Biomarkers and pollutants embedded in the hair structure are protected by the hair scales (epidermis). For this reason, all substances present in the hair are protected from external aggressions (UV light, washing, etc.). 

Once synthesised, the hair is no longer biologically active. Thus, the substances present in the hair are not transformed by biological processes (metabolisation).  

For these reasons, the substances incorporated are more stable over time than other biological samples such as blood or urine. 

A strand of hair can therefore be analysed several years, or even decades, after it has been collected.

However, it is recommended that hair analysis be carried out quickly after collection. Even if they are well protected in the hair structure, the substances can still degrade slowly, depending on their chemical nature.