UltraViolet Germicidal Irradiation (UVGI) is a way of attacking the DNA of pathogens so they can’t reproduce. Viruses are especially susceptible to this because they have no cell walls or cell membrane. Using high energy photons in the UV-C frequency band, it’s possible to damage the DNA in pathogens rendering them unable to reproduce and thus sterile.
With UV light, there are three frequency bands. UV-A (315–400 nm) would be the standard blacklight that makes neon colors all glow-ey. UV-B (280–315 nm) is what gives you sunburn and skin cancer. UV-C (100–280 nm) is germicidal and completely absorbed by the ozone layer in our atmosphere which also means it’s non-naturally occurring. We can produce it though using mercury fluorescent lamps and UV-C LEDs.
With this in mind, it’s also important to wear proper safety equipment and cover up your body, eyes, and any exposed wounds since the UV light can also damage DNA in human cells.
The ability to damage pathogens isn’t dependent on the intensity of the UV bulb but rather the dosage. It’s a cumulative effect much like radiation so as long as you irradiate the surface long enough, it’s possible to render the pathogens on it sterile. This also means that you can adjust how quickly you sterilize equipment since increasing the intensity will increase the dosage rate for pathogens.
The dosage is measured in μWs/cm2 (micro Watt seconds/squared centimeter) or sometimes μJ/cm^2 (micro Joules/squared centimeter). Although the units are intimidating, the formula to obtain it is quite simple:
Dosage = (Intensity at that particular point) x (time exposed)
For example, in this particular HOWTO, I’m using a 50W germicidal fluorescent bulb with a rated output of 2.7W of UV-C light (UV light at 254 nm). This light theoretically puts out 1,364 μW/cm^2 of intensity at a point 7 cm away. I’ll go through the calculations in a bit.
The Covid-19 virus requires 5,000 μWs/cm^2 (5 mJ/cm^2) for sterilization. Based on the above formula, it would take 3.67 seconds of UV irradiation to inactivate the virus. There are other factors to consider though such as UV light warm up time, inaccuracies with the theoretical model, etc which is why you see people irradiating objects anywhere from 1 minutes to 30 Sec.
Theroy is Source: Link
UVGI is a pretty straightforward approach. You take a UV-C fluorescent bulb and shine it on the surface you want to disinfect. Calculating the intensity actually depends on the shape of the light source. If we make some educated assumptions, the intensity calculation can be quite straightforward. These are the two assumptions that are important:
- The power radiated in the UV-C band is uniform across the whole light
- The distance we’re looking at is very small compared to the length of the light (r << L, where r is the distance we’re trying to calculate, L is the length of the light)
With those two assumptions, we can approximate the intensity at some point “r” from the fluorescent lamp as evenly distributed on a cylindrical surface. The formula to calculate the theoretical intensity would then be
(Total UV power) / (Surface area of a cylinder)
or
P / (2 * PI * r * L)
where “L” is the length of the lamp and “r” is the point that we’re trying to calculate the intensity for. We tried to make a beautiful CAD drawing to illustrate this but got frustrated and hand-drew it on a Post-It instead:
So with a 45 cm bulb that radiates 50W of power, assuming ideal conditions, the intensity will be 1,364 μW/cm^2 at a point 7 cm from the bulb. These happen to be the actual values for this build. We can also use this value and calculate the theoretical time it would take to sterilize a particular virus.
You may have noticed that I mentioned I’m using a 50W bulb but it only has 2.7W of power radiated in the UV-C spectrum. This is one of the key specs of the bulbs since the rest of the power is radiated in other frequencies (ie: the beautiful blue light that can be seen by us) as well as given up as heat.
Of course our assumptions are very ideal and we’re dealing with a real-life system. There will be fringe effects at the ends of the lamp, the power may not be evenly distributed, the bulbs take time to warm-up, etc. Hence we can take our ideal model and then add a healthy margin to it to make sure it’s safe. That’s why it may take 3.67 seconds to theoretically inactive SARS-CoV-2 under ideal conditions, but the medical studies are using 60 seconds (1 minutes). Any mistakes would be tragic so everyone is being extremely conservative on the sterilization time estimates.
