Why do traces of pollution end up in the hair?

Hair is continuously synthesised in the scalp at the root level. It is part of the body's means of eliminating endogenous or exogenous substances from the bloodstream. 

A multitude of blood vessels irrigate the hairline. The bloodstream provides anyone necessary for hair synthesis. During its synthesis in the scalp, the hair incorporates the substances present in the bloodstream into its structure. Thus, hair includes traces of exposure to environmental pollutants and biological markers of the body's functioning. 

As hair grows on average 1 cm per month, each centimetre of hair provides information on the accumulation of toxic substances in the body over a period of 1 month. Hair analysis can therefore be used to establish a person's exposome over a period of several months.