If you finish the water in your disposable bottle you might want to think twice about refilling it. The issue is the plastic water bottles contain the chemical BPA, which can leach into the water and quickly grow dangerous bacteria in the bottle's cracks. Kent Atherton, CEO of PuriBloc technology, says, "Even BPA-free products are not safe since manufacturers are now substituting other estrogenic chemicals, not as widely known, which may pose a danger to human health." These chemicals can have a negative effect on human beings' hormonal balances. Also, single-use plastic bottles are mostly made of PET-- which is safe to use, but not reuse, as it also leaches chemicals into your water if heated or if the bottle gets scratched. The other concern about re-using a bottle is very basic-- it comes into contact with your hands and mouth, which are both huge sources of germs. Your best bet is to invest in a BPA-free plastic bottle or one made from glass or stainless steel.