It's generally not a threat, although a strong concentrated smell in the air wouldn't be good. The main thing to be cautious of with ACV is that its benefits are vastly overhyped, and it's not likely to do anything that distilled white vinegar wouldn't do at a fraction of the cost. The subject of ACV comes up so often on the bird nutrition group I belong to that I wrote an article about it:
http://www.littlefeatheredbuddies.com/i ... n-acv.html