🛤️ 🙍 🤵🏻 UV sterilization box for reuse of disposable masks 🎏 🙍🏿 🌿

Recently, there has been great interest in using ultraviolet radiation as a potential method of sterilizing protective equipment for reuse, such as, for example, expensive “N95” masks.

In this article, I would like to describe the assembly of a box for disinfecting objects with ultraviolet radiation and a little analysis of the methods of ultraviolet sterilization.

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Description of the subject of the article

Nuke Box is a box designed to sterilize the surfaces of objects placed in this box. First of all, I mean medical masks, which seem to be lacking everywhere. And although the restored masks are not perfect, but when you have to choose between this and the lack of protection in general, it is better to have at least something than nothing.

Stanford Medicine has just released a study of sterilization methods with dry hot air, steam and UV sterilization and how they affect the performance of N95 masks sterilized by such methods.

Theory of UV Sterilization

Ultraviolet disinfection is a way to damage DNA pathogens so that they lose their ability to reproduce and therefore make them sterile.

Viruses are especially sensitive to this because they do not have cell walls or a cell membrane. Using high energy photons in the short ultraviolet range, DNA of pathogenic microorganisms can be damaged.

Ultraviolet radiation is usually divided into three wavelength ranges.

Long-wave ultraviolet radiation (UV-A) with a wavelength of 315–400 nm is familiar to many from black lamps at discotheques, the radiation of which makes white t-shirts, braids, bracelets, eyes and even smiles of dancers shining in the twilight of the dance floor. The same lamps can be found in banknote detectors.

Medium ultraviolet (UV-B with a wavelength of 280-315 nm) is what gives you a sunburn and, in the long run, skin cancer.

And finally, short-wave ultraviolet (UV-C with a wavelength of 100–280 nm) is bactericidal and completely absorbed by the ozone layer in our atmosphere, which also means that this radiation is not found in the nature around us. We can obtain this ultraviolet artificially using special lamps containing mercury and modern UV-C LEDs.

Keep this information in mind when working with ultraviolet radiation. It is important to use protective equipment and protect your body, eyes and any open wounds from ultraviolet radiation, as it can damage the DNA in human cells ( i.e. cause cancer)

The ability to damage pathogens does not depend on the intensity of the UV lamp, but depends on the dose they receive. This is a cumulative effect, very similar to radiation, so if you irradiate the surface for a long time, you can make it sterile. It also means that you can control the rate of sterilization of equipment, since an increase in radiation intensity will increase the dose rate received by pathogenic microorganisms.

The dose is measured in μW * s / cm ² (microwatt second per square centimeter) or sometimes μJ / cm ² (microjoule per square centimeter) is used. Although the unusual units are scary, the formula for calculating the dose is quite simple:

Dose = (Intensity of radiation at this specific point) x (exposure time)

For example, in my case I use a 10 W bactericidal lamp with a nominal power of 2.7 W in a UV spectrum with a wavelength of 254 nm. This radiation at a point 7 cm from the lamp theoretically gives an intensity of 1364 μW / cm ² . The calculations will be a little later.

Source

UV disinfection is quite easy to use. You take a short-wave ultraviolet lamp and shine it on the surface that you want to disinfect.

In practice, the calculation of the radiation intensity at a particular point depends on the shape of the radiation source used. If we make some obvious assumptions, calculating the intensity can be quite simple. Assume that:

.
( r ) , , (L) ( r << L).

Using such assumptions, we can calculate the theoretical intensity distributed on a cylindrical surface at a distance r from the ultraviolet lamp.

Intensity = Total power of the ultraviolet radiation source) / (Cylinder surface area)

i.e.
Intensity = P / (2 * PI * r * L) , where
P is the power radiated by the lamp, L is the length of the lamp, r is the distance to the point where we want to measure the intensity.

I tried to make a beautiful computer drawing to illustrate this, but was disappointed and instead painted it manually somehow like this:

Thus, under ideal conditions, when using a lamp with a length of 45 cm, and which emits a power of 2.7 W in ultraviolet, the intensity will be 1364 μW / cm ² at a point at a distance of 7 cm from the lamp. These are the actual values for this design. Further, we can use the obtained intensity value and calculate the time theoretically necessary to sterilize the surface from a specific virus.

You may have noticed that I mentioned that although I use a 10 W lamp, its power in a short ultraviolet is only 2.7 W. This is one of the key characteristics of the lamps, since the remaining power is emitted in other spectral ranges of electromagnetic radiation - a beautiful blue light, which we can see, and in the form of heat.

Of course, our assumptions are very perfect, and we are dealing with a real system. At the ends of the lamp, side effects will be observed, the radiation power can be distributed unevenly, the lamps will need time to warm up, etc. Therefore, we can take our ideal model of the sterilizer and design it with such a margin that we will certainly feel safe. That's why it takes 3.67 seconds theoretically under ideal conditions to neutralize SARS-CoV-2, and in practical medical research, numbers from 300 seconds (5 minutes) to 1800 seconds (30 minutes) are called. Any errors will be tragic, so all researchers are very careful in evaluating the required sterilization time.

Before we get started building the Nuke Box, we’ll make two statements about:

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Nuke Box

Below will be described step by step how I will build a sterilization box in which the masks will be located at a distance of no more than 7 cm from the upper and lower sources of ultraviolet radiation.

First of all, it is SAFETY! Please cover with thick clothes the exposed areas of the skin of your body, any existing damage to your skin, and wear safety glasses from ultraviolet radiation. Also try not to spend too much time in close proximity to radiation sources while they are on.

In this manual, I will use factory ultraviolet lamps ( electronic ballasts for igniting bactericidal lamps may differ in high current from lamps for standard lamps) The luminaires can be easily purchased and are supplied with the necessary components (ballast, starter, T8 lamp size) and wires. I will also use ultraviolet bactericidal lamps. In Japanese-made lamps, this will be the GL series (in this case, the Toshiba GL-10), which stands for a bactericidal lamp and is manufactured by Panasonic, NEC and Toshiba. They are available in power ratings from 10 W to 40 W and in various sizes. In the USA, it will be the TUV (Tubular UltraViolet) series from Philips Lighting.

These are special low-pressure mercury fluorescent lamps. Due to the thermionic electron emission of electrons, the heated cathode ionizes mercury atoms, forcing them to emit light with different frequencies, one of which has a length of 254 nm. This is very close to the peak of absorption of DNA at a wavelength of 260 nm and the energy of photons that collide with DNA, damaging it and depriving pathogens of the ability to multiply.

Lamps also emit some visible light and heat. That's what gives them a cool, cyber-punk glow ...

Assembling a sterilization box will actually be quite simple. I will take two solid plastic drawers and lay them on top of each other. I will also fix the wire cord inside and will use spring steel stationery paper clips to fix the masks to the cord.

I start by drilling holes in one of two plastic crates at intervals that correspond to the width of the N95 mask, which is about 15 cm.

Next, install fluorescent lights inside plastic boxes. I decided to drill a hole and cut the cable so that I could install the lights inside and leave their switch on the outside. I did this mainly for my own convenience and because I can work with electricity and soldering. In truth, you don’t need to follow this part of the instructions, and you can just leave the cables intact and pull them out through the junction of the boxes so you don't have to cut or solder the wires.

I drilled several holes in plastic boxes and fixed the fluorescent lamps in place with nuts and bolts.

Then pull the cord through the drilled holes to make a kind of clothesline for fastening inside the box of sterilized masks. I made a double clothesline, because it is important that the mask is oriented so that the lamps shine on it from above and below.

I attached the second lamp to the bottom of the second plastic box. There is no need to mount the cord.

Now we can connect the two drawers, add a kitchen timer and we got a Nuke Box.

I hope this is a useful instruction. I am personally interested in sterilizing masks for reuse. My wife's mother and sister are nurses working in hospitals and clinics in Melbourne and London. My mother-in-law’s clinic does not have enough masks, and probably the same thing happens in the UK. I hope that if they eventually have to reuse their masks, then there is a way to make it safe for them and their patients.

Enhancements

Here are some of the improvements I would make:

More lamp power : this is just a prototype. If we wanted to mass sterilize masks, we would use 40 W lamps. The sterilizer will be able to place more masks and sterilize them faster.

More radiation power : we can also place several radiation sources above and below. By installing two radiation sources and one row of masks, you can double the irradiation intensity of each mask. This will halve the sterilization time.

Big box: I would also try to place as many masks as possible without obscuring each other. Here is an example diagram with two rows of masks. If something like this will be used for mass sterilization, you need to optimize the time and number of masks that can be sterilized in each cycle. Here's a rough example of how I tested inserting two rows of masks into a Nuke Box prototype. You can put 6 masks in this prototype.

If you made a sterilizer with a 40 W full-size fluorescent lamp with several lamps above and below, then you could process a lot more masks each time you start the sterilization cycle.

A comment on the article from myself: two years ago I bought such a UV lamp on Aliexpress.

It is similar to that used in this design, but the kit has a convenient timer for 5.15.30, 60 minutes with a delayed start. And the lamp has the ability to consistently build up a chain of several such lamps.

I also recommend a recent article on Habré Coronavirus 2019-nCoV. FAQ on respiratory protection and disinfection .

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UV sterilization box for reuse of disposable masks

UFO Care Minute

We urge you to be critical of any published information.

Description of the subject of the article

Theory of UV Sterilization

Nuke Box

Enhancements

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