Scientists have harnessed UV as a staple in sterilization for over a century and developed sanitation methods used in hospitals, airplanes, and factories (among others) today. However, UV is dangerous and disinfecting your skin with any kind of UV will lead to damage and increase risk of skin cancer.
Ultraviolet (UV) light has shorter wavelengths compared to visible light. The natural source of ultraviolet radiation on Earth is the sun which contains three types of UV—UVA, UVB and UVC, categorized by wavelength ranges. Most UVA reaches the Earth’s surface and is thought to cause skin ageing for humans; while UVB causes sunburn and therefore damages skin DNA. Studies show that UVA and UVB can eventually cause skin cancer. Fortunately, both UVA and UVB can be blocked out by skin products.
UVC (100 nm—280 nm) is considered the “germicidal” UV as it destroys genetic material. UVC is filtered out by the Earth’s ozone long before it can reach its fragile inhabitants. Scientists have harnessed and artificially produced UVC to kill microorganisms since the late 1800s. In 1878, the sterilization of bacteria exposed to UVC was described in a paper published by Arthur Downes and Thomas Blunt. Nucleic acids strongly absorb wavelengths between 200 nm and 300 nm. This absorbed energy will cause cellular defects that can prevent replication or protein expression resulting in the death or inactivation of the organism. Microorganisms are vulnerable as they have less protection against UV and cannot survive prolonged exposure. Today, UVC is fundamental in the process of sanitizing drinking water and is used in sterilizing hospitals, airplanes, and factories.
For a long time, mercury lamps have been the only option for disinfection. With advances in UV LED technology, there are new options that are energy efficient and allow for infinite on/off switching, with better lifetime performance. This makes the devices smaller, battery powered, portable and with instant full light output--making LEDs a viable alternative over lamps. Wavelength emission by LEDs is determined by the chemistry of the semiconductor material. UVC LEDs are used in small reactors (systems that expose water to UV light, widely used in the treatment of drinking water), allowing for point-of-use applications.
The peak wavelength of a mercury lamp is around 254nm while the peak of the DNA absorption curve is at 265nm. However, absorption (and thereby disinfection and sterilization) occur over a range of wavelengths. One disadvantage of UVC LEDs is that they have very specific and narrow wavelengths. They are manufactured to be tailored to the particular application need. Mercury lamps on the other hand have a much broader spectral distribution that covers a wide range of germicidal wavelengths. Absorption rates vary depending on the microorganism. Although single wavelengths can be used, different microbes have different optimum absorption wavelengths and they can be simultaneously present in the material that is to be disinfected.
A UV germicidal irradiation (UVGI) system is designed to expose sealed environments to germicidal UV. Germicidal light sources emit the desired wavelength and repeated forced fluid (air or water) flow within the environment ensures exposure. The effectiveness of germicidal UV depends on its intensity, wavelength, the length of exposure and the organism’s ability to withstand UV. It is also important to calculate the intensity power and duration of exposure for a given wavelength in order to achieve the desired level of disinfection.
Needless to say, UV is dangerous to humans and we should not be exposed to it. To use UVC safely, special equipment and training are needed. The World Health Organization has issued a stern warning against using UV light to sterilize any part of the skin. The vast majority of UVC lamps and LEDs in the market are only suitable for surfaces of certain materials and not human use. From laboratory tests with animal cells, it only takes seconds to get sunburn from UVC exposure. It will also be detrimental for the eyes to be exposed to these rays. Scientists are continuously doing research on discovering a new type of UV that is less dangerous to handle while still being effective in killing bacteria and viruses.