Buy your lettuce whole. It's just safer. Credit: Andre Baranowski / Getty Images

It's hard to beat the convenience of prewashed, prepackaged greens — rip open the bag, flip it over and you have an instant plateful of vitamins and roughage. But considering the number of reports of deadly diseases lurking amid the leaves, the product's possible environmental toll, and the fact that you're supposed to wash them anyway (making them that much less convenient), you should probably grab that whole head of lettuce instead.

The latest food-borne illness outbreak related to packaged greens — as you've likely read — began last summer. Since then, a dozen people in six states have been infected with Listeria a life-threatening infection that results in 1,600 illnesses and 260 deaths every year, according to the Centers for Disease Control.

An investigation by the CDC and the Food & Drug traced the source back to a Dole processing facility in Springfield, Ohio, which packages salad greens under the labels Dole, Fresh Selections, Simple Truth, Marketside, The Little Salad Bar, and President's Choice Organics (it's not uncommon for organic and nonorganic produce to come from the same facility).

Bagged greens are usually washed thoroughly (some packages boast that their contents are "triple-washed") with a little bleach, but that unfortunately doesn't make any difference if the produce carries disease-causing bacteria. "Listeria is a natural soil inhabitant, and spinach commonly comes in contact with the soil," says Jeri Barak, associate professor of plant pathology and executive member of the Food Research Institute at the University of Wisconsin-Madison. Listeria, like Salmonella and E. coli, can't be rinsed or washed from leaves even if the dirt is, she says.

The reason this matters specifically for bagged greens is that most of them are younger leaves or microgreens that pose a greater risk for contamination. "Data suggests that bacteria (Salmonella and E. coli) can enter wounds, such as the harvesting cut, and travel some distance into a leaf. But fresh produce that's harvested as a head has longer leaves and is more mature," she says. "A consumer has the opportunity to remove part of the head away from the harvest cut and eat leaves in which a pathogen did not reside in the harvest wound."

In addition to piggybacking pathogens on baby leaves, it takes more steps — between harvesting, washing, and processing — to bring prepackaged greens to your store, which provides more opportunity for contamination along the way. Many bags contain more than one type of green, for example, so a crop of contaminated spinach could carry bacteria into many mixed greens bags as well as ones containing spinach only. Researchers have also studied the atmosphere inside bags, which can foster the growth of bacteria.

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To add to this, organic bagged greens are no less likely to make you sick. "It may be a common misconception that organic vegetables are less likely to cause illness, but there's no data to support the conclusion that organic produce is less or more likely to carry pathogenic E. coli," Barak says. "Case in point, the 2006 spinach outbreak of E. coli O157:H7 was organic bagged spinach."

Evidence suggests that health risks posed by the multi-billion dollar bagged greens industry could be growing, in part because people are eating more greens, and more raw produce in particular. But many experts still stop short of recommending you never buy prepackaged greens. The odds are in your favor that you won't be sickened by them given that outbreaks are relatively low — particularly if you're not pregnant, an infant, over 65 years old, or have a compromised immune system.

Still, your best option is to buy mature, unpackaged produce and rinse the leaves thoroughly.