A new study has shown that glyphosate levels in humans have more than doubled since 1993; glyphosate-resistant GMO crops were first introduced into the United States in 1994.

The research, which compared glyphosate levels in the urine of 100 people in California, was conducted by the San Diego School of Medicine and was published in the Journal of the American Medical Association.

"Prior to the introduction of genetically modified foods, very few people had detectable levels of glyphosate," says study author Paul Mills, of the University of California at San Diego School of Medicine. "Our exposure to these chemicals has increased significantly over the years but most people are unaware that they are consuming them through their diet."

Researchers found that detectable amounts of the herbicide increased from an average of 0.2 micrograms per liter to .44 micrograms per liter in 2014-2016. The daily limit set by the EPA is 1.75 milligrams per kilogram.

Mills indicated that the next step for researchers would be to examine the general health of individuals who had higher levels of the herbicide in their urine.

"I am concerned,” he tells Radio New Zealand. “This is one of the reasons I put together this study, because there wasn't such information in the biomedical literature, and I thought we needed it, and we needed to start having some good data to have a conversation around these questions."

Glyphosate, which is most often sold under the brand name Roundup by Monsanto, is used as an herbicide with genetically modified glyphosate-resistant soy and corn crops. It is also used on non-GMO oats and wheat, which are sprayed to dry them out in preparation for harvest.

Use of glyphosate has increased approximately 500 percent since the early '90s, according to the study authors.

"Prior to the introduction of genetically modified foods, very few people had detectable levels of glyphosate," says Mills.

The World Health Organization found that glyphosate was a probable human carcinogen in 2015. Last Tuesday, the European Parliament called for use of the herbicide to be phased out over the next five years throughout the European Union. The non-binding resolution should have been voted on Wednesday, but the vote has been postponed for the time being.

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