I collected snail shells while enjoying the sun in Vancouver, then put them in my checked bag and visited Los Angeles, went thru LAX and IAD and arrived in Buenos Aires. I didn't have a small bag or recipient for the sand though. Now I'm happy I got "a piece" of Vancouver and I have it here till I get the chance to see this beautiful city again. Never though of it as harmful or dangerous or anything, and luggage is going to collect bacteria at the airports in a way or another.