mercury
The higher up a food chain that humans eat, the greater the levels of toxic metals they take in. Thats why people should avoid eating too much of large fish such as tuna. Tuna are predators near the top of their food chains. They have been shown to contain high levels of mercury. In people, heavy metals can damage the brain and other organs. Unborn babies and young children are most affected. Thats because their organs are still developing.