About 2.5 million AEDs are scattered across a country of more than 300 million people; experts calculate that 30 million of the devices are needed just to cover all major metropolitan areas.

And even if your office building or church is equipped with an AED, you might not know it. Unlike fire extinguishers, which are a familiar sight in buildings, AEDs often are not prominently visible and identifiable, and are frequently kept under lock and key.