How to use Ultraviolet light (UVC) to fight COVID-19 effectively in dental clinics: Dr Ajay Bajaj
Dentistry, labelled as the highest-risk profession in COVID-19 times because of the aerosol spread in the clinic, has forced dental professionals to look for tools that guarantee total elimination of the virus risk. Among the various technologies being considered, ultraviolet light (UV-C) disinfection has gained favour due to its efficacy against a broad range of microbial and viral agents found in a variety of environments.
An emergent pneumonia outbreak originated in Wuhan city, China in late December 2019. Researchers quickly isolated a new virus and sequenced its genome (29,903 nucleotides). The infectious agent of this viral pneumonia happening in Wuhan was finally identified as a novel coronavirus (2019-nCOV), the seventh member of the family of coronaviruses that infect humans. On 11th February 2020, WHO named the novel viral pneumonia as “Corona Virus Disease (COVID19)”, while the International Committee on Taxonomy of Viruses (ICTV) suggested this novel coronavirus name as “SARS-CoV-2” due to the phylogenetic and taxonomic analysis of this novel coronavirus.
This initiated the pandemic story and now we all are facing challenges of COVID-19 cross-infection our practices. COVID-19 is a disease that is highly infectious and spreads rapidly in the community.
Transmission routes of SARS CoV-2 in clinics and hospital environments:
Since SARS CoV-2 can be passed directly from person to person by respiratory droplets, evidence suggested that it is transmitted through contact and fomites. In addition, the asymptomatic incubation period for individuals infected with SARS CoV-2 has been reported to be ~1–14 days. To et al. reported that live viruses were present in the saliva of infected individuals by viral culture method.
Droplet and aerosol transmission of SARS CoV-2 are the most important concerns in dental clinics and hospitals because it is hard to avoid the generation of large amounts of aerosol and droplet mixed with patient’s saliva and even blood during dental practice. In addition to the infected patient’s cough and breathing, dental devices such as high-speed dental handpiece use high-speed air to drive the turbine to rotate at high speed and work with running water. When dental devices work in the patient’s oral cavity, a large amount of aerosol and droplets mixed with the patient’s saliva or even blood will be generated. Particles of droplets and aerosols are small enough to stay airborne for an extended period before they settle on environmental surfaces or enter the respiratory tract. Thus, the SARS CoV-2 has the potential to spread through droplets and aerosols from infected individuals in dental clinics and hospitals.
Contaminated surfaces spread Human coronaviruses such as SARS-CoV, Middle East Respiratory Syndrome coronavirus (MERS-CoV), or endemic human coronaviruses (HCoV) can persist on surfaces like metal, glass, or plastic for up to a couple of days. Therefore, contaminated surfaces that are frequently contacted in healthcare settings are a potential source of coronavirus transmission. Dental practices derived droplets and aerosols from infected patients, which likely contaminate the whole surface in dental offices. In addition, it was shown at room temperature that HCoV remains infectious from 2 hours up to 9 days and persists better at 50% compared with 30% relative humidity. Thus, keeping a clean and dry environment in the dental office would help decrease the persistence of 2019- nCoV.
How do we disinfect our clinics and hospitals?
Highly infectious microbial and viral diseases are a major challenge to global health, and, as such, are also a significant risk to global financial stability and security. While vaccines play a key role in preventing viral epidemics and pandemics, once an outbreak has occurred, the implementation of disinfection measures to limit spread becomes paramount.
To disinfect the dental operatories before and after the patients, many protocols have been discussed including –
-simple only natural ventilation with min. 6 air changes per hour (ACH) of 60 min
-HEPA 13/14 air filters with min 12 ACH
-Disinfectant defogging of 30-45 min or
-Ultraviolet germicidal irradiation (UVGI) plus ventilation of 2 min to 6 hours depending upon wattage of the light source, room size and UV light position in the room
Among these different types, UV-C disinfection has gained favour due to its efficacy against a broad range of microbial and viral agents in a variety of environments.
What is ultraviolet light?
Ultraviolet (UV) is that part of electromagnetic light bounded by the lower wavelength extreme of the visible spectrum and the X-ray radiation band. The spectral range of UV light is, by definition between 100 and 400 nm (1 nm=10-9m) and is invisible to human eyes.
Using the CIE classification the UV spectrum is subdivided into three bands:
- UVA (long-wave) from 315 to 400 nm
- UVB (medium-wave) from 280 to 315 nm
- UVC (short-wave) from 100 to 280 nm
UVC is totally absorbed by atmospheric ozone, has minimal penetration to the surface of the Earth and thus has little effect on human health. 90% or more of UVB is absorbed by atmospheric ozone, while UVA passes through the atmosphere with little change. Thus, the solar ultraviolet radiation of importance to human health consists of UVA and UVB. However, UVC can be created artificially by various means.
A strong germicidal effect is provided by the Light in the short-wave UVC band. In addition erythema and conjunctivitis can also be caused by this form of Light. Because of this, when germicidal UV-Light lamps are used, it is important to design systems to exclude UVC leakage and so avoid these effects.
Self-evidently, people should avoid exposure to UVC. Fortunately, this is relatively simple, because it is absorbed by most products, and even standard flat glass absorbs all UVC.
Again fortuitously, UVC is mostly absorbed by dead skin, so erythema can be limited. In addition, UVC does not penetrate to the eye’s lens; nevertheless, conjunctivitis can occur and though temporary, it is extremely painful; the same is true of erythemal effects.
How does UVC destroy germs?
The high energy from short wavelength UVC light is absorbed in the cellular RNA and DNA, damaging nucleic acids and preventing microorganisms from infecting and reproducing.
UVC is strongly absorbed by RNA and DNA bases leading to molecular structural damage via a photodimerization process. This results in virus inactivation, such that they are no longer able to replicate.
The amount of inactivation is directly proportional to the UVC dose, which is received, and this, in turn, is the result of its intensity and duration of exposure. The farther away you keep an object from the light source, the less UVC will reach the target, so only a quarter of the UVC remains when the distance doubles.
The UV light emitted by a source is expressed in watts (W) and the irradiation density is expressed in watts per square meter (W/m2). For germicidal action dose is important. The dose is the irradiation density multiplied by the time (t) in seconds and expressed in joules per square meter (J/m2). (1 joule is 1W.second).
The most efficient source for generating UVC is the low-pressure mercury discharge lamp, where on average 35% of input watts is converted to UVC watts. The radiation is generated almost exclusively at 254 nm viz. at 85% of the maximum germicidal effect and 80% on IES curve.
Radiation wavelengths below 240 nm form ozone, O3 from oxygen in the air. Ozone is toxic and highly reactive; hence precautions have to be taken to avoid exposure to humans and certain materials.
Good brands in the market like- Philips’ low-pressure tubular fluorescent ultraviolet (TUV) lamps have an envelope of special glass that filters out ozone-forming radiation. Hence, Ozone is almost not formed.
Unlike other techniques, UVC photolysis rarely produces potentially dangerous by-products. Air has a low absorption coefficient and hence allows UVC to attack micro-organisms present
UVC radiation has been found very effective on Coronavirus family. Of course, every method of sterilization has its pros and cons.
A few are listed in the table below:
|Pros for UV Sterilization||Cons for UV Sterilization|
|It is convenient to use and no chemicals are needed. Therefore, you won’t leave any chemical residue behind.||UVC is dangerous to humans. Remember to avoid direct exposure to UVC, especially skin and eyes.|
|It is able to kill all kinds of microorganisms, including drug-resistant bacteria, fungi, viruses, spores||It has a major limit! UV only works in its light path and can be blocked by objects. Make sure that the object you want to sterilize, is kept directly in line with the UV light. You can minimize this issue by using multiple UV bulbs to generate UV irradiation from different angles.|
Since UVGI has been proven technique without any residue and quicker option especially between patients, ‘UVC Tower’ – consisting of UV C source was conceptualised.
UVC Tower has 2 UVC tube light of 40 watts each fitted vertically. UVC Tower is on the wheels and hence can be moved anywhere. Typically, you can move it to all corners of the room and achieve comprehensive disinfection.
How to avoid human irradiation from the UVC?
To avoid human irradiation with UVC, a delayed timer has been added. It can be set from 10 sec- 1min. The operator can switch on UVC Tower with pre-decided timer settings and can comfortably move out of the operatory. E.g. You set 15-sec delay onset. Switch on the unit. And move out of the operatory. UVC Tower will switch on only after 15 sec pre-set delay giving enough time to the operator to move out of the operatory.
We can preset timing of UV irradiation. While some of the bacteria, fungi may take several minutes to get inactivated, coronaviruses take about 2- 3 minutes. Now, this can be the major advantage over other methods especially while disinfecting the operatory between 2 patients as one can start with the next patient within minutes saving vital clinical time!
Since UV C Tower is on the wheels, it can be moved to different corners of the room. The same single unit can be used between multiple operatories, passage, waiting-reception area, consultation area disinfection.
There are numerous studies which show that UV C can be very effectively used to sterilise PPE including coverall, N 95 masks!
What is the advantage of Dontics UVC Tower over rooftop UV light fitting?
Inverse Square law:
Ultraviolet light is an electromagnetic wave. 254 nM wavelength light (253.7, more specifically) has inherent energy, which would have an impact on whatever it touches. Just like any other light, even UVC light gets dimmer and dimmer, the farther you move away from the source. The reduction in intensity or power-drop happens very quickly. The simple formula for how fast this energy drops is calculated by power divided by the resultant quantity of distance away from the source raised to the second power (Ed = P/(d^2). Take a look at the graph. Every UVC system has to deal with this rule of physics.
Starting with a "100%" energy value, the energy at four feet is, incredibly, reduced to "6.3%" of the original energy! At ten feet, the energy is only 1% of the initial energy. The key point is that distance matters. The best results of UVC energy happen in fairly close proximity to the target object(s). So, key point one: "Get the dose close."
Rooftop fittings are fixed at one place. Hence radiation does not reach in shadow areas effectively compromising on disinfection.
Distance is an extremely important factor in the efficiency of UV sterilisation. As the distance doubles, radiation weakens to only 1/4th energy. The typical height of a room is 9-10 feet. So a UVC tube source close to the roof will have to cover min. 9-16(diagonal to opposite floor corner of the room) feet of distance.
Typical 40-watt UVC tube source has sufficient radiation up to 6 feet to kill organisms. A central standing UVC Tower can easily irradiate 12x12 feet room very effectively!
Other sources like 20-watt tube can irradiate only up to 2.5 feet and lesser power UVC sources can be effective only for a few inches.
Rooftop tubes have a throw of only 180 degrees losing energy on the wall. UV light loses it’s energy drastically when it hits the wall or any surface and has poor reflection properties for most of the materials. (except aluminium, Chromium etc.) However, both UVC Tower light sources have 240-degree throw giving complete 360-degree powerful coverage.
Also, the dental chair is usually closer to the centre of the clinic around which can be maximum viral load. Keeping Dontics UVC Tower close to the chair gives a maximum dose of UVC radiation in the most required area.
What if I keep the weaker UV source for a longer period?
Long hours exposure of mild UVC can reduce the life of plastics, mica and many other products. Dark coloured things may lose/fade their colour and whiter things get yellower.
UVC Tower is a quick method and hence saving on precious furniture and dental equipment
How do you calculate irradiation time for SARS CoV-2?
We do not know the exact irradiation time needed for SARS CoV-2. However, here is a table for other types of coronaviruses having the potential to infect humans
Table: Summary of UV light studies on Coronaviruses
|Microbe||D90 dose (exposure) required||Source|
|Coronavirus||7 J/m2||Walker 2007|
|Berne virus (Coronaviridae)||7 J/m2||Weiss 1986|
|Murine Coronavirus (MHV)||15 J/m2||Hirano 1978|
|Canine Coronavirus (CCV)||29 J/m2||Saknimit 1988|
|Murine Coronavirus (MHV)||29 J/m2||Saknimit 1988|
|SARS Coronavirus CoV-P9||40 J/m2||Duan 2003|
|Murine Coronavirus (MHV)||103 J/m2||Liu 2003|
|SARS Coronavirus (Hanoi)||134 J/m2||Kariwa 2004|
|SARS Coronavirus (Urbani)||241 J/m2||Darnell 2004|
The table above summarises the results of studies that have been performed on Coronaviruses under ultraviolet light exposure, with the specific species indicated in each case. The D90 value indicates the ultraviolet dose for 90% inactivation.
So even if we compare SARS CoV-2 to highest D90 dose of SARS coronavirus (Urbani),
Desired direct exposure time in seconds is:
Exposure_time = Desired_UV_dose (multiplied by) 4 (multiplied by) pi (multiplied by) (UV_bulb_distance)^2 )/ UV_bulb_power
So, Exposure time= 241 J/m2 (x) 4 (x) 3.14 (x) 1.8 /80 = 68 seconds.
Even if you consider air resistance, tube curvature energy loss, tube temperature and other factors are taken into consideration, it is wise to double irradiation time to 136 seconds i.e. close to 2.5 minutes should be good enough to eradicate most of SARS CoV-2. I am talking about only SARS CoV-2 here. Many other organisms may need a higher dose of UVC radiation, which is not a topic of discussion in this article.
Advantages of DONTICS UVC Tower:
-Quick 2-3 min powerful UV disinfection
-Movable trolley. So reaches to all corners of the clinic. Only one machine for all operatories, reception, lobby, consultation room, pantry etc!
- No residue, no mess. Easy to stack it in a corner
- A delayed timer to avoid any human exposure
-Variable Timer for various needs
-Buzzer signal at the end of the session
We all have to learn to live in this post COVID era until we have effective mass-scale vaccination or herd immunity in place. We have to take extra precautions and DONTICS UVC Tower can be of great help to minimise the viral load from our operatories especially between 2 patients saving precious clinical time without any side effects and appears to be the invisible solution to the invisible pandemic!!
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Dr Ajay Bajaj completed his BDS from GDC and MDS from Nair Hospital Dental College, in 2002, both from Mumbai University. He was awarded 'The best postgraduate student National award' for his dedication and innovations then. Micro-endodontist by profession, Dr Ajay has worked with almost all organisations. He is Co-founder of Endo-scope, the endodontic training society. He has trained 26,000+ dentists in more than 13 countries in 390+ lectures, courses. He has numerous publications in his name. His innovative nature has always helped dentists in the profession to solve the problems.