UV-A is the longest wavelength defined as ultraviolet spanning from 457nm to 315nm. While it has a lower energy than shorter wavelengths, it has greater penetrating power for skin and eyes. It is associated with sunburn, skin aging, and altering DNA in the dermis, epidermis, and even the hypodermis. Higher frequencies of UV-A from 425nm to 457nm usually require special lasers to be generated. UV-A travels well through the atmosphere and is associated with modest germicidal potency. Some UV-A wavelengths can be generated by light emitting diodes (LEDs) and there are several sanitizing systems based upon this wavelength. All such systems require very long exposure periods that can be harmful to certain materials; in particular, polymers like plastics.

UV-B falls between 280nm and 315nm. It is associated with sunburn and skin cancers. It penetrates the dermis and can reach the epidermis. Although UB-B has a higher energy than UV-A, some doctors and scientists believe it is more harmful for longer exposures. Modest exposure of about 15 to 20 minutes of summertime UV-B has the benefit of generating vitamin D3 through a chemical reaction in the skin. Most UV-B is filtered out at sea level by the atmosphere. Sun blocking lotions use waxes or metalized compounds like zinc oxide paste to prevent UV-B from penetrating the skin.

UV-C is blocked from reaching the lower atmosphere by the ozone layer. Ranging from 200nm to 260nm, some UV-C at above 260nm to 280nm can reach very high altitudes, but is not naturally occurring from sunlight at sea level. Within UV-C there are several categories, but the most common distinctions are between the germicidal frequency of 254nm and “far UV-C” falling between 207nm and 222nm. UV-C at 254/257nm is the most commonly used germicidal frequency (wavelength) due to its higher energy and proven efficacy. In general, light emitting diodes (LEDs) cannot generate germicidal wavelengths with sufficient intensity to sanitize wide areas. There are some LEDs being used to “clean air” with a circulation system, but surfaces must still be addressed using other methods.

FAR UV-C references the 207nm to 222nm range and is highly effective in destroying viruses, bacteria, and fungi. Its higher energy is associated with germicidal power while the shorter wavelength is believed to be safe for human exposure because it does not penetrate the skin or eye cornea (lens). It is important to note that assumed safety associated with the lack of penetration is based upon laboratory experiments. Since this frequency band is not naturally occurring in our environment, there is no way to determine with certainty that long-term exposure is not harmful. Still, claims are being made that continuous exposure to 207nm~222nm can be tolerated. UV-C is absorbed by the atmosphere and does not travel well or far at sea level under normal humidity. This implies the far UV-C source must be relatively close to the intended target.

VACUUM UV is often placed in the UV-C category, but it is significantly different because it is powerful enough to ionize substances/elements like oxygen (O2), turning it into ozone (O3). Vacuum UV ranges from 10nm to 200nm, but is usually confined between 180nm and 200nm which can be produced from special UV bulbs. It has very potent germicidal efficacy, but does not travel well through the atmosphere because it is absorbed and blocked when reacting to the atmosphere, creating ozone and other reactions. Because of the ozone effect, vacuum UV-can be effective in controlling dust mites, bed bugs, and certain insect larvae. Ozone is, itself, an effective germicide that destroys viruses, bacteria, fungi, and living organisms. Ozone fumigation is used in extermination and deodorization. In fact, ozone is perhaps the only effective means for removing odors as potent as skunk.

Professor Anne Rammelsberg of Millikan University explains that UV energy initiates a reaction between two thymine molecules within DNA. Although bacteria can normally repair damaged DNA, when the damage is extensive the cell ceases to function. This same response can be engendered in viruses and fungi relative to the wavelength used and the light intensity. There are actually four criteria that determine the germicidal “kill rate:”

1. UV spectrum/wavelength (UV-A, UV-B, UV-C, Vacuum UV)
2. Light intensity – power
3. Proximity to intended surface and/or space volume
4. Duration (exposure time)

In practice, effective sanitizing is a function of these four components taken proportionately. This means that a longer exposure can offset distance from the intended target and wavelength can determine exposure time as can intensity. The objective is to maximize efficiency and effectiveness in the shortest, least disruptive application. Since UV radiation can adversely affect plastics and materials, care must be taken to minimize possible damage. Here, the science is clear… short exposure is better because reactions occur over time.