Right now, you probably don't own a mask because you weren't prepared for the pandemic. I guess you're not prepared either for the nuclear holocaust that will ensue soon enough.
All you need is a smartphone and a small $25 chinese device that plugs in the audio jack of your phone (you didn't buy a phone without an audio jack, right?).
Search for "FTLAB FSG-001 Smart Geiger" on ebay: https://www.ebay.com/sch/i.html?_nkw=FTLAB+FSG-001+Smart+Geiger
And install one of these two apps:
https://play.google.com/store/apps/details?id=kr.co.ftlab.geiger
https://play.google.com/store/apps/details?id=com.optivelox.radmeter
(couldn't find any open source Geiger app on F-Droid, so here's an idea to keep yourself busy during the confinement: make one)
The $25 radiation detector?