Factors influencing levels of trace elements in human hair.

Hair, as a biological tissue, is unique in that it remains isolated from human metabolic activities and indicates concentration profiles of elements in an individual at a particular time period. However, hair concentrations of elements are affected by different factors that are also specific to certain regions and subjects. Many causal factors have been reported for element level changes in hair; however, in the future more new factors are likely to be reported. All the factors causing change in the hair levels of elements are grouped under four main categories: biological factors, personal factors, environmental factors, and analytical and methodological factors. Comparison of results of element levels in hair for certain factors shows different (positive, negative, or specific) trends because of regional variation. Regarding certain factors (age, sex, health, occupation, etc.), the influence causing the change in element levels is obvious, whereas the influence of other factors (structure of hair, height and weight of the subject, etc.) is obscure. It is very important to consider all the factors at the time of investigation for effective interpretation, validity, and application of results of hair analysis.