Hair drug testing is a common method used in employment screening and legal contexts to detect substance use over a longer period than blood or urine tests. The test relies on the biological fact that drug compounds are incorporated into the hair shaft as it grows, providing a historical record of consumption.
The Primary Collection Site and Requirements
The standard procedure for a hair drug test involves collecting a sample of hair directly from the head. The hair is typically sourced from the posterior vertex, or crown, of the scalp, where growth is generally consistent among individuals. The hair is cut as close to the scalp as possible to ensure the most recent growth is included in the sample.
Collectors aim to obtain a sample size roughly equivalent to the thickness of a pencil, or about 100 milligrams of hair, involving 90 to 120 strands. The required length is standardized at 1.5 inches, measured from the root end. This length corresponds to approximately 90 days of drug use history, based on the average head hair growth rate of about one-half inch per month.
Alternative Collection Sites for Testing
When head hair is unavailable, too short, or the individual is bald, a collector will turn to alternative body hair sites. Body hair can be collected from areas such as the chest, underarm, leg, or face. It is acceptable to combine hair from multiple body sites to reach the necessary 100 milligrams, which is collected by weight rather than a specific length.
Body hair grows at a much slower rate than head hair, and its growth cycle is less predictable. Because of this slower growth, a body hair sample cannot be segmented for a month-by-month analysis of drug use. Instead, body hair provides a longer, less precise overview that can span up to a year. Head hair and body hair must never be mixed in a single sample, and the collection site is always documented.
The Scientific Basis of Drug Detection in Hair
The ability of hair testing to detect past substance use stems from the biological process of hair growth and blood circulation. When a drug is consumed, it is metabolized, and both the parent drug compound and its metabolites circulate in the bloodstream. A dense network of capillary blood vessels surrounds the hair follicle at the base of the scalp.
As hair is formed in the follicle, these circulating compounds are incorporated into the hair shaft’s inner core, known as the cortex. Once encased, the drug molecules are structurally locked in place, creating a permanent record of drug presence. Since hair grows outwardly from the scalp, a specific length corresponds to a definable period of time, which is the basis for the typical 90-day detection window.
The detection of a drug’s metabolite, a substance created only after the body has processed the drug, is a more definitive indicator of actual consumption. For instance, laboratories look for the metabolite THC-COOH to confirm the ingestion of cannabis, helping toxicologists distinguish between environmental exposure and systemic use. The hair itself is a durable natural fiber, allowing the chemical history to be preserved long after the substance has cleared from other biological fluids.
Factors That Can Influence Test Results
Several factors unrelated to consumption can complicate the interpretation of hair drug test results. Chemical treatments, such as bleaching, perming, or dyeing, can reduce the concentration of drug metabolites found within the hair shaft. These processes alter the hair’s structure, potentially washing out incorporated drug compounds and leading to a lower-than-expected concentration.
External contamination is also a significant consideration, particularly for drugs encountered in the environment, like cannabis smoke or cocaine powder. Laboratories address this by performing thorough wash procedures on the hair sample before analysis to remove surface contamination. The detection of metabolites, rather than just the parent drug, further helps differentiate between external contact and actual ingestion.
Individual biological variability plays a role in the accuracy of the timeline established by the test. While the average head hair growth rate is half an inch per month, a person’s specific metabolic rate and hair growth speed can vary significantly. Differences in hair color and texture can also affect results, as some drugs bind more readily to the melanin present in darker hair.