RICHMOND, Va. - You may have to be at least 18 to buy cigarettes in the U.S., but children as young as 7 are working long hours in fields harvesting nicotine- and pesticide-laced tobacco leaves under sometimes hazardous and sweltering conditions, according to a report released Wednesday by an international rights group.

The Human Rights Watch report details findings from interviews with more than 140 children working on farms in North Carolina, Kentucky, Tennessee and Virginia, where a majority of the country's tobacco is grown. The group acknowledges that most of what it documented is legal under U.S. law but aims to highlight the practice and urge both governments and tobacco companies to take further steps to protect children from the hazardous harvesting of the cash crop that has built businesses, funded cities and influenced cultures.

"The U.S. has failed America's families by not meaningfully protecting child farmworkers from dangers to their health and safety, including on tobacco farms," said Margaret Wurth, children's rights researcher and co-author of the report. "Farming is hard work anyway, but children working on tobacco farms get so sick that they throw up, get covered by pesticides and have no real protective gear."

Children interviewed by the group in 2012 and 2013 reported vomiting, nausea and headaches while working on tobacco farms. The symptoms they reported are consistent with nicotine poisoning often called Green Tobacco Sickness, which occurs when workers absorb nicotine through their skin while handling tobacco plants.

The children also said they worked long hours - often in extreme heat - without overtime pay or sufficient breaks and wore no, or inadequate, protective gear.

According to the report, U.S. agriculture labor laws allow children to work longer hours at younger ages and in more hazardous conditions than children in any other industry. With their parent's permission, children as young as 12 can be hired for unlimited hours outside of school hours on a farm of any size. And there's no minimum age for children to work on small farms.