"There is currently no scientific evidence to suggest that bagged salad is any more or less risky then a whole head of lettuce." Cutting any fresh produce creates a risk of bacterial growth.

So says a spokeswoman for the U.S. Food and Drug Administration in a Wall Street Journal feature about reviving the prepared salad category; the food safety nuggets are left to the end.

Scientists don’t agree on whether bagged salad has a higher risk of illness than a head of lettuce. Some scientists say it does, because of the possibility that contaminated leaves will spread bacteria to thousands of other leaves during washing and packaging.

For prewashed packaged salads, a second wash at home isn’t recommended for preventing foodborne illness. Addressing the question in 2007, a scientific panel of food-safety experts found the risk of cross-contamination with other foods outweighed any possible benefit from washing packaged salad greens a second time at home.

When washing at home, "there’s a risk that is the sink where you just washed your chicken," says Donald Schaffner, Rutgers University professor of food science.

A table of leafy green related outbreak is available at http://bites.ksu.edu/Outbreaks%20related%20to%20leafy%20greens%201993-2010