Mercury poisoning takes a long time to work its horrific damage. Hiding for decades in the body and brain before its lethal work is accomplished, people think it is safe to use. Early symptoms include: a "burning tongue" sensation, or metallic taste in your mouth, or a burning feeling at the bottom of your feet. First a word, here or there, is forgotten.
Then, a person cannot remember more and more words, or how to do even the simplest daily activities. In its final stage, over 9 or 10 years (sometime less), the brain shuts down. Then the body cannot function properly. A person becomes unable to do basic tasks or even speak. This is mercury doing its deadly work. So, billions of people have been sold decades of advertising and medical lies. There is no safe level of ingestion or any kind of use of mercury!