UPDATE: You can download the code in this notes in this notebook and play with …
Recently there’s been a lot of interest in using ultraviolet light as a potential way to sterilize protective equipment like N95 masks for reuse. Stanford Medicine just came out with a study on sterilization techniques and how they affect the performance of N95 masks with dry heat, steam, and UV sterilization as potential techniques.
In this HOWTO, I’d like to go through the build and a bit of analysis of ultraviolet techniques for sterilization.
Nuke Box is a box designed for sterilizing surfaces. In putting it together, I had medical masks in mind which seem to be a shortage almost everywhere. Although re-used sterilized masks aren’t ideal, when having to decide between that and having no protection, having something is probably better than nothing. In that vein, if masks need to be sterilized, it’s important to try and make them as safe as possible. Also something like this could potentially be useful in developing countries that may need to re-use masks out of necessity and cost.
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 10W 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 3 minutes to 30 minutes.
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)
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. I 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 2.7W 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 10W 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 300 seconds (5 minutes) to 1,800 seconds (30 minutes). Any mistakes would be tragic so everyone is being extremely conservative on the sterilization time estimates.
Before we get to the actual build of the Nuke Box, two things:
- Collaboration: It would be great to work with a lab that can help identify experimental results, especially for Covid-19. If you can help us test the sterilization rates, esp for Covid-19 coronavirus, please let us know. We’d love to collaborate to confirm and also improve the design.
- Liability: This design would fall under the Good Samaritan Law. We can’t take responsibility if anything goes wrong with this design. These are strange times and we’re trying to become knowledgeable about new subjects as quickly as we can.
In this HOWTO, I’ll be building a box where the masks will be located at most 7 cm from the top and bottom light sources.
- The first thing is SAFETY! Please cover exposed skin, any open wounds, and wear UV-blocking eye protection. Also try not to spend too much time very close to the lights while they are on.
- In this particular HOWTO, I’ll be using off-the-shelf fluorescent fixtures. These can be easily bought and come with the required components (ballast, starter, T8 sockets) and wiring. I’ll also be using UV germicidal fluorescent tubes. In Japan, these would be the GL series (Toshiba GL-10 in this case) which stands for germicidal lamp and are made by Panasonic, NEC, and Toshiba. They come in sizes and power ratings from 10W to 40W. In the US, they would be the TUV (Tubular UltraViolet) series by Philips Lighting.
- These are low pressure mercury fluorescent bulbs. The bulb filament ionizes the mercury atoms causing them to give off light at various frequencies, one of them being at 254 nm. This is very close to the peak absorption of DNA at 260 nm and the energy from the photons that hit the DNA damage it and render pathogens unable to reproduce.
- They also give off some amount of visible light and generate heat. That’s what give them the cool, cyber-punkish glow…
- This build is actually going to be quite simple. I’ll be taking two box bottoms and laying them on top of each other. I’ll also be setting up wire cord in a clothesline fashion and using spring-steel paper clips to attach them.
- I start by drilling holes in the bottom box with a spacing that accomodates the width of an N95 mask which is about 15 cm.
- Next is to put the fluorescent fixtures inside the box. I decided to drill a hole and cut the cable to the fluorescent lights so that I could fit the fixtures inside and have the switch on the outside. I mainly did this for my own convenience and because I’m comfortable working with electricity and soldering. Truthfully, you don’t need to do this part and can just leave the cables intact and peaking out the seam so you don’t have to do any cutting or soldering.
- I drilled some holes in the box and added nuts and bolts to keep the fluorescent fixtures in place.
- Then strung the cord through the holes that were drilled to make clotheslines for the mask. Once that’s done, you can just clip the masks to it. I used a double clothesline because it’s important to have the mask oriented so that the lights shine on the top and bottom directly.
- I attached the light fixture to the other matching box bottom. I didn’t add the clotheslines though since this light will shine down on the masks.
- Now we can put the Nuke Box together. Literally. We just put them together.
Slap on a kitchen timer and that’s the Nuke Box. I hope it’s a useful HOWTO. I have a personal stake in sterilizing masks for re-use. My partner’s mother and sister are nurses working at hospitals and clinics in Melbourne, Australia and London, England. My mother-in-law’s clinic doesn’t have enough masks and it’s likely the same in the UK. I’m hoping that if they end up having to reuse their masks, there is a way to make it safe for them and their patients.
Here are some of the improvements I’d make:
More Power: This is just a prototype. If we wanted to bulk sterilize masks, we’d be using the 40W long bulbs (think standard fluorescent lights). They’d accomodate more masks and sterilize them faster.
More Power: We can also put an array of lights on top and bottom. Having two lights installed and a single row of masks would double the intensity on each mask. This would cut the sterilization time in half.
Bigger Box: I’d also try to accomodate as many masks as possible without shading each other. Here’s a rough layout for having two rows of masks. If something like this is going to be used in a mass sterilization environment, you’d need to optimize the time and amount of masks you could sterilize in each cycle. Here’s a rough example of me testing out putting in two rows of masks into the prototype Nuke Box. You could fit in 6 masks total in this prototype box. If you did a full-sized 40W fluorescent box with multiple lamps on top and bottom, you could scale it to do a lot more masks in each run.
Here are more links to references and articles on using UVGI to sterilize masks and PPE:
- N95 Filtering Facemask Respirator Ultraviolet Germicidal Irradiation (UVGI) Process for Decontamination and Reuse, Nebraska Medicine
- Comment: A great read on how Nebraska Medicine is pioneering reusing masks in the face of shortage
- Effects of Ultraviolet Germicidal irradation on N95 Masks, NIH
- Can Facial Masks be Reused, Stanford Medicine
- Inactivation of the Coronavirus that Induces Severe Acute Respiratory Syndrome, SARS-CoV, Journal of Virological Methods
- Inactivation of Viruses on Surfaces by Ultraviolet Germicidal Irradiation, NIH
- Ultraviolet Germicidal Irradiation of Influenza-Contaminated N95 Filtering Facepiece Respirators, NIH
- Building a Crude UV-C Mask/PPE/IPad/Phone Disinfection Chamber (1 hour, $30, minimal tools) Part 1
- UVGI steriliser
I’ll add more as I find them.
Good luck to us all and much love to everyone in the world from Hackerfarm!