Apparently, according to Trusted MD.com, it is NOT recommended to re-fill plastic bottles made from #1 plastic. And many #7 plastic bottles (bottles made from non 1-6 resins or more than one resin) contains Bisphenol A, a harmful xenoestrogen that interferes with human hormonal messaging. (However, not all – Nalgene makes a non BPA bottle that is made of a #7 plastic)
However, numbers 2 (high density polyethylene), 4 (low density polyethylene), and 5 (polypropylene) should be safe.
One particularly important note to take away from the article is that many baby bottles are made from unsafe materials – as far as chemical toxins go, your best bet for your own water and for babies’ milk and juice is glass or #2 HDPE bottles.
For more info on baby-safe bottles, go here.
For a more in-depth look at the different types of plastic, go here.