The latest study to look at the long-term effects of Roundup, a popular weed killer developed by Monsanto in the 1970s, raises questions about the herbicide’s possible contributions to poor health in certain communities.

The study, published Tuesday in JAMA, tracked people over the age of 50 in southern California from 1993-1996 to 2014-2016, with researchers periodically collecting urine samples during that time.

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Researchers led by Paul Mills, professor of family medicine and public health at the University of California San Diego, found that the percentage of people who tested positive for a chemical called glyphosate, which is the active ingredient in the herbicide Roundup, shot up by 500% in that time period. The levels of glyphosate also spiked by 1208% during that time.

Read more: Study Links Widely Used Pesticides to Antibiotic Resistance

Exactly what that means for human health isn’t quite clear yet. There are few studies of the chemical and its effects on people, although animal studies raise some concerns. One trial from the UK, in which rats were fed low levels of glyphosate throughout their lives, found that the chemical contributed to a higher risk of nonalcoholic fatty liver disease, a condition in which fat accumulates in the liver and contributes to inflammation and scarring of the tissue. Mills says that the levels of glyphosate documented in the people in his study were 100-fold greater than those in the rats.

To follow up on these results, Mills plans to measure factors that track liver disease, to see if the levels of glyphosate he found are actually associated with a greater risk of liver problems in people. He heads the Herbicide Awareness & Research Project at UCSD, an ongoing research project in which he invites people to provide urine samples to test glyphosate levels. By gathering more information about people’s exposure, he is planning to tease apart how much of it comes from actually ingesting products sprayed with the chemical, and how much can be attributed to breathing in particles that have been sprayed into the air, especially in farm communities.

Read more: Here’s Which Produce Has the Most Pesticides

In a statement, a Monsanto representative said: “The amounts reported are consistent with prior reports from the U.S. and Europe and do not raise health concerns. Since food is often grown using pesticides, trace amounts can sometimes be found in people’s urine, which is one way our bodies get rid of non-essential substances.”

For now, Mills says the findings should make people more aware of what they are ingesting along with their food. While Roundup was developed to eliminate most weeds from genetically modified crops — and thus reduce the amount of pesticides sprayed on them — recent studies have found that many weeds are now resistant to Roundup. That means growers are using more Roundup, which could only exacerbate potential negative health effects on people who consume those products. Eating organically grown produce may help to reduce exposure to some pesticides and herbicides, but it’s not a guarantee that the products are completely free to potentially harmful chemicals.

“From my perspective it’s remarkable that we have been ingesting a lot of this chemical over the last couple of decades,” says Mills. “But the biomedical literature hasn’t said much about its effects on people. That’s a gap that we endeavored to address and bring more awareness to with this study.”

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