Jennifer
They dont put preservatives on lettus but they do spray pesticides on most fruits and veggies. There are varities of pesticides (it may be one kind that you are allergic to or even just 1 element they use to make it) used and washing the food before serving dosent help because it is absorbed into the ground and grows into the fruit or veg. Dont think that washing your food is eliminating these chemicals from your diet because it is not. Try organic lettus, that may solve your problem. Good Luck...:D