Challenges and Opportunities of UV Disinfection

Dr. Simon Rankel

Electromagnetic radiation in the wavelength range between 200 nm and 280 nm refers to the UV-C part of ultraviolet radiation. The natural source of UV radiation is the Sun. Most of the UV-C that the Sun generates is blocked by the ozone layer in the atmosphere. Among man-made UV-C sources, the most common are gas discharge lamps. For a long time, mercury vapor lamps dominated the UV lamp market. Now, the latest advances in LED technology are bringing UV-C LEDs into the disinfection "game," specifically ones in the 260 to 280 nm range.

Although UV-C LEDs are still relatively expensive and less energy efficient as established UV-C sources, they can already be efficiently used in well-designed disinfection systems, especially for point of use applications. The great strengths of UV-C LED solutions are miniaturization, robustness, better control of heat flow, and precise tuning of wavelengths.

The term "pathogens" covers infectious microorganisms that cause diseases: bacteria, viruses, parasitic protozoa, and fungi. For more than 140 years, it has been known that UV radiation has a bactericidal effect, meaning it can inactivate or kill microorganisms. UV radiation accomplishes this by causing photochemical destruction of their nucleic acids (DNA and RNA); this means that the pathogens are, in most cases, not killed but are made incapable of replication. In contrast to a sterilization where all forms of microbiologic life are eliminated, disinfection is the process of reducing/inactivating harmful microorganisms to some extent. By inactivating the pathogens, they are no longer able to infect other organisms.

The UV disinfection method where UV radiation is used to inactivate the pathogenic microorganisms is called Ultraviolet Germicidal Irradiation - UVGI. Let's take a look at the key parameters that determine whether UV radiation can inactivate pathogens. This process depends on the wavelength of the radiation, the microbial sensitivity of pathogens, and the amount of radiation energy to which the organism has been exposed.

Figure 1 shows that the germicidal effectiveness curve of UV radiation for E. Coli bacteria peaks around 260-265 nm. This curve can vary between microorganisms but the region that shows maximum absorption of UV by their DNA is usually between 250 and 280 nm.

Low pressure mercury lamps have a peak at 253.7 nm, which makes them very convenient for UVGI. That´s why those lamps are often called germicidal lamps.

UVGI delivers a significant advantage as compared to chemical disinfection: UV radiation leaves no harmful chemical substances or by-products behind. Therefore, UVGI can be used in a variety of applications: in water, air, and surface disinfection.

The most developed and worldwide accepted UV-C application is water disinfection. The concept of airborne infection via droplets and the use of UVGI to disinfect the air was first discovered in the 1930's (1) Today, air disinfection can be accomplished by several methods: irradiating the air in a room, either in parts or the whole room, or irradiating the air that passes through a specially designed enclosed air-circulation system (like ventilation or air-conditioning) (1). For airborne route pathogens that eventually fall and contaminate surfaces, surface disinfection enters the picture. To obtain optimal results, UV air and UV surface disinfection need to go hand in hand as both are important in fighting pathogens.

The amount of UV radiant energy that microorganisms are exposed to is a crucial determinant of their inactivation. The interesting question is: What amount of radiant energy finally reaches the germs, if it reaches them at all?

To quantify UV radiation, there are different common radiometric quantities: the radiant power (radiant flux) of the UV source, which is the radiant energy emitted per unit time (W); irradiance, which is power perceived by a surface per unit area (W/m2); and radiant exposure, which is radiant energy perceived by a surface (J/m2); this is also called UV dose, which is crucial for understanding the UVGI mechanisms.

In antimicrobial testing, term log reduction is used, which refers to the magnitude of change in inactivated or killed microorganism numbers by using a logarithmic scale. 3-log reduction means that the number of microorganisms in a sample has been reduced by a factor 1000 and only 0.1 % of the initial number remained after the exposure to a specific UV dose. Different microbes have different sensitivities to UVGI and require different doses of radiation for the same fraction of inactivation. Table 1 shows how the UV dose affects the level of pathogen reduction, depending on the type of microorganism.

Dose values and inactivation levels normally correspond to lab results obtained under specific conditions, which may differ from "real" situations. Usually, higher doses than proposed are applied in the field. At the moment, many researchers are working on determining the lethal UV dose for the SARS-CoV-2 virus. Currently, there are no lab results generally available.

Once the dose of a specific log reduction for a certain microorganism is known, the next question arises: What level of reduction is required to prevent spreading of the specific virus? EPA guidelines on disinfection state that higher than or equal to a 6-log reduction in less than 10 minutes is needed to claim disinfection for the mentioned pathogens. Leaving too many viable pathogens behind can mean quick exponential growth again.

The two most important aspects of the UV-C disinfection process are its efficiency and safety. If we want efficient disinfection, we need to know our space and surfaces in every detail, understand the capabilities and limitations of UV source used, and know the pathogens we are trying to eliminate. Also, we need to keep an eye on safety issues that we do not harm anyone when applying UV radiation. One of the main criteria that need to be addressed is shadowing, which significantly affects the effectiveness of UV-C disinfection. It can be either macroscopic (germs behind objects) or microscopic (where tiny germs can be hidden in the cracks of the surface material). In both cases, the germs cannot be inactivated if they are in shadowed areas.

To grasp the complexity of this situation, let´s imagine a common room in a hospital that needs to be disinfected by UV-C disinfection system or a robot (see fig. 2)

First, there is the question of safety. As direct exposure to UV light has an impact on living organisms, all persons or other living creatures who shouldn't be exposed need to leave the room before disinfection starts. We should also take into consideration the material in the room as repeat exposure to UV-C radiation can lead to long-term deterioration of surfaces (especially polymeric).

Second, to make UV disinfection effective, the space that is supposed to be disinfected needs to be cleaned at the start, e.g. the absorption properties of liquids like blood will reduce inactivation success. All unnecessary objects creating macroscopic shadows should be removed. Rags on the floor or the interior of wardrobes are covered locations and the germs cannot be inactivated if they cannot be reached by the UV radiation.

There are different surface types and lots of crevices where bacteria and viruses can grow. When considering whole room environmental disinfection, it is necessary to use systems that can reach and are effective on every surface, including the floor, regardless of distance. For instance, there is a large difference in the UV-C irradiance at the Earth´s surface vs above the ozone layer of the atmosphere. We can draw an analogy to the situation occurring in our room. The measurement position with respect to the objects or substances in between the UV source and the irradiated surface is very important.

Third is the issue of exposure time. As irradiance is inversely proportional to the square of the distance from the UV source and the radiant power from the source is usually fixed, this distance and duration of exposure is crucial to achieve the needed dosage. Multiple UV sources can be combined; multiple systems installed on many corners, walls, ceiling, or moving robots would increase the available radiant power. With this approach, we can be assured that the log reduction corresponding to the required UV dose is achieved for specific type of pathogen in the room. To optimize disinfection results and use less time, it's ideal if the source moves close to the surfaces. The angle of the UV source tilt towards the irradiated object is also important as perpendicular surfaces will absorb more energy and micro shadowing due to the surface material texture can be approached more efficiently.

Coming back to UVGI safety issues, it´s important to note that there is a difference between the biological effect and the penetration depth of UV radiation. While the UV-C wavelength range is biologically the most active radiation, it is less dangerous to people because it is absorbed by the dead outer layer of skin. UV-B and UV-A radiation, on the other hand, penetrate further into the tissue. Safe practices are critical as an overexposure to 254 nm radiation can result in sunburn-like effects (erythema) and a painful eye condition known as "welder's flash" (photokeratitis) (1)

UV radiometers – also called irradiance and dosage sensors – are commonly used for UV irradiation and dosage measurement over the selected spectral band for disinfection applications. To achieve reliable results, it is crucial to select the right sensor and use it correctly. One way to measure the radiant power of the UV source is by using an appropriate integrating sphere connected to other sensors and a spectrometer. And finally, the proper calibration is what assures that the results obtained are accurate and reliable. Specific field conditions might require some special measurement approach or custom solutions.

Inactivating pathogens like a SARS-CoV-2 virus on surfaces with UV radiation can be an effective, safe, and environmentally friendly disinfection method, but it is essential to find a comprehensive approach that takes all limitations into account. There is a substantial need to improve regulations concerning the use of UV radiation for disinfection applications, especially in the healthcare industry. Developing and applying easy to use measurement equipment will be pivotal in this aspect.

Source 1: The History of Ultraviolet Germicidal Irradiation for Air Disinfection, N.G. Reed, Public Health Rep. 2010 Jan-Feb; 125 (1): 15-27