Mercury occurs naturally in the environment and can also be released into the air through industrial pollution. When mercury falls from the air and accumulates in streams and oceans, it transforms into methylmercury. It is this type of mercury that can be harmful to your unborn baby or young child. The methylmercury is then absorbed by the fish as they feed in these waters, which is why it is considered potentially unsafe to eat too much of certain fish species. Certain types of fish and shellfish absorb more methylmercury than others and at different rates depending on what they eat.