The Lethal Effect of Ultraviolet Radiation on Microbial Growth

Ultraviolet radiation (UV light) is a type of electromagnetic

energy. Like all electromagnetic energy, UV travels in waves and is distinguishable from all others by its wavelength. Wavelength is the distance between adjacent wave crests and is typically measured in nanometers (nm) (Fig. 2.52).

Ultraviolet light is divided into three groups catego- rized by wavelength:

1. UV-A, the longest wavelengths, ranging from 315 nm to 400 nm

2. UV-B, wavelengths between 280 nm and 315 nm

3. UV-C, wavelengths ranging from 100 nm to 280 nm. (These wavelengths-more specifically, 240 nm-280 nm-are most detrimental to bacteria. Bacterial expo- sure to UV-C for more than a few minutes usually results in irreparable DNA damage and death of the organism. For a discussion on the mutagenic effects of UV and DNA repair, refer to Exercise 10-5.)

   The germicidal effect of UV-C is related to time of

exposure, lamp intensity, and distance to the target (the inverse square law applies here-intensity diminishes by the reciprocal of square of the distance: dis~ nce' ). It also must have "line of sight" to the surface being decontaminated. That is, it doesn't penetrate well, bend around corners, or trickle into crevices. Lastly, its effectiveness is diminished by dust, organic material, etc. Taken together, UV-Casa germicidal agent has its limited applications.