Hair follicle drug testing is a forensic method used by employers, legal systems, and government agencies to detect drug use over an extended period. Unlike urine tests, which cover only a few days, this method provides a historical record of substance use for up to three months. A common question is whether chemical hair treatments, such as dyeing or bleaching, can interfere with the accuracy of the results. This concern stems from the possibility that the chemical processes might alter the hair structure enough to compromise the drug evidence stored within the hair shaft.
The Science Behind Hair Follicle Testing
The ability of hair to serve as a long-term record of drug exposure begins within the hair follicle, a structure embedded beneath the scalp. When a substance is consumed, it enters the bloodstream and circulates throughout the body. The hair follicle is surrounded by a network of blood vessels that transfer drug molecules into the growing hair strand.
As the hair strand forms, drug molecules and their breakdown products are transferred from the blood supply directly into the hair shaft’s inner core, the cortex. These substances become permanently trapped within the keratinized structure as the hair grows outward, creating a timeline of exposure. The test is performed on the hair cut as close to the scalp as possible, not the follicle itself.
Laboratory analysis distinguishes between the parent drug (the original substance consumed) and the drug metabolite (a byproduct created when the body processes the substance). Metabolites are important because their presence confirms the drug was ingested and processed internally, ruling out external contamination. Laboratories test for both the parent drug and the metabolite to provide the most accurate result. This dual-testing approach helps prevent false positives, as surface contamination is typically removed by washing procedures before testing begins.
How Chemical Treatments Alter Drug Detection
Chemical hair treatments, including dyeing, bleaching, perming, and relaxing, physically or chemically damage the hair shaft, impacting the concentration of incorporated drug metabolites. These treatments use strong alkaline or oxidative agents designed to open the hair’s protective outer layer, the cuticle, to penetrate the inner cortex. This process can inadvertently cause drug metabolites embedded inside the hair to leach out or degrade.
Studies have consistently shown that cosmetic treatments can reduce the measured concentration of drug substances within the hair sample. The degree of this reduction is directly related to the type of treatment and the extent of the damage inflicted on the hair structure. For example, hair dyeing may cause a slight-to-moderate reduction in drug concentration. However, more aggressive treatments like bleaching, which involves powerful oxidizing agents, typically cause a more significant reduction in drug content.
Research has found that drug content reduction after cosmetic treatment often falls within the range of 40% to 60% for many common substances. This reduction in concentration can potentially lower the amount of a drug or metabolite below the laboratory’s cut-off level, leading to a negative result even if drug use occurred. However, the effect of these treatments is highly variable and does not guarantee a negative result, especially in cases of chronic or heavy drug use where the concentrations are initially high.
Limitations and Timeframes of Hair Drug Testing
The standard timeframe for a hair drug test is a 90-day window, determined by the average growth rate of human head hair. Head hair grows at approximately 0.5 inches (1.3 cm) per month. The standard sample collected for analysis is 1.5 inches (3.9 cm) of hair cut closest to the scalp, corresponding to the last three months of growth.
A hair test cannot detect drug use that occurred within the most recent seven to ten days preceding the test. This is because the hair containing the drug has not yet grown long enough to emerge from the scalp for collection. For this reason, a urine test is often preferred if the goal is to determine very recent substance use.
If head hair is unavailable or too short, body hair (chest, armpit, or leg hair) may be collected as an alternative sample. Body hair has a slower and less predictable growth cycle than scalp hair, meaning it cannot pinpoint a specific 90-day window. A body hair sample provides a general indication of drug exposure over a much longer period, sometimes covering up to a year.