http://www.islandone.org/Launch/boron-sharp-article.htm
Boron combustion (including air) is a very rich research field, particularly in rocketry.
They don't seem to think it's a problem leaving boron combustion products in the atmosphere.
This diagram, in particular : http://www.islandone.org/Launch/boron-sharp-article_files/slurry-lfrj.gif
