Laboratory studies commissioned by the Environmental Working Group in Oakland found the chemical perchlorate, the explosive ingredient in rocket fuel and a well-known groundwater contaminant, in four of 22 samples of lettuce traced to growers in Southern California or Arizona.
A typical serving of the contaminated lettuce would contain four times the level of perchlorate considered safe in drinking water by the U.S. Environmental Protection Agency, the study concluded. The standards are under review, and health officials said it's unclear whether perchlorate in produce poses any significant health risk.
At high doses, perchlorate can disrupt activity of the thyroid gland, which in a pregnant woman could upset delicate hormone levels crucial for normal development of the fetus, theoretically producing deficits in brain function and motor skills.
Most of the lettuce sold in the United States from November to March comes from an Arizona area where crops are irrigated with water from the Colorado River, long suspected of being contaminated by underground plumes of perchlorate from industrial and military sites.
Lettuce on store shelves from April to October comes from growing regions that are not likely to pose a problem.
Edith Garrett, president of the industry group International Fresh-Cut Produce Association said: "This was based on a very small sample.” "I'm not sure if anybody can extrapolate any suggestions at all from this small of a sample, and there's no other data similar to this."