Consumer Reports examined more than 200 packages to find out. The salad greens were bought in the New York metro area and covered 16 brands, including Dole, Earthbound Farm Organic, and Fresh Express.
The tests, done at an outside lab, didn't find disease-causing bacteria like E. coli, listeria, or salmonella. But they did detect other bacteria.
"Our tests found total coliforms and enterococcus, bacteria that are indicators of poor sanitation and fecal contamination," said Consumer Reports' Kim Kleman. "There are no federal standards for these organisms in salads, but there should be."
Of the 208 bags tested, there were relatively high levels of total coliforms in 39 percent of the salads and enterococcus in 23 percent.
Most brands had at least one package with elevated levels. But even within the same brand, results varied widely.
"It didn't matter whether the salads came in a clamshell or a bag. But the ones with higher levels of bacteria tended to contain spinach or be within five days of their use-by date," said Kleman.
The scary part is that even rinsing them at home won't get rid of all the bacteria, though it will remove dirt. Your best bet is to buy the freshest produce you can. Consumer Reports found the cleanest greens were at least six days away from their use-by date.
NOTE: The bacteria in the bags were not E. coli, listeria or salmonella, the bacteria that cause food poisoning. So while the bacteria found in this report is unsettling, the bags remain safe at this point.