Excellent question! You’ve got to keep that noggin protected.

The official response:

Section 11 of The Occupational Safety General Regulations states:

Hazard to head

11 Where a person is exposed to a hazard that may injure the person's head, an employer shall ensure that protective equipment is worn that is appropriate to the hazard and that complies with CSA Standard
(a) CAN/CSA-Z94.1-92 (R1998), "Industrial Protective Headwear"; or Clause 11(a) amended: O.I.C. 2000-130, N.S. Reg. 52/2000. ”
 

More information is available in the Personal Protective Equipment: A guide to Part 3 of the Occupational Safety General Regulations.