Hair is often used for drug testing because a lot of what we ingest deposits residue in the hair. This residue remains on the hair shaft, as the hair grows out, leaving a pattern---similar to rings on a tree. Because this residue remains on the shaft indefinitely, it is possible to tell not only what was ingested but when, based on where it is deposited on the hair shaft. The longer the hair, the more information available.
How Far Drug Tests Measure
Human hair grows at an average of 0.5 inch a month and labs require 0.5 to 1.5 inches for testing. Each 0.5 inch represents approximately 30 days growth, so a 1.5-inch sample will show up to 90 days of possible drug use.
Types of Hair Tests
There are hair drug tests and hair alcohol tests. Hair is also tested to determine the presence of toxin and poisons.
Hair Drug Tests
Hair drug tests look for actual drug residue present in the hair shaft. If there is not enough scalp hair available, then hair is harvested from the body. If body hair isn't available, hair follicles are extracted.
Hair Alcohol Tests
Alcohol does not deposit directly into the hair. Instead, it leaves markers that are only present when alcohol is consumed. The number of markers directly corresponds with the amount of alcohol consumed.
Drawbacks to Hair Testing
Hair testing does not show the most recent drug use because that hair hasn't grown out yet. Also, ethnic hair type, shampoos and other factors can skew the results---resulting in a false positive (or negative).
Tags: hair shaft, alcohol consumed, hair available, Hair Drug Tests, hair grows, residue remains, Types Hair