CHICAGO — Ana Pérez was on the production line at a printing press outside of the city when a forklift operator rolled over her right foot. Hobbled by the pain, she said, an assembly line supervisor let her see a doctor, who provided her pills for the swelling, a bandage and then sent her home.

That accident three years ago was only the beginning of a puzzling process that had her seeking legal aid for workers' compensation and foot surgery — a quest that continues today and has left her unable to work, she said, because she can no longer properly stand. Perhaps if she were treated like a full-time employee of the factory, entitled to health benefits, and not a temporary worker, she added, her experience would have been different.

"If they were to value people, they would respond when we have accidents. They would try to treat people as humans because I don't think we are animals," Pérez, 56, said recently in Spanish through a translator. "We are people, like they are."

Pérez's experience as a temp worker for more than 13 years echoes that of others who are placed by staffing agencies into these mostly low-wage jobs — sometimes for a day or a week, but even months or years. After millions of full-time positions were shed when the Great Recession began a decade ago, the U.S. economy is rebounding, in part, on the backs of temp workers who have helped to fill in the gaps.

And it's at a rate suggesting they are not only increasing — but are increasingly permanent.

"This is absolutely the new frontier in the American labor force," said Keith Cunningham-Parmeter, a labor and employment law professor at Willamette University in Oregon, who has studied how companies such as Amazon and Uber employ people.

These workers represent the underbelly of the U.S. workforce, holding an integral role in the country's ever-evolving, on-demand economy — and whose precariousness has left them vulnerable to a system of exploitation and abuse, advocates say.

They are hired in temporary roles alongside full-timers, boxing up online orders, putting together car parts, making the flimsy aluminum trays sold in dollar stores or whatever else is needed in a pinch.

Ana Perez, 56, of Chicago was a temp worker for more than 13 years. Jake Heller / NBC News

But they can also be the targets of racial discrimination, wage theft and sexual harassment, workers and labor groups said. They face a greater risk than full-time workers of getting injured on the job, and enjoy far fewer workplace benefits, such as healthcare, paid leave and overtime, studies show.

Tim Bell, an organizer with advocacy group the Chicago Workers' Collaborative, said he's in favor of legislative reforms that mandate temp workers are treated in the same way permanent ones are under the law.

"They can work for a company and give all their soul to make a product," he said, "and at the end of the day, they're disposed of like a plastic cup."

Rise of Temp Work

After America's longest and deepest recession lasted from late 2007 to mid-2009, the temp worker industry snapped back with the rest of the economy — to more than 3 million workers in July of this year, according to federal labor data compiled by the W. E. Upjohn Institute for Employment Research. That was a net gain of 481,400 jobs.

Recent studies indicate that freelancing, independent contracting and temp work are on the rise, although most Americans still hold traditional, full-time employment. In July, "temporary help services" workers made up just over 2 percent of all non-farm jobs in the country, federal data show.

What is noteworthy remains the prevalence of such temp workers across a range of industries. No longer are they merely office workers answering phones — they're packing boxes and operating machinery in warehouses, and employed as security guards, janitors and nursing assistants.

A 2016 study by Harvard and Princeton researchers dug into federal employment numbers and found something striking: "94 percent of the net employment growth in the U.S. economy from 2005 to 2015 appears to have occurred in alternative work," which includes temp workers, on-call workers, independent contractors and freelancers.

Researchers noted several reasons, including employees choosing to have jobs that give them more flexibility in their work schedules and employers wanting to focus on their core business, therefore outsourcing certain positions instead of directly hiring for them.

"We were kind of invisible and just a number, easily replaced."

Federal labor statistics have been tracking this growth. A 2013 projection says employment services, which include temp and placement agencies, could rise to 4 million workers by 2022.

But Susan Houseman, a senior economist at the W. E. Upjohn Institute for Employment Research, said the temp worker increase could eventually hit a limit, as jobs move overseas and companies turn to automation.

She added that exact data on temp workers can be unreliable given how inherently transitional it is and that less credible staffing agencies aren't responding to surveys.

For some workers, they hope that signing on as a temp worker can translate into a full-time job. The American Staffing Association, which represents many of the country's 20,000 staffing and recruiting companies, says that about one-third of workers are eventually offered some type of permanent position by a client.

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But a 2017 report by the advocacy group the National Economic and Social Rights Initiative says that four out of five workers never saw a temp job lead to being directly hired.

"We were kind of invisible and just a number, easily replaced," Jasmine Jordan, a Chicago temp worker, told NBC News. "And your hard work always goes unnoticed."

'Imbalance' of Power

The colorful streets of Chicago's Little Village neighborhood were hushed before dawn on a recent morning. There were just tamale cart sellers and workers lining up for breakfast. Some huddled in staffing agencies or on sidewalks for their transportation to job sites outside of the city.

Illinois is one of the country's major distribution hubs. In the past two years, Amazon has opened five distribution centers in the state and is planning another four.

Employers in need of human capital can find them in these mostly immigrant enclaves such as Little Village, or "La Villita," dubbed the "Mexico of the Midwest."

When Pérez, a Salvadoran immigrant and mother of three adult sons, entered temp work, she wasn't only sent to a printing press. She said her staffing agency, Labor Temps, which has several offices in northern Illinois, placed her at separate factories that assembled newspapers, candies, cartons and sausages.

"That was a horror because they drop the sausage meat to the floor and they just pick it up and throw it into the grinder again," she said.

Other workers described conditions they said were unsafe or working for a company that seemed indifferent. But many did not want to complain, they said, for fear of being labeled a DNR by their temp agency — or "Do Not Return."

"I had suffered accidents at other jobs, but I would never say anything. I stayed quiet," said Enedina Zacarias. "Once I fainted after being hit on the head with a box, but I never said anything."

Other workers mentioned how being Latino was favorable because employers saw them as hard-working, while black workers were not wanted because they did speak up.

"It feeds into a narrative, historically in this country, of how African-Americans are perceived, and these companies carry those biases through when they're asking for workers from temp agencies," Bell said.

After her foot injury, Pérez said, she could no longer use her car and relied on buses and less often, vans driven by men called "raiteros" — a Spanglish term for someone who gives rides to job sites.

White vans pick up temporary workers outside an office in Chicago. Jake Heller / NBC News

In 2006, Illinois cracked down on raiteros, making it illegal for temp agencies to charge workers for rides or suggest drivers that do. But a 2013 ProPublica investigation exposed how the system of "underworld labor brokers" is still prevalent. And workers groups also told NBC News it persists.

The drivers, they said, get an immediate cut from workers' wages — about $8 round trip — which can add up when factory workers pull in less than the state minimum wage of $8.25 an hour after taxes. Pérez said the printing press position paid her $8.25 an hour.

The Staffing Services Association of Illinois, which represents staffing agencies, said Labor Temps takes safety seriously and conducts a walk-through before any workers are assigned to a client.

"Should there be an injury they do a follow up investigation," Ryan Keith, an association spokesman, said in a statement. "They have systems in place to get any injured employee the necessary care. Ana Pérez received medical care after she was injured and returned to work at Labor Temps for a year and a half."

He added that Labor Temps has never used raiteros: "Staffing companies cannot charge for transportation under state law, and they provide transportation for free when it is needed."

Chris Williams, a Chicago attorney who is suing six temp agencies, three currency exchanges and two van drivers on behalf of workers, said a "serious power imbalance" thrives between workers and raiteros — with staffing companies essentially looking away while still profiting from workers.

It can unfold this way, he said: An employer contracts from a staffing agency for workers. Workers without transportation rely on raiteros, who can drive them to their shifts. The raitero holds the paychecks from the employees and then goes to currency exchanges to take out what they're owed.

"It's wage theft on steroids," Williams said. "It might be a little bit from this worker's check, a little bit from this worker's check. In the grand scheme of things, it's hundreds, if not millions of dollars."

Changing the Industry

In recent years, a handful of states have moved to expand workers' rights to include those in temp positions.

In 2014, California enacted a law that puts companies on the hook for temp workers who are ripped off or underpaid by their agencies.

Massachusetts has a temp worker right-to-know law that requires agencies give their workers important details about the job they're going to do, for whom, what their wages will be and their ability to apply for workers' compensation.

Illinois also has amended legislation pending called the Responsible Job Creation Act. It seeks to protect temp workers from transport driver abuse; requires workers be given a proper breakdown of their wages; prohibits workers from being paid less than the state or federal minimum wage after meals, equipment and transportation costs are deducted; and advises labor service agencies to help temp workers get permanent employment when an employer client is hiring.

The bill, however, no longer includes a more forceful provision that would have made employers who use temp workers' pay them the same as permanent ones. It is awaiting the signature of Gov. Bruce Rauner, a Republican and former businessman who previously chaired a private equity firm. His spokeswoman said the bill remains under review.

George Gonos, a visiting professor at the Center for Labor Research & Studies at Florida International University, said temp workers already make less than their permanent counterparts — in some cases 20 percent or more lower.

"Permanent employees can't always go to their bosses freely and confidently to ask for more money, so how can temp ones be expected to?" he asked.

The Illinois Search & Staffing Association, another group that represents staffing agencies, said it has not taken a public stance on the bill, but that it supports "the highest ethical and professional standards in the industry," including if the bill becomes law.

Worker advocates worry that any protections could also be eroded on the federal level after the Republican-controlled House introduced the Save Local Business Act in July. The act would roll back an Obama administration-era standard that found a company has joint responsibility for any labor violations incurred by a contractor or franchise operator.

Business groups are lobbying for the standard to be reversed, arguing that it allows contract workers to unfairly sue employers for actions that were perpetrated by the temp agencies.

Toby Malara, the American Staffing Association's government affairs counsel, said the issue isn't necessarily the need for more regulations, but ensuring that laws already on the books are properly enforced.

"Of course you're going to have the bad actors and we're the first in line to say they need to be regulated and taken care of, but there are also staffing firms that just aren't aware of some of the changes and there has to be education," Malara said. "We're very confident that ASA members [adhere to our code of ethics] and follow laws."

Cunningham-Parmeter, the labor professor, said that while temp jobs offer flexibility, they fail workers in this regard: guaranteeing a financial safety net for the future.

"What's going to happen to all of these workers when they reach retirement age?" he said. "They're going to have zero in their retirement account, and that could end up being a drain on society in other ways."

For now, Pérez can't think that far. She remains in limbo over her workers' compensation claim. One of her sons is disabled and living with her, she said, after he was shot on his Chicago block — a victim of collateral damage in a drive-by.

"Sometimes I think that I will leave, but for what? Why leave?" Pérez asked. "Life did not treat me well here, but we have to move forward."

Jake Heller reported from Chicago, and Erik Ortiz reported from New York.