According to the free online medical dictionary:
http://medical-dictionary.thefreedic....com/Code+Grey
Code Gray
A message announced over a hospital’s public address system, indicating the need for an emergency management response to
(1) A combative person with no obvious weapon
(2) Real or perceived act of terrorism from conventional, nuclear, biological or chemical agents, or other security emergency
(3) Severe weather
(4) Severe air pollution