The concern is that the BPA can leach from the plastic of the bottle into the milk or water or whatever is in the bottle, and then it will be ingested, especially when heat is involved - so the washing, sterilising and heating of bottles for feeding. The recommendation is to only use bottles and teats for about three months before replacing them with new ones, and throw the old ones away - do not use second hand bottles. And replace any bottles and teats straight away that show signs of damage. I will admit to breaking all of this advice - I only first found out about BPA after having my third baby, so it was a bit late by then - I had used second hand bottles and teats (borrowed from my SIL), used them for the entire time my babies were bottle fed, then kept them and used them again, along with buying a few new ones along the way.