UV-C light disinfection of n-95 face masks

In an effort to prolong the life of N-95 face masks, many institutions have been using UV-C light disinfection.  It is being looked at by 3M and they list testing results on their masks. FDA recommends following the guidelines of the manufacturer for UVC decontamination procedures. CDC references a article from the Infectious Disease Journal providing guidance on UVC decontamination. The CDC lists UVC as a method of N-95 respirator decontamination. 

Here are some safety notes to note up front.  UV-C light is hazardous.  Do not expose skin or look at the light as it will cause damage. The light must be turned on remotely without individuals present in the room. The lamps can get hot, and ozone may be produced.

There are concerns with making sure all parts of the mask are disinfected. There are also concerns that the UV-C light will cause the material to photo degrade and loose its effectiveness.

Not all UV lights are equivalent (notes from Nancy Rolstad and Wikipedia UV)

• UV-A lights (the ones used in a tanning bed) do not have sufficient germicidal properties (long-wave UV) It is associated with skin aging
• UV-B light is a shorter wavelength of light associated with skin burning.
• UV-C lights are necessary for disinfection. (aka UVGI germicidal irradiation or shortwave UV)
• UV-C light bulbs efficiency is reduced over time. A UV meter is necessary to measure the quality of the light source. 
• UV-C light is absorbed by the ozone layer and the atmosphere. That is why the sunlight that reaches the earth does not have good disinfecting properties for COVID-19.

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The University of Nebraska has excellent guidance on the use of UV-C light for COVID-19 disinfection including information on a COVID-19 UV-C decontamination process They also looked at using the UV-C lights in biological safety cabinets to disinfect masks.  The University of Nebraska produced specific information on UV-C light respirator decontamination. The AMA has a brief description of the process. 

Stanford University also has a UV-C COVID-19 disinfection process.

In Science Daily: Researchers report a 99.9% reduction in surface COVID-19 levels with a 30 second dose of UV-C light.  The dose was not specified.

U of M's CIDRAP has weighed in on N-95 reuse. Current methods are for an emergency use only to extend the amount of stock available. The UV-C light does begin degrading the material. Under the emergency circumstances they recommend 5 cycles at 1 J/cm2 per cycle.

Conversion of milliwatts/cm^2 to Joules/cm^2 Milliwatts to Joules converter

Papers:

Journal of Occupational and Environmental Health - Effects of UVGI on N95 Respirator filtration performance and structural integrity.  - July 2015.

Applied Environmental Microbiology - This was a study on the use of biological indicator strips to verify sterilization with plasma, UV-C, and blue light.

American Journal of Infection Control - UVGI decontamination of respirators with influenza virus - 2018.

ECRI - N-95 respirator decontamination by UVC for COVID-19.  Contains extensive list of peer reviewed references. 

NIH publication on UV-C disinfection for COVID-19

American Medical Association - Tackling the shortage of N-95 respirators with Novel Decontamination.

Omaha hospital using ultraviolet light (UV-C) to disinfect masks - 3:32 min. 

Duke Health - Benefits of UV-C disinfection of patient rooms  - 2:17 min. 

DIY disinfection of masks with 30W UV-C light (Not FDA approved) 11 min.
I am working with a health physicist to verify the calculations.