The device is a lightweight box with UV-C bulbs installed inside, lined with reflective material, fitted with two doors and a removable frame. To treat PPE (here N95 masks), they are hung on the frame that is inserted into the center of the box from one side, treated with light, then removed from the opposite side. Users can create a physical barrier between the two sides, with a protocol to keep treated and untreated materials separate, so one person per side can continuously operate the device.
This design can be modified to make devices of different sizes. The critical elements are the power of the UV-C emitted from the bulb, its distance from the material to be treated, the time that material is exposed to the UV-C light, and the elimination of shadows in the material so all the material is directly exposed to the UV-C light.
If you wouldn’t risk your grandma’s life on your own calculations of these details, bring experts onto your build team.
This design uses two 33″ UV-C bulbs with manufacturers’ ratings of 15W emitted UV-C radiation and 40W of operational electric power. WATTAGE OF ELECTRICITY IS NOT THE SAME THING AS WATTAGE OF UV-C GIVEN OFF. CONSULT MANUFACTURER SPECS TO GET BULBS THAT GIVE OFF APPROPRIATE UV-C. You MUST understand how to interpret manufacturer’s specifications like these for your specific bulbs.
Testing UV-C Levels
Test your final device to ensure it emits effective levels of UV-C. Radiometers capable of measuring UV-C, like this example, are one way to test the UVGI output of a device if you know how to use them. Another is to use color-changing card dosimeters like these, which indicate the amount of UV-C radiation emitted by a device. Many medical facilities use these card dosimeters. UV-C bulbs degrade after extensive use, so testing them periodically is the only way to insure you are getting effective doses of UVGI.
In this design one bulb is mounted per side so both sides of the PPE are treated simultaneously, avoiding the need to flip PPE halfway through a treatment. A timer is wired and set with the exposure time required to reach at least the 1 Joule/cm2 threshold established by the scientific literature as necessary to be effective.
We strongly recommend treating each batch for longer than the calculations say are required to reach 1 Joule/cm2. For example, our radiometer readings for our specific bulbs suggest we can use a treatment time of 2:40 (two minutes forty seconds) to reach 1 Joule/cm2. We installed a five-minute timer, almost doubling the treatment time. We want to be sure we’re being effective, and we find value in telling first responders and health care workers we’re using almost double the treatment called for. Cooking masks for other people is the easy part. Remember that the people whose faces are going in them need to trust you.
External: 48″ high, 16″ wide, and 12″ deep
(from the perspective of an open door).
Internal: 46.5″ high, 14.5″ wide, and 12″ deep
(doors are mounted externally)
Dimensions were determined by using one 33″ bulb emitting 15W of UV-C on each side. Each bulb is positioned about 5.5″ away from the center of the cabinet (the approximate center of the mask). To determine the width of the box, account for the size of bulb mounting hardware as it sticks out from the interior sides. Our design does not locate masks above or below the ends of the bulbs. That risks getting a lower exposure to the bulb’s output.
Revise your box dimensions according to the dimensions and power of bulbs you use. Since 33″ bulbs tend to be easily available, scaling up is easily done by adding more boxes next to each other instead of building big, unwieldy devices. When the emergency is over, they can be easily dismantled and stored.
We intentionally left out details on electrical wiring.
If you don’t know how to do that safely, find someone who does.
Device construction details
Sides: ¾” plywood cut to 46.5″ x 12″
A box 12″ deep allows N95 masks to stretch securely with full exposure to a single bulb on each side.
Top and bottom pieces: ¾” plywood cut 16″ x 12″
Install top pieces on top of the side pieces, giving an exterior width of 16″ and an interior width of the box of 14.5″.
The important factor is that bulbs be about 5.5″ from the center of the cabinet, as determined by the height of the mounting hardware. Adjust the width of your box if your mounting hardware or bulb power are different.
Use any shape and size bulb mount you like, but be sure to take the final distance between the bulb and your PPE into account as you calculate your necessary treatment time.
Using a router and chisel to carve out the mounting location for bulb mounts. Bulbs are mounted in the center of the side panels.
Bulb mount installed on a side panel before the reflective foil is installed.
Aluminum radiant barrier foil is the reflecting material used inside the box. It is strong and comes in 48″ wide rolls, which makes it easy to work with. It is attached to the inside of all pieces with spray adhesive. Do not use stainless steel as a reflective material. It is a poor reflector of UV-C. Read about it if you need to be convinced.
Aluminum radiant barrier foil is attached to the inside of all pieces with spray adhesive.
Box and PPE Frame
Apply weather stripping to keep UV light from spilling out. We used a stapler, as the adhesive backing isn’t very strong.
Doors: ½” plywood. Dimensions: 16″ x 48″
Cover the interior door surface in reflective foil.
Note that ½” plywood can warp, so consider using ¾” for these if you don’t mind the extra weight. If operators do not look through the gap while the bulbs are on, they should easily avoid exposure to the harmful light.
Mount some sort of latch to secure the door while lights are on. We used barrel latches, but anything secure is fine.
Frames for holding N95 masks: 1″ x 2″ pine
External dimensions 46″ x 11.5″
Cut side pieces to 46″, top and bottom to 10″
This size fits within the cabinet and stretches out the N95 masks. We used wood glue and cabinet screws to connect the pieces. Use a pilot hole to prevent splitting the ends of these pieces.
After wrapping the frame in reflective foil, make a jig to set locations of hooks and drill pilot holes spaced 5.5” and insert small hooks to hold mask straps.
Vent holes can be cut in the top and bottom for air circulation in the cabinet during each treatment. This will reduce the accumulation of a burnt odor masks might take on over repeated cooks (an issue found in some studies), and they also provide a safe visual indicator that the bulbs are on (so the operators don’t accidentally open the box and risk eye damage).
We use a 2″x4″ across the base on the “dirty” side to reduce air entering from that side, and it also helps lateral stability. If additional ventilation is wanted, a small fan can be placed near an opening at the bottom of the “clean” side. Experts in the field told us that masks can take on a burnt odor after repeated treatments, but we’ll share that we cooked our own masks for 30 minutes (the length of six treatments all at once), and we didn’t notice an odor. Since then, we’ve cooked our own masks several times and have not noticed any odor.
You can install these devices most anywhere. Here are a few pictures of our installation in the Columbia Richland Fire Rescue Department’s trailer. You’ll see the plastic barrier that separates the dirty and clean sides. The fire department’s written procedures for use are on the “Process For Use” page.