You might have never heard of glyphosate before, but you probably have some in your garage or basement. It’s the active ingredient in herbicides and weed killers, and now scientists in the US have found that people exposed to it have biomarkers (a naturally occurring molecule or gene found in body fluids or tissues) linked to the development of cancer and other diseases.

In their study, the researchers analyzed urine levels in farmers across the US and found that the high levels of the pesticide were linked with signs of a reaction in the body called oxidative stress that causes damage to DNA. Cancer initiation and progression have been linked in previous studies to oxidative stress by increasing DNA mutations or inducing DNA damage.

“Glyphosate is the most widely applied herbicide worldwide, and its use has been associated with increased risks of certain hematopoietic cancers,” the study reads. “Our findings contribute to the weight of evidence supporting an association between glyphosate exposure and oxidative stress in humans and may inform evaluations of the carcinogenic potential of this herbicide.”

Glyphosate and cancer risks

Since it was invented in the 1970s, glyphosate has been used in hundreds of commercial products. One of the best-known ones is the herbicide Roundup, manufactured by Monsanto. The product has been under scrutiny by scientists around the world, especially since the World Health Organization said in 2015 it could be carcinogenic to humans.

Last year, researchers from the Center for Disease Control and Prevention (CDC) found that 80% of urine samples taken from children and adults had traces of glyphosate. They analyzed over 2,300 urine samples and found 1,885 had detectable traces of glyphosate, with concentrations of oxidative stress biomarkers rising alongside increasing urinary glyphosate levels.

Although WHO’s International Agency for Research on Cancer stated in 2015 that glyphosate is “probably carcinogenic to humans, this assessment is not shared by the U.S. Environmental Protection Agency (EPA), which says there’s “no evidence that glyphosate causes cancer in humans.” Meanwhile, the European Food Safety Authority agrees — but only for now. The EU is now reviewing these new findings and is considering whether or not to include them in their new glyphosate assessment, whose conclusions are due in July.

In 2019, another study by the University of Washington found that using glyphosate increases the risk of non-Hodgkin Lymphoma by 41%. The analysis of human epidemiological studies “suggests a compelling link” between exposures to glyphosate-based herbicides and a higher risk for non-Hodgkin lymphoma, the researchers argued.

Herbicide manufacturers have rejected all the accusations while agreeing to settlements of lawsuits. In 2020, Bayer, which bought Monsanto in 2018, settled most of the current and possible future lawsuits for $10 million. Bayer said back then that the “extensive body of science indicates that Roundup doesn’t cause cancer and isn’t responsible for the illnesses.”

Responding to the new study, Bayer told The Guardian the study has “significant methodological limitations” that affect its reliability. The company said the increased oxidative stress found by the researchers could have been caused by other factors not related to glyphosate. “The study doesn’t support that glyphosate is the cause,” the company said.

The new study was published in the Journal of the National Cancer Institute.