Short answer
A standard hair test covers about 90 days. Labs cut 1.5 inches (3.8 cm) of head hair at the scalp. Head hair grows about 0.5 inch (1.27 cm) per month. That equals 3 months of growth, so the sample records drug use from that period.
How the 90-day window is set
The window comes from hair length, not from a lab rule about time. A 1.5-inch sample covers 90 days. A 2-inch sample covers about 4 months. Most labs use 1.5 inches as the standard.
If head hair is shorter than 1.5 inches, a lab may collect body hair instead. Body hair grows slower, about 0.25 inch per month. The window for body hair can reach 6 to 12 months.
how long after smoking once does thc appear in hair
Single use compared with daily use
One use may not show up at all. Levels stay below the cutoff in many cases. Daily use over months builds higher deposits in the hair shaft. Those levels pass the cutoff at a higher rate.
Cutoff levels
Labs report hair results in picograms per milligram (pg/mg). Many labs use a screen cutoff near 1 pg/mg for the THC metabolite, THC-COOH, and a lower cutoff for confirmation. Numbers differ by lab and by panel, so ask the lab for its cutoff.
Detection also depends on dose, product potency, body weight, metabolism, and hair color. Hair color matters because melanin binds THC-COOH.
What does not clear a hair test
Shampoo, bleach, and dye do not remove THC-COOH from hair. Bleach and dye can damage the hair shaft and lower the measured level. Labs look for damage and may reject the sample or call it invalid. No shampoo or detox product has proof that it clears a hair test.
Where hair tests are used
Hair tests are used by some courts, probation offices, child custody agencies, and private employers. DOT-regulated testing for truck drivers, pilots, and rail workers uses urine. Hair testing is not part of DOT rules.
What a hair test cannot show
A hair test cannot name the day of use or the amount used. A positive result means THC-COOH was present above the cutoff at some point in the covered window. It does not prove impairment at the time of collection.