For decades, the government has encouraged farmers across the United States to spread sewage sludge on their cropland and pastures. But now there’s a growing awareness that sludge fertilizer can contain heavy concentrations of “forever chemicals” linked to cancer, birth defects and other health risks.

This sludge is a byproduct of the nation’s wastewater-treatment plants. It’s the solid stuff that remains after city sewage is treated. But because it’s essentially concentrated waste, those toxic chemicals, known as PFAS, can become concentrated in it, too.

Here are the key findings from The New York Times’s examination of sludge fertilizer use and the consequences for farmers and the food supply: