Plastic is made from petro-chemicals and these chemicals do in fact leach. When making an Ecobrick we are simply using plastic that would have ended up in oceans or land, which would have then leached chemicals. By containing it in an ecobrick (plastic bottle), we are preventing the contained plastic from leaching, or at least delaying the leaching process by another 100 years or more. There is no such thing as an overuse of an ecobrick, since the plastic is here anyway!