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Levels of the herbicide glyphosate have soared in older Americans in 23 years, according to a study led by University of California researchers.

The study wasn't designed to detect any potential harm from the increased exposure, but it will help with future studies to determine if any such link exists, said Paul J. Mills, a UC San Diego professor of family medicine and public health.

The study used data from the long-running and influential Rancho Bernardo Study of Healthy Aging, established in 1972. It was published as a research letter Tuesday in the Journal of the American Medical Association. The study can be found at

Environmental activists have attacked the widespread use of glyphosate as causing cancer and other diseases. Some studies have suggested a potential link, but none have actually demonstrated a causal effect in humans. Other studies have found no correlation.

Opponents say animal research shows the potential for harm.

"There's a lot of animal work, but few if any on people," Mills said. "And I was surprised to see that, given how much the chemical is in the environment, and that's what inspired us to just start researching it so we can fill in that gap."

Unlike other studies on glyphosate, the data from the Rancho Bernardo study has been collected for decades, providing a time window to examine glyphosate exposure and any effects in individuals. Also importantly, glyphosate levels were directly measured; other studies relied on interviews to assess exposure.

The study examined urinary levels of glyphosate in 100 people from 1993 to 2016. Very few had detectable levels in 1993, but by 2016 70 percent had detectable levels, Mills said. Of those with detectable amounts, in 1993-1996, the average level was 0.203 nanograms per liter. By 2014-2016, the level had risen to 0.449 nanograms per liter.

Ongoing research at UCSD is conducted at the Herbicide Awareness & Research Project. Go to for more information.

Glyphosate is widely available as the active ingredient in Roundup, sold by Monsanto, a St. Louis-based agricultural company. Roundup is considered safe by federal regulators within normal levels of use. It enables farmers to quickly kill weeds without using more toxic herbicides. Monsanto also sells crops seeds that are genetically modified to resist Roundup.

Scott Partridge, vice president of global strategy for Monsanto, said glyphosate has an extensive safety record.

"Glyphosate has been around for 40 years, and it's been studied more than any agricultural herbicide in history," Partridge said. "It has been more studied than any chemical that's ever been used in agriculture, with over 600 studies peer-reviewed articles, medical journal publications, over this 40-year period."

Regarding the JAMA study, Partridge said the highest levels of exposure equate to a tiny fraction of what the Environmental Protection Agency has established as the safe limit.

Moreover, Partridge said glyphosate's widespread use has made it possible for farmers to reduce plowing, which disturbs topsoil and contributres to erosion, and use "no-till" agriculture, in which most of the surface is left intact.

"It has done remarkable things throughout the globe, from the most advanced farming techniques to relatively primitive farming, to help to help farmers and growers around the globe control those difficult-to-manage weeds and grasses, so nutrients aren't expended on nonproductive vegetative matter," Partridge said.

Critics point to a 2014 study reporting that rats disproportionately developed tumors after exposure to Roundup and maize genetically genetically altered to resist Roundup.

However, that study has been widely condemned by other scientists for various flaws, such as using too few rats to draw a valid result. And the rats themselves come from a strain known to develop cancer.

Moreover, the study, led by French researcher Giles-Eric Seralini, was a republication of a 2012 study that was retracted. The original study was given to reporters under questionable circumstances. Reporters were forbidden to show the study in advance of publication to independent scientists for comment, a normal practice in science to ensure accurate reporting.

Seralini and supporters say Monsanto is conducting a PR campaign to suppress unfavorable information, and that their critics can't be trusted because they have relationships with Monsanto.

The critics, such as UC Berkeley biologist Michael Eisen, say opposition is fueled by an anti-scientific rejection of genetically modified crops, despite their record of safety.

The allegations that Monsanto has suppressed information are contained in documents obtained by a law firm suing Monsanto over claims that Roundup causes non-Hodgkin's lymphoma. The documents can be viewed at

And earlier this year, Seralini led another study using the same 2012 data, reporting that rats given an ultra-low dose of Roundup developed nonalcoholic fatty liver disease, a major and growing health concern.

Mills said the concerns were great enough for the World Health Organization to list glyphosate as a carcinogen or probable human carcinogen in 2015.

However, the next year WHO issued another report stating that "glyphosate is unlikely to pose a carcinogenic risk to humans from exposure through the diet."

That statement didn't rule out potential risk from exposure by direct application, such as in agriculture.

The 2016 WHO statement said the group studying the issue concluded that "glyphosate is not carcinogenic in rats but could not exclude the possibility that it is carcinogenic in mice at very high doses."