London, May 27 (IANS): The concentration level of antibiotics in rivers around the world exceed 'safe' levels by up to 300 times, says a study.

The 'safe' limits 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", said the study.

Sites where antibiotics exceeded 'safe' levels by the greatest degree were in Bangladesh, Kenya, Ghana, Pakistan and Nigeria, while a site in Austria had the highest concentration of the European sites monitored.

For the study, the researchers looked for 14 commonly used antibiotics in rivers in 72 countries across six continents and found antibiotics at 65 per cent of the sites monitored.

"The results are quite eye opening and worrying, demonstrating the widespread contamination of the river systems around the world with antibiotic compound," said Alistair Boxall, Professor at University of York.

Metronidazole, used to treat mouth infections, exceeded safe levels by the biggest margin, with concentrations at one site in Bangladesh 300 times greater than the 'safe' level.

In the River Thames, London, the researchers detected a maximum total antibiotic concentration of 233 nanograms per litre (ng/l), whereas in Bangladesh the concentration was 170 times higher.

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, said study.

The researchers compared the monitoring data with 'safe' levels, which is depending on the antibiotic, range from 20 ng/l to 32,000 ng/l.

For the study, the research team sent out 92 sampling kits to partners across the world who were asked to take samples from locations along their local river system.

Some of the world's most iconic rivers were sampled, including the Chao Phraya, Danube, Mekong, Seine, Thames, Tiber and Tigris, said the researchers.

"Solving the problem is going to be a mammoth challenge and will need investment in infrastructure for waste and wastewater treatment, tighter regulation and the cleaning up of already contaminated sites," Boxall said.

The study is scheduled to be presented at the two-day annual meeting of the Society of Environmental Toxicology and Chemistry (SETAC) in Helsinki, Finland, starting on May 27.