On Tuesday, November 22, 2016 at 9:02:10 AM UTC, Brian Gaff wrote:
Heat and cold cycling in the place where it lives?

Are they rubber, or are they polyurethane? If it's polyurethane it could
be hydrolysis, the process that makes shoe soles fall apart when used after
a long period of storage - happens a lot at weddings apparently.

I bought a proper dust mask last year, used it a bit then put it in the shed.
When I got it out last week the elastic in the strap had totally crumbled
into black dust. The rubbery bit that fits on the face looked good as new.