Consumers have grown to love convenience salads, from tubs of pre-washed baby spinach to bags of chopped romaine.
There's only one problem with these modern-day conveniences: They're regularly implicated in foodborne illness outbreaks.
The latest, a nationwide flare-up of E. coli, has sickened 84 people in 19 states and hospitalized 42. Most of the victims grew ill after eating chopped romaine lettuce from a farm near Yuma, Arizona.
Such outbreaks are rare overall, but more common in certain types of foods. Data from the Centers for Disease Control and Prevention suggest that leafy greens cause roughly a fifth of all foodborne illnesses.
And food safety experts say convenience greens – those handy bags of pre-chopped and pre-washed salads – carry an extra risk because they come in contact with more people and machinery before they arrive on your plate.
Recent industry efforts and federal rules have attempted to reduce outbreaks. But the risks will never completely disappear, experts say.
"We're always going to have these cases, unfortunately, because consumers have gotten used to this product," said Bill Marler, a prominent food safety lawyer who represents several patients sickened by the Yuma lettuce. "The product has risks, in my opinion."
Federal regulators haven't yet uncovered the source of this latest lettuce outbreak. But the Centers for Disease Control and Prevention and the Food and Drug Administration are urging consumers to throw out romaine that could be from Yuma, where most lettuce is grown during the winter season.
Most of the 84 people grew ill after eating at restaurants that use bagged, pre-chopped lettuce in their salads. This strain of E. coli, known as 0157: H7, produces a toxin that can disrupt liver function. The majority of victims are women, a reflection of the fact that women generally eat more salads.
Government regulators have long known that greens and lettuces pose a particular food-safety risk. According to one CDC analysis, leafy vegetables were responsible for 22 percent of foodborne illnesses between 1998 and 2008, the latest period for which detailed attribution data is available.
A more recent analysis of outbreak data from 2013 concluded that "vegetable row crops" – lettuces plus broccoli, asparagus, celery and some other vegetables – account for 42 percent of E. coli infections. In the past four months, E. coli infections linked to leafy greens in Canada and the U.S. have caused 151 illnesses and two deaths.
"Leafy greens continue to be a problem, and we've looked at leafy greens and fresh produce with concern," said Robert Tauxe, the director of the CDC division that responds to foodborne illness outbreaks. "Back 15 to 20 years ago, there was a huge concern in food safety around foods of animal origin … But beginning about 10 years ago, the produce side has become more and more prominent."
Contamination can occur on the farm when birds make frequent flights overhead or low-lying fields flood with contaminated water. E. coli can also be spread by farm workers who don't wash their hands or via farm equipment that has manure on it.
Once the greens are picked, they move to a packaging plant, where they're exposed to more workers and more equipment. Product from multiple farms is often bagged in the same facility, which further increases the odds of cross-contamination.
While packers frequently rinse lettuce with a chlorine wash to kill pathogens, studies have shown those sprays are only partly effective. The same is true of washing fruits and vegetables at home, Tauxe said, because pathogens "cling" to the surface of produce and can even enter the inside of a leaf or fruit after they've been cut open.
There's no "kill step" that destroys pathogens for foods eaten raw, as there is for a well-done burger or a glass of pasteurized milk.
"This is why it's so important that the people who grow food do everything they can to minimize contamination," said Sandra Eskin, who heads the food safety project at the Pew Charitable Trusts. "Lettuce grows in the dirt. It's eaten raw. There's no opportunity to cook it to kill bacteria."
By all accounts, farmers and regulators have made progress toward making lettuce and leafy greens safer. Since 2006, when E. coli from fresh spinach sickened nearly 200 people and hospitalized 100, the produce industry has launched several initiatives to tighten farm safety rules for leafy greens and lettuces.
In 2011, Congress passed the Food Safety Modernization Act which included new standards for irrigation water quality, worker hygiene and equipment sanitation, went into effect for large farms this January. Smaller farms will have to comply with the rules by early 2020.
But despite these efforts, the number of outbreaks and infections linked to leafy greens has largely remained flat over the past 10 years, with 11 outbreaks and 242 illnesses per year on average, according to CDC.
Eskin and Tauxe say they believe the new rules will help — but they will not eliminate the risk completely.