www.salford.gov.uk/204014_report.doc
A study was done in the United Kingdom, referenced above, indicating that the inner leaves of a tightly-packed head of lettuce (such as romaine, iceberg, and some others) do not show evidence of microbiological contamination.
So I've concluded it's safe to wash only the outer leaves if the inner leaves show no signs of contamination or insect infestation.
Do read the report and see if you agree with my conclusion.
