TT #82 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for which hairs count as part of the affected area.
Sifra — Vayikra 13, Midrash Sefer
English: “In the lesion” includes a hair whose root is inside the lesion even if its tip extends beyond it. It excludes a hair rooted outside the lesion that merely crosses over it.
Textual application
The hair’s root determines whether it belongs to the lesion for this rule: a hair rooted inside counts even if it extends beyond the edge, while one rooted outside does not count merely because it crosses the lesion.