Tests on 15 bottled water brands from Canada and 48 from Europe found some products were contaminated with trace amounts of the toxic metal antimony.
The researchers, from the University of Heidelberg in Germany, said they strongly suspected PET bottles were the source of the contamination. Water in PET contained up to 30 times more antimony than water packaged in glass, they said.
The team confirmed the theory by adding 'pure' water to PET bottles and monitoring antimony contamination.
All levels found were well below the guidelines for antimony in drinking water. Yet, professor William Shotyk, who led the research, said the continuous release of antimony from the PET containers to the water was "bothersome".
Antimony levels doubled to 630 parts per trillion in bottled water that had been stored for three months, indicating contamination increases with a product's lifetime.
The levels found in water were not an immediate health threat to consumers, though Shotyk added: "There is unlikely to be a beneficial effect of Sb [antimony] contamination."
Antimony trioxide is used as a catalyst in the manufacture of PET, and PET typically contains several hundred mg/kg of the metal. For comparison, most of the rocks and soils at the surface of the earth contain less than 1 mg/kg.
Little is known about the character and effects of antimony, although the University of Heidelberg said last year the element's "environmental significance clearly outweighs the attention it has received to date".
It said there had been few comprehensive studies on antimony, despite the fact that "most humans are now being chronically exposed to low levels of antimony in food, water and the air".
Antimony is recognised as a hazardous substance by authorities in US and Europe. Heidelberg's Institute of Environmental Geochemistry held the 1st International Workshop on 'Antimony in the Environment' in May last year.