UVGI

Ultraviolet Germicidal Irradiation (UVGI)

UVGI is a sterilization method that uses (UV) ultraviolet light at sufficient wavelength to break down micro-organisms. It is used in a variety of applications, such as food, air and water purification. UV has been a known mutagen at the cellular level for more than 100 years. The 1903 Nobel Prize for Medicine was awarded to Neils Finsen for his use of UV against tuberculosis 1. A common example of UVGI is the sterilization tank used in barbershops to disinfect combs and brushes.

UVGI replicates the low wavelength form of UV that is harmful to forms of life at the micro-organic level. It is effective in destroying the nucleic acids in these life forms so that their DNA is unhinged. This eradicates them or destroys their ability to reproduce.

The wavelength of UV that causes this effect is rare on Earth as its atmosphere blocks it 2. Using a UVGI device in certain environments like circulating air or water systems, create a deadly effect on micro-organisms such as pathogens, viruses and molds that are in these environments. Coupled with a filtration system, UVGI can remove harmful micro-organisms from these environments.

The application of UVGI to sterilization has been an accepted practice since the mid-20th century. It has been used primarily in medical sanitation and sterile work facilities. Increasingly it was employed to sterilize drinking and wastewater, as the holding facilities were enclosed and could be circulated to ensure a higher exposure to the UV. In recent years UVGI has found renewed application in air sanitization.

How UVGI Works Ultraviolet light is electromagnetic radiation with wavelengths shorter than visible light. UV can be separated into various ranges, with short range UV (UVC) considered “germicidal UV.” At certain wavelengths UV is mutagenic to bacteria, viruses and other micro-organisms. At a wavelength of 2,537 Angstroms (254 nm)3 UV will break the molecular bonds of micro-organisms, either destroying them, rendering them harmless or prohibiting growth and reproduction. It is a process similar to the UV effect of higher wavelengths (UVB) on humans, such as sunburn or sun glare. When this happens a person’s cells such as collagen are damaged at the cellular level. Micro-organisms have less protection from UV and cannot survive prolonged exposure to it.

A UVGI system is designed to expose environments such as water tanks, sealed rooms and forced air systems to germicidal UV. Exposure comes from bulbs that emit germicidal UV electromagnetic radiation at the correct wavelength, thus irradiating the environment. The forced flow of air or water through this environment ensures the exposure.

UVGI Effectiveness UVGI is a highly effective method of destroying micro-organisms. Since the Earth’s atmosphere absorbs most of the UV from the sun, germicidal UV is very rare in all circumstances. When concentrated in a closed environment such as a water holding tank or duct system it is lethal over time to all micro-organisms.

The effectiveness of germicidal UV in such an environment depends on a number of factors: the length of time a micro-organisms is exposed to UV, power fluctuations of the UV source that impact the EM wavelength, the presence of particles that can protect the micro-organisms from UV, and a micro-organism’s ability to withstand UV during its exposure.

In many systems redundancy in exposing micro-organisms 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 micro-organisms and will irradiate resistant micro-organisms more than once to break them down.

The effectiveness of this form of sterilization is also dependent on line of sight exposure of the micro-organisms 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 sterilization.

A separate problem that will effect UVGI is dust or other film coating the bulb, which can lower UV output. Therefore bulbs require annual replacement and scheduled cleaning to ensure effectiveness. The lifetime of germicidal UV bulbs varies depending on design.

Creating UVGI Germicidal UV is delivered by a mercury-vapor lamp that emits UV at the germicidal wavelength. Many germicidal UV bulbs use special transformers to ensure even electrical flow to the bulbs so the correct wavelength is maintained. Since germicidal UV has a narrow bandwidth, power fluctuations will be ineffective in irradiating environments.

Potential Dangers of UVGI At certain wavelengths (including UVC) UV is harmful to humans and other forms of life. In most UVGI systems the lamps are shielded or are in environments that limit exposure, such as a closed water tank or closed air circulation system. Limited exposure mitigates the risk of exposure.

Another potential danger is the UV production of ozone. UVC light from the sun is partly responsible for the earth’s ozone layer in the stratosphere, but ozone at the atmospheric level can be harmful to a person’s health 4. The EPA designated .05 parts per million (ppm) of ozone to be a safe level.

Uses for UVGI Air Purification: UVGI can be used to sterilize air that passes UV lamps via forced air. Air purification UVGI systems can be freestanding 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 micro-organisms past the lamps. Key to this form of sterilization is placement of the UV lamps and a good filtration system to remove the dead micro-organisms. 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 drainpan of cooling system will keep micro-organisms from forming in these naturally damp places.

Water Purification: Water purification via UVGI is used in most water sterilization processes, such as purification, detoxification and disinfection. Its use in wastewater treatment is replacing chlorination due to that chemical’s toxic by-products 5.

Food and Beverage Protection

Other Uses

1. http://nobelprize.org/nobel_prizes/medicine/laureates/1903/ 2. http://news.softpedia.com/news/UV-Radiation-What-UVA-UVB-and-UVC-Rays-Are-and-How-They-Affect-Us-30345.shtml 3. “Mathematical Modeling of Ultraviolet Germicidal Irradiation for Air Disinfection;” 4. Kowalski, Bahnfleth, et al., Quantitative Microbiology 2, 249–270, 2000 5. http://www.americanairandwater.com/uv-water-applications.htm