Pollution can seem like a vague, general problem, but sometimes it is specific and personal. People with asthma living in some major cities know to keep tabs on the ozone report in the weather forecast, for example. And frequent anglers should be keenly aware of how much of their catch they put on the dinner table because of mercury contamination in fish. Mercury is a problem for marine fish, as well—particularly the ever-popular tuna.

Mercury emitted by burning coal finds its way from the atmosphere into seawater, but an additional step is necessary to weaponize the heavy metal. Bacteria convert mercury into methylmercury by attaching a carbon atom and three hydrogens, making a molecule that freely wanders into biological tissue and hangs around.

Since predators are made up of all the many critters they eat, this mercury accumulates to greater and greater levels at each step in the food chain. The meaty tuna sits at the top of its food chain, and that means it can contain a lot of accumulated mercury. Because of how much tuna is consumed in the US, the fish actually accounts for about 40 percent of the mercury ingested from fish.