If an adult is not breathing, the immediate action that should be taken is to start cardiopulmonary resuscitation (CPR). Before starting CPR, one should ensure that the person is unresponsive and make sure that they are lying flat on their back on a hard surface.
To initiate CPR, follow these steps:
1. Call for emergency medical services (EMS) immediately.
2. With the person lying on their back, place the heel of one hand in the center of the chest and the other hand on top of the first.
3. Interlock fingers and apply pressure to the chest using the heel of the hand, pushing down at least 2 inches.
4. Deliver 30 chest compressions at a rate of 100-120 per minute.
5. Open the airway by tilting the head back and lifting the chin.
6. Pinch the nose shut and give two breaths into the mouth, watching for the chest to rise with each breath.
7. Repeat the cycle of 30 compressions and 2 breaths until help arrives or the person starts breathing on their own.
Performing CPR can help maintain the flow of oxygenated blood to vital organs and improve a person's chances of survival in cardiac arrest situations when done correctly and promptly.