Supermarket eggs gleam with apparent cleanliness, and nothing might seem more wholesome than breaking one of them into a frying pan.

Think again. The Humane Society of the United States plans to release on Thursday the results of an undercover investigation into Kreider Farms, a major factory farm that produces 4.5 million eggs each day for supermarkets like ShopRite.

I’ve reviewed footage and photos taken by the investigator, who says he worked for Kreider between January and March of this year. In an interview, he portrayed an operation that has little concern for cleanliness or the welfare of hens.

“It’s physically hard to breathe because of the ammonia” rising from manure pits below older barns, said the investigator, who would not allow his name to be used because that would prevent him from taking another undercover job in agriculture. He said that when workers needed to enter an older barn, they would first open doors and rev up exhaust fans, and then rush in to do their chores before the fumes became overwhelming.