UVC AS A DISINFECTANT
Since Niels Finsen won the 1903 Nobel Prize for discovering that ultraviolet (UV) light could kill germs, science has recognized the germicidal effects of the ultraviolet area of the electromagnetic spectrum. When biological organisms are exposed to deep UV light in the range of 200 nm to 300 nm, the energy is absorbed by DNA, RNA, and proteins. Absorption of this energy can lead to damage to the DNA and RNA, rupture of cell walls, and even death of the organism. While death of the germ is possible, it is widely accepted that it is not necessary to kill pathogens with UV light, but rather apply enough UV light to prevent the organism from replicating.
WHAT TO UNDERSTAND...
Key terminology and concepts to understand the efficacy of UVC and viral inactivation
UVC SPECTRUM
UV light covers the range of electromagnetic radiation between visible light starting at 400 nm and x-rays starting at 10 nm. The UV spectrum is separated into four parts: UVA (315 nm to 400 nm), UVB (280 nm to 315 nm), UVC (200 nm to 280 nm) and UV Vacuum (100 nm to 200 nm). No natural UVC radiation reaches the surface of the earth due to absorption by the atmosphere and microorganisms have not developed a way to prevent genetic damage. Fortunately these wavelengths are available to us through artificial sources lamps and LEDs.
DOSAGE
For surface disinfection applications, the desired dose is the amount of energy required to damage the virus and render it inactive. Dose is calculated as the product of the UVC source irradiance and exposure time.
The way the units work is 1 Joule (J) of energy delivered = delivering 1 Watt (W) of energy for 1 second. In the UV world, we usually measure things in small increments, i.e., thousandths of a Joule or Watt. These are shown as ‘milli-Joules' (i.e., ‘mJ’ or 1/1,000 of a Joule), and milli-Watts (i.e., ‘mW’ or 1/1,000 of a Watt).
VIRAL INACTIVATION
Absorption by DNA or RNA (specifically by thymine bases) is known to cause inactivation of the DNA or RNA double helix strands through the formation of thymine dimers. If enough of these dimers are created in DNA, the DNA replication process is disrupted, and the cell cannot replicate. The UV doses required to prevent replication are orders of magnitude lower than required to kill, making the cost of UV treatment to prevent infection commercially viable and safe for frequent use.
UVC SAFETY
There are currently no work place related rules and regulations set by OSHA (Occupational Safety and Health Association) in regard to UVC environmental health and safety but as any of us who have spent too much time in the sun know, too much exposure to UV radiation can cause harm to the skin and eyes. This potential damage will vary with the wavelength of UV light and time of exposure.
Unlike UVA and UVB, the shorter wavelength of UVC light cannot penetrate the atmosphere so the only sources of UVC light are artificial. The energy delivered from these point sources falls off exponentially over the distance past the scattering length of the light. This means that the greater the distance between the UVC source and a cell, the lesser the dose the cell is exposed to. In addition, the absorption length of UVC radiation in human skin is extremely short so that almost no UVC radiation can reach the living cells due to the absorption that occurs in the dead cell layers of our skin. With those considerations in mind, the National Institute for Occupational Safety and Health (NIOSH) still recommends limiting UVC exposure at wavelength of 254 nanometers and an intensity of 100 microwatts per square centimeter to less than 1 minute per 8 hour period.
With proper safety design considerations and controls, UVC exposure can be virtually eliminated or reduced to safe levels for every day use.