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Americans spent a record $161.5 billion on e-commerce sites last year, an increase of 13 percent from 2010 according to Internet research firm comScore. A growing number of consumers are choosing the Internet as their retail shopping destination and e-commerce sales could account for 9 percent of all retail sales by 2016 says Forrester Research.

Online shopping offers consumers convenience, speed and perks like free shipping. But few consumers think twice about the individuals who package and ship their Internet purchases.

Mac McClelland, a human rights reporter at Mother Jones magazine, went undercover last year at a warehouse fulfillment center. She shared her short but eye-opening experience in a recent issue of the magazine under the startling headline "I Was a Warehouse Wage Slave". Her expose sheds light on a little known industry.

McClelland cannot disclose the name or location of where she worked but she says in the accompanying video that the working conditions in her warehouse were similar to other major fulfillment centers. Her 10-to-12 hour work day consisted of running around a "multiple story several-hundred-thousand-square-foot warehouse," searching for orders with a scanner in hand that would determine how long it should take her to find the item (usually 20 to 30 seconds, she notes). She says she was required to pick at least 1,500 items per day, which required her to climb metal staircases or get down on her hands and knees to hunt for someone's purchase.

Her 3,000 coworkers — of all ages — would often cut their timed lunch and rest breaks short because every minute was precious for a worker who needed to pick thousands of orders per day to keep his or her job.

Here's how McClelland describes the warehouse where she worked:

McClelland's experience proves not to be unique. Former warehouse employees are starting to speak out against the harsh and sometimes dangerous working conditions at these fulfillment warehouses.

Unsatisfactory working conditions at warehouses and assembly plants overseas have become a catalyst for improving workers' rights. Last year Apple was pilloried in the press after news reports revealed that Foxconn workers in China worked long hours on their feet performing highly repetitive tasks, lived in cramped quarters and often worked seven days in a row. Foxconn workers make Apple's iPhone and iPad gadgets. Under pressure to reform the working conditions at Foxconn, the Chinese tech supplier and Apple agreed to reduce workers' hours and boost wages and Apple said it would publish monthly audits of factories that produce Apple products. Apple also became the first technology company to join the Fair Labor Association.

McClelland says the government and online retailers do not keep reliable statistics about the employees hired at fulfillment centers here in America. Industry insiders could offer only vague estimates of the number of employees in this industry, citing the high turnover rate and large number of temporary workers at these warehouses, she says. According to McClelland, temp workers account for 15 percent of warehouse pickers, movers, packers and unloaders and are typically paid $3 less on average than permanent workers, who are paid a few dollars above minimum wage. Temp workers can "be temporary for years," McClelland adds.

McClelland did not report on warehouse employee conditions to stop consumers from shopping online but to change the way the major online retailers — Amazon.com (AMZN), Staples (SPLS), Walmart (WMT), Dell (DELL) and Sears (SHLD) — respond to and respect their employees. She acknowledges that these jobs provide income for individuals desperate for work in a weak economy but argues that online corporations should not be absolved of their duties to ensure a safe and nonthreatening work environment.

"A lot of people are sustaining permanent physical injuries from doing these jobs," she says in the video. "This model is not healthy. It's not like these companies don't have the money to treat their employees a little better. It could be more ergonomically safe…with more relaxed and reasonable goals."

Working conditions at online fulfillment centers could soon become a hot topic at the state level. Amazon, the biggest online retailer in the U.S., plans to open new warehouses in several states this year and next, including two in New Jersey, one in Virginia, two in Tennessee and one in Indiana. Amazon has come under intense scrutiny after several news organizations revealed that Amazon employees performed their work in extreme heat, where the temperature would sometimes rise above 100 degrees. The Morning Call of Allentown, Pa. reported that Amazon paid to have ambulances on standby outside the facilities at two Pennsylvania plants. The Seattle Times published a four-part series about Amazon which included stories from employees that they were forced to work long hours and who struggled to keep up with unreasonable demands and expectations from Amazon managers who constantly pressed for higher production.

An Amazon spokesperson provided this response to Yahoo!:

McClelland says she was fortunate that her brief experience as a warehouse picker was just that -- temporary and short lived.

Whether industry-wide changes come to the workers who pick and ship online orders every day, every second, lies in the hands of both consumers and online retailers.

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