Hair follicle drug testing has been around for almost 40 years, but it has always been the subject of controversy as scientists and employers weigh in on its accuracy, benefits and cost.

Several private testing companies provide hair testing, and the FDA approves it. However, it is not currently recognized as a reliable sample for Department of Transportation (DOT) or other federally regulated programs. Since 2015, the Department of Health and Human Services (DHHS) has been in the process of evaluating hair testing as an addition to federal drug testing guidelines, and that process is still ongoing. There was some hope that DHHS would release recommendations in early 2020, but that has not yet materialized.

As this long-anticipated Final Rule regarding drug testing on hair continues to face delays, here are the facts about hair testing.

1. Drug Use Can Be Detected In Hair for A Very Long Time

While urine tests can detect drug use over the previous three days, hair tests uncover drug use over a much more extended period of time, usually at least 90 days. Theoretically, hair tests can reveal drug use over several years. What they can't do is pinpoint the exact day when the drug use occurred. Hair drug tests are also unable to detect recent drug use, as it takes five to seven days for drugs to accumulate in hair follicles.

2. Drugs Bind to Darker Hair in Higher Concentrations

One of the controversies around hair testing is that it could be racially biased. One study revealed that hair tests on non-Caucasians detected 2 to 12 times as much cocaine as it did on their Caucasian counterparts, given the same dose. However, another study found that although cocaine did bind more to dark hair than to light hair, there was no difference between the amount detected on African-American hair and darker hair from other races. Courts have generally ruled that hair tests do not discriminate.

3. You Can't Cleanse Your Hair of Drugs

Despite the numerous products that claim they can wash drugs from your hair follicles, nothing can remove the drugs from your hair other than abstinence. You can safely wash, style and even dye your hair without affecting the outcome of the test.

4. Second-hand Smoke Will Not Affect Hair Testing Results

While this does worry some people and can often be offered up as an excuse for a failed drug test, there is no evidence that inhaling marijuana through secondhand smoke will result in a positive test result. There isn't enough THC in this kind of passive exposure to bind to the hair follicles.

5. Hair Tests Can Identify Prolonged Drug Use Over Time

Unlike urine tests, which can identify the single use of an illicit drug, hair tests provide almost a timeline or a permanent record of drug use. Hair tests are designed to detect long-term drug use, and as hair continues to grow, they can even demonstrate when someone has stopped or started taking drugs. What they can't prove, however, is very recent drug use.

6. Any Hair Will Do

In order to perform the test, technicians require a sample only 1.5 inches long, so in the absence of head hair, they can use body hair if it is the required length. The hair follicle must be "live," however, and come straight from the testing subject's body or head. Hair recovered from a brush, for example, would not work for a hair test.

7. Hair Tests are Nearly Impossible to Cheat

Most of the standard methods people use to cheat a drug test do not work with hair. Chemicals can't mask or remove drugs from the hair follicle or shaft. Participants also can't cheat through substitution since the lab technician harvests the hair sample directly, and there is no need for the privacy that can allow an opportunity for substitution as there is with urine tests.

8. Drugs Aren't Detected in Hair Until 5 to 7 Days After Use

Because it can take up to a week for hair follicles to grow past the scalp, hair drug tests will not be able to determine drug use until 5-7 days after use. This is why they are not used to ascertain recent drug use, as in a post-incident test or similar situations.

9. Hair Testing is Very Accurate

To ensure accuracy and to confirm results, laboratories conduct two tests on approximately 120 strands of the test subject's hair. Before testing the hair, however, technicians screen for contaminants and chemically wash the sample. Because the laboratory technician takes the sample, there is no opportunity for adulteration or substitution as there is in some other methods of drug testing.

10. Hair Testing Can Detect the Presence of Most Drugs

Although hair tests cannot detect alcohol use, they can confirm the presence of many drugs. These include cocaine, cannabis and opiates (including heroin, codeine, morphine, oxycodone, oxymorphone, hydrocodone and hydromorphone). It can also detect amphetamines, including methamphetamines, ecstasy, and phencyclidine (PCP).

11. Hair Testing Can Detect Occasional and Long-Term Drug Use

Metabolites form as drugs are ingested and then processed by the body. They are eventually carried in the bloodstream into the hair follicle and shaft and become part of hair cells as hair grows. Hair testing detects the drug metabolites that become trapped in hair strands. For this reason, it can determine whether drug use is intermittent and occasional or ongoing and long-term.

12. It Doesn't Hurt

Hair is not yanked from the scalp, nor is the follicle disturbed. Generally, a technician will cut the strand of hair very close to the scalp, and it will not be noticeable. It is a quick and painless test.