The temporary staffing business has been booming, far outpacing the slow increase in permanent jobs.

Temp work typically features low-pay, poor working conditions and no benefits, according to an investigation by ProPublica. Employers can use temp agencies to avoid workers' compensation claims, unemployment taxes and union drives.

Temp workers sometimes must pay work-related fees that result in pay below the minimum wage. They suffer higher rates of job injuries. In fact, the Occupational Safety and Health Administration director has called their working conditions "ghastly."

The rise of temp work has created what's being called "temp towns," poor cities where it's almost impossible to get anything other than temporary work, according to ProPublica.

The U.S. has more temp workers than ever, 2.7 million, the Labor Department reported in June.

The American Staffing Association says it's even larger. One out of 10 workers get jobs through temporary-staffing agencies every year.

Some temp agencies, such as Adecco and Manpower, are some of the largest employers in the country.

Most of the increase in temp work is in blue-collar jobs such as work in factories and warehouses.

"We're seeing just more and more industries using business models that attempt to change the employment relationship or obscure the employment relationship," said Mary Beth Maxwell, a top official in the Labor Department's Wage and Hour Division, according to ProPublica. "While it's certainly not a new phenomenon, it's rapidly escalating. In the last 10 to 15 years, there's just a big shift to this for a lot more workers in 2013 which makes them a lot more vulnerable."

U.S. staffing companies employed an average of 2.86 million temporary and contract workers per day in the first quarter of 2013, up 2.9 percent from the same period last year, according to the American Staffing Association. The January through March period marked 13 consecutive quarters of year-to-year staffing job growth since the recession ended in 2009.

Many economists say temp work will continue increasing as companies use temps to avoid paying for health insurance.

Obamacare, which will require businesses with over 50 employees to offer health insurance to full-time employees starting next year, will be a boon for the temp staffing industry, Time magazine reported. Increasing stock values of temp firms reflect that benefit.

The agencies, Time reported, can also market themselves as labor and legal experts to small- and medium-size businesses.

The weak economy, with its high unemployment rate, is their main advantage, Susan Houseman, an economist at the W.E. Upjohn Institute, told Time.