Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses ultraviolet (UV) light, especially UV-C light (180–280 nm), to kill or stop germs. UVGI works mainly by breaking the DNA of microorganisms, which stops them from performing necessary life processes.

UVGI is used in many areas, such as cleaning food, surfaces, air, and water. UVGI devices can stop harmful germs like bacteria, viruses, fungi, molds, and other disease-causing organisms. Recent research has shown that UV-C light can also stop SARS-CoV-2, the virus that causes COVID-19.

UV-C light has different effects depending on its wavelength. Many germicidal lamps, such as low-pressure mercury (LP-Hg) lamps, emit UV light around 254 nm, which can be dangerous to humans. Because of this, UVGI systems are mostly used in places where people are not directly exposed, like hospital surface cleaning, upper-room UVGI, and water treatment. Recently, wavelengths between 200 and 235 nm, called far-UVC, have been used more often for surface and air disinfection. These wavelengths are safer because they do not penetrate human tissue as much. Their effectiveness comes from damaging DNA in two ways: by creating pyrimidine dimers and causing oxidative damage through DNA photoionization.

It is important to note that UV-C light from the sun does not reach Earth’s surface because the ozone layer in the atmosphere absorbs most of it.

History

The development of UVGI began in 1878 when Arthur Downes and Thomas Blunt discovered that sunlight, especially its shorter wavelengths, slowed the growth of microbes. Later, in 1885, Émile Duclaux found that different bacteria reacted differently to sunlight. In 1890, Robert Koch showed that sunlight could kill Mycobacterium tuberculosis, suggesting UVGI might help fight diseases like tuberculosis.

Studies in the 1890s identified which wavelengths of UV light were most effective at killing germs. It was found that the UV part of sunlight had the strongest germ-killing power. Research showed that UV-C was more effective than UV-A and UV-B.

In 1914, scientists observed that UV light caused changes in Bacillus anthracis when exposed to low doses. In the late 1920s, Frederick Gates measured how UV light killed Staphylococcus aureus and Bacillus coli, finding that 265 nm was most effective. This matched the way nucleic acids absorb light, suggesting DNA damage was the main reason bacteria were killed. By the 1960s, research confirmed that UV-C caused thymine dimers in DNA, which stopped microbes from surviving. These discoveries helped create modern UVGI for disinfection.

UVGI was first used for air disinfection in the 1930s. In 1935, William F. Wells showed that UV light at 254 nm could quickly kill airborne bacteria like B. coli. This built on earlier ideas about how germs spread through the air. Before this, UV studies focused on germs in liquids or solids, not in the air.

In 1936, high-intensity UVGI was used in a hospital operating room at Duke University. It reduced post-surgery infections from 11.62% without UVGI to 0.24% with UVGI. Later, UVGI "light curtains" were used in hospitals and infant wards to stop respiratory infections.

UVGI methods changed from "light curtains" to upper-room UVGI, which placed UV lights above people’s heads. Though it needed good airflow, it worked well in preventing infections. Between 1937 and 1941, Wells used upper-room UVGI in schools to stop measles. In schools without UVGI, 53.6% of students got sick, but only 13.3% did in schools with UVGI.

Richard L. Riley, a student of Wells, studied airborne infections and UVGI in the 1950s and 1960s. His experiments in a hospital TB ward showed UVGI could kill airborne germs and stop tuberculosis spread.

Despite early success, UVGI use dropped in the late 20th century due to better infection control methods, inconsistent results, and safety concerns. However, recent issues like drug-resistant bacteria and the COVID-19 pandemic have renewed interest in UVGI for air disinfection.

UV light was first used to disinfect drinking water in 1910 in Marseille, France. A prototype plant closed after problems with reliability. In 1955, UV systems were used in Austria and Switzerland. By 1985, about 1,500 UV water plants operated in Europe. In 1998, scientists found that protozoa like cryptosporidium and giardia were more vulnerable to UV light than thought, leading to widespread use in North America. By 2001, over 6,000 UV water plants were in Europe.

Over time, UV costs have decreased as new methods for water and wastewater disinfection were developed. Many countries, including the US and the UK, have created rules and guidelines for using UV to clean drinking water.

Method of operation

UV light is a type of invisible light with shorter wavelengths than visible light but longer wavelengths than X-rays. UV light is divided into different types based on its wavelength. Short-wavelength UV (UV-C) is called "germicidal UV" because it can kill germs. Wavelengths between about 200 nm and 300 nm are strongly absorbed by DNA and RNA in living things. This absorption can cause damage, such as the formation of pyrimidine dimers. These dimers can stop cells from copying themselves or making needed proteins, which can kill or disable the organism. Recent studies show that these dimers can glow under certain conditions.

• Mercury-based lamps that operate at low pressure produce UV light at a specific wavelength of 253.7 nm.
• UV-C LED lamps produce UV light at wavelengths that can be chosen between 255 nm and 280 nm.
• Pulsed-xenon lamps produce UV light across all UV wavelengths, with the most light near 230 nm.

This process works in a way similar to, but more strongly than, how UV-B light causes sunburn in humans. Microorganisms have less protection against UV light and cannot survive long exposure to it.

A UVGI system is designed to expose areas like water tanks, rooms, and air systems to germicidal UV. This exposure comes from lamps that emit germicidal UV at the correct wavelength, which shines the UV light into the environment. The movement of air or water through the environment ensures that the UV light reaches the air or water.

Effectiveness

The effectiveness of germicidal UV light depends on two main factors: the UV dose, which is the amount of UV light energy that reaches the microbe (measured as radiant exposure), and how sensitive the microbe is to the specific UV light wavelengths, as shown by the germicidal effectiveness curve.

The UV dose is measured in light energy per area, known as radiant exposure or fluence. Fluence is calculated by multiplying the light intensity (irradiance) by the time the microbe is exposed to the UV light.

Irradiance depends on several factors: the brightness of the UV source (radiant intensity), the distance between the source and the microbe, the reduction in light caused by filters (such as dirty glass), the reduction in light caused by the medium (such as cloudy water), the presence of objects that block UV light, and the use of reflectors that bounce UV light through the medium multiple times. If microbes are not free-flowing, such as in a biofilm, they may block each other from receiving UV light.

In 2006, the U.S. Environmental Protection Agency (EPA) published guidelines for UV dose levels used in drinking water treatment. UV dose is not always measured directly but can be estimated using:
– Flow rate (how long the water or air is exposed to UV light),
– Transmittance (how much light reaches the target),
– Turbidity (how cloudy the water or air is),
– Lamp age, fouling, or outages (which reduce UV intensity).

UV bulbs must be cleaned and replaced regularly to maintain effectiveness. The lifespan of germicidal UV bulbs varies by design. Some materials used in bulbs can absorb UV light. Cooling the lamp with airflow can also reduce UV output. UV dose calculations should use the end-of-life (EOL) value, which is the point when the lamp produces 80% of its initial UV output. Some shatter-proof lamps are coated with a material that contains glass shards and mercury if broken, but this coating can reduce UV output by up to 20%.

UV source intensity is often given as irradiance at a distance of 1 meter, which can be converted to radiant intensity. UV intensity decreases with distance, following an inverse square law (it decreases rapidly as distance increases and increases rapidly as distance decreases). Unless the UV dose is calculated exactly 1 meter from the lamp, the intensity must be adjusted for distance. UV dose should always be calculated at the farthest point from the lamp in the target area. Reflecting UV light, such as with aluminum (which has the highest reflectivity), can increase fluence by allowing the same light to pass through the medium multiple times.

In static applications, exposure time can be extended to achieve an effective UV dose. In water or air disinfection systems, exposure time can be increased by enlarging the illuminated volume, slowing fluid or air movement, or recirculating the fluid or air through the UV area multiple times. This ensures more microorganisms are exposed to UV light and resistant microbes are irradiated more than once.

Microbes respond differently to UV light wavelengths, as shown by the germicidal effectiveness curve. For example, E. coli is most effectively killed by UV light with a wavelength of 265 nm, a pattern that applies broadly to most bacteria. For a 90% kill rate of most bacteria and viruses, UV doses between 2,000 and 8,000 μJ/cm² are typically required. Larger parasites, such as Cryptosporidium, require lower doses for inactivation. The EPA has approved UV disinfection as a method to achieve inactivation credits for Cryptosporidium, Giardia, and viruses in drinking water treatment. For a 90% reduction of Cryptosporidium, the EPA’s 2006 guidelines require a minimum dose of 2,500 μW·s/cm².

The term "sterilization" is often incorrectly used to describe UV treatment. While sterilization (complete elimination of all microbes) is theoretically possible in controlled environments, it is difficult to prove. Companies often use the term "disinfection" instead to avoid legal issues. Instead, they may advertise a "log reduction," such as a 6-log reduction (99.9999% effective), which accounts for DNA repair processes (photoreactivation and base excision repair) that microbes can use to repair UV-induced damage.

Safety

UVGI systems often use ultraviolet (UV) light that can be dangerous to humans, causing both short-term and long-term health issues. Short-term effects may include eye damage, such as photokeratitis (sometimes called "snow blindness"), and skin redness (erythema). Long-term exposure may increase the risk of skin cancer.

The safety of UV light depends on its wavelength. Most UV light from the sun includes UV-A and UV-B, but not UV-C. UV-B can reach deep into living tissue and is the most harmful, increasing cancer risk.

Many standard UVGI systems, like low-pressure mercury lamps, emit UV-C light and also some UV-B. This makes it hard to identify which wavelength causes harm. However, longer UV-C wavelengths can still cause eye and skin damage. Because of this, UVGI systems are often used in places where people have limited direct contact with the light, such as in upper-room air cleaners or water disinfection systems.

To protect users, UVGI systems include safety measures:
– Warning labels: These inform users about UV dangers.
– Interlocking systems: Devices like closed water tanks or air units have automatic shut-off features if the system is opened. Some systems also have UV-blocking viewports.
– Protective equipment: Safety glasses meeting ANSI Z87.1 standards block UV-C. Clothing, plastics, and most glass (except fused silica) also block UV-C.

Since the early 2010s, researchers have explored far-UVC light (200–235 nm) for use in entire rooms. This type of UV light is safer because it does not penetrate deeply into the body. It affects only the outer layer of skin (stratum corneum) and the surface of the eye, which lack cells that can cause cancer. Studies show far-UVC does not cause skin redness or eye damage even at levels much higher than regular UV sources.

Exposure limits for UV-C light have changed over time as research improved. Organizations like ACGIH and ICNIRP set guidelines to protect against health risks. These limits, called Threshold Limit Values (TLVs), help define safe UV emissions.

The UV-C range is 100–280 nm, with current limits applying to 180–280 nm. These limits address immediate risks like skin redness and eye damage, as well as long-term risks like cancer. In 2022, updated TLVs were set for specific wavelengths:
– For 222 nm UV-C (from KrCl excimer lamps): 161 mJ/cm² for eye exposure and 479 mJ/cm² for skin exposure over eight hours.
– For 254 nm UV-C: 6 mJ/cm² for eye exposure and 10 mJ/cm² for skin exposure over eight hours.

UV light can affect indoor air by creating ozone and other harmful pollutants, such as particulate matter. This happens when UV light breaks molecules into smaller radicals, which react with volatile organic compounds (VOCs) to form oxidized VOCs (OVOCs) and secondary organic aerosols (SOA).

UV wavelengths below 242 nm can also produce ozone, which is harmful when inhaled in large amounts. Ozone can irritate the eyes and lungs and worsen conditions like asthma. The specific pollutants formed depend on air chemistry and UV source characteristics. Ventilation and filtration help reduce these pollutants and maintain indoor air quality.

UV-C light can break chemical bonds, causing materials like plastics, rubber, and insulation to degrade quickly. Plastics labeled "UV-resistant" are tested for UVB, not UVC, since UVC does not reach Earth’s surface naturally. When UV-C is used near these materials, they may be protected with metal tape or aluminum foil.

Applications

UVGI can be used to clean the air when the UV light is left on for a long time. In the 1930s and 1940s, an experiment in public schools in Philadelphia showed that placing UV lights in the upper part of rooms could greatly reduce how easily measles spread between students.

UV and violet light can stop the ability of SARS-CoV-2 to cause infection. The amounts of virus found in the mucus of people with COVID-19 are completely destroyed by levels of UV-A and UV-B light similar to those from sunlight. This suggests that the lower number of SARS-CoV-2 cases in summer might be partly because sunlight helps kill the virus.

Many devices that produce UV light can be used to kill SARS-CoV-2, which may help stop the spread of infection. SARS-CoV-2 is killed by many types of UVC light, and the light with a wavelength of 222 nm works best.

How well UV light disinfects depends on the strength of the light and how long it is used. For this reason, it may not work as well on moving air or when the light is placed perpendicular to the air flow, because the time the air is exposed to the light is shorter. However, many scientific studies have shown that using UVGI with fans and HVAC systems can improve its effectiveness. These systems help move air around the room, exposing more air to the UV light. Some UVGI systems are freestanding units with protected UV lamps that use fans to push air through the light. Other systems are built into HVAC systems so that air circulates past the UV lamps. To work well, UV lamps must be placed correctly, and the system must have a good filter to trap dead germs. For example, HVAC systems can block the view of UV light in some areas, but placing UV lamps near the coils and drain pans of cooling systems can help prevent germs from growing in damp places.

UV light can disinfect water without using chemicals. Even tiny germs like Cryptosporidium and Giardia, which are hard to kill with chemicals, can be reduced by UV light. UV light can also remove chlorine and chloramine from water through a process called photolysis, which needs a stronger dose than normal disinfection. However, UV light does not remove dissolved chemicals, particles, or organic matter from water. The largest water treatment plant in the world, the Catskill-Delaware Water UV Disinfection Facility in New York City, uses 56 UV reactors to treat up to 2.2 billion gallons of water each day.

UV light can be combined with ozone or hydrogen peroxide to create hydroxyl radicals that break down small amounts of pollution through a process called advanced oxidation.

It was once believed that UV light worked better on bacteria and viruses, which have more exposed DNA, than on larger germs like Giardia, which have protective coatings. However, recent studies show that UV light can also help kill Cryptosporidium, making it a useful method for treating drinking water. Giardia is very sensitive to UV-C light when tested for infectivity, even though it can survive high UV-C doses when tested for other methods. Some germs can survive high UV-C doses but are sterilized at lower doses.

UV water treatment systems can be used for well water and surface water. Compared to other water treatment methods, UV systems are less expensive, require less labor, and do not need highly trained workers. Chlorine treatment is effective against larger germs and leaves a protective chemical in the water, but it is costly and requires careful handling of chemicals. Boiling water is the most reliable method but is labor-intensive and expensive. UV treatment is fast and uses about 20,000 times less energy than boiling.

UV light works best on very clear water, such as water purified through reverse osmosis. Particles in water are a problem because germs hidden inside them are not exposed to UV light and are not killed. UV systems can be paired with a pre-filter to remove larger germs and improve water clarity. The flow rate of water is also important: if the water moves too quickly, it may not get enough UV exposure, and if it moves too slowly, the UV lamp may overheat. A disadvantage of UVGI is that water treated with chlorine remains protected from new germs until the chlorine evaporates, but UV-treated water is not protected and must be handled carefully to avoid contamination.

In 2006, a project at the University of California, Berkeley, designed a low-cost water disinfection system for areas with limited resources. The design was open source and could be adapted to local needs. In 2014, Australian students created a system using foil from potato chip packages to reflect sunlight into a glass tube to disinfect water without electricity.

The size of a UV system depends on three factors: the flow rate of water, the power of the UV lamp, and how clear the water is. Manufacturers use complex computer models to design UV systems, which are tested with harmless germs like MS2 or T1 bacteriophages to predict how well the system will work. For example, all public water systems in the United States must follow guidelines from the EPA for UV treatment.

The flow of water in a UV system is determined by the shape of the chamber, the speed of the water, and the type of turbulence model used. The UV light distribution depends on water quality, the type of lamp (power, germ-killing ability, light output, length), and the clarity and size of the quartz sleeve around the lamp. Special computer software simulates how water and UV light move through the system. Once a 3D model of the chamber is created, it is divided into small cubes for detailed analysis.

Important areas, such as corners, the surface of the quartz sleeve, or near moving parts, are analyzed with a finer grid, while other areas use a coarser grid. After the grid is created, millions of virtual particles are simulated moving through the system. Each particle has details about its path, and the results are collected after the system. This process helps calculate how much UV light the water receives, how much pressure is lost, and other system-specific details.

After modeling, systems are tested by professionals to ensure the model accurately predicts real-world

UV Sources

Germicidal ultraviolet (UV) light for disinfection is most often produced by a mercury-vapor lamp. Low-pressure mercury vapor emits strongly at 254 nm, a wavelength that is very effective for disinfection. The best wavelengths for disinfection are near 260 nm.

Mercury vapor lamps are divided into low-pressure (including amalgam) or medium-pressure types. Low-pressure UV lamps are very efficient, producing about 35% UV-C light, but they have lower power, typically 1 W/cm power density (power per unit of arc length). Amalgam UV lamps use a mixture of materials to control mercury pressure, allowing them to operate at higher temperatures and power densities. They last up to 16,000 hours and produce about 33% UV-C light, slightly less than low-pressure lamps. Their power density is approximately 2–3 W/cm. Medium-pressure UV lamps operate at much higher temperatures, up to about 800 degrees Celsius. They emit a range of light wavelengths and produce high radiation, but their UV-C efficiency is 10% or less. Their typical power density is 30 W/cm or more.

Depending on the type of quartz glass used in the lamp, low-pressure and amalgam UV lamps emit radiation at 254 nm and also at 185 nm. The 185 nm radiation can create ozone.

UV lamps used for water treatment include specialized low-pressure mercury-vapor lamps that produce UV light at 254 nm or medium-pressure lamps that emit a range of wavelengths from 200 nm to visible and infrared light. These lamps do not touch the water. Instead, they are either placed inside a quartz glass sleeve within the water chamber or mounted outside the water, which flows through a transparent UV tube. As water moves through the chamber, it is exposed to UV rays, which are absorbed by particles like microorganisms and dirt in the water.

Recent advances in LED technology have made UV-C LEDs commercially available. These LEDs use special materials to emit light between 255 nm and 280 nm. The wavelength of light they produce can be adjusted by changing the material used in the LED. As of 2019, UV-C LEDs were less efficient at converting electricity to UV-C light compared to mercury lamps. However, their small size allows for use in compact systems, such as point-of-use devices and medical equipment. The low power needs of LEDs have also enabled UV disinfection systems powered by small solar cells to be used in remote or under-resourced areas.

UV-C LEDs do not necessarily last longer than traditional germicidal lamps in terms of total operating hours. Instead, their performance depends on design and usage patterns. In intermittent use, UV-C LEDs may remain functional for longer periods than traditional lamps. LED performance decreases with heat, while the output of traditional lamps like filament or HID lamps depends on temperature. Engineers can design LEDs to have specific outputs and degradation rates based on size and cost.