Hi,  I've never gotten a headache from hot hide glue.  The smell is
mild and easy to get used to, even if not pleasant.  However, many
things associated with the use of hide glue give me a headache.

The dust in any part of an old player action is composed of various
metal oxides generated by strings, pins and rods as well as dry rotted
cloth, rubber, tanning agents from decomposed leather, flaking wood
finishes, paints, etc.  Plus a player piano is like a giant lung and
over the decades has taken in everything from mold to coal dust and oil
residue from inefficient house furnaces.  And then there's insect dust,
moth bodies, and mouse droppings.  Don't forget dangerous lead oxide
from the tubing and lint from thousands of piano rolls.

Cracking open an old piano for the first time and taking it apart
for rebuilding has caused me a couple days of headaches and/or sinus
infections.  I learned to use a respirator during breakdown, cloth
stripping and tubing replacement.  In the rebuilding process, think of
the fumes from new cloth, types of volatile glues other than hot glue,
finishes, polishes, wood and glue dust, chemicals, etc., that you
breathe in addition to the hot glue.  I'm suggesting that the culprit
might be some particles or vapor you are exposed to in addition to a
pot of glue.

Tim Gautreaux