Mercury (汞), an unusual element, is poisonous and can be found just about everywhere. It can also exist as an organic compound, the most common of which is methylmercury (甲基汞). While we can take steps to avoid both elemental and inorganic mercury, it is much harder to avoid methylmercury.
Most of the mercury in the environment comes from coal-burning power plants. The concentration of mercury in the air from power plants is very low, so it is not immediately dangerous. However, the mercury is then washed out of the air by rainstorms and eventually ends up in lakes and oceans.
The mercury in the water doesn't immediately get absorbed by fish, as elemental mercury doesn't easily spread into cells. However, methylmercury spreads into cells easily, and certain bacteria in the water change the elemental mercury to methylmercury. Methylmercury in the water spreads into plankton (浮游生物). Small animals eat the plankton and absorb the methylmercury in the plankton and then large animals eat small animals and absorb the methylmercury. Because each animal passes the methylmercury much more slowly than it absorbs it, methylmercury builds up in the animal over time and is passed on to whatever animal eats it.
As people became aware of the concentration of mercury in fish, many reacted by removing seafood from their diet. Scientists have studied the effects of mercury. By determining the levels of mercury consumption that cause any of the known symptoms of mercury poisoning, they were able to identify a safe level of mercury consumption. The current recommendation is for humans to take in less than 0. 1 microgram of mercury for every kilogram of weight per day. This means that a 70-kilogram person (about 155 pounds) could safely consume 7 micrograms of mercury per day.
Nutritionists recommend that, rather than remove fish from our diet, we try to eat more of the low-mercury fish and less of the high-mercury fish. Low-mercury species tend to be smaller fish while high-mercury species tend to be the largest carnivorous (食肉的) fish. Awareness of the particulars of this problem, accompanied by mindful eating habits, will keep us on the best course for healthy eating.
