What constitutes a CHAZIZAH regarding tefillin? Is a spec of dust, dandruff, loose skin particle? What about water after a shower remaining on your hair? Please be elaborate. Does it mater where the chazizah is?
A chatzitzah is something that comes between the Tefillin and the arm or the head.
A skin particle of fleck or dandruff is not a chatzitzah, because it does not form a ‘partition’ that separates the Tefillin from the body. A partition is only something that isn’t naturally on the skin, and that a person cares about (and would remove it from his bare skin). See also Shulchan Aruch Orach Chaim 28, together with commentaries (Magen Avraham, Shulchan Aruch Harav, Mishnah Berurah) concerning putting Tefillin on a bandage.
Some say that water after the shower or mikva can constitute a chatzitzah (Birkei Yosef), and a person should make sure to dry his hair before putting on Tefillin – also for fear of damaging the Tefillin. However, this is something of a Chiddush, and there is no need to worry about hair that is only damp.