In 2002, the Environmental Working Group (EWG) reported that "Test results never before made public show that leafy vegetables grown with contaminated irrigation water take up, store and concentrate potentially harmful levels of perchlorate, a thyroid toxin that is the explosive main ingredient of rocket and missile fuel." The studies in question revolved around the Colorado River, where high contamination with perchlorate was found. Unfortunately, vegetables store a higher dosage of the chemical than water.
In some tests, the perchlorate dose in the vegetables was 65 times the amount in the water. The Colorado River is the primary source of irrigation water for farms in Arizona, California and Nevada. These states grow most of the lettuce sold in the U.S., which is why the focus is on lettuce rather than other vegetables. Aerospace and defense contractor Lockheed Martin is the major user of perchlorate in the area. It was put forth by the EWG that the company knew about vegetables absorbing the perchlorate as early as 1997. The state responded by setting perchlorate levels for water at six parts per billion, although many environmentalists argue this amount is still too high to be safe for human consumption.