Every time I see a story about people releasing helium balloons (sometimes with messages attached), I see a letter to the editor about the dangers of doing so. Is there any scientific evidence to back this up?
What are the dangers that they state in the letters? 
heh i am no expert but i think you may find this interesting
cotraption (A) discarded balloon chokes duck (B} which expires and pollutes pond© which kills mino (D) which drives away pied billed Grebe (E) which distresses birdwatcher (F) who contributes to enviromental NGO (G) which lobbies members of parliment (H) which passes a non pollution law in parliment(I) which cuts the profits of various companies (J) which lays off 25% of its staff due to lower profits (K) which includes Stanley Finchley(L) who now has more time to spend with his daughter Steffanie walking in the park (M) who lets go of balloon(N) which goes up into the stratosphere and pops (O) and drops into pond§ where it catches the attention of duck (Q)
The balloons deflate, faal to earth, and choke animals. I mean, what are the odds? If you released several thousand balloons and they fell to earth in a concentrated area, there MIGHT be a danger, but I can’t see how a few dozen or even a couple of hundred spread out over thousands of square miles can do harm.
Any experts out there who can explain/verify this?
It is possible to create a dangerous situation by releasing helium balloons, but you have to really work at it. 
Helium is an inert gas, so that’s not going to cause any problems. Balloons are typically made of biodegradable materials so they’re not going to be a major choking hazard for nest-building turtles and such. A quick google search on [“helium balloon” danger] came up empty on other possible significant hazards. I’ll bet the danger stories are a cover for litter control zealotry.