There does not seem to be much about helmets as far as armor class or items PCs can purchase. However, once in a while the adventurers will stumble on a really nice helmet, possibly magical… and the Druid may want to wear it (maybe it offers a +1 to AC.)
I'm not sure if helmets count as armor since they don't appear in the table of armor items. Would a Druid ever willingly wear a helmet made of metal? Are helmets considered part of the armor?
Best Answer
According to a dictionary:
As you can read the first definition, a helmet is armor.
If you are still unsure, the description of Plate might be helpful:
All of the mentioned pieces make the suit of armor, and every piece is armor itself.
According to page 65 of Player's Handbook: Druids will not wear armor or use shields made of metal.
Furthermore, accordingly to an email from Wizards:
This means:
A druid will not wear a metal helmet