Toxicologists have a saying: ‘The dose makes the poison.” In other words, there is no such thing as ‘toxic” or ‘non-toxic”-it always depends on how much of a substance you consume.

So what’s a toxic level of mercury in your diet? This has long been a concern, because many fish contain measurable levels of mercury, which can cause profound neurological disease and death if consumed in sufficient amounts. The issue gained new urgency last week when a study in the journal Nature showed that mercury concentration at the ocean surface has tripled since the beginning of the industrial era.

How does mercury get into fish, anyway?

In the 19th and 20th centuries, factories dumped massive amounts of methylmercury-the most dangerous form of mercury, bonded to carbon and hydrogen-directly into waterways. The most infamous example occurred in Japan in the 1950s and ’60s, when industrial mercury poisoned more than 2,000 people who ate fish from Minamata Bay. (The neurological syndrome caused by severe mercury poisoning is called ‘Minamata disease” after the tragedy.)

Mercury dumping has been a problem in the United States, too. I grew up a […]