Balloon releases are done by many for celebrations such as weddings, or for comfort such as a way to feel closer to a loved one that has passed. Here are a few guidelines to make sure it is done in the best interest of everyone.
First and foremost, it is not legal everywhere, so check with the local regulations of where you plan on releasing them.
If everything checks out ok, here are some recommended things, several of them FAA (Federal Aviation Administration - United States) and CAA (Cival Aviation Authority - United Kingdom) regulations regarding the subject.
-Use standard 100% Latex "party" balloons. They easily decompose, and are less of a potential hazard than alternatives.
-Do not use any fasteners or such to close them or attach to them. If you are sending a message with the balloon it is best to either place it on a small piece of paper inside the balloon or write it directly on the balloon with a marker.
-Use biodegradable string instead of ribbon and keep it very short, or preferably use none at all. One idea I have is bring a scissors and instead of releasing the whole thing cut the string at the balloon to release it.
-Do not release them within 5 nautical miles of an airport. 5 nautical miles equals about 5.7 standard miles, or 9.26 kilometers.
-Do not release them tied together but rather release individually.
-If you are releasing a large number of balloons, notify your local aviation authority of time, date, location, etc. Some places will have a form you need to fill out to avoid a fine or prosecution.
Large numbers or a group tied together can show up on radar or surprise an unknown pilot causing them to swerve to avoid the unknown object and possibly become off course.
As for the debate on weather they are a hazard for planes, I have spoke to several people in aviation, and as long as the regulations are followed there is almost no threat. There is actually a contest at an airshow that involves prop planes challenged to hit the balloon and word is no one has ever successfully hit one on purpose.
A balloon sucked into a jet engine will be pretty much disintegrated and cause no damage. As long as you are not near an airport they will not even reach the height of cruising altitude for commercial aircraft. Balloons max out at an altitude of about 5 miles or 26,400 feet and burst, while cruising altitude is usually above 30,000. As for smaller aircraft with a lower cruising altitude, the biggest threat would be a ribbon or something tied to the balloon which could cause damage to the planes hull or propellers.
There is also debate on birds eating the pieces and dying from them. While it is possible, it is very unlikely that they would mistake a balloon for food, and even if they peck at it they would most likely realize it wasn't before swallowing it. The real danger to birds and other wildlife is getting tangled up in ribbon.
So go ahead and enjoy your balloon release's just be smart about it.