Chemical hair analysis may be considered for retrospective purposes when blood and urine are no longer expected to contain a particular contaminant, typically three months or less.

Hair testing is an increasingly common method of assessment in substance misuse, particularly in legal proceedings, or in any situation where a subject may have decided not to tell the entire truth about his or her substance-using history.

[13] Microscopic hair analysis has a long tradition of being used in crime fiction as well; it was originally popularized in the Sherlock Holmes series before being widely used by the police.

[14] In 1994, the Justice Department created a task force which would eventually review 6,000 cases by 2004, focusing on the work of one particularly zealous examiner, Michael Malone.

[12] These reviews came after reports that sloppy work by examiners at the FBI lab was producing unreliable forensic evidence in court trials.

[12] Kirk L. Odom was convicted of rape in Washington, DC in 1982 by no physical evidence except microscopic hair analysis performed by the FBI Crime Laboratory.

[12] Combined with a witness's identification in a line-up (another technique which modern research has shown to be much less reliable than previously thought), Odom was sentenced to twenty or more years in jail.

By 2015, these cases included as many as 32 death penalty convictions, in which FBI experts may have exaggerated the reliability of hair analysis in their testimony and affected the verdict.

The review concluded that in 257 of these 268 trials (95 percent), the analysts gave flawed testimony in court that overstated the accuracy of the findings in favor of the prosecution.

"[18] In 2017, new Attorney General Jeff Sessions, appointed by President Donald Trump, announced that this investigation would be suspended, at the same time that he announced the end of a forensic science commission that had been working to establish standards on several tests and to improve accuracy; it was a "partnership with independent scientists to raise forensic science standards".

[20] Independent scientists, prosecutors, defense counsel and judges criticized ending the commission, saying that the criminal justice system needed to rely on the best science.

[21] Another notable case that received media attention since was Anthony Broadwater, who had been convicted of raping Alice Sebold in 1982, and was formally exonerated in 2021 after finishing his time in 1998.

[22] Analysis of hair samples has many advantages as a preliminary screening method for the presence of toxic substances deleterious to health after exposures in air, dust, sediment, soil and water, food and toxins in the environment.

The advantages of hair analysis include the non-invasiveness, low cost, and the ability to measure a large number of, potentially interacting, toxic and biologically essential elements.

Hence, head hair analysis is increasingly being used as a preliminary test to see whether individuals have absorbed poisons linked to behavioral or health problems.

Several studies, including the analysis of Ludwig van Beethoven's hair, have been conducted in conjunction with the National Institutes of Health and Centers for Disease Control and Prevention.