But plasma companies locate their collection centers disproportionately in destitute neighborhoods, according to Heather Olsen, who, as a graduate student researcher at Case Western Reserve University, examined 40 years of data on collection centers across the country. “They’re surgically placing these,” she said.

Healthy people can donate plasma twice a week, up to 104 times a year. The plasma industry says that most people do not donate so frequently and that there are minimal health risks involved, but other researchers disagree. One 2010 study found that paid donors who sell their plasma frequently have fewer proteins in their blood, which some experts say could put them at risk for infections and liver and kidney disorders. In the short term, plassers have reported fatigue, tingling sensations, anemia and blacking out.

Nonetheless, many are grateful for the opportunity. The money that comes from it can be the only form of cash income for families living in extreme poverty, as Luke Shaefer and Kathryn Edin note in their book “$2 a Day.”

A number of people at the CSL collection center in North Philadelphia last November confirmed that the money they received there was their only income; they were putting it toward food, rent and bus fare. Some, like Kevin Hayway, were veteran plasma sellers; he estimated that he had sold his plasma more than 100 times a year for the past three years. Robert Jenkins said he had learned about the center when he was living at a homeless shelter; residents there recommended it as a good place to make fast cash. He planned to spend the money on food, starting at a McDonald’s across the parking lot. First-time donors like Mr. Jenkins are paid the most, around $50; after five donations, payment is based on body weight, plus bonuses.

Mr. Shaefer, the poverty scholar, said the solution was not to ban the practice, but instead for policymakers to have a real debate about the risks. He suggested they might consider enforcing a minimum wage for plasma sellers.

“Ideally,” he said, “I’d like to have a good discussion about what a fair price is.”

His idea has some precedent : Selling whole blood used to be reputable, with professional blood sellers living in collective boardinghouses and even forming a union in the 1930s, according to Rose George’s book “Nine Pints.” During World War II, a biochemist figured out how to separate plasma, which can be dried, from whole blood, which was perishable and difficult to ship to the front. Soon after, the plasma industry was born.