A hair follicle drug test analyzes a small sample of hair to determine a person’s history of substance use. Unlike urine or blood screens, which capture recent consumption, this test offers a longer look-back period. By analyzing substances trapped within the hair structure, the test provides a historical record of drug exposure. This long-term perspective makes the hair test a common choice for employment screening, legal cases, and monitoring abstinence.
The Standard Detection Window
The industry standard for a hair drug test is designed to provide a history of substance use over the past three months. This look-back period is achieved by analyzing a specific length of hair sample taken from the scalp.
This standardized length is based on the average rate of human head hair growth, which is approximately 0.5 inches per month. By isolating the 1.5 inches closest to the scalp, testing facilities analyze the hair that grew during the 90 days prior to collection. This standard offers a consistent snapshot of a person’s substance use pattern for a defined duration.
The Science Behind Drug Incorporation
Substances consumed by a person enter the hair shaft through a complex biological process, creating a permanent record of exposure. Once a drug is ingested, it is metabolized by the body and enters the bloodstream. From the blood, the substance and its metabolites diffuse into the actively growing cells at the base of the hair follicle.
As the hair forms, these molecules become entrapped within the keratin matrix of the hair strand. The hair then grows out of the scalp, carrying the chronological record of drug exposure along its length. Drugs can also be incorporated through external routes, such as diffusion from sweat and sebaceous gland secretions that coat the hair shaft.
The test specifically looks for the parent drug and its metabolites, which are the breakdown products created by the body. For example, when testing for cocaine, laboratories look for the drug itself and its primary metabolite, benzoylecgonine. Certain drugs, particularly basic compounds, tend to bind more readily to melanin, the pigment that gives hair its color, which can influence the final concentration measured in the sample.
Factors That Adjust the Timeline
While the 90-day window is the established standard, several biological and external factors can shift this timeline. The most significant variable is the individual rate of hair growth, which can vary by as much as 30% between people. Slower-growing hair may stretch the detection window beyond 90 days for a 1.5-inch segment, while faster growth results in a slightly shorter window.
The actual length of the hair sample collected also directly dictates the timeline; if a person’s hair is shorter than 1.5 inches, the window is proportionally reduced. The type and amount of substance used also play a role, as some compounds incorporate into the hair more efficiently than others. Regular or heavy use is more likely to leave a detectable concentration than infrequent or low-dose use.
External factors such as cosmetic treatments can also influence the results. Processes like bleaching, dyeing, or perming can chemically degrade or wash out drug molecules trapped within the hair shaft, potentially lowering the measured concentration. To account for this, accredited laboratories employ rigorous wash procedures to remove external contamination, such as secondhand smoke residue, before analyzing the internal drug content.
Using Body Hair vs. Head Hair
In cases where an individual has insufficient head hair, or if a longer detection window is required, body hair can be collected and analyzed. Hair from alternative sites, such as the chest, armpit, or leg, can be used as a substitute for the scalp sample. However, the interpretation of body hair results differs significantly from head hair analysis.
Body hair grows slower and has longer, variable dormant phases compared to head hair. Because of this unpredictable growth cycle, a body hair sample cannot be segmented to provide a precise month-by-month history of use. Instead, body hair provides a general overview of substance exposure, with a detection window typically representing an ambiguous history of up to 12 months.