Charles Ellicott Commentary Leviticus 13:30

"then the priest shall look on the plague; and, behold, if the appearance thereof be deeper than the skin, and there be in it yellow thin hair, then the priest shall pronounce him unclean: it is a scall, it is leprosy of the head or of the beard." — Leviticus 13:30 (ASV)

Behold, if it be in sight deeper. — Better, Behold, if its appearance is deeper. The first symptom of its existence is the same as usual—the depression of the affected spot.

And there be in it a yellow thin hair. — While the symptom of leprosy in other parts of the body consisted of the hair turning white on the affected spot, in the case of this disease breaking out on the head or chin, it was indicated by the naturally black hair changing its colour into a golden appearance and becoming short. According to the administrators of the law during the Second Temple, the colour of the hair became like that of the new plumage of young pigeons after they have lost their first feathers, or, in other words, like the appearance of thin gold.

By the “thin hair,” those authorities who came in contact with the disease understood small or short hair. Hence, they laid down the following rule: The condition of the hair constituting one of the signs of leprosy is its becoming short; but if it is long, though it is as yellow as gold, it is no sign of uncleanness. Two yellow and short hairs—whether close to one another or far from each other, whether in the center of the affected spot or on its edge—regardless of whether the diseased spot precedes the yellow hair, or the yellow hair precedes the diseased spot—are symptoms of uncleanness.

It is a dry scall, even a leprosy. — The ancient canons laid down by those authorities in the time of Christ, who were responsible for the official treatment of this disease, define the word (nethek), which is translated “dry scall,” as a condition on the head or chin that causes the hair on those affected parts to fall out by the roots, so that the place where the hair was is left completely bare.