Ultraviolet Disinfection

L ethal to microorganisms, ultraviolet radiation in the range of 2250 to 3020 angstroms is used in a variety of disinfection applications , a process referred to as ultraviolet germicidal irradiation (UVGI). Since the first UVGI system was successfully implemented for disinfecting the municipal water system in Mar-seilles, France, 1 in 1909, the disinfec-tion of medical equipment using UVGI has been a common and reliable practice. But unlike water-and equipment-disinfection applications, the disinfec-tion of air streams using UVGI has a history of varying success and unpredictable performance. The first laboratory studies on UVGI of air in the 1920s showed such promise that the elimination of airborne disease seemed possible. In 1936, Hart used UVGI to sterilize air in a surgical operating room. 2 In 1937, the first application of UVGI for a school ventilation system dramatically reduced the incidence of measles, with subsequent applications enjoying similar success. 3 Experiments by Riley and O'Grady 4 resulted in the elimination of tuberculosis (TB) bacilli from hospital-ward exhaust air. A plethora of designs that were more imitative than engineered followed these early applications. The result was a mixture of successes and failures. This experience is reflected in various guidelines that decline to sanction the use of UVGI as a primary system. A 1954 study on the use of UVGI showed a failure to reduce disease in London schools. Although limited data are available to determine the causes of earlier design failures, the apparent cloning of UVGI systems without regard to operating conditions probably doomed many installations from the start. A review of current industry practices indicates that information on the design of UVGI systems lacks the detail necessary for engineers to ensure performance. This article addresses the factors that determine the design

Mathematical models of the response of populations of microorganisms exposed to ultraviolet
germicidal irradiation (UVGI) are developed that include two-stage response curves and shoulder
effects. Models are used to develop a C++ computer program that is capable of predicting the
performance of UVGI air disinfection systems. The algorithms are based on models for 1) the
intensity field of UVGI lamps, 2) the intensity field due to UVGI reflective enclosures, and 3) the
kill rate of microorganisms to UVGI exposure as they pass through the modeled intensity field.
The validity of the UVGI lamp model is established by comparison with lamp photosensor data.
The validity of the overall predictive model is established by comparison of predictions with
laboratory bioassays for two species of airborne pathogens – Serratia marcescens and Bacillus
subtilis. First stage rate constants, second stage rate constants, and the defining shoulder
parameters are determined for Aspergillus niger and Rhizopus nigricans based on bioassay data,
and it is shown how predictions using only single stage rate constants can deviate significantly
from predictions using the complete survival curve. A dimensional analysis of UVGI systems
identifies nine dimensionless parameters responsible for determining the effectiveness of any
rectangular UVGI system. A factorial analysis of the dimensionless parameters based on data
output by the program identifies the most critical parameters and the inter-relationships that
determine UVGI system effectiveness. Response surfaces are generated using program output to
illustrate the inter-relationships of the dimensionless parameters. The optimum values of the
dimensionless parameters are summarized that result in optimized performance. Economic
optimization is demonstrated by a series of examples that calculate life cycle costs, and principles
of economic optimization are summarized. Conclusions are presented that will produce more
energy-efficient and effective designs and a proposed model for improved UVGI systems is
presented.

This paper summarizes an improved methodology for predicting the rate of airstream disinfection for UVGI systems that will enable effective designs and lower energy costs. This approach uses radiative view factors to define the three-dimensional intensity field for lamps and reflective surfaces inside enclosures. Lamp photosensor data for a variety of lamps are shown to agree more closely with the view factor model than with models using the Inverse Square Law. The intensity field due to reflectivity from internal surfaces is determined by assuming diffuse reflectivity. An analytical method is used to determine the inter-reflection component of intensity due to multiple internal reflections. The superposition of these components yields a three-dimensional intensity field matrix that can be used to calculate disinfection rates for any given microbial rate constant. Results from laboratory bioassays using S. marcescens in various duct configurations have corroborated model predictions within ±15% in most cases.

Ultraviolet light is mainly divided into three wavelength ranges UV-A (315-400nm), UV-B (280-315nm) and UV-C (100-280nm). Among these, the short-UV (UV-C) is found to have germicidal properties and at the same time is harmless to humans. UV-C light can be produced using tube-lights, smartphone flashlights, torches or any light source by incorporating a biochemical reaction using a fused amorphous silica (SiO2) based dichroic mirror filter accompanied by Ultraviolet Germicidal Irradiation and Diffraction Grating splitting of white beams. This arrangement allows UV-C rays to pass while blocking other higher wavelengths. When such light falls on surfaces, it gets absorbed by nucleic acids of microorganisms. The absorbed energy can result in defects like formation of pyrimidine dimers via a photochemical reaction. Dimerization prevents replication by forming covalent bonds between adjacent bases in the DNA, resulting in the inactivation of the organism. Such apparatus can be used to disinfect hands, groceries, electronic equipment, closed rooms and public places. This review work is intended to analyze this cutting-edge technology and its lucid implementation in everyday life, which is most relevant in today's time. This paper has also scrutinized its potency to eliminate bacteria and novel viruses like SARS CoV-2.

Ultraviolet genomic modeling is a method for evaluating the ultraviolet sensitivity of microorganisms by reading and interpreting the complete microbial genome. Base counting software is used to identify short sequences of amino acids that have a high photodimerization potential. The specific sequences that can result in photodimers include thymine-thymine (TT), thymine-cytosine (TC or CT), cytosine-cytosine (CC), and purine-pyrimidine dimers that form under certain conditions. A mathematical algorithm is used to compute the relative UV sensitivity for each microbe. Correlation of the genomic model with known ultraviolet susceptibilities of over 160 microbes has resulted in a general model that can be used to predict the UV sensitivities of bacteria, RNA viruses and DNA viruses with predictive accuracies over 80% for bacteria and over 90% for viruses. Since current UV bioassays are estimated to have an error range of +/-30%, the genomic model may form a benchmark by which the accuracy of bioassay testing may be gauged. An extensive list of predicted UV sensitivities is presented for microbes that do not currently have published UV susceptibilities, including many nosocomial and zoonotic pathogens and bioweapon agents. Other applications of the model are discussed including the use of the genomic model to assess the survival of microbes in an airstream or water stream via PCR technology. Extension of the ultraviolet genomic model to UVA and UVB applications is feasible and the possibility of applying the model to human skin cancer research is discussed. Further extension of the genomic model to ionizing radiation and potential applications are discussed.

Respiratory droplets are coronaviruses primary transmission mode and thus the majority of coronavirus risk mitigation strategies focus on the control of air. However, surface contact remains an alternative infection route. Hence, it remains a concern that the SARS CoV-2 virus can remain viable on surfaces for several hours. How effective is UV-C light against the coronavirus and other pathogenic viruses of concern? This paper undertakes a detailed review of the research.

Published in the Journal of Validation Technology. Link: https://www.ivtnetwork.com/article/shining-invisible-light-viral-pathogens-virucidal-contamination-control-strategies-using-uv-

This paper presents the results of research on the performance of air-cleaning and air-disinfection systems used for protecting buildings against intentional releases of biological agents. The air-cleaning technologies addressed include dilution ventilation, filtration, and ultraviolet germicidal irradiation. A 40-story commercial office building is modeled using typical occupancy levels and leakage rates for doors, walls, and floors. A steady-state single-zone model is used to predict steady-state conditions resulting from the use of various levels of air cleaning. A transient single-zone model is used to predict transient indoor concentrations from which inhaled doses and estimated casualties are predicted. A transient multizone model is used to evaluate contaminant dispersion and estimate potential casualties. Three design-basis attack scenarios are simulated using various biological weapon agents. Predicted casualties are estimated using an epidemiological model of the dose response curves for each of the agents. The effect of increasing levels of air cleaning is examined under the attack scenarios to evaluate their effectiveness. Results indicate that high levels of protection are possible for building occupants with moderate and affordable levels of air cleaning when filtration is combined with ultraviolet germicidal irradiation. Results also suggest that diminishing returns occur when increased levels of air cleaning are applied. It is hypothesized that the maximum useful size of any air-cleaning system is defined and limited by building physical characteristics alone.

A kinetic study was conducted to account for the changes occurring in color, firmness,
weight loss, and total phenolic content of tomatoes subjected to combined ultrasound
technology (40 kHz, 13.78 W/L) and ultraviolet‐C radiation (639.37 μw/
cm2) treatment for 28 min. Combined treatment delayed ripening, increased firmness,
weight loss, and induced total phenolic content retention of tomatoes compared
to untreated samples. The change in color parameters of hue angle and chroma
was described using the fractional conversion and zero‐order models, respectively.
Fractional conversion kinetic model was also found suitable for firmness degradation,
while the zero‐order and first‐order models were appropriate for change in weight
loss and total phenolic content retention prediction, respectively. Activation energy
for measured parameters ranged from 121.29 to 199.70 kJ/mol at a temperature
range of 33.76–37.12ºC, and showed highest sensitivity to treatment temperature in
reduced ranking for firmness degradation, weight loss, total phenolic content retention,
hue angle, and chroma.
Practical applications
The research provides a new method for postharvest handling of fruits and vegetables.
The method utilizes ultrasound technology and ultraviolet‐C radiation in combination
and finds potential application in industries for shelf life extension during the
storage, processing, and supply chain management of fruits and vegetables. However,
the method may result in undesirable changes like weight loss, physical softening, or
bioactive content degradation. Optimized parameters of the method are essential for
loss minimizations and cost‐effective practical applications. A thorough understanding
of the influence of temperature of the system, treatment time, ultrasound acoustic
energy density, and ultraviolet‐C dosage on the quality properties of tomatoes
and their change kinetics through fundamental approach modeling as presented in
this research, is a step toward optimization of the technology.

A dimensional analysis of an ultraviolet germicidal irradiation (UVGI) air disinfection system within a diffusely reflective enclosure is performed using the Buckingham pi theorem. The eight dimensionless parameters obtained include the duct aspect ratio, the lamp aspect ratio, the reflectivity, and terms that incorporate the UV dose, airflow, duct dimensions, and lamp location coordinates. Computer simulation of the dose absorbed by an airborne microbe is used to predict system performance for several thousand combinations of the dimensionless parameters. Statistical analysis of these results is performed to determine the significance of each of the dimensionless parameters and their interactions. Conclusions regarding performance are summarized. The most significant parameters that determine performance of diffusively reflective UVGI systems are shown to be the dose, the reflectivity, the duct geometry, the lamp aspect ratio, lamp location, and various combined functions of these parameters. Practical implications of this research may include the improvement or optimization of UVGI air disinfection systems for induct and recirculation unit applications, with a consequent improvement in energy efficiency.