Hopefully whatever is leeching out of the bottles isn't carcinogenic.

The material the bottle is made out of should be printed on it somewhere (probably the bottom, by the recycling triangle). PET or PETE is considered safe by the FDA, as well as a few others, but there are some that are known to leach. Also, reusing bottles and heating bottles are generally poor ideas, unless you know the material it's made from is designed for that.