Hair grows on average 1 cm per month. The root of a hair is about 1/2 cm deep in the scalp. This 5 mm segment is not accessible during hair collection and it will take at least 15 days after exposure for the traces of exposure to be present in the collected hair. It is therefore recommended to wait at least 1 month after the exposure date to perform a hair analysis.
For example, for a hair sample taken on 15 July :
- The segment from 0 cm (scalp level) to 1 cm corresponds to the period from 1st June to 1st July
- The segment from 1 to 2 cm corresponds to the period from May 1 to June 1
- The segment from 2 to 3 cm corresponds to the period from April 1 to May 1