The EPA's super strict safety levels for two chemicals in drinking water are way lower than other countries' rules — like thousands of times lower — so some people question whether the science behind them really adds up.

The EPA says super tiny amounts of a chemical called PFOA in water could be harmful, but our current tools can't reliably detect levels that low — we can only measure down to a level 1,000 times higher.

The EPA just made the safe limit for two harmful chemicals in drinking water way stricter—dropping from 70 to just 4 tiny parts per trillion—but some scientists are saying that big drop isn't really backed by solid science.