Add this to the list of "things you can buy online": a stranger's breast milk.

Online milk exchanges — unregulated sites where breast milk is bought, sold, and donated — have taken off in recent years, with multiple websites facilitating transactions between moms with milk to spare and those in need. Despite the long list of milk-seeking ads by adult men — some simply pervy, others interested in the purported health powers of breast milk — researchers say the bulk of the market is moms unable to breastfeed or produce enough milk.

Milk from untested sources, like the ones on online markets, comes with risksThe long list of the benefits of breast milk — ranging from protection against childhood infections, to a lower risk of diabetes and obesity, to higher IQ and income as an adult — have made lactation a high-stakes game for new moms. But milk from untested donor sources, like the ones on online markets, comes with risks. Viruses, including HIV and cytomegalovirus (CMV), can be transmitted, and without pasteurization, there is also the potential for harmful levels of bacteria like Staphylococcus or even Salmonella. In 2013 a research group led by Dr. Sarah Keim of the Nationwide Children's Hospital found that about three-quarters of samples bought online would have failed the standards for bacteria levels at a donor milk bank, and a follow-up study released this month, also led by Keim, showed that 10 percent of online samples were diluted with cow's milk or formula — a potentially serious problem for newborns with a dairy allergy.

But traffic to online markets has continued to rise as more moms look to the internet for help. Across the US, online transactions have more than doubled in the past three years, from around 22,000 in 2012 to about 55,000 today, according to Jesse Kwiek, Associate Professor of Microbiology at the University of Ohio and a co-author of the recent Keim study.