Hair analysis may refer to the chemical analysis of a hair sample, but can also refer to microscopic analysis or comparison. 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. Its most accepted use is in the fields of forensic toxicology and in environmental toxicology. This can be used to test for the presence of certain drugs in a subject's system, such as during a pre-employment screening. Several alternative medicine fields also use various hair analyses for environmental toxicology, but these uses are controversial, evolving, and not standardized. Microscopic hair analysis has traditionally been used in forensics as well, which goes beyond simple comparison of hair color. Analysts examine a number of different characteristics of hairs under a microscope, usually comparing hair taken from a crime scene and hair taken from a suspect. However, DNA testing of evidence has cast microscopic hair analysis into disrepute. DNA analysis has shown many claimed hair matches were in fact faulty, even ones conducted by what the field considered experts. This led to the overturning of many convictions that relied on hair analysis. Since 2012, the US Department of Justice has conducted a study of cases in which hair analysis testimony was given by its agents, and found that a high proportion of testimony could not be supported by the state of science of hair analysis.
Biology of hair Hair consists of various characteristics that can be of use in a forensic examination and analysis. To the core, hair is composed of keratin, water, minerals, and lipids. Hair consist of two distinctions parts, the hair shaft and the hair root. The shaft is composed of 3 layers, first being the medulla which is the inner core (if present), the second being the cortex which is the middle layer composed of multiple cells, and then lastly the cuticle which is the outer layer. The hair root is the bulbous portion of the hair shaft that resides in the hair follicle with the surrounding cells in the hair follicle that allow for production of hair and its growth. There are three phases of hair growth; anagen, catagen and telogen. In the anagen phase, the hair root actively creates the hair shaft. In the catagen and telogen phases, it regresses and gets ready for the following cycle. As hair shedding is a natural process, hairs tend to be found in the anagen phase when collected for analysis rather than the regression phase . This can cause issues in analysis as the follicular tag which is located on the hair follicle must be present to be able to analyze it with DNA analysis. If no follicular tag is present, the hair is sent for mitochondrial DNA (mt-DNA), as this is the only way to truly identify there source of hair. In theory, all human hairs are suitable sources for mitochondrial DNA.
In forensic toxicology Chemical hair analysis is used for the detection of many therapeutic drugs and recreational drugs, including cocaine, heroin, benzodiazepines and amphetamines. Hair analysis is less invasive than a blood test, if not quite as universally applicable. In this context, it has been reliably used to determine compliance with therapeutic drug regimes or to check the accuracy of a witness statement that an illicit drug has not been taken. 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. Post-mortem hair sample analysis can also be performed, to allow for determination of long-term drug use or poisoning. It is also used by employers, who test their employees. Hair analysis has the virtue of showing a 'history' of drug use due to hair's slow growth. Urine analysis might detect drugs taken in the past 2–3 days; hair analysis can sometimes detect use as far as 90 days, although certain cosmetic treatments (e.g. dyeing hair) can interfere with this. Notably, basic drugs get incorporated into hair to a greater extent than neutral or acidic drugs, e.g. amphetamines and cocaine are present in higher concentrations in hair compared with benzodiazepines and cannabinoids. Large-scale drug screening (or urine, hair and other samples) is usually done using enzyme-linked immunosorbent assays (ELISA). Positive ELISA findings are followed by confirmatory testing with liquid chromatography–mass spectrometry (LC-MS) or gas chromatography–mass spectrometry (GC-MS). Chromato-mass-spectrometry is less likely to result in false positive findings than ELISA, but the former requires expensive equipment and highly trained personnel. The judicial admissibility of the test in the United States is guided by the Daubert standard. A notable court case was United States v. Medina, 749 F.Supp. 59 (E.D.N.Y.1990).
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