Abstract: An indoor air purifier has a container with a layer of soil supported above a lower air plenum. Room air is drawn downwardly through the layer of soil and discharged from the plenum back into the room. Contaminant gases in the air are adsorbed in the soil and metabolized by microorganisms, producing carbon dioxide and water, and substantially reducing noxious and toxic materials in the air. The soil may be divided into two layers, and the air purifier operated in either of two modes. During normal operation, air is drawn through both layers of soil for removal of particles and destruction of contaminants by microorganisms. Alternatively, air may be drawn through only one layer of soil at a substantially higher flow rate for removing particulate materials. Plants may be grown in the upper portion of the soil to aid in air purification and to provide an attractive room decor.