The change in policy for fresh spinach and iceberg lettuce marks the first time the F.D.A. has allowed any produce to be irradiated at levels needed to protect against illness.

Industry leaders are questioning whether the weak produce-tracking rules that many of them once championed are more a curse than a blessing, several food safety experts say.

Federal health officials said that they hoped to announce soon the source of a salmonella outbreak associated with raw tomatoes that has sickened at least 167 people in 17 states.

McDonald’s, Wal-Mart and other chains have halted sales of some tomatoes as health officials work to trace the source of a salmonella food poisoning outbreak.