Balloons, their strings and labels, when let go of, fall back to earth and can be ingested by grazing animals, birds or marine creatures, so the Marine Conservation Society http://www.mscuk.org suggests we don't use balloons. So-called biodegradeable ones may take 4 years to disapper - that's not very biodegradeable, and they could do damage til then.
Sky lanterns similarly cause fires and litter and sometimes havoc when they fall back to land. More and more councils are not allowing these to be let off on council land ,
http://www.balloonsblow.org has a few alternative ideas,
