Michigan’s biggest chemical disaster of the 20th century began with a small mistake.

Workers at Michigan Chemical Corp. in the mid-Michigan town of St. Louis confused bags of a magnesium oxide cattle feed supplement with bags of a flame retardant called polybrominated biphenyl (PBB), which the company produced and stored in nearly identical bags. The bags, marked as feed supplement but filled with PBB, went to a Farm Bureau feed center near Battle Creek where it was mixed into cattle feed and sent to farms around the state.

Soon, livestock statewide were succumbing to terrible illnesses: Tens of thousands of animals began dying, bearing offspring with gross deformities, losing their ability to walk straight and shaking uncontrollably.

But it was nearly a full year after the mixup before PBB was identified as the culprit. By then, nearly all living Michiganders — 9 million people — had consumed the chemical through meat or milk. It was later found to be linked to high levels of exposure to breast and liver cancer and kidney and thyroid problems.

Then-Gov. William Milliken initially listened to officials in the Department of Agriculture and other state agencies who were heavily influenced by agricultural industry interests. State officials didn’t notify the public until seven months after the problem was found and spent years more downplaying the scope of the problem. The federal government, deeming the crisis a Michigan problem, didn’t intervene.

As fears mounted and researchers studied the chemical’s effects, public pressure built for lawmakers to address the crisis. Milliken ultimately realized he’d been misled and by 1977 advocated for and signed legislation to limit the allowable amount of PBB in food supplies. Farms began quarantining livestock. Thousands were ordered to be executed en masse and deposited in gigantic graves in Kalkaska and later, in Mio in Oscoda County.

Eventually, legislation passed that would significantly limit allowed amounts of PBB and compensate farmers for their losses.

But the delays made it hard for many farmers to recover what they’d lost.

Over the course of a long-term health study, researchers have found that PBB has epigenetic consequences, meaning it can pass along harmful health effects from generation to generation. Many farm families exposed to high levels of PBB over the first few years of the crisis in the 1970s are still living with the consequences.