It can happen to all of us: you look in the mirror and find you have a bad hair day. This realization might elicit a stress response and you curse your hair for making you look ridiculous. Although it might not be relevant to you at that time, hairs actually contain very important information about chronic stress exposure.

Hair analysis has already been used for decades in the field of (forensic) toxicology: hairs found at crime scenes serve as evidence, and hair samples are widely used to investigate use or abuse of illegal or recreational drugs. Recently, scientific evidence has shown that hairs also contain levels of cortisol, an important hormone that is produced in response to stress. The hair shaft is made up of three layers, from the outside: the cuticle, cortex and medulla. The mechanism of how cortisol enters the hair shaft is still unclear. It has been suggested that cortisol enters the hair at the level of the medulla, through passive diffusion from blood, but sweat and sebum are also likely candidates. Hair has a fairly predictable growth rate of about 1 centimeter per month. Therefore the 1 cm segment closest to the scalp approximates the previous month's cortisol production, the second closest 1 cm segment approximates the production during the month before that, and so on.

Recently, Groeneveld et al analyzed children’s hair-cortisol levels at the time they first went to elementary school. Hair samples were taken 2 months after school entry and divided into two 2-cm sections. The first section had grown out immediately following school entry, reflecting the first 2 months of school attendance. The second section had grown out prior to school entry and was used as a baseline measure. Results showed that the stressful event of starting elementary school caused an increase in hair cortisol level, compared to baseline levels before school entry.

A great methodological advantage of hair-cortisol analysis is that it can be used to measure chronic stress exposure. Cortisol levels after acute stress can be approximated by a single sample of saliva or blood. However, due to daily fluctuations the assessment of long-term (chronic) cortisol exposure is difficult. Hair-cortisol analysis overcomes this methodological problem, as it offers a rather stable retrospective way of measuring cortisol.

However, in the analysis of hair samples certain pitfalls need to be taken into account. First of all, after six months (corresponding to 6 cm) hair segments can no longer be reliably measured due to ‘wash-out’ effects. These wash-out effects can be caused by perming, dyeing, bleaching, or ultraviolet irradiation in prolonged air exposure. Moreover, hair growth rate differs depending on position on the scalp, so that consistent sampling from one location is needed. Finally, there are ethnic differences in hair growth rate, with African hair growing the slowest and Asian hair growing the fastest (and Caucasian hair somewhere in between). The good news is that natural hair color or hair-washing frequency does not seem to affect hair cortisol levels.

To conclude: try not to stress too much about bad hair days. Heightened cortisol levels might haunt you for a long time.