Share thisThe germicidal effects of ultra violet (UV) radiation have been known and researched since the late 19th century. In the early part of the 20th century, at least one medical equipment manufacturer (Westinghouse) was producing a sterilizing unit for hospital operating rooms which capitalized on the sterilization properties of ultra violet light.
UV for sterilization fell into disfavor by the early 1900’s with the development of chemical sterilization techniques using chlorine, ozone, peroxides, etc. Within the last few decades, however, UV has seen resurgence based on economics and from its lack of toxic chemical byproducts.
In fact, enough interest in the use of UV for a wide variety of applications has resulted in the formation of the International Ultra Violet Association, which was founded in 1999. In addition, the Environmental Protection Agency has decided to include UV disinfection in forthcoming US drinking water regulations.
The Physics of UV Disinfection
Ultra violet radiation consists of high energy photons which occupy the 200 to 400 nanometer (2000 to 400 Angstroms) wavelengths of the electromagnetic spectrum. (See Figure 1) UV light sits just below soft X-ray radiation and just above visible light. UV energy does not directly kill a pathogen, but rather causes a photochemical reaction within its genetic structure which inhibits the organism’s ability to reproduce. (The same phenomena human skin cells to burn and die...not so good if you are a single cell organism.)
The amount of energy delivered by a photon is inversely proportional to its wavelength. Planck’s Law of Radiation provides us with a simple mathematical expression to determine the amount of energy of a given wavelength:
E = h * c/l
Where:
E = -amount of energy in Joules (1 Watt-second)
H = -Planck’s Constant (6.6261 x 10-34 m2 kg/sec)
C = -the speed of light (299792458 meters/sec)
l = -wavelength (meters)
Intuitively, as the amount of energy increases, the germicidal effectivity would also increase. Empirical analysis of the survivability of various pathogens over the UV spectrum have concluded that a wavelength of 253.7 nanometers to be the most effective.
Many types of germicidal lamps are available today, but the most common is a Type UV-C, low pressure mercury arc lamp available in tubes that range from a few inches to 60 inches in length, similar in appearance to a common fluorescent light. A UV-C lamp produces UV light in the range of 200 to 280 nanometers.
Additional empirical work has been conducted to determine the amount of exposure time and UV intensity required to kill various pathogens. Table 1, (available from numerous sources) describes the product of intensity and exposure time required to kill specific pathogens using a standard UV-C source producing 253.7 nm wavelength output.