Hair element analysis is a non-invasive test that offers insight into present and past exposure to toxic metals, as well as the average status of some necessary elements. It only requires a small amount of hair, 0.25 grams, to perform the test. Hair is an excretory tissue, meaning it does not have any functional purpose and instead contains elements that are of use for diagnosis. When combined with other laboratory values and symptoms, it can help physicians to identify physiological issues linked to imbalances in essential and toxic element metabolism. As a result of the concentration of elements in hair being hundreds of times higher than in blood or urine, it is the preferable tissue to detect elements such as arsenic, aluminum, cadmium, lead, antimony and mercury. The CDC recognizes the value of hair mercury levels in measuring exposure to the neurotoxic methylmercury found in fish.