A compound that has been banned in Europe and Japan for a long time due to environmental and human health concerns is still being used in the USA as an ingredient in some soda drinks.
    
Brominated vegetable oil (BVO) was originally developed as a flame retardant for use in products such as foam. It was found to also help keep flavors mixed in drinks - how that leap was made, I have no idea.
    
Brominated flame retardants have been found to be building up the bodies of animals, including humans. There is concern the bromine in brominated vegetable oil could be doing the same. Even if it isn't, bromine is quite toxic to aquatic life and whatever bromine that doesn't accumulate in our bodies winds up in our waterways; many of which are becoming chemical cocktails.
   
Brominated vegetable oil was actually banned in the USA in 1970, but its use was again allowed after studies from an industry group demonstrated a level of safety and the FDA was petitioned by a soda-related industry group to get BVO back into some  beverages.
    
According to Wikipedia, BVO is one of a handful of substances that the USA Food and Drug Administration (FDA) have defined as interim food additives. An "interim food additive" is one where its safety has been called into question.
    
It's meant to be a temporary status pending further investigations, but a great deal of time has passed for it to maintain that classification.
    
Environmental Health News recently ran an interesting article on the topic, covering the history of this dubious ingredient, some results of various tests and cases where folks have ingested huge amounts of sodas containing BVO and experienced some nasty side effects.
   
It's not as though there isn't anything to replace brominated vegetable oil - it seems the Europeans have been using hydrocolloids, a natural compound, for years.
  
Brominated vegetable oil aside, I don't think it's any startling revelation that most soda drinks aren't really good for us or the environment. 
   
While I believe if someone wants to consume a product that could do them harm over the long term, that is their call, I also believe the person should be fully informed. If that product also has a broader environmental impact, then it's additional food (or drink) for thought.
   
If this is an issue that concerns you, check the label of your favorite soda for brominated vegetable oil or BVO - the article I referred to above also lists a few brands that use it. You might want to check out any other mysterious ingredients while you're at it.
 
It's interesting to note that on the Center For Disease Control (CDC) page for bromine, in the section entitled "How you could be exposed to bromine", it's not mentioned that brominated vegetable oil is used in some soda drink products. The only reference to food on that page is in relation to contamination.