The Air Force ignored decades of warnings from its own researchers in continuing to use a chemical-laden firefighting foam that is a leading cause of contaminated drinking water for at least 6 million Americans, including thousands of people south of Colorado Springs. Multiple studies dating back to the 1970s found health risks from the foam, and even an agreement 16 years ago between the Environmental Protection Agency and the foam's main manufacturer to stop making the substance did not curtail the Air Force's usage. Until drinking water tests announced by health officials this year revealed contaminated wells here, the Air Force did almost nothing to publicly acknowledge the danger of the firefighting chemical.