On Tuesday, the Guardian released the results of a nine-month investigation conducted jointly with Consumer Reports (CR) which showed alarming levels of heavy metals like arsenic, lead and chemicals from plastic PFAS (per- and polyfluoroalkyl substances) in drinking water samples across the U.S.

According to the Guardian, millions of people face serious water quality problems in the U.S. because of contamination, deteriorating infrastructure and inadequate treatment at water plants.

As part of the study, CR and the Guardian selected 120 volunteers to provide tap water samples which were then tested for heavy metals like lead and arsenic, contaminants and PFAS — a group of compounds found in hundreds of household products that are linked to learning delays in children, cancer and other health problems.

The samples came from water systems that service more than 19 million people.

Here are four key findings from this report:

-A total of 118 of 120 samples analyzed had concerning levels of PFAS, arsenic or lead exceeding safety thresholds set by CR scientists and other health experts.

-Almost every sample had measurable levels of PFAS and more than 35% of samples contained the potentially toxic "forever chemicals," at levels that exceeded CR's  maximum safety threshold.

-About 8% of samples contained arsenic at levels above CR's recommended maximum.

-One tested water sample in New Britain, Connecticut, had a lead concentration of 31.2 ppb — more than double the U.S. Environmental Protection Agency's (EPA) action level of 15 ppb, and 25 ppb higher than the water quality report sent to people who use the water.