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Introduction to Ultraviolet Germicidal Irradiation (UVGI)

What is UV light?


Ultraviolet (UV) light is an invisible light with a wavelength shorter than that of visible light, but longer than x-rays. This means electromagnetic waves with a wavelength between 400 nm to 10 nm. UV light is more energetic than visible light and has a shorter wavelength, letting it penetrate more readily through obstacles.

 

Where UV light can be found?


There are many sources of UV light including the natural source and man-made sources, e.g. black light, UV lamp, UV LEDs, UV laser, etc. However, the sun is considered as the most important and primary source of ultraviolet light.

 

Naturally, the sun emitted ultraviolet radiation in its sunlight spectrum combining with many other wavelengths of light spectrum including radio waves, infrared, visible light, x-rays, gamma rays and cosmic rays. The Earth's ozone layer blocks 98.7% of this UV radiation from penetrating through the atmosphere. 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA.

 

How many subtypes are there in UV light?


UV light can be divided into three subtypes based on its ranges of wavelength, which are UV-A, UV-B and UV-C. UV-A band is beginning from 320 to 400 nm of wavelength. It is 280-320 nm for UV-B band and 200-280 nm for the UV-C, the shortest wavelength band of UV light.

 

UV-A, the long wave UV, as known as black light, is responsible for skin tanning and applied in medicine to treat certain skin disorders. It is the most part of UV light that reaches the Earth's surface.

 

UV-B, a small, but dangerous part of sunlight, is mostly absorbed by the Earth's atmospheric ozone layer. Prolonged exposure could result in unhealthy effects on the skin and eyes.

 

UV-C, the short wave UV, which includes optimal germicidal ultraviolet at 253.7 nm wavelength, is used for air, surface and water disinfection. However, exposure to UV-C causes skin redness and eye irritation and, therefore, this should be any precaution sign when UV-C light is applied in occupied areas.

 

What is UVGI?


UVGI stands for Ultraviolet Germicidal Irradiation, is defined as the use of ultraviolet (UV) wavelengths of light in the germicidal range (200-300 nm) for the disinfection of air and surfaces. Microbes are uniquely vulnerable to the effects of light at wavelengths at or near 2537 Angstroms (253.7 nanometers) due to the resonance of this wavelength with molecular structures. Looking at it another way, a quanta of energy of ultraviolet light possesses just the right amount of energy to break organic molecular bonds. This bond breakage translates into cellular or genetic damage for microorganisms. The same damage occurs to humans, but is limited to the skin and eyes.

 

This UVGI system, equipping with certain germicidal ultraviolet light (UVC at 253.7 nm) and well-engineered sufficient dosage of UVC exposure, can effectively kills germs, such as bacteria, viruses, mold, fungi and spores that transmit infections, cause allergies, trigger asthma attacks or cause other unhealthy effects.

 

Brief History of Ultraviolet Disinfection


The story of ultraviolet light begins with Isaac Newton and his contemporaries. In 1672, Isaac Newton published a series of experiments with prisms that resolved sunlight into its constituents colors, red through violet. The effects of sunlight on man, microorganisms, and chemicals become a matter of treat interest and experimentation in the 1800s. However, it was not until 1900s that the first quartz lamp for UV was developed and UV drinking water disinfection was implemented in 1906.

 

By the 1950s it had been well established that UV irradiation was effective at disinfecting both air and surfaces, and new engineering applications were being developed. Furthermore, bacterial growth on cooling coils had been recognized ad a potential health problem as early as 1958. Grun and Pitz detailed the first UVGI system, designed specifically for disinfecting the surfaces of air handling equipments, in 1974. Many applications including the hospital applications in which the UV lamps are specifically placed upstream of the cooling coils and downstream of the filters.

 

Although UVGI systems had been in use in hospitals since 1930s but it wasn't until 2003, that the Centers for Disease Control (CDC) formally acknowledge that UVGI systems were effective and could be used in hospitals. In the same year, the influential American Society of Heating Refrigerating and Air Conditioning Engineers (ASHRAE) formed a task group to focus on UV air and surface treatment, which become the standing Technical Committee TC 2.9 in 2007.

 

Year

Event

Reference

1892

UV component of sunlight identified as biocidal

Ward (1892)

1904

First quartz lamp for UV developed

Lorch (1987)

1906

UV first used to disinfect drinking water

Recklinghausen (1914)

1909

First European applications for UV water disinfection

AWWA (1971)

1916

First USA applications of UV for water disinfection

AWWA (1971)

1932

UV germicidal peak at 253.7 nm isolated

Ehrismann (1932)

1936

First overhead UV system in hospitals

Wells and Wells (1936)

1937

First upper air application in schools

Wells (1938)

1954

First air conditioner application

MRC (1954)

1994

CDC acknowledges UV effectiveness for Tuberculosis control

CDC (2005)

1999

WHO recommends UVGI for TB control

WHO (1999)

2003

CDC formally sanctions UVGI use in hospitals

CDC (2003)

2003

ASHRAE forms UV air and surface treatment committee

Martin et al. (2008)

2005

Federal government specifies UV for cooling coil disinfection

GSA (2003)

 

What's the problem?


Airborne contaminants including bacteria, viruses, fungi, spore and surface mold colonies exist in today's indoor air environment, accounting for indoor pollution. Furthermore, in order to achieve energy saving, no matter for heating or cooling purpose, buildings are designed to have containment of indoor air inside, which makes ventilation much less than outside and results in many times greater of air pollution than the outdoor.

 

The coils and air ducts of HVAC (heating, ventilation and air conditioning) systems often are the incubators or breeding grounds for countless invisible pathogenic, allergenic and toxic contaminant causing allergies, asthma as well as other respiratory related health problems.

 

Germicidal ultraviolet light, at certain wavelength, is able to help keeping the indoor environment healthier and the HVAC systems clean, in top performance shape and realizing savings for medical expenses. Additionally, unlike the physical HEPA (high efficiency particulate air) filter, installation of this UVGI system will save your electronic and maintenance bills since the pressure drop is reduced.

 

UVGI and its applications


UVGI systems can be categorized into three parts, which are air, surface and water disinfection. Many sub-systems are shown in the table below.

 

Classification 

Systems 

Air disinfection

In-duct air disinfection

 

Recirculation units

 

Upper room systems

 

UV barrier systems

 

Overhead tank disinfection

Surface disinfection

Equipment & packaging disinfection

 

Cooling coil disinfection

 

Lower room disinfection

 

Overhead surgical site disinfection

 

Area/room disinfection

 

Food surface disinfection

 

Hand wands

 

Mold remediation systems

 

These systems above can be elaborated into simple applications below.

 

 

  • Direct air and surface disinfection by uncovered UV lamps, which irradiate air and surfaces in direct view of the lamp.
  • Direct air and surface disinfection by uncovered UV lamps, which irradiate air and surfaces in direct view of the lamp with reflector to intensity and direct the UV light to a specific area.
  • Indirect air disinfection by enclosed UV lamps, which irradiate the air that passes through the UV system or chamber drawn by a fan or the natural air convention.
  • HVAC coils surface disinfection by UV light. Keeps the coil clean of microbial growth. In 2005, Federal government specifies this application for cooling coil disinfection
  • HVAC air duct disinfection by UV light. High output UVGI systems are engineered for the cold and high velocity air in the HVAC air ducts which helps improving indoor air environment.
  • In-room UV fixtures: direct air and surface and indirect air irradiation for operating rooms, laboratories, offices, schools, etc.
  • Portable UV: portable, recirculation, units, easy to carry to different locations. Designed for single room air disinfection. Utilize fans for air circulation and enclosed UV lamps.
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    Factors relating UVGI system effectiveness


    Laboratory tests have achieved extremely high rates of mortality under idealized conditions. In actual applications, many factors can alter the effectiveness of UVGI, including following:

     

     

  • Exposure time (the air velocity must allow for a sufficient dose).
  • Room air mixing (for non-powered applications like upper units).
  • Power levels.
  • The presence of moisture or particulates provides protection for microbes.
  • Dust settling on light bulbs can reduce exposures. Maintenance is necessary.
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    One especially effective application of UVGI is the control of microbial growth in air handling unit cooling coil and filter assemblies. The constant exposure has been found to be very effective at controlling fungal growth, either because the spores are inactivated, or perhaps because mycelia growth cannot be sustained under continuous exposure.

     

    Certain types of UVGI designs seem to provide a much higher rate of disinfection than standard models operating at nearly identical spectrums, the difference being the result of improvements in the electrical power controls and regulation of internal plasma temperature, resulting in the generation of a more constant energy density at a distance from the light source.

     

    Viruses are especially susceptible to UVGI, more so than bacteria, but are also very difficult to filter. Some studies have shown that viruses are more sensitive to ultraviolet radiation at wavelengths somewhat above the normal UVGI broadband wavelength of 2537 A (Rauth 1965; Setlow 1961). A combination of filtration for bacteria and spores, with UVGI for viruses may be an optimum combination if all components are sized appropriately.