We both eat a lot of fish. But now even freshwater fish have me worried.

I know. In the latest studies, hundreds of fish taken from about three hundred freshwater streams across the US found every one was contaminated with mercury. Worse, more than a quarter of the fish had levels that exceed federal recommendations.

People didn't have to worry about this before the industrial age. But after 1850, when industry exploded, that all changed. The main source of mercury release in the US is coal-burning power plants.

Their smoke stacks release mercury into the atmosphere, and then it settles into soil. From there, the mercury is washed into streams and rivers, which then carry it into lakes and reservoirs. There, bacteria chemically alter it into methyl mercury, the most toxic form of mercury.

So how does it get into fish?

Well, microscopic invertebrates in lakes called zooplankton are tainted when they eat algae that have absorbed methyl mercury. Small fish are exposed when they eat zooplankton and large fish, like largemouth bass, are contaminated when they eat small fish.

This process is called biomagnification, which means that a toxic compound like mercury is found at higher and higher concentrations as it moves up a food chain. In other words, while the mercury levels are low in algae, they can be very high in large fish.

So where do we stand now that more freshwater fish in the U.S. are contaminated? Well, mercury has negative effects on the nervous and cardiovascular systems of humans especially children.

That's why some states have recommended that pregnant women and children not eat fish from certain bodies of water.

While mercury levels may be low enough for most of us to continue eating freshwater fish... when will it not be okay?

That's something to ponder every time we turn on our lights.