Can using weed killer give you cancer?

In this July, 9, 2018, file photo, plaintiff DeWayne Johnson looks up during a brief break as the Monsanto trial in San Francisco. (AP)

A long-awaited jury decision last week has put new attention on the potential health risks of a popular weed killer — and the debate over those risks.

Researchers say the jury is still out on whether the popular weed killer Roundup poses risks to the average consumer.

However, scientists are piecing together a picture that shows the chemicals in the weed killer are becoming increasingly common in humans.

DeWayne Johnson is an extreme example of exposure to glyphosate, the main ingredient in Roundup and related products sold by agricultural biotech giant Monsanto.

The California school groundskeeper sprayed a high-concentration version of the product around school property between 20 and 30 times a year over the course of about four years.

Twice, he accidentally got drenched in the herbicide, he testified during his trial.

Two years into his time as groundskeeper, he was diagnosed with non-Hodgkin’s lymphoma and, later, with a more aggressive form of the cancer.

A San Francisco Superior Court jury decided the cancer was caused by the exposure to glyphosate and ordered Monsanto to pay $289 million in damages.

The company is appealing the decision, but if it stands it could set a precedent for the hundreds of other cases in the United States brought against Monsanto by people with non-Hodgkin’s lymphoma.

How much exposure is harmful?

Johnson sprayed the herbicide out of 50-gallon drums as part of his job. Farmers also use it in high quantities.

But the more typical consumer might take a spray bottle of Roundup out to the backyard a couple of times a month to combat annoying weeds.

For most of them, there’s much less risk of significant harm from Roundup products — but exactly how much less is still unclear.

“There is consensus, among non-industry scientists, that there is no safe level of exposure to glyphosate. That is, no level established that comes with no possible harm,” said Paul Mills, a public health professor at the University of California San Diego (UCSD) who has studied the prevalence of glyphosate exposure in humans.

The World Health Organization’s International Agency for Research on Cancer concluded in 2015 that glyphosate was a probable cause of cancer in humans.

California’s Office of Environmental Health Hazard Assessment has concluded it’s known to cause cancer.

But the U.S. Environmental Protection Agency (EPA) has concluded that it’s not likely to cause cancer in humans, as has the UN’s pesticide review group and the European Food Safety Authority.

In court in San Francisco, Monsanto pointed to the latter findings. The plaintiff’s lawyers pointed to the former.

But how concerned should the average consumer be?

One of the many problems in determining health effects on humans is that experiments measuring the effects of different exposure levels can’t be conducted on people, said Ariena van Bruggen, a plant disease epidemiologist at the University of Florida’s Emerging Pathogens Institute who has studied the environmental and health effects of glyphosate.

But van Bruggen told Healthline that studies have found correlations between exposure to glyphosate and various diseases.

She pointed to studies showing sugar cane cutters exposed to the chemical were found to suffer renal damage and that women in rural communities where crop-dusting planes dropped the herbicide have later experienced shortened pregnancies.

“As circumstantial, correlational evidence accumulates and points in the same direction, together, these observations can become corroborating evidence,” van Bruggen said.

She added that these studies have often been dismissed by politicians and scientists, although she noted some of these critics have documented ties to Monsanto.

A paper published last year found that the “acute toxic effects” of glyphosate are low but that there’s data “raising the possibility of health effects associated with chronic, ultra-low doses.”

J. Glenn Morris, director of the Emerging Pathogens Institute, told Healthline the compound may affect our intestinal flora and may be tied to reproductive impacts and health problems.

However, he said, the data is “all very soft” and needs more and longer-term studies before anything can be determined with certainty.

It’s not just the spraying

What we do already know is that for most of us, exposure to glyphosate comes more through eating foods that have been exposed to it in the fields.

Exposure from accumulation in drinking water is also an issue in rural, farming areas.

And we know that a significant proportion of the population has low levels of glyphosate in their bodies, that concentrations of the chemical accumulate over time, that those exposure levels have been growing, and that they’re likely to continue to increase since glyphosate is being used in greater quantities in farming.

Crops such as soybeans are often genetically modified to not be affected by Roundup, which allows the herbicide to be sprayed on fields and kill weeds without killing the crop.

But since Roundup gets sprayed on everything in the field, some glyphosate residue remains on the food we eat.

“What we’ve found — and others have found — is when you screen urine samples of people, a third to a half will have fairly low levels of glyphosate,” said Morris. “So we’re probably seeing fairly constant exposure, primarily through foodstuff.”

A paper that UC San Diego’s Mills co-authored last year found that 70 of the 100 people they studied outside San Diego had detectable levels of glyphosate in their urine.

The average levels in urine had more than doubled in two decades, from 0.203 micrograms per liter (µg/L) in 1993-1996 to 0.449 µg/L in 2014-2016. (Genetically modified crops that are resistant to Roundup were introduced in the U.S. in 1994.)

“The concentrations are often higher in the urine of farmers, but not that much higher, indicating that indirect chronic exposure through water or food is perhaps as important as direct exposure,” van Bruggen said.

She added that the amounts of glyphosate recommended for spraying by homeowners is low and that only accidentally high exposures “would lead to noticeable direct toxicity.”

But, she said, “such high exposures do occur, for example when applicators are not protected properly, as might be common for homeowners.”

Asked for comment, the Leukemia & Lymphoma Society said it “does not comment on active litigation, but is aware of the lawsuits related to glyphosate and non-Hodgkin’s lymphoma. We are closely monitoring ongoing large epidemiologic studies that we expect will provide additional insights.”

The American Cancer Society said it relies on the International Agency for Research on Cancer (IARC) and National Toxicology Program to determine carcinogenicity. They directed Healthline to a page on its website that lists the groups’ findings, including IARC’s that glyphosate probably causes cancer.

The NTP is re-evaluating the chemical and expects to issue a report next year.

How concerned should you be?

Given how much is still unknown, Morris said to not worry too much about what you’re eating for now.

He said he might “hesitate slightly before going for a GMO product” but that “there isn’t enough to make me change any eating habits right now.”

“It’s reasonably safe,” Morris said, “and to get up to the level to cause illness it takes a whole lot of it. So you’re not going to get that through food.”

But there’s still some cause for concern.

Part of the reason long-term studies are still so needed despite Roundup being around for decades is that the way it’s used, and thus our exposure to it, is always changing.

Over the years, weed species develop tolerances to glyphosate as the most resistant individuals survive and pass on their resistance to future generations.

This resistance means that farmers have to use more Roundup to kill weeds. That, combined with the fact that more farms are using it each year, means that more glyphosate may be finding its way into our bodies, Morris said.

It’s possible the same logic might apply to homeowners, van Bruggen said.

“It is not known how often homeowners spray, but they may spray repeatedly because the effect of glyphosate on plants is not immediate but takes a few days. Homeowners may spray again just to be sure.”

What effect prolonged but low-level exposure could have is still largely an open question, as is whether those effects might vary widely from person to person.

“That is the golden question: What are the actual health consequences from the chronic exposure over the past 20-plus years?” Mills told Healthline. “Surprisingly, little human clinical or epidemiological research has been done on this.”

It does appear that some people should be more cautious than others.

Someone with a weaker liver might be more susceptible to glyphosate’s observed impacts on the liver, for example, van Bruggen said.

Small children and immune-deficient adults may also be more susceptible to impacts, she added.

The state of California has tried to warn people who might come into contact with potentially carcinogenic compounds through its Proposition 65, which requires warning labels on products found to cause cancer, birth defects, or reproductive harm.

But a week after the Johnson ruling, a judge in Sacramento ruled that California couldn’t require a label on glyphosate products because of what he saw as a “heavy weight of evidence” showing the herbicide was safe.

Some experts disagree with that finding.

“I think that warning labels are appropriate because there is sufficient evidence of potential carcinogenicity and reproductive harm,” said van Bruggen.

She would even like to take it a step further.

“I also think that there is no need for glyphosate to be available to the general public,” she said. “Consumers often spray too much and too frequently, which is dangerous for their health in the long run.”

But significant gaps remain in our knowledge, and long-term studies are still needed.

Mills said UC San Diego is currently working to fill some of those gaps, and the initial findings are “suggestive of a relationship” between prolonged glyphosate exposure and non-cancer illnesses.

For now, though, the evidence is still accumulating.

“How do you convey the idea that [there] may be a risk but don’t worry about it?” asked Morris.

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