The knowledge that electromagnetic radiation with wavelengths in the Ultra Violet C band (210 nm to 280 nm) kills bacteria is well established with the first recognition of this discovery resulting in a Nobel prize awarded to Niels Ryberg Finsen for Physiology or Medicine in 1903. It is now known that DNA of living organisms is highly absorbent at 260 nm.
Pathogens are a serious issue in hospitals, killing over 100,000 persons per year in the USA. Pathogens are spread from infected individuals through touch and through the air. Removal of pathogens is very difficult. Filtration is commonly used but does not remove the pathogen; filtration makes matters worse by providing sites for the pathogens to multiply as filtration does not kill them. UVC using mercury and fluorescent lamps have been used but cost, production of ozone, maintenance and heat are barriers to large scale adoption.
Life on this planet evolved in the absence of UVC, specifically 260 nm wavelength of light. The ozone layers shields the planet form this wavelength of electro-magnetic radiation. All life forms are extremely sensitive to this wavelength and cells are destroyed with small doses of this radiation. In simple organisms, this means immediate death, in complex organisms, it means various cancers and cellular destruction leading to death.
The first attempts in removing pathogens from the air were to filter them out as disclosed by M. WILEY et al in U.S. Pat. No. 3,347,025. Filters at that time were not fine enough to be helpful as well as the pathogens were not killed. Filters do block bacteria but not all pathogens as well as the filter surfaces become sites for the pathogens to grow in numbers.
Attempts with varying levels of success have been made to use UVC to kill pathogens to date.
Traditionally UVC is created through the mercury arc lamps and through gas discharge tubes. These sources produce a complex spectrum of electromagnetic radiation and some of it is in the UVC region and a smaller part of this UVC is exactly in the region that is of interest in killing pathogens.
The production of UVC by above means also produces ozone as a side effect of the use of high strength electromagnetic field necessary to operate the lamps. In this application ozone has to be removed as it is has a negative impact on human health. This adds to the cost and the complexity of the equipment.
An alternative method of removing pathogens from the air is to use HEPA filters as disclosed by Mark A. Tuckerman et al in U.S. Pat. No. 5,616,172. This method has a draw back as the pathogens are not killed but blocked and the filter surfaces are used by the pathogen to multiply. Periodic replacement of the filters can solve this but at a cost of very frequent maintenance. Smaller pathogens, like viruses, cannot be removed by this method.
Further improvement on this filter based method has been disclosed by William Morrow et al in U.S. Pat. No. 5,656,242, where UVC produced from tube lights is used to kill the pathogen that is deposited on the filter surfaces. Although an improvement, the method requires a large number of tube lights, regular replacement of these tube lights as they have rapidly declining power curve with time and they need to be operating twenty four hours a day and very day.
Another method found in U.S. Pat. No. 7,407,633 uses UV light to create ozone to kill pathogens. While effective in killing most forms of bacteria, ozone still needs to be removed.
US Pre-Grant Patent Application Publication (PG-Pub) 2005/0249630 A1 by Olubunmi Ayodele Odumuye et al discloses a method wherein UV light produced by tube lights is used to purify the air or other carriers of pathogens such as water. This method suffers from the drawbacks of all of the above methods, namely it is costly and produces ozone.
US PG-Pub 2010/0320440 A1 teaches producing electromagnetic radiation in UVC, specifically 365 nm, aimed at killing pathogens. This method is effective provided there is sufficient time to overcome the low power of these devices and is confined to purifying water in small containers over hours.
In U.S. Pat. No. 6,555,011 by Zamir Tribelsky et al, claim is made of an apparatus whereby the gas or liquid to be disinfected is contained in a parabolic chamber that is then exposed to electromagnetic radiation of many types, the claim is broad without specific information on how to implement such a solution using lasers and claim is confined to use in a parabolic chamber.
US PG-Pub 2011/0142725 A1 by Xuanbin Liu et al discloses an air purification method that uses electromagnetic radiation through a glass with a coating of a photo catalyst and the light source claim limited to VCSEL, led, lamps but not solid state lasers.