Why Do Smoke Detectors Contain Radioactive Materials?

One of the most common types of smoke detectors is called an ionization detector. This type of smoke detector contains a small quantity of Americium-241, a radioactive element that emits alpha particles. The alpha particles emitted by the americium ionizes the air molecules (oxygen and nitrogen) allowing electric current to flow between a set of metal plates hooked up to a battery. When smoke is present, the ions attach to the smoke particles and are neutralized. This reduces the electric current between the plates which triggers the detector to sound the alarm.

Before you worry, the alpha particles emitted by the americium do not travel very far in air and are not any danger to you. This is as long as you don't DISTURB the americium. Americium can be very dangerous if inhaled. So do not tinker with the smoke detector or try to examine/play with the americium.