U.S. researchers say they have discovered high concentrations of lead in the Indian Ocean despite leaded gasoline having been slowly phased out worldwide.

While leaded gasoline usage has decreased drastically in the last few decades, lead is still pervasive in the environment, researchers at the Massachusetts Institute of Technology reported.

MIT ocean geochemist Ed Boyle has been tracking lead and other trace elements in Earth's oceans for the past 30 years and recently has analyzed water and coral samples from the Indian Ocean, using the coral to trace the history of anthropogenic lead over the last 50 years.

Lead concentrations in the Indian Ocean are now higher than in the northern Atlantic and northern Pacific oceans, Boyle and students from MIT's Trace Metal Group said.

One reason, Boyle said, could be that Asian and African countries were behind North America and Europe both in industrialization and then in phasing out leaded gasoline.

The Indian Ocean has had less time than the Atlantic and Pacific to dissipate lead pollution as a result, he said.

Reconstructing a history of lead in the Indian Ocean over the last 50 years, the researchers found lead levels began to increase in the mid-1970s, consistent with the region's pattern of industrialization and leaded gasoline use.

"It is an indication of the human footprint on the planet that essentially all the lead in the oceans now is from human activities," Boyle said in an MIT release. "It's very hard to find a trace of the lead that's there naturally."

Today, 185 countries have stopped using leaded gasoline.

Studies have shown lead can cause neurological and cardiovascular damage.