An air ionizer is a device which uses high voltage to ionize, or electrically charge molecules of air. These machines can be designed either to generate specifically charged ions (all positive or all negative), or to create both polarities indiscriminately. Allergens such as dust, bacteria, mold, smoke etc which contains positive charge are bonded by negative ions produced by the ionizer. Once the particles have been bonded (opposites attract), it becomes too heavy to float around which can easily be inhaled. As a result, they become large pieces of dirt on the ground where they can be cleaned by normal means.
Studies had shown nature surroundings such as mountains and waterfalls produce large amounts of negative ions. It is no coincidence that throughout history, mankind has gone to these places to rest and recuperate, particularly from respiratory diseases.