I lived in Portland when Mt.St.Helens erupted. Probably the best "worst case
scenario" for air filter testing you could find. My 66 Beetle with oil bath
cleaner was fine. Cop cars and other state and county vehicles had variations
of  industrial air cleaners taped to the outsides or pulling intake air from
the trunk or interior. Amsoil dealers quickly sold out of air filters and it
was reported that the filter worked very well under those harsh conditions.

Ken Hunter
82 Westy 1.9D
84 Jetta 1.6D
89 Mitsubishi van