Ultraviolet germicidal irradiation (UVGI) is a well established and reliable means of disinfection and has proved to be an effective method to prevent the spreading of many infectious diseases. UVGI treatment involves exposing contaminated space to radiation from UV lamps. Shorter wavelength light-emitting Low-pressure mercury (Hg) discharge lamps are commonly used for UVGI applications. These lamps emit shortwave ultraviolet-C (UV-C, 100–280 nanometer [nm]) radiation, primarily at 254 nm. Upon irradiation at a specified dosage and duration, the UV light penetrates the organism's cell walls and disrupts their deoxyribonucleic acid (DNA), making reproduction impossible. Based on the previous research data the optimum UV wavelength range to destroy bacteria is between 250 nm and 270 nm. UVGI is employed in applications such as air water disinfection, surface, and material disinfection.

Ultraviolet (UV) radiation covers the wavelength range of 100–400 nm, which is a higher frequency and lower wavelength than visible light. UV radiation comes naturally from the sun, but it can also be created by artificial sources used in industry, commerce and recreation.

The UV region covers the wavelength range 100-400 nm and is divided into three bands:

  • UVA (315-400 nm)
  • UVB (280-315 nm)
  • UVC (100-280 nm).

Even from the earlier days, It was a well-known fact that electromagnetic radiation or sunlight has some effect on microorganisms. During the year 1877, two researchers Arthur Downes and Thomas Blunt[1-3] in their studies found that, upon exposure to sunlight, a test tube containing Pasteur’s solution showed resistance to bacterial multiplication. Their study also mentions that an increase in exposure duration resulted in maintaining the test tube bacterial free for several months. This study was considered to be a breakthrough in Light-based disinfection method research and also called this as “one of the most influential discoveries in all of photobiology” in an article published by Hockberger in 2002 [4].

Downes and Blunt continued their research and performed tests to unravel the underlying mechanism of bacterial growth inhibition under sunlight. Although their experiments did not point out the exact cause, they put forward valid suggestions. According to them the sun’s radiation might destroy an inside molecule or might trigger a chemical reaction which results in the formation of a toxic molecule or both phenomena happening together. They arrived at this conclusion based on the observation that the sunlight and oxygen must exist together in order to inhibit the bacterial growth. Dowens and Blunt also investigated the influence of factors such as dependent on intensity, duration, and wavelength on bacterial neutralization. John Tyndall, another researcher, successfully reproduced Dowen’s and Blunt’s experiments and confirmed the results in 1881 [5,6].

James Jamieson [7], an Australian bacteriologist- during the year In 1882, had a different argument, He argued that the underlying mechanism behind bacterial growth inhibition is due to the elevated temperature and not because of any chemical reactions. The basis of his argument was based on the results he obtained while experimenting with Bacillus terms. In his study, direct heating of the bacterial solution to around 51 degrees celsius and the solution put under sun radiation for 3-4 hrs, yielded almost similar results and both showed diminishing bacterial growth. But other researchers after carefully examining this new argument came up with proof that even though the sun rays consist of heat-producing infrared rays, they are not the main reason behind bacterial growth inhibition or toxicity.

By the end of 1882, it became clear that the inactivation of a microorganism or any fraction of it depends on the radiation exposure parameters such as radiation intensity (W•m2 ), exposure duration (s) and wavelength of received radiation. After these discoveries researchers focussed their attention on decoding the wavelength dependence of the germicidal action of light.

Crucial results were obtained from a study conducted by Marshall Ward during the period 1892-1895 [8-11] on wavelength sensitivity and response. The experiment results proved beyond any doubt that the rays responsible for damaging the bacterial cells are of shorter wavelengths ie.,UV-violet-blue. In his study the Inhibition of bacterial growth happened throughout the UV-violet-blue region with a sharp cut-off at the borderline between blue and green light was observed. And also the study did not show evidence of any effect in any other region of the visible or the infrared spectrum.

Another study done in 1904 by Hertel [12,13] established the effect of wavelength and dosage on pathogens and bacteria. The growth inhibition effects were inversely proportional to the wavelength and directly proportional to the dose of radiation. (As an example, bacteria were killed after only ten seconds with 210 nm irradiation at relatively low intensity, whereas death required a 10-fold greater dose at 232 nm). He ordered the spectral regions based on the effectiveness of their germicidal action. The region of greatest effectiveness was found to be the UV-C (100–280, nm), UV-B (280–315 nm), UV-A (315–400 nm), visible (400–700 nm), and infrared (700–106 nm).

Below table summarises the timeline of inventions and discovery including key events in UVC technology research.

YearKey events
Downes and Blunta discover the ability of sunlight to prevent microbial growth. It is later shown that the ability of light to inactivate microorganisms is dependent on the dose (intensity 3 time) and wavelength of radiation and the sensitivity of the specific type of microorganism
1930 Gatesb publishes the first analytical bactericidal action spectrum with peak effectiveness at 265 nm, very near the 254 nm output of low-pressure Hg germicidal lamps.
1933 Wellsc presents the concept of airborne infection via “droplet nuclei”—evaporated droplets containing infectious organisms that can remain suspended in the air for extended durations.
1935 Wells and Faird demonstrate the ability of UVGI to efficiently inactivate airborne microorganisms and prove the concept of infection via the airborne route.
1937 Wells et al.e use upper-room UVGI to prevent the epidemic spread of measles in suburban Philadelphia day schools where infection outside the school is unlikely.
1940s to 1950s    Several studiesf,g are unable to reproduce Wells et al.’s success in using UVGI to prevent the spread of measles in schoolchildren, contributing to the disillusionment with and abandonment of UVGI for air disinfection. These failures have since been attributed to infections occurring outside the irradiated schools.
1956–1962Rileyh exposes guinea pigs to air originating from an occupied TB ward and proves that TB is spread via the airborne route. A group of guinea pigs receiving infected air via a UVGI irradiated duct were not infected, while a group receiving air via a non-irradiated duct were infected. 
1969–1972Riley and colleaguesi–l conduct model room studies evaluating the use of upper-room UVGI to reduce the concentration of aerosolized test organisms in the lower room. They also show that air mixing between the upper and lower room is imperative for effective disinfection and confirm that UVGI is less effective at high humidity. 
1974–1975Riley et al.m determine virulent tubercle bacilli and BCG to be equally susceptible to UVGI and measure the disappearance rate of aerosolized BCG in a model room with and without upper-room UVGI. Upper-room UVGI is shown to be highly effective in disinfecting the lower room, quantitatively demonstrating the potential of upper room UVGI to reduce TB infection.
1985–1992After decades of decline, there is an unexpected rise in TB in the United States, leading to a renewed interest in UVGI for air disinfection.
1990s to presentNew in-depth efforts are undertaken, aimed toward quantitatively examining UVGI efficacy and safety and providing guidance for the proper use of UVGI
Table referred from “Nicholas G. Reeda, The History of Ultraviolet Germicidal Irradiation for Air Disinfection” Public Health Reports / January–February 2010 / Volume 125

Effect of temperature and humidity 

K. H. Chan, in 2011[19], conducted studies on the effect of temperature and humidity on SARS Coronavirus proved environmental parameters that can favor the virus spread. In the study the dried virus on smooth surfaces retained its viability for over 5 days at temperatures of 22–25°C and relative humidity of 40–50%, that is, typical air-conditioned environments. However, virus viability was rapidly lost (>3 log10) at higher temperatures and higher relative humidity (e.g., 38°C, and relative humidity of >95%). 

COVID-19 Coronavirus Ultraviolet Susceptibility

The global pandemic causing viruses , commonly known as SARS-CoV2 and MERS-CoV possesses a distinctive morphology and protein structure. This viruses come under the group of enveloped viruses with nonsegmented, single-stranded, and positive-sense RNA genomes. From the family of coronaviruses, six coronaviruses have been known to infect human hosts and cause respiratory diseases. Among them, severe acute respiratory syndrome coronavirusSevere Acute Respiratory Syndrome Coronavirus (SARS-CoV2) and Middle East respiratory syndrome coronavirus (MERS-CoV) has been identified as highly pathogenic and are responsible for the global pandemic situation happening around the world. The transmission occurs mainly through droplet (direct or close contact with an infected individual) and contact transmission (when an infected individual leaves viral particles on a surface or fomite) [14].

The transmission through droplets can be avoided by practicing social distancing, but it is very difficult to control the spread through surface or material contacts. Since the exchange of materials/goods/ packaged items is unavoidable in daily life it becomes very crucial to address this issue. A recent study [15] done on transmission of SARS-CoV2 through surfaces of plastics and cardboards found that remnants of the viruses were found to live on cardboard for over 24 hours and on plastic surfaces for 3 days. At present around the world a series of research has been initiated to address the issue of surface contact transmission. Sterilization methods such as ultraviolet germicidal inactivation (UVGI)alcohol based solutions, solution fogging, etc are being extensively investigated. Among various other techniques UVGI method has emerged as one of the best and easiest way to disinfect surfaces. Excitation with UV rays can induce mutations inside microorganism cell structure and that can ultimately inactivate the organism. UVGI is currently used for mass disinfection for office spaces, public spaces, public vehicles, aeroplane cabins, hospitals, parcels and courier items, N95 respirator disinfection and many more. Since SARS-CoV2 is a new strain which came into light only in 2020, there have been no in depth studies reporting the inactivation effect of UV-C radiation on the virus. However after 2002 SARS outbreak which was caused by another strain of virus from the same family known as SARS-CoV1 was studied and many literatures are available with proofs and findings on effect and nature of UV-C inactivation of SARS-CoV1.

In a research paper published in 2020 [16] april by Paolo Arguelles (Cornel university) titled “Estimating UV-C Sterilization Dosage for COVID-19 Pandemic Mitigation Efforts” Paolo compared SARS-CoV1 and SARS-CoV2 and established genomic relation between the two. With the help of mathematical curve fitting techniques to existing literature on SARS-CoV-1, an equation is proposed to determine the amount of exposure time required to effectively inactivate the novel coronavirus given UV-C bulb wattage and distance.

Also he inferred that the two pathogens SARS-CoV1 and SARS-CoV2 are close genomic relatives with 79% sequence identity [10], suggesting that the strains have comparable UV-C inactivation responses. The calculated dimerization probabilities of the two coronavirus strains were found to be effectively identical, differing by only 1.48%. These results suggest that SARS-CoV-1 is a sound proxy for SARS-CoV-2 in the context of UV-C inactivation.


For a given UV-C dosage D (expressed in J/m2) viral concentration decays exponentially as a function of time. Useful parameters such as applied dosage can be numerically extracted from experimental data by fitting the survivorship curve to a decaying exponential with the form:

S = e−KIt ------------------ (1)

For a given UV-C dosage D (expressed in J/m2) viral concentration decays exponentially as a function of time. Useful parameters such as applied dosage can be numerically extracted from experimental data by fitting the survivorship curve to a decaying exponential with the form:

S = e−KIt ------------------ (1)

Combining (1) and (2) yields the simplified relation:

D = It

This numerical method is applied to multiple studies to extract the approximate dosages for various stages of inactivation

Miriam E.R. Darnell in his study conducted in 2004 on SARS-CoV deactivation using UV-C found that Exposure of virus to UVC light resulted in partial inactivation at 1 min with increasing efficiency up to 6 min, resulting in a 400-fold decrease in infectious virus. In contrast, UVA exposure demonstrated no significant effects on virus inactivation over a 15 min period. And the experiment data show that UVC light inactivated the SARS virus at a distance of 3 cm for 15 min.

Summary of UV light studies on Coronaviruses

Dosage and Exposure time

The dosage values mentioned in the above table are obtained from the various studies performed on Coronaviruses under UV-C light source. The table also shows the type of species observed and corresponding exposure dosage. 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 x 4 x 3.14 x (UV source 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.

The above mathematical estimation is based on the article published by Dr Ajay Bajaj [18] in his article titled “How to use Ultraviolet light (UVC) to fight COVID-19 effectively in dental clinics”

Comparison table for various UV-C sources

Mercury UV lamps can be selected for sterilization devices that are supposed to run continuously. These lamps offer low cost and high power. When compared to LEDs mercury lamps require warm-up time and it is not suitable for on-off cyclic operations. Cyclic on and off operations can drastically reduce the lamp’s service life to no more than a year.

UVC LEDSs offer instant, high intensity light output that can be cycled on and off to provide immediate sterilization action. LEDs have 5-10 years of service life without affecting its performance. Appropriate cooling fins must be provided to efficiently remove the heat generated inorder to improve the performance and life. So long term cyclic operations LEDs stands ahead of the lamps

Limitations of UVGI

Although UV-C method of disinfection is chemical free and a convenient method, it is dangerous for healthy human cells. It processes severe health risk to eyes and skin cells. As a number of studies beyond any doubt have proved that UVC can disinfect surfaces, there are significant concerns in few applications like disinfection of N95 respirators, textiles and garments. In such applications the shadowing effect of fibers can hinder with complete disinfection. Which implies that the UV only works when the surfaces are directly exposed and if the light paths get blocked by objects, it is advised to use multiple bulbs at different angles to minimise the shadowing effect.
UVC LEDSs offer instant, high intensity light output that can be cycled on and off to provide immediate sterilization action. LEDs have 5-10 years of service life without affecting its performance. Appropriate cooling fins must be provided to efficiently remove the heat generated inorder to improve the performance and life. So long term cyclic operations LEDs stands ahead of the lamps


UVGI is an effective method of disinfection and with clear research data it is a convenient and smart weapon against SARS-CoV-2. Even Though there is no solid proof of 100% disinfection or deactivation of the Covid 19 virus yet, based on the few studies done in 2020 promises a good amount (approximately 9-99%) of deactivation of the viruses. While developing a UV-C based disinfection system one should consider the danger effects of UVC and the shadow effects and factors that can reduce the disinfection rates. By Carefully setting the design factors such as the, pro[er wavelength, dosage, exposure time, placement of bulbs/LEDS at suitable angles to minimise the shadow effect, a good sterilization equipment can be manufactured which will help prevent the spread of viruses through surface contacts.


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