Calculating exposure time to inactivate coronavirus by UV light

Sorry for a layman question. I'm having difficulties understanding some basic math apparently.


According to a [paper](https://www.tandfonline.com/doi/full/10.1080/02786820500428575), for a coronavirus, the UVGI dose for 90% inactivation is 339–423 μW sec/cm2.


I have a UV lamp with the following specifications:


* it's 15 watts
* it takes *1h30* min to 90% inactivate coronavirus in a 20 square meters room with a 3-meter ceiling.


But that doesn't make any sense if you believe the paper's number.


This is how I calculate the necessary exposure time:


* the surface of the floor, ceiling, and walls in the specified room is 94 square meters (or 940,000 square centimeters.)
* 15 watts is 15,000,000 microwatts
* from the 339–423 μW sec/cm2 in the paper, take the highest number


So since our surface is 940,000 cm2, we multiply 423 \* 940,000 and that gives us 397,620,000 μW sec/surface of the room.
Since our lamp is 15,000,000 microwatts, we divide 397 million by 15 million, and we get 26 (watts provided by our lamp) \* sec/surface of the room.


So 26 seconds is the necessary time exposure, but it should be 5400 secs as specified in the manual. Is there an error somewhere?




yes, there are several errors in your calculation.


Germicidal lamps specify an effective power output in UVC. This lamp


<https://www.osram.de/ecat/PURITEC%20HNS%20UV-C-UV-C%20lamps%20for%20purification-Ultraviolettstrahler-Industrie-Spezialbeleuchtung/de/de/GPS01_1028570/ZMP_1022255/>


consumes 16W of electricity but has a UVC output of 4W. You need to calculate with the 4W value.


Then you need to calculate the intensity of your irradiation on the surface you want to inactivate the virus on. The distance from the lamp to the surface is key, as the 4W UVC power remains contant over the surface of the light cone that shines from it. In the lamp's data sheet it says that this is valid for up to 3m from the lamp. (after 3m it most likely decreases due to other effects.) So you need to calcuate the distance from your lamp, and you need to calculate your powerdistribution over the surface of the sphere/cylinder (depending on your geometry), with the lamp in the center. It does not do to calculate the surface of the room as a whole. Given the irradiation intensity on the surface of that sphere you can now calculate the time needed until the UVC output of the germicidal lamp manages to reach the 90% inactivation dose for a given virus. For corona viruses different values for that dose are given in different papers. Some say 10.3mJ/cm^2 for 99%, some quote 0.6mJ/cm^2 for 90% inactivation. This one gives a good overview: <https://www.nature.com/articles/s41598-020-67211-2.pdf>


I recommend to choose a simple geometry for calclation of the irradiation power. Living rooms are quite tricky. Spheres are better.


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Likely related disinfecting questions are, apparently, daily becoming more of interest, and while not always best presented, are certainly worthy of scientific-based comments.


My suggestion, perhaps staining a virus-infected surface, with say a B-12 based photocatalyst (see, for example, [Enhanced photocatalytic activity of a B12-based catalyst co-photosensitized by TiO2 and Ru(ii) towards dechlorination](https://pubs.rsc.org/en/content/articlelanding/2018/ra/c7ra13037f#!divAbstract)), that releases hydroxide radicals capable of decomposing organics (including DNA) with UV light irradiation (or strong sunlight), is, coming from a non-expert in biocides, a suggested path. However, there are supporting sources (see this 2010 article available for free online: ['Applications of Photocatalytic Disinfection'](https://www.hindawi.com/journals/ijp/2010/764870/) and this 2015 paper ['Advances in photocatalytic disinfection of bacteria: Development of photocatalysts and mechanisms'](https://www.sciencedirect.com/science/article/pii/S1001074215002247)).


Upon complete decomposition of all present stained organics, the solution becomes colorless (likely measurable).


Interestingly, the article mentions measurable results in 30 minutes with UV irradiation. My experience with sunlight acting on aqueous B-12, and also intensely colored B-complex solutions, demonstrates surprising results (decoloration) in a few hours of sunlight.


In your particular case, add B-12 to a disinfecting alcohol mix (for example, one containing over 50% isopropanol, or [especially ethanol](https://chemistry.stackexchange.com/questions/127253/antibacterial-properties-of-ethanol-and-isopropanol/127307#127307) for addressing a virus concern, but have at least, 10% water) onto a white surface and apply your UV product (or, say irradiation by a natural sunlight electric bulb) for testing (take periodic pictures). Note, decoloration by itself, is only a chemistry-based presumption of disinfecting effectiveness. 


My proposed solution replaces the estimation issue with visible results, at least with respect to decoloration, by applying a doable photocatalytic spray. If your UV lamp is 'safe and useless', perhaps you can convert it into something that is part of a safe and effective protocol.


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The effectiveness of the lamp diminishes at a rate of twice the rate of the increase in distance. You have a kill rate o lets say 100 at 1 ft. You will have a kill rate of 50 at 2 ft an a kill rest of 25 at 3 ft you don't realy have any major worries about skin issues with the small lamps, if you are not sleeping nude under them. The ozium is as big a health issue as eye burn linet your exposure to breathing the sweet air produced by the lamp it as effective at killing germs and viruses as the light more effective at killing bacteria and mold. Protect your eyes with a welding eye shield for prolonged exposure and at least a good par of sunglasses for anything more than a minutes of direct eye exposure (looking at the light directly). Protect your pet's also signs of excessive eye exposure is red burning eyes that feel like thay are full of corse sand the same as if you have been watching someone weld without eye protection. If you are a few feet away, you don't have much skin worries with the small low power lamps. UVC will not penetrate your clothes.


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