After being chopped, the salad leaves are washed in chlorine, effectively sterilising them.The amount of chlorine used is around 20 times more concentrated than you would find in you average local swimming pool.
The salad leaves are then packaged in Modified Atmospheric Packaging . This process removes most of the oxygen out of the plastic bag and replaces it with carbon dioxide. This will keep the salad looking fresh for around ten days. But unfortunately though its appearance is maintained, the nutrient content rapidly diminishes.
Cheaper to buy a lettuce and wash it yourself.