If you are a Verizon customer on the East Coast, odds are good that your cellphone or tablet arrived by way of a beige, windowless warehouse near Tennessee’s border with Mississippi.

Inside, hundreds of workers, many of them women, lift and drag boxes weighing up to 45 pounds, filled with iPhones and other gadgets. There is no air-conditioning on the floor of the warehouse, which is owned and operated by a contractor. Temperatures there can rise past 100 degrees. Workers often faint, according to interviews with 20 current and former employees.

One evening in January 2014, after eight hours of lifting, Erica Hayes ran to the bathroom. Blood drenched her jeans.

She was 23 and in the second trimester of her first pregnancy. She had spent much of the week hoisting the warehouse’s largest boxes from one conveyor belt to the next. Ever since she learned she was pregnant, she had been begging her supervisor to let her work with lighter boxes, she said in an interview. She said her boss repeatedly said no.

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