Paper is very absorbent of environmental elements... US currency is identified by the region it was issued from... licking the stamp or envelope leaves saliva, which can and will be used in a court of law...

When finding a needle in the haystacks, it is beneficial to know which haystack to look at... when finding a radio active needle, ie one that emits, again reducing the number of haystacks that have to be exhaustively searched is beneficial...

Paranoid... no just read the news...