Nearly half of all U.S. teenagers working in the retail or service sectors have performed workplace tasks that are prohibited by federal child labor laws, according to a new national study.

In a telephone survey of 866 teenagers between the ages of 14 and 18, 52 percent of males and 43 percent of females said they had operated box crushers, slicers, and other dangerous workplace equipment, or had performed other tasks that are prohibited by law for their age group -- such as driving a delivery truck or serving alcohol.

The study, conducted by researchers at the University of North Carolina's Injury Prevention Center and published in the March issue of Pediatrics, also found that one-third of working teens said they did not receive any on-the-job safety training. Nearly 30 percent said they have worked at least one day a week without an adult supervisor, while others had worked late on a school night or alone after dark.

According to federal regulations, employees under age 16 can only work after 7 a.m. and before 7 p.m., except from June 1 through Labor Day, when they are allowed to work until 9 p.m. They are also prohibited from working more than three hours on a school day, or 18 hours in a school week, according to Youth Rules!, a Labor Department website that seeks to raise awareness of federal and state laws governing young employees.

Some states have stricter laws regarding working hours and permissible tasks for teenage workers than those set out by federal regulations.

Carol Runyan, the director of UNC's Injury Prevention Research Center who led the study, said she was shocked to discover the lack of supervision for teen workers.

"When teens are working without anyone else around, particularly after dark, that's a concern because that is when robberies and homicides can happen," Runyan said.

In many cases, employers simply aren't aware of federal child labor laws, while others chose to ignore them, Runyan said.

As many as 70 teens die every year from work-related accidents, and hundreds of thousands more are injured, according to the Labor Department.

While Runyan acknowledged that child labor laws can be confusing since they differ slightly at the state and federal levels, she said, "it is the responsibility of employers to know the laws and make sure their work environment is safe. They need to do a better job."

She added that state labor departments and parents of teen workers can play a major role in raising awareness among employers.

"Labor departments should make sure there is understanding of the laws and enforce them when necessary," Runyan said.

By familiarizing themselves with the regulations, parents can also send a strong message to employers that they are paying attention, she said.

"To the extent that adults can get involved, they can really help their child navigate the employment process," Runyan said, adding that young workers often don't know their rights as an employee or feel intimidated about confronting an employer.