LONDON, May 27 (Xinhua) -- Concentrations of antibiotics can be exceedingly high in some of the world's rivers, much greater than "safe" level, according to a study released on Monday by the University of York.

A team led by researchers from the university analyzed water samples from rivers in 72 countries across six continents.

The results showed that antibiotics were found at 65 percent of the sites monitored, and metronidazole, a kind of antibiotic, exceeded "safe" levels by the biggest margin, with concentrations at one site in Bangladesh 300 times greater than the "safe" level.

Aside from metronidazole, the most prevalent antibiotic was trimethoprim, which was detected at 307 of the 711 sites tested and is primarily used to treat urinary tract infections.

The "safe" level, the team used, was established by the AMR Industry Alliance. The "safe" limits were most frequently exceeded in Asia and Africa, but sites in Europe, North America and South America also had levels of concern showing that antibiotic contamination was a "global problem", according to the team.