A recent ProPublica analysis found that at least 840,000 temp workers across the U.S. work blue-collar jobs earning them less than $25,000 a year. These aren’t day laborers, ProPublica’s Michael Grabell reports, but regular employees of temp agencies working in the supply chains of some of America’s largest companies, such as Walmart and Nike.

“The rise of the blue-collar permatemp helps explain one of the most troubling aspects of the phlegmatic recovery,” Grabell writes. “Despite a soaring stock market and steady economic growth, many workers are returning to temporary or part-time jobs. This trend is intensifying America’s decades-long rise in income inequality, in which low- and middle-income workers have seen their real wages stagnate or decline. On average, temps earn 25 percent less than permanent workers.”

Grabell joined editor-in-chief Steve Engelberg in our Storage Closet Studio to discuss how the temp sector has ballooned, accounting for nearly 20 percent of the total job growth since 2009; how the temp system insulates companies from many employer responsibilities while pushing workers’ pay below minimum wage; and how some of the country’s biggest retailers have come to rely on (and profit from) this growing work force.

You can read Grabell’s latest report, The Expendables: How the Temps Who Power Corporate Giants Are Getting Crushed, and more on temp agencies on our series page. You can also listen to all of ProPublica’s podcasts on iTunes and Stitcher.