Kale is heralded for its ample supplies of calcium, magnesium, potassium, Vitamin K, and various healthful phytochemicals and anti-oxidants. But the superfood is hiding a nasty secret: dangerous levels of heavy metals.

In a recent study, molecular biologist Ernie Hubbard found that kale—along with cabbage, broccoli, cauliflower, and collard greens—is a hyper-accumulator of heavy metals like thallium and cesium. What’s more, traces of nickel, lead, cadmium, aluminum, and arsenic are also common in greens, and this contamination affected both organic and standard produce samples.