Farther than the eye can see: disinfecting light

Light illuminates darkness…but it can do so much more. In healthcare and medical technology, light therapy has long been used to treat a variety of skin conditions or to disinfect rooms. Two new projects at the Ulm University of Applied Sciences (TH Ulm), led by Professor Martin Hessling, now study how light can effectively kill germs like viruses and bacteria.

Professor Hessling, at TH Ulm you are currently working on two projects, each using light to destroy viruses or bacteria. Can you briefly describe the projects?

Martin Hessling: We study several health-relevant microorganisms and viruses within the scope of these two projects. One project investigates disinfection systems designed to inactivate airborne SARS-CoV-2 viruses in indoor environments. A specific low dose of UV radiation is expected to prompt a 90 percent coronavirus reduction. To provide evidence of this, we have analyzed data from the past 50 years because the effects of UV radiation on these types of viruses has been studied extensively and is well documented. Commonly used air filter types also use UV light or UV light filters, but they often have not yet been tested for their effects on coronaviruses. The currently circulating SARS-CoV-2 virus strain must not be examined in a "conventional" laboratory, because it is only cleared for S3 laboratory installations due to its biosafety level. We also don’t work with the current SARS-CoV-2 virus strain but use similar viruses that also spread through airborne transmission of aerosols.

The second project uses visible blue, and violet light to eliminate bacteria that can be found on breathing tubes in intensive care units. Antibiotic-resistant bacteria often colonize on these devices and can travel into the patient's lungs via the tube. The best-known example is the methicillin-resistant Staphylococcus aureus (MRSA). As you might imagine, this can lead to complications and possibly even death in patients whose lungs may already be compromised and weakened, which includes patients with COVID-19. In further research, we plan to test whether viral transmissions can be reduced via this method.

If UV light is so effective at killing germs, why wouldn’t you develop lamps for classrooms or hospital rooms and kill at least some types of airborne viruses?

Hessling: UV light does not just inactivate bacteria and viruses it can also be damaging to humans. If you installed conventional mercury-vapor lamps – a common disinfection application - you could destroy pathogens such as coronaviruses very quickly and effectively. Unfortunately, they also damage human cells and can cause skin irritation and even skin cancer. This makes them unsuitable to emit UV radiation in rooms with humans in it. However, another type of ultraviolet light called far-UVC damages microorganisms but appears to be safe to use around people. This is because the light is rapidly absorbed by the outer (non-living) layers of human skin, preventing it from reaching any living skin cells. One conceivable option in the future is to use it to irradiate entire rooms while people are in them. However, we need further studies on this as it is still unclear what kind of damage it causes to the eyes, for example. And what happens if someone sleeps with their mouth open in an irradiated hospital room? How does this light affect the colonization by beneficial microorganisms of our skin? All of this has yet to be investigated. Apart from that, far-UVC lamps are currently also very difficult to obtain, which makes it unlikely that they can be used on a large-scale for now.

Why is that?

Hessling: Until now, it was not necessary to irradiate rooms where people are staying. The rooms are typically treated with the more cost-effective UVC lamps, which is done when nobody is in the room. So far, there has not been a huge demand for far-UVC lamps.

If these lamps are not yet available, the only option for nursing homes, schools, daycare centers and businesses are air purifiers. Are they as effective as manufacturers say they are in removing virus-carrying aerosols?

Hessling: This is part of our current research. As I have already mentioned, manufacturers cannot safely guarantee that their air purifiers can also protect from SARS-CoV-2 viruses because they simply have never been tested on these strains. That being said, it shows promise that these systems use the “old” low-pressure mercury-vapor lamps. From our point of view, that should work well. However, when it comes to air purifiers, you must also look very closely at their testing conditions and requirements and review the virus strains and virus-carrying aerosols they were tested against. But I personally think it makes sense to use them in the abovementioned facilities. Of course, air purifiers still don't provide 100 percent virus protection. If two students sit too close to each other, the air may not have a chance to first flow through the filter before it reaches the other person.

To provide more security, we are currently conducting a master's thesis, which develops a plausibility check of air purifiers. We have set up a test track here in the laboratory where we atomize bacteria and viruses, put them in the air purifier and then check how the concentration has changed. We work with bacteriophages that are similar to the SARS-CoV-2 in structure. So far, the results are very promising.

Your studies on the "lighted" endotracheal tube, which you have already briefly discussed, are also very promising. How does this idea work?

Hessling: We are working closely with Professor Spellerberg from the University Hospital of Ulm in this setting. Her idea was to use light for disinfection "from inside the person". We assume that mechanical ventilation in an intensive care unit usually will save the lives of patients in respiratory distress. While that is mostly the case, it can also lead to major problems in some instances. If a patient is ventilated for more than five days, bacteria can move to the lungs via the endotracheal tube. This leads to pneumonia, which can increase the risk of death in some critically ill and weakened patients. We equip the endotracheal tube with blue LEDs – visible light and therefore harmless to human cells – and irradiate the pathogens to inactivate them. This has already worked successfully in the laboratory, allowing us to soon take the next step and test this process on pigs.