1947: Perfluorinated compounds are produced at a 3M plant in Cottage Grove, Minn.
1962: DuPont issues an internal memo raising health concerns.
1967: The Naval Research Laboratory patents Aqueous Film-Forming Foam to fight shipboard fires. "This firefighting foam is now used on all U.S. Navy aircraft carriers and by major airports, refineries, and other areas where potentially catastrophic fuel fires can occur," the lab says on its website.
About 1970: The Navy's foam is adopted by the Air Force for airfield use, replacing earlier foams that were less effective but nontoxic.
1979: An Air Force study using perfluorinated chemicals similar to those in firefighting foam finds it damaged "thymus, bone marrow, stomach, mesentery, liver, and testes in the male rats."
1980: A study finds high concentrations of fluorochemicals in the blood of plant workers at a manufacturing plant, though researchers found no attributable health effects.
1981: Another Air Force study finds the chemical in the firefighting foam is harmful to female rats. The Air Force says the study was needed to show "the potential hazard in Air Force women."
1983: An Air Force study on the effects of firefighting foam chemicals on mouse tissue found they "caused impairment of clone-forming (cell replication) ability after treatment with concentrations that were non-toxic in suspension."
1985: An Air Force study finds that perfluorinated compounds could be harmful to cellular growth. "This would imply that these perfluorinated acids are producing toxicity through a membrane interaction."
1991: The Army Corps of Engineers tells Fort Carson to quit using the firefighting foam at the post, saying it "must be replaced with nonhazardous substitutes."
1993: An Air Force study finds that rats exposed to firefighting foam chemicals suffer liver effects.
1997: An Army study tells soldiers to treat the firefighting foam as hazardous waste. "In large volumes, AFFF foam can be harmful to the environment. AFFF solution should not be allowed to flow untreated into the ecosystem, or into the sewage systems in large quantities."
1997: A Navy study attempts to break down the toxic chemicals of firefighting foam using bacteria. The experiment fails.
2000: The EPA and manufacturer 3M issue joint statement warning of the chemicals' dangers.
2002: 3M finishes phasing out its production of perfluorinated compounds.
2005: A landmark settlement is reached between DuPont and residents in the mid-Ohio Valley over water contamination near a manufacturing plant. It established the C8 Project that tested the blood of 69,000 people and led researchers to say the chemicals are associated with six health conditions: kidney and testicular cancers, diagnosed high cholesterol, ulcerative colitis, thyroid disease and pregnancy-induced hypertension.
2005-06: An EPA draft assessment finds a "suggestive" link to cancer, and a follow-up review finds one such chemical is "likely to be carcinogenic to humans."
2006: A study finds levels of one type of perfluorinated compound "greatly exceeded general population medians," largely due to drinking water contamination from a nearby chemical manufacturing plant.
2006: The eight leading manufacturers of perfluorinated compounds commit to ending production of the chemicals by 2015 as part of an EPA stewardship program.
2007: The Air Force says in 2007 it first learned from the EPA that firefighting foam might be dangerous. The service doesn't take action until a stronger warning in 2009.
2009: The EPA issues its first provisional health advisories about perfluorinated compounds that say "epidemiological studies of exposure to (the chemicals) and adverse health outcomes in humans are inconclusive at present."
2011: An Army study finds the chemical in firefighting foam causes immune system damage. "However, autism risk cannot be determined from these data alone."
2012: A study shows perfluorinated compounds are associated with reduced vaccine effectiveness among children ages 5 and 7.
2013: Water districts - largely those serving 10,000 customers or more - begin an EPA-led effort to test their water for perfluorinated compounds through 2015.
2014: Reduced vaccine effectiveness is found in the mid-Ohio Valley population.
2015: A statement authored by 14 leading scientists on perfluorinated compounds, called The Madrid Statement, warns of the dangers these chemicals pose.
May 2016: The EPA tightens its guidance regarding PFCs, issuing a health advisory for water containing 70 parts per trillion or more of perfluorinated compounds.
August: A study is published that finds firefighting sites that used the chemical-laden foam were one of the greatest predictors of nearby water contamination.