Hair can yield DNA evidence, if hair is pulled out by the root, as in some violent struggles, it will contain root pulp which is a good source of nuclear DNA (nDNA), the type of DNA most often used in forensics.  The hair shaft does not contain nDNA, so the 100-150 strands of hair most people lose daily will not contain a root or nDNA, but it does have mitochondrial DNA (mDNA).  MDNA, which cannot be used for individualization, can narrow the source of the hair down to a certain family group (mDNA is passed from mother to all offspring).

Hair can also be used to run drug tests, but these tests are only presumptive and have to be confirmed by blood or urine tests.  Hair absorbs chemicals, such as marijuana smoke and arsenic, from the environment and once the chemical is in the hair there is no way of telling if it got there from consumption or from exposure from the environment.

Age cannot be determined from hair, generally the only age determination that can be made from hair is if came from an infant or from an elderly person.  The sex of the person the hair came from can also not be determined via hair examination unless there is nDNA  evidence such as the root.

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