UV-C bulbs use a dab of mercury plus a glowing hot filliment in a low pressure tube of glass to generate ultraviolet light that peaks around 253.7 nano meters, the  optimal germicidal wavelength in the UV-C part of the spectrum.  Not to be confused with UV-A or UV-B, UV-C has the ability to quickly damage DNA and RNA.  UV-C is what's doing the microbe killing.  UV-C is also the light blocked by our ozone layer that would otherwise greatly increase the rates of cataracts and skin cancer in mammals like us humans.  This may be why sunshine hitting the earth lacks the germ killing abilities some would expect to find there.  Perhaps that hole in the ozone layer we made years ago with aerosols and freon would make light that hits the earth more germicidal.  But you have to go to Antarctica and live under the hole to enjoy these benefits.  And the added risk of skin cancer and cataracts.   Back to the bulb.  Mercury when energized is really good at producing light at 253.7nm.  This is what makes these bulbs germicidal.  Fluorescent bulbs at the offices would be germicidal too if it weren't for the phosphorus in the bulb and UV blocking filter they have.  Dont go opening a bulb trying to weaponize it against germs.  The mercury they contain is deadly on it's own to humans even in trace amounts.  Just buy a germicidal bulb.  They're used widely in water purification systems for aquariums and well water to large scale municipal water treatment systems.  They're also used in air purication systems.  However, UV-C may not be effective in these systems.  Killing a germ with UV-C light depends on 3 things.  Intensity of the light in the 253.7 nm part of the spectrum, how far the germ from the light source and how long the germ is exposed to the light.  Airborne germs moving fast in a mass of air at  varying distances past a dinky light may not damage the germs at all.  Therefore some UV-C based air purifiers systems may not work at all.  This includes the portable models as well as the more expensive systems added to your home HVAC system.  I would look to see what reliable 3rd party science based testing has been done on these devices before you potentially waste your money on something that will be ineffective.  Or possibly  detrimental your health if it produces Ozone.  Ozone is created when UV strikes oxygen atoms at a specific wavelength.  It can be managed by filtering light at that wavelength but that's another discussion.  

Systems with enough intensity to work may generate too much ozone, something you don't want to breath. Ozone in your air feels like your breathing too much chlorine.  Like when cleaning a shower with chlorine bleach and no ventilation. 

Back to the DIY germ killing machine.

For this project choose a fluorescent UV-C germicidal bulb.  From Amazon or elsewhere.  The higher the wattage bulb, the more intense the germicidal light.  100w bulb doesn't doesn't equal 100w of UV-C light.  The bulbs I've seen produce around 25% of there light in this range.  But more watts means more germ killing.  Back to the weapon.  Purchase your germicidal bulb.  Buy a matching ballast something neccessary for all fluorescent lights due to how they work.  They change their electrical resistance as they' operate.  The ballast, the part that hums in the light above you at the office keeps this in balance.  Chose a ballast with enough capacity for the bulb or bulbs in your design.  Wire the bulb and ballast together.  The wiring diagram is usually right on the ballast.  Wire a plug to the other end for the 110v AC wall outlet.  ZIp tie it all to a stick.  Add foil behind the bulb to reflect the light and enhance its intensity applied to your target.  Metal foil tape used on duct works well.  But not duct tape.  Aluminum is great at reflecting UV-C light.  Don your UV safety glasses or appropriately rated sunglasses, plug your contraption in to the wall.  Preferably not while holding it for this first test and never while standing in a pool nor with both hands could create a deadly to you electrical circuit through your heart.  Pick it up one handed at first.  If it doesn't electrocute you, start slaying microbes.  Slowly sweep the light near the surface you're sanitizing.  The calculation for how much exposure is required to do the trick is a more technical discussion.  Depending on the three factors mentioned intensity, proximity, and exposure time, it could take seconds to several minutes to do the job.  One bulb manufacturer cited 6 inches for 10 seconds from a 32 total watt bulb kills 99. (something) % of the microbes.  Keep in mind something as insignificant as a box of tissues, a crumb or a particle of dust is all it takes to shield the microbe from damage.  Allowing it to remain viable and potentially pissed off for up to 17 days in the case of the coronavirus.  Keep your weapon pointed at the germs, avoid prolonged contact on the skin and always wear shades while it's powered up.

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Update April 4th 2020.

Thank you for your comments and I appreciate the skeptical feedback.  If anyone can suggest improvements or sees errors  I've made please speak up.   It really helps us all a great deal.

The text below is my response to comment rooted in healthy skepticism,  I've copied it here so it doesn't get lost.

I know there's a lot of misinformation swirling about regarding COVID-19. Some of it comes from people trying to profit from this horrible situation. I am not one of them. I have nothing to gain here by promoting this and I'm going to keep it that way. I don't want people wasting time trying to figure out some non existent selfish motive before coughing up $30. I believe the sooner people have access to something like this the safer they're likely to be. I encourage people to build their own but I'd rather just give them one if that's not an option. And I will continue to build these and give them away as long as I can and for as long as I think it's helping. The science behind this thing isn't so easy to explain or to understand for anyone including me. I don't think anyone who knows me would describe me as soapbox kind of guy. I don't like the soapbox. I can't wait to get off the damn thing.

So here's an answer to a comment I received regarding my build on Hackaday from someone skeptical about whether these UV-C lights will work on COVID-19. The answer is a 99.99% yes but with some reasonable assumptions made. Nothing is 100% when it comes to this stuff. I'm a skeptic, normally. I hope others are too.

COVID-19 is new. Little direct research has been done on this new arrival to our world. So we make some assumptions. Reasonable assumptions are the foundation of science. I and many others are making the assumption that COVID-19 is susceptible to UV-C at some level just like the other 6 versions of Corona viruses are that are known to infect humans. Anything with DNA or RNA can be damaged by UV-C light in the 253,7nm wavelength of the spectrum as long as it can get to it. Different microbes including viruses die or are disabled at different levels of Fluence., the measure of this exposure. Time of exposure, distance from the light, and intensity all are factors. So I'm making the assumption that the RNA of COVID-19 can be deactivated or destroyed at some level of exposure by UV-C, just like other versions of Corona as well all other microbes we humans worry about.. And I'm assuming that it will take something less than or equal to what is necessary to kill all microbes we know about to do so. This is conservative or at least reasonable. The list of these microbes include fungus and bacteria likely to be far more resistant to UV-C. So I choose to believe exposing COVID-19 to the level of UV-C to kill every other microbe will do the trick. And that level is most often cited as 10 seconds of UV-C exposure at a wavelength of 253.7 nanometers from a 32 watt bulb 6 inches away will result in a Log 4 reduction of ANY microbial population. It will kill or deactivate 99.99% of the microbes on a surface. Nothing is 100%. I am not profiting from any of this. and I don't want to . With all the misinformation flying around such as Antibiotics killing viruses it's tough enough to pick through and determine what's real. I have nothing to gain by people believing me. I just happen to know something that will help with 99.99% effectiveness. It's cheap. It's easy to build and use. And it's safe to use unless you're a complete idiot. I don't want to come out the other side of this pandemic, assuming I will, knowing I didn't do my best to tell people a way for them to easily help themselves. It would be like not yelling fire in a movie theater. I'm just communicating what I know. You do what you want with the information. But get the hell out of the burning theater while you decide.

Here are some links to more information about UV-C .

Ted Talk on the subject: https://www.ted.com/…/david_brenner_a_new_weapon_in_the_fig…

Link to reference articles: https://www.forbes.com/…/these-uv-sanitizers-can-kill-off…/… https://www.medicalnewstoday.com/articles/324284 https://www.sciencemag.org/…/could-ultraviolet-lamps-slow-s… https://www.scientificamerican.com/…/how-does-ultraviolet-…/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3831650/…

Other: https://www.nobelprize.org/…/medi…/1903/finsen/biographical/ https://www.cnbc.com/…/cdc-coronavirus-survived-in-princess…