It is up to the final judgment of the individual making the decision. Not ALL things that are called emergencies are GENUINE emergencies.
It depends on the nature of the emergency. A more accurate question is needed here.
Depending on the emergency you will either give advanced life support, evacuate the area, tend to people in need, shut off your gas supply to your house, stay away from power lines, get to higher...
Something that increases human alertness, causing one to panic, can include natural disasters, etc.
Only when instructed by a senior ranking officer (be it fire/ambulance or police)