Dole announced last week that it’s temporarily suspending operations at an Ohio production facility after its bagged salads were linked to a multi-state outbreak of listeria. At least one person has died, and a dozen more have been hospitalized in six states.

The Centers for Disease Control issued a warning that “consumers do not eat, restaurants do not serve, and retailers do not sell packaged salads produced at the Dole processing facility in Springfield, Ohio.” (The contaminated salad was sold under various brand names: Dole, Fresh Selections, Simple Truth, Marketside, The Little Salad Bar, and President's Choice Organics. Salad that should be discarded can be identified by the letter “A” at the beginning of the manufacturing code found on the package.)

But what about leafy greens in general?

They’re packed with essential vitamins, but they’re also tied to an outsized number of food-related deaths. In 2006, an E. coli outbreak caused by contaminated spinach killed three people and sickened more than 200 others in 26 states. Several studies have found that various changes in food production in recent decades increases the likelihood that contaminated foods will be distributed widely. One study, published last year in the journal Foodborne Pathogens and Disease, found that in the 1970s, there were about 3 multi-state outbreaks per year, compared with 14 per year between 2000 and 2010. (Some 48 million people get foodborne illnesses in the United States annually, and about 3,000 of them die.)

In a review of nearly four decades of food outbreaks reported to the Centers for Disease Control since the 1970s, scientists found outbreaks of illnesses associated with leafy vegetables were larger than those attributed to other food types.

This is surprising in part because of the cultural space spinach occupies. How could a vegetable that’s so nourishing, so vibrant, so low-calorie—and apparently so appealing that Popeye sometimes sucks it out of his pipe—be dangerous? The 2006 E. coli outbreak wasn’t just a one-off, either. Leafy vegetables were also responsible for the most deaths from foodborne illness between 2003 and 2012, during which time more E. coli cases were reported than in the previous 20 years. (The increase doesn’t necessarily mean food safety is worsening; it may also be attributable to more diligent monitoring by food-safety officials.) Seven people died from illnesses traced back to leafy vegetables in that time, compared with six people who died after eating contaminated fruit, and five who died after eating unsafe beef.

It’s worth noting that far more people were sickened by beef than by leafy vegetables between 2003 and 2012—mostly ground beef, some steak. Illnesses related to beef consumption represented more than half of all outbreaks that could be traced to a specific food source in that time, whereas leafy greens were implicated in nearly one-quarter of identifiable outbreaks. But the people who got sick from beef were more likely to recover than those who ate bad leafy greens.

Another troubling difference between foodborne illness from beef versus, say, spinach, is that there are several ways consumers can make meat consumption safer for themselves—including better handling of raw meat, cooking it to a safe temperature, and thorough hand-washing—whereas it’s less clear what can be done to improve the safety of leafy greens.

“Further research is needed to identify interventions to reduce the risk of illness caused by consumption of raw produce,” the authors of a CDC report wrote in August. “Ways must be found to encourage implementation of interventions such as decreasing contamination in the production of produce, vaccinating cattle, and irradiating foods.”

After the 2006 E. coli outbreak linked to spinach, investigators were able to identify environmental risk factors, but they still couldn’t say precisely how the contamination originated. On top of that, there seemed to be nothing the consumers who ate raw spinach could have done to protect themselves. Washing produce “would not have prevented the recent E. coli outbreak,” the FDA said after its investigation.  

A closer look at the farming practices and consumer behaviors paints a slightly clearer picture, as Emily Bazelon wrote for Slate that year.

Spinach has taken off in popularity since the industry figured out how to sell pre-packaged baby spinach. Sales of bagged salad have risen 14 percent a year in the last decade, to $3 billion annually. To produce the bags, processing plants take greens from different farms, put them through three different chlorinated baths, dry and seal them in plastic, and then ship them to a market near you. The chlorination doesn't get rid of E. coli: To do that, you need to heat the leaves and treat them with an organic acid, which would probably make them go limp. So, by mixing greens from different farms without treating them for contamination, the processing of bagged spinach spreads E. coli once it’s present in a particular field.

In November, the agency released new produce safety rules, several years in the making, that were established as part of the 2011 Food Safety Modernization Act. They mandate minimum standards for water quality, employee health and hygiene, and biological elements used in soil (like compost and manure)—plus additional rules related to equipment, tools, facilities, and more. The FDA says the new regulations represent a “monumental shift in food safety,” though farms and businesses, depending on their size, have anywhere from one year to four years to comply with various aspects of the new rules.

In the meantime, the agency still encourages people to wash their leafy vegetables, even when salads are advertised as pre-washed—and even though officials have acknowledged that washing doesn’t always kill deadly bacteria. Several studies have found that soaking leafy greens in cool water or a vinegar-lemon mixture for about three minutes, followed by a water rinse, can reduce the risk of foodborne illnesses. The pre-soak, more than any sort of rinsing by itself, appears to be the key, several scientists found—though some vegetables respond better than others to this sort of sanitizing. (At least one study found that a vinegar mixture wasn’t significantly better than cool, clean water alone.) Other food experts recommend skipping bagged greens altogether, and instead buying lettuce and other leafy vegetables in their most bunched up, cabbage-y form—so you can peel off and discard outer layers before washing.

Leafy greens are nutritious and, most of the time, perfectly safe to eat. Still, the best thing to do, for those who love spinach and are still looking for some peace of mind, may be to pick the recipes that call for cooked greens instead. In 2010, a microbiologist at the New York Institute of Technology found it takes two minutes in boiling water for 99 percent of E. coli bacteria to be obliterated from a spinach leaf. And if boiled spinach isn’t your thing, there’s always lasagna.