I run some air filters in the house. Out of curiosity, I took some readings with my radiation detector (a bGeigie from Safecast, which I understand detects mostly alpha particles). For whatever reason, the dirty air filters give readings significantly higher than the background. Whatever they are filtering out is apparently radioactive.

Another interesting thing I've noticed is that my bGeigie gets the highest readings on city streets. Tends to be lowest out in the forest.