BBC News - Rice from China and Taiwan had the highest lead levels, but Dr Tongesayi stressed that all of the samples significantly exceeded the PTTIs.
Dr Tongesayi has also worked on quantifying arsenic contamination – and is in effect working his way through the heavy metals one by one to determine their prevalence.
The problem, he said, is the range of agricultural practices around the world.
"If you look through the scientific literature, especially on India and China, they irrigate their crops with raw sewage effluent and untreated industrial effluent," he explained.
"Research has been done in those countries, and concerns have been raised because of those practices, but it's still ongoing."
Dr Tongesayi also said that the increasing practice of sending electronic waste to developing countries – and the pollution it leads to – exacerbates the problem.
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