Mikvaos 9:3

The following are not interpositions: matted hair on the head, in the armpits and on a man’s genitals. Rabbi Eliezer says the rule is the same for men and for women: if one finds something annoying, it’s an interposition; if one doesn’t find it annoying, it’s not an interposition.

Mikvaos 9:4

More non-interpositions include: secretions inside the eye; the scab on a wound; wet juice; moist excrement on one’s body; dirt under one’s nail; a dangling fingernail; the downy hair of a child. These neither contract nor convey ritual impurity. A membrane that forms over a wound both contracts and conveys ritual impurity.