Mercury patch testing is usually done with corrosive mercury salts that cause the skin to redden and possibly swell. Hair mercury levels are not an accurate indicator of mercury exposure. Hair testing has never been standardized to provide meaningful information. Traces of mercury are normally found in the hair, whether the person has mercury fillings or not. Because hair is dead tissue and has no blood supply, it does not reflect the concentration of mercury circulating within the body.
Hair is also subject to shampoos, rinses, dyes, and other products that can contain mercury.
One wire from the device goes to a brass cylinder covered by moist gauze, which the patient holds in one hand. A second wire is connected to a probe, which the operator touches to various points inside the mouth. This completes a low-voltage circuit, and the device registers the flow of current, which the operator misinterprets as abnormal. Mercury Poisoning is very rare and requires considerable exposure, usually at work. People don't get dangerous amounts from tooth fillings no matter how many they have.