"...3. When contemplating an award of this decoration, the key issue that commanders must take into consideration is the degree to which the enemy caused the injury. The fact that the proposed recipient was participating in direct or indirect combat operations is a necessary prerequisite, but is not sole justification for award. 4. Examples of enemy-related injuries which clearly justify award of the Purple Heart are as follows: (a) Injury caused by enemy bullet, shrapnel, or other projectile created by enemy action. (b) Injury caused by enemy placed mine or trap. (c) Injury caused by enemy released chemical, biological, or nuclear agent. (d) Injury caused by vehicle or aircraft accident resulting from enemy fire. (e) Concussion injuries caused as a result of enemy generated explosions. 5. Examples of injuries or wounds which clearly do not qualify for award of the Purple Heart are as follows: (a) Frostbite or trench foot injuries. (b) Heat stroke. (c) Food poisoning not caused by enemy agents. (d) Chemical, biological, or nuclear agents not released by the enemy. (e) Battle fatigue. (f) Disease not directly caused by enemy agents. (g) Accidents, to include explosive, aircraft, vehicular, and other accidental wounding not related to or caused by enemy action. (h) Self-inflicted wounds, except when in the heat of battle, and not involving gross negligence. (i) Post-traumatic stress disorders. (j) Jump injuries not caused by enemy action. 6. It is not intended that such a strict interpretation of the requirement for the wound or injury to be caused by direct result of hostile action be taken that it would preclude the award being made to deserving personnel. Commanders must also take into consideration the circumstances surrounding an injury, even if it appears to meet the criteria..." More examples at link.