In less than a decade, kale has gone from a garnish on salad bars to a staple item in salads, smoothies, and side dishes across the US. It's been turned into chips, crushed into powder, and added to popular items like popcorn and soda.

It's also one of the most pesticide-contaminated vegetables in the US, according to the nonprofit Environmental Working Group (EWG), which lists kale among its "Dirty Dozen" guide to the most contaminated food products.

Despite not making the list in 2018, kale ranks third this year, behind strawberries and spinach. The news, though potentially worrying for salad fans, could have even greater implications for America's agricultural workers.

Kale samples contained traces of weed-killer

After analysing 2017 tests from the US Department of Agriculture, the EWG found that the average kale sample contained traces of more than five different pesticides, despite the fact that the samples were thoroughly washed.

More than 90% of the samples showed detectable levels of at least two pesticides, and the most contaminated sample had 18 different pesticide residues.

While scientists are still figuring out the link between pesticides and human disease, studies have linked certain pesticides to cancer,diabetes,infant autism, and attention deficit hyperactivity disorder (ADHD) in children and young teens.

When it comes to kale, the EWG found that 60% of the samples tested positive for an herbicide called DCPA, which is marketed under the name Dacthal.

The chemical was introduced in the US in the late 1950s as a weed-killer for strawberries, vegetables, beans, and cotton. By the 1990s, it was mostly used to tame weeds on golf courses and residential lawns.

By 2005, the product's US manufacturers had suspended most uses of Dacthal amid concerns that it was leaching into groundwater.

A few years later, the European Union prohibited all uses of the herbicide on crops. Today, the only products that contain Dacthal in the US are sweet potatoes, eggplant, turnips, and kale.

Thus far, the EPA has found "suggestive evidence" that Dacthal could cause cancer. The organisation has listed the herbicide as a possible carcinogen based on a two-year study that linked DCPA to thyroid and liver tumours in rats, but the effects on humans haven't been tested.

As of 2018, the California Office of Environmental Health Hazard has determined that even a lifetime exposure to Dacthal in drinking water would not present an adverse health effect in humans.

Animal studies have shown, however, that Dacthal may disrupt thyroid hormone pathways and produce neurological effects like decreased motor activity.

The EWG analysis also found that 30% of the kale samples contained bifenthrin and cypermethrin, two insecticides that have been classified by the EPA as possible human carcinogens based on studies in mice.

Excessive exposure to these chemicals could cause nausea, headaches, and neurological issues, such as tingling and numbness.

Pesticides could be even more harmful to farmers

Though contaminated vegetables may wind up at restaurants and grocery stores, farmers are routinely exposed to higher quantities of pesticides.

Workers who grow kale run the risk of coming into direct contact with herbicides like Dacthal. Those spraying pesticides on kale crops may also inhale chemicals that could jeopardize their health.

The EPA states that respiratory and skin contact with Dacthal is "expected among agricultural and horticultural professionals" who work with the chemical, though it doesn't identify the risk of exposure.

A 2017 study found that exposure to certain pesticides may increase farmers' risk of non-Hodgkin's lymphoma, a cancer that develops in the lymphatic system.

By the EPA's own estimates, around 10,000 to 20,000 agricultural workers are diagnosed with pesticide poisoning each year. More than a decade ago, the organisation stated that "risks to workers still exceed EPA's level of concern" even with its worker protection standards in place.

Should we be worried about eating kale?

For those still worried about putting kale in their salads or smoothies, there isn't much to fear. There's far more evidence to suggest that the vegetable actually benefits your health.

A single cup of raw, chopped kale gives you more than 200% of your daily allowance of vitamin A and nearly 700% of your daily allowance of vitamin K. It's also full of vitamins like B6, calcium, vitamin C, and potassium.

Some studies even suggest that cruciferous vegetables like kale can protect against certain types of cancer.

In the future, scientists could discover new health risks related to pesticides in food. If that happens, the consequences for farmers may be even more devastating than the effects on consumers.

This article was originally published by Business Insider.

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