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Germicidal ultraviolet light inactivates microorganisms by disrupting their cellular membranes and by damaging their DNA or RNA. UV light at 253.7nm wavelength (germicidal UV or UVC) causes adjacent thymine molecules on the DNA to dimerize (bonding two adjacent bases together instead of across the DNA strands). The thymine dimers are very stable. If enough of these defects accumulate on a microorganism's DNA its replication is inhibited, rendering the microorganisms harmless because if the pathogens cannot reproduce can’t cause diseases.
High UV dose delivered to microorganisms leads to the disruption of the cellular membrane and the death of the cells resulting in effective UV disinfection of the exposed air, water or surface. See UV Dose table for various microorganisms.
The electromagnetic spectrum encompasses radio and infrared waves - with wavelengths longer than visible light - to ultraviolet, X-rays and gamma rays - with wavelengths shorter than visible light. Ultraviolet (UV) radiation is the portion of the spectrum with wavelengths below that of the visible region. The UV part of the spectrum is subdivided into UVA, UVB and UVC regions in order of decreasing wavelengths and increasing energies.
Germicidal UV used for disinfection is generated by specially designed ultraviolet lamps which produce radiation at 253.7nm wavelength.
Ultraviolet wavelengths below 200nm break down oxygen molecules in the air and produce Ozone. The monochromatic germicidal UV lamps emitting energy at 253.7nm wavelength do NOT produce Ozone.
by Edward A. Nardell, M. D. Harvard Medical School
What is UV or ultraviolet light? Ultraviolet light is part of the spectrum of electromagnetic energy generated by the sun. The full spectrum includes, in order of increasing energy, radio waves, infrared, visible light, ultraviolet, x-rays, gamma rays and cosmic rays. Since UV is not visible, it is technically not "light", but use of the term "ultraviolet light" is so widespread that, it will be used here. Most sources of light generate some UV. For air disinfection, UV is generated by electric lamps that resemble ordinary fluorescent lamps.
What is germicidal UV? This is UV of a specific wavelength (253.7nm) known to kill germs. In-room germicidal UV is aimed at the upper room air so that only airborne microbes are directly exposed. Room occupants are exposed only to low levels of reflected UV - levels below that known to cause eye irritation. Germicidal UV has been used safely and effectively in hospitals, clinics and laboratories for more than 60 years. UV does not prevent transmission of infections (e.g. colds) by direct person to person contact.
Is UV harmful? We are all exposed to the UV in sunlight. UV exposure can be very harmful, or harmless, depending on the type of UV, the type of exposure, the duration of exposure, and individual differences in response to UV. There are the following types of UV based on the wave length measured in nanometers (nm):
UV-V ultraviolet with wavelength below 200nm is known as Vacuum UV or UV V.
UV-C (200 - 280nm) - Also known as "shortwave" UV, UVC or C-band UV includes germicidal (253.7nm wavelength) UV used for air disinfection. Unintentional overexposure causes transient redness and eye irritation, but does not cause skin cancer or cataracts. Note: Never expose eyes or skin to UVC light from any source. Prolonged UV exposure can cause burns and temporary loss of vision or blindness.
UV-B (280 -315nm) - A small, but dangerous part of sunlight. Most solar UV-B is absorbed by the atmospheric ozone layer. Prolonged exposure is responsible for some types of skin cancer, skin aging, and cataracts (clouding of the lens of the eye).
UV-A (315 - 400nm) - Longwave UV, also known as "blacklight", the major type of UV in sunlight, responsible for skin tanning, generally not harmful, used in medicine to treat certain skin disorders.
Why is UV-B harmful while UV-C (germicidal UV) is not? - The difference has to do with the ability of UV rays to penetrate body surfaces. UVC has an extremely low penetrating ability. It is nearly completely absorbed by the outer, dead layer of the skin (stratum corneum) where it does little harm. It does reach the most superficial layer of the eye where overexposure can cause irritation, but it does not penetrate to the top of the lens of the eye and cannot cause cataracts. UVC is completely stopped by the ordinary eye glasses and by ordinary clothing.
How much UV exposure is considered safe? The National Institute for Occupational Safety and Health (NIOSH) has established safe exposure levels for each type of UV. These safe exposure limits are set below the levels found to cause eye irritation, eye being the body part most sensitive to UV. For germicidal UV (253.7nm) the irradiance limit is set to 0.2µW/cm².
How can people be certain they are not overexposed to UV? When upper room UV is first installed it must be checked with a sensitive UV meter to make sure reflected UV is less than 0.2µW/cm² at eye level. UV air cleaners must be installed well above eye level - usually 7 feet above the floor. UV tubes (lamps) within the air cleaners should not be directly visible from within 30 feet. Safety is assured if UV measurements at eye level meet NIOSH standards.
What are the symptoms and signs of UV overexposure? UV overexposure causes an eye inflammatory condition known as photokeratitis. For 6 to 12 hours after an accidental overexposure the individual may feel nothing unusual, followed by the abrupt sensation of foreign body or "sand" in the eyes, redness of the skin around the eyes, some light sensitivity, tearing, and eye pain. The acute symptoms last 6 to 24 hours and resolve completely without long-term effects. Overexposure of the skin resembles sunburn but does not result in tanning.
The American Conference of Governmental Industrial Hygienists (ACGIH) Committee on Physical Agents has established a TLV (Threshold Limit Value) for UV-C exposure to avoid skin and eye injuries. For 254nm UV, this TLV is 6mJ/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. This TLV is interpreted as if eye exposure in rooms is continuous over eight hours and at the highest eye-level irradiance found in the room. In those highly unlikely conditions, a 6mJ/cm² dose is reached under the ACGIH TLV after just eight hours of continuous exposure to irradiance of 0.2 µW/cm². Thus, 0.2µW/cm² is widely interpreted as the upper permissible limit of irradiance at eye height.