a safe level. lamps designed to release uv and higher frequencies are doped so that any uv light below 254 nm wavelengths will not be released, to minimize ozone production. a full-spectrum lamp will release all uv wavelengths and produce ozone when uv-c hits oxygen (o2) molecules. the american conference of governmental industrial hygienists (acgih) committee on physical agents has established a threshold limit value (tlv) for uv exposure to avoid such skin and eye injuries among those most susceptible. for 254 nm uv, this tlv is 6 mj/cm2 over an eight-hour period. the tlv function differs by wavelengths because of variable energy and potential for cell damage. this tlv is supported by the international commission on non-ionizing radiation protection and is used in setting lamp safety standards by the illuminating engineering society of north america. when the tuberculosis ultraviolet shelter study was planned, this tlv was interpreted as if eye exposure in rooms was continuous over eight hours and at the highest eye-level irradiance found in the room. in those highly unlikely conditions, a 6.0 mj/cm2 dose is reached under the acgih tlv after just eight hours of continuous exposure to an irradiance of 0.2 μw/cm2. thus, 0.2 μw/cm2 was widely interpreted as the upper permissible limit of irradiance at eye height. according to the fda, a germicidal excimer lamp that emits 222 nm far uv light instead of the common 254 nm uv light is safer for mammalian skin. uvc radiation damage to materials uvc radiation is able to break chemical bonds. this leads to rapid aging of plastics and other material, and insulation and gaskets. plastics sold as "uv-resistant" are tested only for the lower-energy uvb since uvc does not normally reach the surface of the earth. when uv is used near plastic, rubber, or insulation, these materials may be protected by metal tape or aluminum foil. uses air disinfection uvgi can be used to disinfect air with prolonged exposure. in the 1930s and 40s, an experiment in public schools in philadelphia showed that upper-room ultraviolet fixtures could significantly reduce the transmission of measles among students. in 2020, uvgi is again being researched as a possible countermeasure against covid-19. uv and violet light are able to neutralize the infectivity of sars-cov-2. viral titers usually found in the sputum of covid-19 patients are completely inactivated by levels of uv-a and uv-b irradiation that are similar to those levels experienced from natural sun exposure. this finding suggests that the reduced incidence of sars-cov-2 in the summer may be, in part, due to the neutralizing activity of solar uv irradiation. various uv-emitting devices can be used for sars-cov-2 disinfection, and these devices may help in reducing the spread of infection. sars-cov-2 can be inactivated by a wide range of uvc wavelengths, and the wavelength of 222nm provides the most effective disinfection performance. disinfection is a function of uv intensity and time. for this reason, it is in theory not as effective on moving air, or when the lamp is perpendicular to the flow, as exposure times are dramatically reduced. however, numerous professional and scientific publications have indicated that the overall effectiveness of uvgi actually increases when used in conjunction with fans and hvac ventilation, which facilitate whole-room circulation that exposes more air to the uv source. air purification uvgi systems can be free-standing units with shielded uv lamps that use a fan to force air past the uv light. other systems are installed in forced air systems so that the circulation for the premises moves microorganisms past the lamps. key to this form of sterilization is placement of the uv lamps and a good filtration system to remove the dead microorganisms. for example, forced air systems by design impede line-of-sight, thus creating areas of the environment that will be shaded from the uv light. however, a uv lamp placed at the coils and drain pans of cooling systems will keep microorganisms from forming in these naturally damp places. water disinfection a portable, battery-powered, low-pressure mercury-vapour discharge lamp for water sterilization cutaway model of uv disinfection unit used in water treatment plants ultraviolet disinfection of water is a purely physical, chemical-free process. even parasites such as cryptosporidium or giardia, which are extremely resistant to chemical disinfectants, are efficiently reduced. uv can also be used to remove chlorine and chloramine species from water; this process is called photolysis, and requires a higher dose than normal disinfection. the dead microorganisms are not removed from the water. uv disinfection does not remove dissolved organics, inorganic compounds or particles in the water. the world's largest water disinfection plant treats drinking water for new york city. the catskill-delaware water ultraviolet disinfection facility, commissioned on 8 october 2013, incorporates a total of 56 energy-efficient uv reactors treating up to 2.2 billion u.s. gallons (8.3 billion liters) a day. ultraviolet can also be combined with ozone or hydrogen peroxide to produce hydroxyl radicals to break down trace contaminants through an advanced oxidation process. it used to be thought that uv disinfection was more effective for bacteria and viruses, which have more-exposed genetic material, than for larger pathogens that have outer coatings or that form cyst states (e.g., giardia) that shield their dna from uv light. however, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism cryptosporidium. the findings resulted in the use of uv radiation as a viable method to treat drinking water. giardia in turn has been shown to be very susceptible to uv-c when the tests were based on infectivity rather than excystation. it has been found that protists are able to survive high uv-c doses but are sterilized at low doses. developing countries a 2006 project at university of california, berkeley produced a design for inexpensive water disinfection in resource deprived settings. the project was designed to produce an open source design that could be adapted to meet local conditions. in a somewhat similar proposal in 2014, australian students designed a system using potato chip (crisp) packet foil to reflect solar uv radiation into a glass tube that disinfects water without power. wastewater treatment ultraviolet in sewage treatment is commonly replacing chlorination. this is in large part because of concerns that reaction of the chlorine with organic compounds in the waste water stream could synthesize potentially toxic and long lasting chlorinated organics and also because of the environmental risks of storing chlorine gas or chlorine containing chemicals. individual wastestreams to be treated by uvgi must be tested to ensure that the method will be effective due to potential interferences such as suspended solids, dyes, or other substances that may block or absorb the uv radiation. according to the world health organization, "uv units to treat small batches (1 to several liters) or low flows (1 to several liters per minute) of water at the community level are estimated to have costs of us$20 per megaliter, including the cost of electricity and consumables and the annualized capital cost of the unit." large-scale urban uv wastewater treatment is performed in cities such as edmonton, alberta. the use of ultraviolet light has now become standard practice in most municipal wastewater treatment processes. effluent is now starting to be recognized as a valuable resource, not a problem that needs to be dumped. many wastewater facilities are being renamed as water reclamation facilities, whether the wastewater is discharged into a river, used to irrigate crops, or injected into an aquifer for later recovery. ultraviolet light is now being used to ensure water is free from harmful organisms. aquarium and pond ultraviolet sterilizers are often used to help control unwanted microorganisms in aquaria and ponds. uv irradiation ensures that pathogens cannot reproduce, thus decreasing the likelihood of a disease outbreak in an aquarium. aquarium and pond sterilizers are typically small, with fittings for tubing that allows the water to flow through the sterilizer on its way from a separate external filter or water pump. within the sterilizer, water flows as close as possible to the ultraviolet light source. water pre-filtration is critical as water turbidity lowers uv-c penetration. many of the better uv sterilizers have long dwell times and limit the space between the uv-c source and the inside wall of the uv sterilizer device. laboratory hygiene uvgi is often used to disinfect equipment such as safety goggles, instruments, pipettors, and other devices. lab personnel also disinfect glassware and plasticware this way. microbiology laboratories use uvgi to disinfect surfaces inside biological safety cabinets ("hoods") between uses. food and beverage protection since the u.s. food and drug administration issued a rule in 2001 requiring that virtually all fruit and vegetable juice producers follow haccp controls, and mandating a 5-log reduction in pathogens, uvgi has seen some use in sterilization of juices such as fresh-pressed. technology lamps a 9 w germicidal lamp in a compact fluorescent lamp form factor main article: germicidal lamp germicidal uv for disinfection is most typically generated by a mercury-vapor lamp. low-pressure mercury vapor has a strong emission line at 254 nm, which is within the range of wavelengths that demonstrate strong disinfection effect. the optimal wavelengths for disinfection are close to 260 nm.: 2–6, 2–14 mercury vapor lamps may be categorized as either low-pressure (including amalgam) or medium-pressure lamps. low-pressure uv lamps offer high efficiencies (approx. 35% uv-c) but lower power, typically 1 w/cm power density (power per unit of arc length). amalgam uv lamps utilize an amalgam to control mercury pressure to allow operation at a somewhat higher temperature and power density. they operate at higher temperatures and have a lifetime of up to 16,000 hours. their efficiency is slightly lower than that of traditional low-pressure lamps (approx. 33% uv-c output), and power density is approximately 2–3 w/cm3. medium-pressure uv lamps operate at much higher temperatures, up to about 800 degrees celsius, and have a polychromatic output spectrum and a high radiation output but lower uv-c efficiency of 10% or less. typical power density is 30 w/cm3 or greater. depending on the quartz glass used for the lamp body, low-pressure and amalgam uv emit radiation at 254 nm and also at 185 nm, which has chemical effects. uv radiation at 185 nm is used to generate ozone. the uv lamps for water treatment consist of specialized low-pressure mercury-vapor lamps that produce ultraviolet radiation at 254 nm, or medium-pressure uv lamps that produce a polychromatic output from 200 nm to visible and infrared energy. the uv lamp never contacts the water; it is either housed in a quartz glass sleeve inside the water chamber or mounted externally to the water, which flows through the transparent uv tube. water passing through the flow chamber is exposed to uv rays, which are absorbed by suspended solids, such as microorganisms and dirt, in the stream. light emitting diodes (leds) compact and versatile options with uv-c leds recent developments in led technology have led to commercially available uv-c leds. uv-c leds use semiconductors to emit light between 255 nm and 280 nm. the wavelength emission is tuneable by adjusting the material of the semiconductor. as of 2019, the electrical-to-uv-c conversion efficiency of leds was lower than that of mercury lamps. the reduced size of leds opens up options for small reactor systems allowing for point-of-use applications and integration into medical devices. low power consumption of semiconductors introduce uv disinfection systems that utilized small solar cells in remote or third world applications. uv-c leds don't necessarily last longer than traditional germicidal lamps in terms of hours used, instead having more-variable engineering characteristics and better tolerance for short-term operation. a uv-c led can achieve a longer installed time than a traditional germicidal lamp in intermittent use. likewise, led degradation increases with heat, while filament and hid lamp output wavelength is dependent on temperature, so engineers can design leds of a particular size and cost to have a higher output and faster degradation or a lower output and slower decline over time. water treatment systems sizing of a uv system is affected by three variables: flow rate, lamp power, and uv transmittance in the water. manufacturers typically developed sophisticated computational fluid dynamics (cfd) models validated with bioassay testing. this involves testing the uv reactor's disinfection performance with either ms2 or t1 bacteriophages at various flow rates, uv transmittance, and power levels in order to develop a regression