Small arms primers contain an initiating explosive mixture. By design, this explosive is easily initiated by impact energy. Two key safety issues with primers are dusting and mass detonation.

“Dusting” is the term describing minute particles of the primer explosive mix. The dust from a primer contains the same initiating and explosive qualities of the primer itself, including initiating sensitivity to friction (impact). At the time of manufacture, the explosive mixture of the primer has a binding ingredient. After manufacture, handling of the primer can result in some of the mixture breaking apart, forming explosive dust. Over time the primer dust can form a thin layer in and around the handling and storage areas of primers. Friction or spark can initiate the dust. Proper cleaning is the best way to avoid a problem. 

A mass quantity of primers loose in a container can detonate with the power of the total explosive quantity of the individual primers. If an initiating force ignites just one primer, the explosive energy can create a "mass detonation" of all the primers.

When primers are manufactured, they are placed directly into packaging designed to prevent propagation and mass detonation. Always store primers in their original containers. When reloading, limit the quantity of primers you remove from the factory packaging.