This year the Environmental Working Group expanded it's "Dirty Dozen" list to highlight green beans and leafy greens (kale and collard greens) because the vegetables did not meet traditional criteria to make the original list, but the group said they were "commonly contaminated with highly toxic organophosphate insecticides."
The insectisides have been tied to toxic nervous system effects and have been largely removed from crops over the past decade, however the EWG says they are not banned and still show up on these vegetables.
To find out which fruits and veggies were part of the group's "Clean 15" list of least contaminated produce, visit the Environmental Working Group.