Hair Colour, Texture and Style: Understanding Racial Bias in Hair Drug Testing

Hair drug testing is widely used in family law and safeguarding cases to help assess patterns of substance use or exposure over time. However, interpreting the findings involves more than examining the concentrations of a substance detected in a sample.

Individual hair characteristics, including natural colour and texture, can influence how certain drugs become incorporated into hair and how samples are prepared for analysis. These differences are important because two people with similar patterns of drug use may not necessarily produce comparable findings.

If these factors are overlooked, particularly when results are assessed against standardised cut-off levels such as the Society of Hair Testing (SoHT) published levels, there is a risk of misleading conclusions and potential racial bias.

At Forensic Testing Service (FTS), our forensic toxicologists consider relevant individual characteristics alongside the analytical findings, providing expert interpretation that helps legal and safeguarding professionals understand what the evidence can, and cannot, establish.

How Hair Colour Can Influence Drug Test Findings

Natural hair colour is determined by pigments called melanin, which can influence how certain drugs become incorporated into hair.

Some substances, including cocaine, have an affinity for melanin, meaning they can bind to these pigments. As darker hair generally contains higher concentrations of melanin, two individuals with similar patterns of drug use may produce different measured drug concentrations if one has dark brown or black hair and the other has blonde or red hair.

This is an important consideration when interpreting hair drug testing findings. A higher concentration does not necessarily indicate more frequent or heavier drug use, and relying on numerical cut-off levels alone may lead to misleading conclusions.

However, not all drugs interact with melanin in the same way. Cannabis, for example, has little to no affinity for melanin, meaning that natural hair colour is not expected to influence its incorporation into hair in the same way as drugs such as cocaine.

Chemical hair treatments, including bleaching and dyeing, can affect the quality of the sample, alter the structure of the hair and influence the concentrations of drugs detected during analysis. These treatments can limit the specificity of our interpretations, as reduced concentrations may not accurately reflect the extent of drug use. In some circumstances, minimal drug use may go undetected, while more frequent drug use may be ‘masked’ by the effects of chemical hair treatments.

The importance of considering these factors is also reflected in Re H (A Child) (Hair Strand Testing) [2017] EWFC 64, which emphasises the need for laboratories to report all findings and for hair drug testing results to be understood within the full context of the case. This includes recognising how individual characteristics, such as hair colour and condition, may influence the findings. It is at the interpretation stage that these details matter.

At FTS, our forensic toxicologists consider relevant hair characteristics and cosmetic treatments when interpreting analytical findings, helping to ensure that conclusions reflect the individual circumstances of the case.

to ensure that conclusions reflect the individual circumstances of the case.

Hair Texture and Segmented Hair Analysis

Hair texture is another important consideration when interpreting hair drug testing findings, particularly when segmented hair analysis is used to assess patterns of drug use over time.

At FTS, we frequently receive questions about whether differences in hair texture affect growth rates, particularly whether Afro-Caribbean hair grows more slowly than naturally straight hair. Our reporting scientists explain that this is a common misconception, with no statistically significant difference in average scalp hair growth rates between naturally straight and curly or coiled hair. However, the perception may arise from the challenges of straightening and accurately measuring coiled hair when preparing samples for segmented hair analysis.

Segmented hair analysis involves dividing a scalp hair sample into smaller sections, each representing an approximate period of growth. Scalp hair can be segmented on an approximate month-by-month basis, allowing changes in drug use or exposure to be assessed over time. However, individual segments cannot establish an exact date or point in time when drug use occurred.

During sample preparation, hair samples must be kept as straight as possible to ensure each section is uniform in length and can be measured precisely. However, this becomes more challenging with curly or coiled hair, which can be difficult to keep straight throughout the process. In some cases, it may not be possible to straighten coiled hair completely without using chemical or heat treatments that could damage the sample.

As a result, sections of coiled hair may represent a slightly longer period than equivalent sections of naturally straight hair. This difference is taken into account during our expert interpretation of the test results, recognising that the timeframes presented in our results tables are approximate and may be influenced by the challenges of measuring coiled hair during segmentation.

Although curly or coiled hair occurs across all racial and ethnic backgrounds, this limitation of the sample preparation process may disproportionately affect people of colour, particularly Black individuals. Importantly, the potential impact relates to the estimated detection windows rather than the analytical findings themselves and is accounted for at FTS during expert interpretation.

How Hairstyles Can Influence Hair Drug Testing

Hairstyle is another factor that can influence the interpretation of hair drug testing findings. How hair is worn, particularly when it is covered or locked in place, can affect how easily drugs contaminate the hair externally and whether hair that would ordinarily shed naturally is retained. These differences are important considerations when assessing drug concentrations and patterns of use over time.

For example, when hair is worn in dreadlocks or locs, individual hair strands that would ordinarily shed over time can remain locked within the hairstyle. This means that locs may contain a higher proportion of non-growing hair than would typically be expected, which is relevant when considering the impact of a process known as carry-over.

At any given time, approximately 85–90% of scalp hair is actively growing, while the remaining 10–15% is generally in a dormant phase, known as the telogen phase. Carry-over can occur when drugs and their metabolites from previous use remain present in non-growing scalp hair. This means that some hair within a sample may reflect drug use from an earlier period than the actively growing hair surrounding it.

Normally, hair in the telogen phase eventually enters the exogen phase, when it is naturally shed. However, when hair is worn in locs, these non-growing hairs may remain trapped within the hairstyle rather than falling away.

As a result, a sample containing a higher proportion of non-growing hair can create a bias towards more historic periods of drug use than would otherwise be expected. This potential influence on the drug profile is considered by FTS forensic toxicologists during expert interpretation, ensuring that findings are assessed in the context of the individual's hairstyle and hair characteristics.

Why Expert Interpretation Matters

Hair drug testing findings require careful interpretation by an experienced forensic toxicologist. While results tables provide important analytical information, they do not always present the full evidential picture. Comparing drug concentrations between individuals, or even between samples from the same person, without considering the factors that may influence those findings can lead to misleading conclusions.

For example, a social worker comparing two sets of hair drug test results would not necessarily be expected to recognise the full evidential picture or understand every factor that could influence the interpretation of those findings.

This is why expert interpretation is so important. At Forensic Testing Service (FTS), our forensic toxicologists draw on scientific evidence, statistical analysis and years of experience to provide informed opinions that consider the individual circumstances of each case. While these opinions are not irrefutable, they help explain what the analytical findings can, and cannot, establish.

Hair drug testing is a reliable forensic technique, but interpreting the findings requires more than examining numerical concentrations or applying standardised SoHT cut-off levels. There is a misconception that hair testing is ‘not an exact science’, when the reality is that forensic toxicology is a complex field requiring specialist knowledge and expertise beyond the raw numbers presented in a results table. The analytical testing provides scientific findings, while our expert conclusions are formed on the balance of probabilities, taking account of the circumstances and limitations relevant to each case.

As this article demonstrates, natural hair colour, texture and hairstyle can all influence the findings or the periods represented by a sample. These are just some of the factors considered during expert interpretation and overlooking them can introduce bias into the conclusions drawn.

A one-size-fits-all approach risks disproportionately affecting people of colour, particularly where differences in drug incorporation or sample preparation are not adequately considered. For example, two individuals with different natural hair colours may have similar patterns of cocaine use but produce different measured drug concentrations. Without expert interpretation, someone with darker hair could be incorrectly assessed as having used cocaine than someone with lighter hair, simply due to exposure. similar levels of use. Equally, the findings for the individual with lighter hair could lead to an underestimation of their drug use. Both outcomes can have significant implications for families involved in legal and safeguarding proceedings.

This is why FTS advocates for reliable, evidence-based and equitable interpretation of hair drug testing findings. The results tables alone do not tell the full story.

Frequently Asked Questions About Hair Drug Testing

Does natural hair colour affect drug test findings?

Yes, natural hair colour can influence the concentrations of certain drugs detected in hair. For example, cocaine can bind to melanin, which is generally present in higher concentrations in darker hair. This means that individuals with similar patterns of drug use may produce different measured concentrations, making expert interpretation important.

Does curly or coiled hair grow more slowly than straight hair?

No. It is a common misconception that curly or coiled hair grows more slowly than naturally straight hair. According to our reporting scientists, there is no statistically significant difference in average scalp hair growth rates between these hair types. However, curly or coiled hair can be more challenging to straighten and measure during segmented hair analysis, which may affect the approximate periods represented by each section. This is considered during expert interpretation.

Can hairstyles affect hair drug testing?

Yes. Dreadlocks or locs can retain hair that would otherwise naturally shed, potentially increasing the proportion of non-growing hair within a sample. This can introduce a bias towards more historic periods of drug use, which forensic toxicologists should consider when interpreting the findings.

Need Expert Advice on Hair Drug Testing?

At Forensic Testing Service, our forensic toxicologists provide expert interpretation of hair drug testing findings, considering individual characteristics and the wider circumstances of each case. If you have questions about a test report or need guidance on the most appropriate testing approach, contact our team at This email address is being protected from spambots. You need JavaScript enabled to view it. or call 01924 480272.