A hair follicle drug test is designed to detect drug use over an extended period. Unlike urine tests, which capture drug use only in the most recent few days, hair analysis provides a historical record of substance exposure. This capability makes it a common tool in employment screenings, legal proceedings, and probation monitoring to determine patterns of use rather than a single instance. The primary concern is understanding the specific timeframe for which drug use remains detectable in the hair.
How Hair Follicle Testing Works
The mechanism behind hair testing relies on the biological process of hair growth and the body’s circulation. When a drug is ingested, smoked, or injected, it enters the bloodstream and circulates throughout the body. Each hair follicle possesses its own blood supply, and as the hair is formed, drug compounds and their metabolites are transferred from the bloodstream and incorporated into the hair shaft’s inner structure.
Once the drug metabolites are trapped inside the hair shaft, they remain there as the hair grows out from the scalp. The hair is essentially a biological record of the substances present in the body during its formation. To collect a sample, a technician cuts a small bundle of hair, typically 100 to 120 strands, as close to the scalp as possible, isolating the most recent growth segments for analysis.
The Standard 90-Day Detection Window
The industry standard for hair drug testing is a 90-day detection window, or approximately three months. This timeframe is based on the average rate of human head hair growth. Head hair grows at a relatively consistent rate of about 0.5 inches (1.25 centimeters) per month.
To capture a 90-day history, the laboratory analyzes the 1.5 inches of hair that was cut closest to the root. Since each half-inch corresponds to roughly one month of growth, the 1.5-inch segment provides a reliable picture of drug use over the preceding three months. Only the first 1.5 inches from the scalp are typically tested to maintain the standard three-month detection period.
Variables That Change Detection Length
The standard 90-day window can be influenced by several biological and external factors, which can either extend the detection period or alter the concentration of drug metabolites.
Hair Type
One significant variable is the type of hair collected, as body hair, such as that from the chest, legs, or armpits, grows at a much slower and more irregular rate than head hair. Because of this slower growth cycle, body hair can provide a detection window spanning up to 12 months, but it cannot be segmented to pinpoint specific months of use like head hair can.
Drug Chemistry
The chemical properties of the substance used also affect how long it remains detectable. Drugs classified as weak bases, including cocaine, opioids, and amphetamines, bind strongly to the melanin pigment found in the hair shaft. Individuals with darker hair, which contains more melanin, may show higher concentrations of these specific drugs in their hair compared to those with lighter hair. Conversely, cannabinoids like THC are non-basic and lipophilic, meaning they bind more to the hair’s lipids and oils, and their incorporation is less dependent on melanin.
Cosmetic Treatments
Cosmetic treatments represent an external variable that can reduce metabolite concentrations in hair. Chemical processes like bleaching, dyeing, and perming use strong chemicals that damage the hair’s structure. This damage can cause drug metabolites trapped within the hair shaft to leach out, potentially reducing their concentration. Bleaching, an aggressive oxidative treatment, can reduce drug levels by 40 to 80%, while dyeing may cause a reduction of 20 to 40%.
Understanding Test Limitations
A limitation of hair drug testing is the inability to detect very recent use, often referred to as the “recent use gap.” It takes time for the hair containing the drug metabolites to grow out of the follicle and emerge above the scalp, where it can be sampled. This growth delay means that drug use within the most recent 7 to 10 days before the sample is collected will generally not be detectable.
Laboratories are equipped to address concerns about external contamination, such as exposure to secondhand smoke or residual drug powder on the hands. Testing protocols involve a rigorous washing procedure, often a three-step chemical wash, to remove any drugs deposited on the hair’s surface. After this decontamination, the lab analyzes the sample for the presence of drug metabolites that are trapped inside the hair shaft, confirming the substance was ingested and circulated internally. Because the drug is encased within the hair shaft, using common “detox” shampoos or other topical remedies is ineffective at altering a positive result.