So, on a crisp Monday morning in November, she traveled 40 minutes by bus to CSL Plasma, a blood plasma collection center wedged between a Dollar Tree and a Wells Fargo bank in a strip mall in North Philadelphia.

“What always brings me here is money,” Watson, 46, said, as she waited in line to get her vitals taken. “I’m doing it for him, I guess you could say.” She earns about $30 each time she donates.

The plasma business is booming in the United States, with the number of collection centers like this one more than doubling since 2005, and global sales roughly quadrupling since 2000 to be more than $21 billion in 2017. Many developed countries have banned paying people for their blood, but not the United States. Blood products made up 1.9 percent of all American exports in 2016, more than soybeans, more than computers.

Plasma — the golden liquid that transports red and white blood cells and proteins through our bodies — is something of an elixir. It’s used to create lifesaving medicines for people with hemophilia, immune disorders, burns and other painful conditions, and it cannot be replicated in a lab.

The market for those medicines is “projected to grow radiantly by 2023,” according to a report from Market Research Future. But there is an underside to all that growth: The industry depends on the blood of the very poor.

Ethicists, sociologists, business executives and “plassers” themselves, as the donors are sometimes called, are increasingly asking: Is the business exploitative, taking advantage of desperate people? Or is it beneficial, offering much-needed income to those who have few avenues to make money? Should we encourage people to sell their lifeblood so frequently, or make it harder to do so?

The Plasma Protein Therapeutics Association, a trade group, disputes the idea that the industry depends on desperate people.

“When I go to centers, what I see in those centers is people of all walks of life. You see mothers, you see students, you see employed people, you see unemployed people,” said Jan Bult, the group’s president and chief executive.

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 most people do not donate so frequently and 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 Jenkins are paid the most, around $50; after five donations, payment is based on body weight, plus bonuses.

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.

In the decades after the war, a growing body of research suggested that the blood supply would be healthier and safer if people voluntarily donated. In the 1970s, the Food and Drug Administration began requiring that whole blood be labeled either a voluntary or paid donation. Blood that had been paid for came to be seen as both morally and physically tainted, according to George; today, the Red Cross will not use it. The FDA requirement never applied to plasma, perhaps because it is broken down and processed before entering another person’s body.

The medicines manufactured from plasma do save lives. Seth Kaufman, a 44-year-old marketing executive and volunteer for the Immune Deficiency Foundation, had been sick most of his life, in and out of the hospital with relentless ear and sinus infections.

Finally, when Kaufman was 36, his condition was diagnosed as an immunoglobulin deficiency, meaning his blood lacked crucial antibodies, and he was prescribed a medicine derived from human plasma. Now he can work and travel and play with his children without fear of contracting a debilitating lung infection.

“When I tell you that this changed my life, it’s a massive understatement,” Kaufman said.

I wanted to see exactly what type of screening donors went through to provide the plasma for people like Kaufman, so I donated plasma at a CSL Plasma clinic in Linden, New Jersey, in a strip mall next to a shuttered Pay Half store.

I watched a 23-minute safety video; my finger was pricked and my temperature was taken; I answered 63 questions, from whether I had lived with someone with hepatitis in the past 12 months to whether I had ever received money for sex; and my arms and ankles were examined for needle sticks.

This screening is robust but not fail-proof: In a survey of plasma donors at a CSL Plasma clinic in Ohio, Olsen found that 13 percent reported they had misled clinic workers about their health in order to donate.

For three hours of my time and 689 mL of my plasma, I made $50, which was given to me on a prepaid debit card that charges a small fee when used.

It made me wonder: Why, if money was changing hands, do plasma collection centers still use the language of donation?

Harriet Washington, a bioethics lecturer at Columbia and the author of the book “Medical Apartheid,” said the semantics can mask the more troubling realities of the industry.

“It’s really important to understand that the profit motive has sometimes been shown to outweigh concern for people’s health,” Washington said. “The problem with using words like ‘donors’ is that it creates the assumption of beneficence.”

Hayway, who donates twice weekly, said he originally came for the money, but he liked that his plasma was helping people, too. He’s just not sure he’s getting a fair shake.

A donation of plasma, for which he will be paid about $30, will yield roughly $300 worth of wholesale immunoglobulin, according to Roger Kobayashi, a clinical professor at the UCLA School of Medicine.

“I know they get a lot of money,” Hayway said of the companies that run the plasma centers. “I would say they should pay more, but that’s just my opinion.”