A quantitative analysis that measures the concentration of iron (Fe) deposited in the hair shaft. It is based on the principle that minerals, including iron, are deposited in hair as it grows at a rate of approximately 1 cm per month, making the hair a long-term (several-month) record of exposure and storage levels. This assay is used complementarily to assess long-term iron status, monitor slow changes, and evaluate potential environmental or occupational iron exposure.