An American nonprofit organization, the Environmental Working Group (EWG), have found contaminated freshwater fish in almost every state of the United States by a recent study, the country's media CBS reported on Monday.

These fish were contaminated by per- and polyfluorinated substances (PFAS), a human-made toxic chemicals that last long in the environment and have been called "forever chemicals," which are linked to cancer and other illnesses, according to CBS.

PFAS can be used in a variety of products, including clothing, furniture, food packaging, heat-resistant non-stick cooking surfaces and insulation of electrical wire, according to the U.S. Centers for Disease Control and Prevention (U.S. CDC).

The chemicals can move through soils and contaminate drinking water sources and do not break down in the environment. They can be built up in rivers and lakes and in many types of animals.

High levels of PFAS may lead to multiple health issues, including kidney and testicular cancer, higher cholesterol, and high blood pressure or pre-eclampsia in pregnant women, according to the health agency.

Before this, a team of American researchers have analyzed over 500 samples of fish across the U.S. from 2013 to 2015, and found out that the median level of PFAS in fish samples was 9,500 nanograms per kilogram, according to a study published in the journal Environmental Research.

"Nearly every single (fish) sample had PFAS in it, and at levels that consuming a single serving of fish would be equivalent to a year of contaminated water," said CBS, citing EWG's senior scientist David Andrews, who led the research.

The CBS said that the country's federal government is proposing "strict regulations" limiting PFAS in drinking water, suggesting 90 percent lower than previous recommendations, adding that the states have made their own rules for the time being.