Numerous studies have determined glyphosate, the active ingredient in Roundup, to be safe. However, several others have linked it to human health ailments. Critics say they fear that glyphosate is so pervasive in the environment that extended exposure even to trace amounts can be harmful.

U.S. consumer groups, scientists and food companies are testing substances ranging from breakfast cereal to breast milk for residues of the world's most widely used herbicide on rising concerns over its possible links to disease.

The focus is on glyphosate, the active ingredient in Roundup. Testing has increased in the last two years, but scientists say requests spiked after a World Health Organization research unit said last month it was classifying glyphosate as probably carcinogenic to humans.