV,
Fluence = 2.5 – 3.9 J/m2
D37 Inactivation time (floor level) = 4.42 minutes
D37 Inactivation time (desk level) = 2.5 minutes
Filoviridae can cause severe hemorrhagic fever in humans and nonhuman primates. So far, three genera of this virus family have been identified: Cuevavirus, Marburgvirus and Ebolavirus.
Fluence = 7.4 J/m2
D37 Inactivation time (floor level) = 8.39 minutes
D37 Inactivation time (desk level) = 4.74 minutes
Picornaviridae is a large family of viruses where primary sites of replication are epithelial cells and lymphoid tissues of upper respiratory and gastrointestinal tracts. In the upper respiratory tract enterovirus infections can cause the common cold. In the gastrointestinal (GI) tract, some enteroviruses can escape to infect other organs (For example, Hepatitis A). Also, foot and mouth disease virus (FMDV, genus Apthovirus). FMDV is very stable in the environment and is easily transmitted over long distances. In FMDV-free regions of the world occasional incursions and outbreaks cause major disruptions of agriculture and huge economic losses.
Fluence = 45 – 53 J/m2
D37 Inactivation time (floor level) = 60.09 minutes
D37 Inactivation time (desk level) = 34 minutes
Retroviridae constitute a large family of viruses that predominantly infect both human and animal vertebrates. They are positive-stranded enveloped RNA viruses that reverse transcribe their RNA into a DNA intermediate during viral replication, hence the name ‘retroviruses’. Retroviral infections can cause a wide spectrum of diseases ranging from malignancies to immune deficiencies and neurologic disorders.
Fluence = 67 – 110 J/m2
D37 Inactivation time (floor level) = 124.71 minutes
D37 Inactivation time (desk level) = 70.49 minutes
Li, Chun-Chieh Tseng & Chih-Shan. Inactivation of Virus-Containing Aerosols by. Study. Taipei: Aerosol Science and Technology, 2007. Document.
New Coronavirus (SARS-CoV-2) and the Safety Margins of Plasma Protein Therapies. 2020. Website. 08 07 2020.
Riley, Monica. Correlates of Smallest Sizes for Microorganisms. n.d. Website. 08 07 2020.
Sagripanti, C. David Lytle and Jose-Luis. Predicted Inactivation of Viruses of Relevance to Biodefense by Solar Radiation. Study. Harford County: Edgewood Chemical Biological Center, 2005. Document.
Soo-Ji Kim, Do-Kyun Kim, Dong-Hyun Kang. “Using UVC Light-Emitting Diodes at Wavelengths of 266 to 279 Nanometers To Inactivate Foodborne Pathogens and Pasteurize Sliced Cheese.” n.d. American Society for Microbiology. Document. 08 07 2020.
Chun-Chieh Tseng & Chih-Shan Li (2007) Inactivation of Viruses on Surfaces by Ultraviolet Germicidal Irradiation, Journal of Occupational and Environmental Hygiene, 4:6, 400-405, DOI: 10.1080/15459620701329012