Why is the amount of pollutant in the hair related to the level of exposure?

Hair is a very good source of information about exposure to environmental pollution. Significant exposure to a pollutant results in an increase in its concentration in the blood. It also results in the incorporation of pollutants and their metabolites into the hair at higher levels. 

Several scientific studies on drug use in humans and on pesticide exposure in rats have established with certainty that the amount measured in the hair is proportional to the dose to which the person has been exposed. 

Therefore, a high concentration in the hair indicates a high level of exposure.