The bagged lettuce found in most grocery stores is another, more complicated matter. The lettuce leaves, even those labeled organic, are likely to have been washed in a weak chlorine solution. This is both good and bad news. The good news is that the chlorine is there to destroy any harmful bacteria that can lead to sickness. The bad news is that most folks would prefer not to eat something that has been washed in chlorine, no matter how diluted.
Would rewashing your bagged lettuces help? Well, besides the fact that it defeats the reason for buying the pre-washed lettuce to begin with, there is also the possibility of contaminating the lettuce in your own kitchen. It's very possible that your kitchen is not nearly as clean as a lettuce processing plant, which uses industrial strength cleaners and has rigorous cleaning protocols.
The bottom line is that many foods, and especially raw foods like lettuce, aren't sterile, and for the most part that's okay. While no one wants to buy contaminated lettuce, you can find a middle ground. I recommend buying your lettuce from a smaller producer whose farming practices you trust. Keeping them refrigerated and using them up quickly are also good ways to keep bacteria at bay.
Don't assume, though, that just because you purchase your lettuce at the farmers' market that it isn't washed in the chlorine solution. Many organic famers use this method to assure that no harmful bacteria is making its way into your salads. If this practice is a concern for you, be sure to engage one of the people at the stall about their washing and testing practices.
(Image: Dana Velden)