Brian Cockfield wrote an article about this hack - thank you!

Some of the comments left were very useful and I appreciate them.

I had focused on what the CDC recommended, https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/decontamination-reuse-respirators.html and the use UVGI. This was not universally accepted by WHO or CDC organization in other countries.

I set up a simple experiment with my UVC lamp, a UVC sensor with amplifier and oscilloscope.  It has about 40dbV dynamic range to measure over.

I put a transparent glass slide over the UVC sensor and exposed it to UVC, it effectively blocked the UVC light. Both the N95 and KN95 mask blocked the UVC light.  The cheap safeway mask did show UVC at -30dbV, a little got through.

If the mask is designed for filtration of smallest particles and the if the material is not transparent to UVC,  the dense forest of filtering fibers isn't gong to allow UVC to pass through. It cannot sanitize what it cannot reach.

There is practical information on this in other applications when using UVC.  Air-filtration and Water-filtration units that utilize UVC have prefilters whose function is to block the larger particles, The larger particles need to be removed as they will block UVC from reaching pathogens.

Again, thanks for the useful comments.