Shutterstock/Halfpoint Your drinking water may contain more of a toxin than you'd think.

A new study out Tuesday in the journal Environmental Science and Technology Letters looked at a national database that monitors chemical levels in drinking water and found that 6 million people were being exposed to levels of a certain chemical that exceed what the Environmental Protection Agency considers healthy.

The chemicals, known as poly- and perfluoroalkyl substances, or PFASs, are synthetic and resistant to water and oil, which is why they're used in things like pizza boxes and firefighting foam. They're built to withstand the environment.

But PFASs also accumulate in people and animals and have been observationally linked to an increased risk of health problems including cancer. And they can't be easily avoided, like with a water filter, for example.

The kind of PFASs that are considered the most harmful are rarely used in the US, but other countries like China still use them, and that could have effects elsewhere, experts say.

"What is emitted from China could eventually land at very long distance away, such as here," Xindi Hu, lead author of the study and a doctoral student at the Harvard TH Chan School of Public Health, told Business Insider. "I want people to be aware that this is not a single country's problem, this a global problem, and different countries need to work together if they want to tackle it."

Here are the areas that were monitored, with everything above the 70 ng/L falling above what's considered a healthy lifetime exposure:

Hu said her next step is to build a model that can better identify which areas have PFAS contamination so that the EPA can better monitor the areas that are the highest priority, like those near manufacturing plants.