According to an article on the Huffington Post website, balloons are among the top 3 most harmful pollutants threatening marine wildlife, but they also harm domestic animals such as cattle, dogs and sheep.
Seabirds are most at risk. Deflated balloons are mistaken for jellyfish or squid and are eaten by the birds. Not able to digest the balloon, it will clog the bird's stomach and the bird will eventually die of starvation. Choking is also a threat.
The ribbons or strings that are attached to the balloon are a hazard to birds because the birds can become entangled in them when the balloon lands in a tree. They may also use the materials to build a nest causing a risk factor for the hatchlings.
In its natural state, latex is biodegradable, but when processed for balloons and treated with chemicals, plasticizers and dyes, it can no longer be classified as natural. It can take a balloon from six months to four years to decompose. Shiny mylar balloons can take even longer. Think of the damage it can cause in that time.
How can you help? Explain the dangers of releasing helium balloons to your children. Don't plan balloon releases at any of your celebrations, and if you know of someone who is planning this, please discourage them from doing so.