ultraviolet germicidal irradiation ultraviolet germicidal irradiation ultraviolet germicidal irradiation (uvgi) is a disinfection method that uses short-wavelength ultraviolet (ultraviolet c or uv-c) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their dna, leaving them unable to perform vital cellular functions. uvgi is used in a variety of applications, such as food, surface, air, and water purification. a low-pressure mercury-vapor discharge tube floods the inside of a biosafety cabinet with shortwave uv light when not in use, killing microbes on irradiated surfaces. uv-c light is weak at the earth's surface since the ozone layer of the atmosphere blocks it. uvgi devices can produce strong enough uv-c light in circulating air or water systems to make them inhospitable environments to microorganisms such as bacteria, viruses, molds, and other pathogens. recent studies have proven the ability of uvc light in inactivating the novel coronavirus (covid-19). uvgi can be coupled with a filtration system to sanitize air and water. the application of uvgi to disinfection has been an accepted practice since the mid-20th century. it has been used primarily in medical sanitation and sterile work facilities. increasingly, it has been employed to sterilize drinking and wastewater since the holding facilities are enclosed and can be circulated to ensure a higher exposure to the uv. uvgi has found renewed application in air purifiers. history in 1878, arthur downes and thomas p. blunt published a paper describing the sterilization of bacteria exposed to short-wavelength light. uv has been a known mutagen at the cellular level for over 100 years. the 1903 nobel prize for medicine was awarded to niels finsen for his use of uv against lupus vulgaris, tuberculosis of the skin. using uv light for disinfection of drinking water dates back to 1910 in marseille, france. the prototype plant was shut down after a short time due to poor reliability. in 1955, uv water treatment systems were applied in austria and switzerland; by 1985 about 1,500 plants were employed in europe. in 1998 it was discovered that protozoa such as cryptosporidium and giardia were more vulnerable to uv light than previously thought; this opened the way to wide-scale use of uv water treatment in north america. by 2001, over 6,000 uv water treatment plants were operating in europe. over time, uv costs have declined as researchers develop and use new uv methods to disinfect water and wastewater. several countries have published regulations and guidance for the use of uv to disinfect drinking water supplies examples include the us. and in the uk. method of operation low-pressure & medium-pressure mercury-vapor lamp compared to e. coli germicidal effectiveness curve.: fig 2.1 uv light is electromagnetic radiation with wavelengths shorter than visible light but longer than x-rays. uv is categorised into several wavelength ranges, with short-wavelength uv (uv-c) considered "germicidal uv". wavelengths between about 200 nm and 300 nm are strongly absorbed by nucleic acids. the absorbed energy can result in defects including pyrimidine dimers. these dimers can prevent replication or can prevent the expression of necessary proteins, resulting in the death or inactivation of the organism. recently, it has been shown that these dimers are fluorescent. mercury-based lamps operating at low vapor pressure emit uv light at the 253.7 nm line. ultraviolet light-emitting diode (uv-c led) lamps emit uv light at selectable wavelengths between 255 and 280 nm. pulsed-xenon lamps emit uv light across the entire uv spectrum with a peak emission near 230 nm. uvc led emitting 265 nm compared to e. coli germicidal effectiveness curve.: fig 5.5 this process is similar to, but stronger than, the effect of longer wavelengths (uv-b) producing sunburn in humans. microorganisms have less protection against uv and cannot survive prolonged exposure to it. a uvgi system is designed to expose environments such as water tanks, rooms and forced air systems to germicidal uv. exposure comes from germicidal lamps that emit germicidal uv at the correct wavelength, thus irradiating the environment. the forced flow of air or water through this environment ensures exposure of that air or water. effectiveness the effectiveness of germicidal uv depends on the duration a microorganism is exposed to uv, the intensity and wavelength of the uv radiation, the presence of particles that can protect the microorganisms from uv, and a microorganism's ability to withstand uv during its exposure. in many systems, redundancy in exposing microorganisms to uv is achieved by circulating the air or water repeatedly. this ensures multiple passes so that the uv is effective against the highest number of microorganisms and will irradiate resistant microorganisms more than once to break them down. "sterilization" is often misquoted as being achievable. while it is theoretically possible in a controlled environment, it is very difficult to prove and the term "disinfection" is generally used by companies offering this service as to avoid legal reprimand. specialist companies will often advertise a certain log reduction, e.g., 6-log reduction or 99.9999% effective, instead of sterilization. this takes into consideration a phenomenon known as light and dark repair (photoreactivation and base excision repair, respectively), in which a cell can repair dna that has been damaged by uv light. the effectiveness of this form of disinfection depends on line-of-sight exposure of the microorganisms to the uv light. environments where design creates obstacles that block the uv light are not as effective. in such an environment, the effectiveness is then reliant on the placement of the uvgi system so that line of sight is optimum for disinfection. dust and films coating the bulb lower uv output. therefore, bulbs require periodic cleaning and replacement to ensure effectiveness. the lifetime of germicidal uv bulbs varies depending on design. also, the material that the bulb is made of can absorb some of the germicidal rays. lamp cooling under airflow can also lower uv output. increases in effectiveness and uv intensity can be achieved by using reflection. aluminum has the highest reflectivity rate versus other metals and is recommended when using uv. one method for gauging uv effectiveness in water disinfection applications is to compute uv dose. epa published uv dosage guidelines for water treatment applications in 1986. uv dose cannot be measured directly but can be inferred based on the known or estimated inputs to the process: flow rate (contact time) transmittance (light reaching the target) turbidity (cloudiness) lamp age or fouling or outages (reduction in uv intensity) in air and surface disinfection applications the uv effectiveness is estimated by calculating the uv dose which will be delivered to the microbial population. the uv dose is calculated as follows: uv dose (μw·s/cm2) = uv intensity (μw/cm2) × exposure time (seconds) the uv intensity is specified for each lamp at a distance of 1 meter. uv intensity is inversely proportional to the square of the distance so it decreases at longer distances. alternatively, it rapidly increases at distances shorter than 1 m. in the above formula, the uv intensity must always be adjusted for distance unless the uv dose is calculated at exactly 1 m (3.3 ft) from the lamp. also, to ensure effectiveness, the uv dose must be calculated at the end of lamp life (eol is specified in number of hours when the lamp is expected to reach 80% of its initial uv output) and at the furthest distance from the lamp on the periphery of the target area. some shatter-proof lamps are coated with a fluorated ethylene polymer to contain glass shards and mercury in case of breakage; this coating reduces uv output by as much as 20%. to accurately predict what uv dose will be delivered to the target, the uv intensity, adjusted for distance, coating, and end of lamp life, will be multiplied by the exposure time. in static applications the exposure time can be as long as needed for an effective uv dose to be reached. in case of rapidly moving air, in ac air ducts, for example, the exposure time is short, so the uv intensity must be increased by introducing multiple uv lamps or even banks of lamps. also, the uv installation should ideally be located in a long straight duct section with the lamps directing uvc in a direction parallel to the airflow to maximize the time the air is irradiated. these calculations actually predict the uv fluence and it is assumed that the uv fluence will be equal to the uv dose. the uv dose is the amount of germicidal uv energy absorbed by a microbial population over a period of time. if the microorganisms are planktonic (free floating) the uv fluence will be equal the uv dose. however, if the microorganisms are protected by mechanical particles, such as dust and dirt, or have formed biofilm a much higher uv fluence will be needed for an effective uv dose to be introduced to the microbial population. inactivation of microorganisms the degree of inactivation by ultraviolet radiation is directly related to the uv dose applied to the water. the dosage, a product of uv light intensity and exposure time, is usually measured in microjoules per square centimeter, or equivalently as microwatt seconds per square centimeter (μw·s/cm2). dosages for a 90% kill of most bacteria and viruses range between 2,000 and 8,000 μw·s/cm2. larger parasites such as cryptosporidium require a lower dose for inactivation. as a result, us epa has accepted uv disinfection as a method for drinking water plants to obtain cryptosporidium, giardia or virus inactivation credits. for example, for a 90% reduction of cryptosporidium, a minimum dose of 2,500 μw·s/cm2 is required based on epa's 2006 guidance manual.: 1–7 strengths and weaknesses advantages further information: disinfectant uv water treatment devices can be used for well water and surface water disinfection. uv treatment compares favourably with other water disinfection systems in terms of cost, labour and the need for technically trained personnel for operation. water chlorination treats larger organisms and offers residual disinfection, but these systems are expensive because they need special operator training and a steady supply of a potentially hazardous material. finally, boiling of water is the most reliable treatment method but it demands labour and imposes a high economic cost. uv treatment is rapid and, in terms of primary energy use, approximately 20,000 times more efficient than boiling. disadvantages uv disinfection is most effective for treating high-clarity, purified reverse osmosis distilled water. suspended particles are a problem because microorganisms buried within particles are shielded from the uv light and pass through the unit unaffected. however, uv systems can be coupled with a pre-filter to remove those larger organisms that would otherwise pass through the uv system unaffected. the pre-filter also clarifies the water to improve light transmittance and therefore uv dose throughout the entire water column. another key factor of uv water treatment is the flow rate—if the flow is too high, water will pass through without sufficient uv exposure. if the flow is too low, heat may build up and damage the uv lamp. a disadvantage of uvgi is that while water treated by chlorination is resistant to reinfection (until the chlorine off-gasses), uvgi water is not resistant to reinfection. uvgi water must be transported or delivered in such a way as to avoid reinfection. hazards to humans warning for optical radiation applies to devices that emit uv light. uv light is hazardous to most living things. skin exposure to germicidal wavelengths of uv light can produce rapid sunburn and skin cancer. exposure of the eyes to this uv radiation can produce extremely painful inflammation of the cornea and temporary or permanent vision impairment, up to and including blindness in some cases. common precautions are: warning labels warn humans about dangers of uv light. in home settings with children and pets, doors are additionally necessary. interlock systems. shielded systems where the light is blocked inside, such as a closed water tank or closed air circulation system, often has interlocks that automatically shut off the uv lamps if the system is opened for access by humans. clear viewports that block uvc are available. protective gear. most protective eyewear (in particular, all ansi z87.1-compliant eyewear) block uvc. clothing, plastics, and most types of glass (but not fused silica) are effective in blocking uvc. another potential danger is the uv production of ozone, which can be harmful when inhaled. us epa designated 0.05 parts per million (ppm) of ozone to be