A single strand of hair acts as a biological diary of your diet, health, and travels.

Hair is primarily composed of the protein keratin, which grows from follicles supplied by a dense network of blood vessels. As new cells form, they incorporate chemical elements and isotopes circulating in the blood. Because hair grows at a relatively constant rate of about 1 centimeter per month, it provides a physical timeline of an individual's life.A key tool in this analysis is stable isotope ratio analysis (SIRA). Isotopes of hydrogen and oxygen in drinking water vary by geographic location due to climate and elevation. When a person drinks local water, these isotopes are locked into the hair shaft, allowing researchers to map their movements across different regions with high accuracy.This forensic method has been used to solve historical mysteries and criminal cases. For example, analysis of hair from the 1845 Franklin Expedition explorers confirmed high levels of lead poisoning, which likely contributed to their demise. Similarly, carbon and nitrogen isotopes can reveal shifts between vegetarian and meat-heavy diets over specific months.Modern toxicology also uses hair to detect long-term exposure to heavy metals or drugs. Unlike blood or urine tests that show recent consumption, a 12-centimeter strand of hair can provide a full year of physiological history. This makes hair an invaluable resource for both archaeological research and modern forensic science.