This week we will look at the history of disinfecting objects with ultraviolet light. This is a continuation of last week’s column where we looked at the basics of UV illumination covering both wavelength and fluorescence, and now we will report on what new research is being carried out to help stem the pandemic we are suddenly in.
Since short wave UV light has the highest energy per photon in the UV spectrum, it has the greatest power to kill germs that reside on surfaces. This was first announced back in January 1878 when Arthur Downes and Thomas Blunt published a paper “On the Influence of Light upon Protoplasm” in the Royal Society of London reports, where they described the sterilization of bacteria when exposed to light of various colors by using simple boxes equipped with light filters. More experimentation followed and a few years later the 1903 Nobel Prize for Medicine was awarded to Niels Finsen, a Danish scientist, for his use of UV against lupus vulgaris, the bacteria that causes tuberculosis of the skin.
Because UV light with wavelengths between 200 nm and 300 nm are strongly absorbed by nucleic acids, the absorbed energy can change the composition of bacteria cells including those with pyrimidine dimers. These modified dimers prevent replication of the cell and can prevent the expression of necessary proteins, resulting in the death or inactivation of the living organism.
Using UV light to disinfect drinking water is an old process, dating back to 1910 when a prototype water treatment plant was constructed in Marseille, France. Six years later, the first US application of UV water disinfection was used in Henderson, Kentucky in a plant supplying water from the Ohio River. The equipment there used three mercury vapor arc lamps, all driven by a 220-volt direct current generator, where the water to be sterilized was run past the lamps housed within a large pipe. Running for many years, this location could clean 3 million gallons of water a day.
UV lamps first became commercially available in 1930 and following the end of the Second World War in 1945, germicidal UV lamps became more and more commonly used to disinfect air. They were commonly found in public restrooms and as a kid I remember seeing the peculiar purple UV bulb through the air fins inside of a hand dryer at our local E.J. Korvettes store.
What kind of bulbs can emit shortwave UV light? The most common is the low pressure mercury bulb. Invented by Peter Cooper Hewitt, American electrical engineer and inventor, in 1901, the lamp found widespread industrial use whenever UV illumination was needed. A high pressure variation of this bulb can still be found in streetlights or floodlights today. They are the ones that have a distinctive violet color, as opposed to the yellow sodium types.
Tiny UV bulbs like the one in the hand dryer mentioned above are really low pressure mercury bulbs and have a thin quartz envelope outside in order to allow the transmission of short wavelength light. As mentioned last week, ordinary window glass will absorb the generated UV light. You can easily tell when that type of bulb is operating because the short wave UV radiation converts tiny amounts of the oxygen present in the air to ozone, O3, which has a pungent odor and which also has a deleterious effect on any microbes present. But because ozone is a highly reactive and unstable gas capable of damaging living cells, such as lung cells, its production, even from common hand dryers, has been regulated in many states.
Besides mercury bulbs, ultraviolet light can also be generated by an LED (Light Emitting Diode). Some new solid state devices available on the market today include ones that can make wavelengths between 255 and 280 nanometers which do not have enough energy to produce ozone but can destroy bacteria. In time, however, you will see LEDs based on diamond structures that will emit even shorter rays.
Some systems, such as the Skytron UV cleaning robot, are being used now to quickly disinfect jails, planes, buses, busy factories and operating rooms. They look like R2-D2 carrying a bank of UV lamps. Some versions come equipped with pulsed-xenon tubes that also emit short wave UV and are the cousins of the electronic flash that older cameras used to have where you would have to wait for the energy storage capacitor to charge up before you could take a photo.
But can short wave UV light work on the new pandemic we are seeing? COVID-19 is a new breed, and as such there is a dearth of studies on its sensitivity to UV illumination. To be scientific, we can look to previous coronaviruses, such as SARS and MERS, for insight. Studies on these show that UV light does inactivate those viruses, so it’s not unreasonable to expect that it will have a similar effect on COVID-19.
For the past 4 years, a group at Columbia University Medical Center led by physicist David Brenner has tested various wavelengths of UV light and has found a certain band that eliminated bacteria and viruses on surfaces but did not harm lab mice. They are now shifting emphasis to COVID-19 and will have data soon according to their website.
In China, whole buses are being lit up by rows of purple UV lights each night, back at the yard, where large short wave UV emitters “scrub” the internal surfaces clean.
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