Workers at the Mountaire Farms poultry plant in rural Robeson County, North Carolina, say that about four months ago, something changed. At the time, they couldn’t immediately identify the strange new chemical they’d begun to smell floating around the production lines. But one day things were running as expected, and the next there was a sharp, suffocating kind of odor that one worker said felt like it “invaded your brain.” “It stings your eyes and throat; it’s like a burning in your nasal cavity,” said Sharon, an immigrant who has worked at Mountaire’s Lumber Bridge plant for more than a...